The provisions of these Specifications are intended for the design, evaluation and rehabilitation of highway tunnels. These Specifications are intended for the design of tunnels constructed using cut-and-cover, bored, mined and immersed tunnel construction methodologies.
Provisions are not included in these Specifications for water conveyance, utility, transit or rail tunnels or for shafts. For tunnel elements not explicitly covered herein, the provisions of these Specifications may be applied, as augmented by the Engineer with additional design criteria where required.
Construction specifications consistent with these design Specifications are not included. There is a listing of suggested construction specification sections included in Appendix B.
Structures internal to tunnels that support roadways over ventilation plenums, roadways or other openings in the tunnel shall be designed in accordance with the AASHTO LRFD Bridge Design Specifications (hereafter referred to as the LRFD Specifications) including all applicable interim changes and as modified or supplemented herein. The load effects of these internal structures shall be applied to the tunnel lining, walls or other supporting members in accordance with these Specifications.
Retaining walls for retained cut approaches to tunnels shall be designed in accordance with the LRFD Specifications.
Support and ancillary structures such as ventilation, control and administrative buildings are not covered by these Specifications. These structures shall be designed in accordance with local building codes.
These Specifications are not intended to supplant proper training and experience or the exercise of judgment by the Engineer, and provide only the minimum requirements necessary for public safety. The Owner or the Engineer may require the sophistication of design or the quality of materials and construction to be higher than the minimum requirements. The design of tunnels is strongly dependent upon the geologic setting, site conditions and construction methodology, and this fact is considered in the Specifications. The concept of ground/structure interaction is emphasized for mined and bored tunnels; however, it is also applicable to cut-and-cover and immersed tunnels.
The concept of safety through redundancy and ductility is emphasized for tunnel elements subject to repeated loads and load reversals.
The design provisions of these Specifications employ the Load and Resistance Factor Design (LRFD) methodology. The load factors have been calibrated using structural analysis modeling for a limited number of loading conditions that take into
C1.1
These Specifications are modeled after the LRFD Specifications and the AASHTO Guide Specifications for LRFD Seismic Bridge Design. The philosophy and guidance provided in those documents are carried forward and implemented in this document.
Whenever the LRFD Specifications are referenced in this document, the reference is to the latest edition including all applicable interim changes.
The term “notional” is often used in these Specifications to indicate an idealization of a physical phenomenon, as in “notional load” or “notional resistance.” Use of this term strengthens the separation of an engineer’s “notion” or perception of the physical world in the context of design from physical reality itself.
The term “shall” denotes a requirement for compliance with these Specifications.
The term “should” indicates a strong preference for a given criterion.
The term “may” indicates a criterion that is usable, but other local and suitably documented, verified and approved criterion may also be used in a manner consistent with the LRFD approach to tunnel design.
The load factors specified in Section 3 of these Specifications have been calibrated to provide designs with member proportions consistent with the current practice in tunnel design.
The calibration is based upon analyses performed for a circular bored tunnel. Additional calibration for different tunnel cross-sectional geometry and loadings from a variety of ground conditions would be useful in further validating and refining the load factors.
The primary loads on structural components of tunnels are groundwater and earth loads. For immersed tunnels, loads imposed by transporting
account ground/structure interaction. The commentary is not intended to provide a
complete historical background concerning the development of these Specifications, nor is it intended to provide a detailed summary of the studies and research data reviewed in formulating the provisions of the Specifications. However, references to some of the research data are provided for those who wish to study the background material in more depth.
The commentary directs attention to other documents that provide suggestions for carrying out the requirements and intent of these Specifications. However, those documents and this commentary are not intended to be a part of these Specifications.
The Specifications direct the Engineer to utilize other documents in the development of designs. When this occurs, the most current edition of those documents should be utilized. Those documents referenced in the Specifications are intended to be part of these Specifications by reference.
Unless otherwise specified, the Materials Specifications referenced herein are the AASHTO Standard Specifications for Transportation Materials and Methods of Sampling and Testing.
immersed elements from the fabrication site to the tunnel location can also govern the design of these tunnels. For pre-fabricated linings used in bored tunnels, construction-imposed loading can govern the design. The determination of groundwater, earth, transportation and other construction loads varies based on the in-situ conditions, level of testing during subsurface investigations and ground conditions that may have great variation. There are little data available to establish a statistically significant sampling in order to calibrate these Specifications based upon structural reliability theory. As such, judgment and past experience were also used to select the load factors.
These Specifications are an initial attempt to codify and standardize highway tunnel design. As such, as future data that is produced in a systematic fashion in accordance with these Specifications becomes available, recalibration may be implemented based on statistical evaluation of these data.
For definitions not shown, see the LRFD Specifications.
Bored Tunnel – A tunnel constructed utilizing a tunnel boring machine.
Calibration – The selection of load and resistance factors to achieve a specified goal such as uniform reliability, as is the case with the bridge design specifications, or member proportions consistent with past practice, as is the case with these Specifications.
Collapse – A major change in the geometry of the tunnel lining or other structural component rendering it unfit for use.
Component – Either a discrete element of the tunnel or a combination of elements requiring individual design consideration.
Contract Documents – Drawings, specifications, reports and memoranda that provide direction and/or guidance for the construction of a tunnel and that form a contractual basis for the work to be performed.
Contractor – Entity responsible for the construction of the tunnel and associated construction engineering.
Cut-and-Cover – Sequence of construction in which a trench is excavated and the tunnel or conduit section is constructed and then covered with backfill. (AASHTO, 2010)
Design – Proportioning and detailing the components and connections of a tunnel.
Design life – Period of time on which the statistical derivation of transient loads is based: 150 years for these Specifications.
Engineer – Agency, design firm or person responsible for the design of the tunnel and/or review of design related to field submittals.
Evaluation – Determination of the load carrying capacity of a component or components of an existing tunnel.
Extreme Event Limit States – Limit states relating to events such as earthquakes, flooding, vehicle fire, vehicle and vessel collision, with return periods in excess of the design life of the tunnel.
Force Effect – A deformation, stress or stress resultant (i.e., axial force, shear force, torsional or flexural moment) caused by applied loads, imposed deformations, temperature changes or volumetric changes.
Immersed Tunnel – A tunnel constructed from prefabricated elements constructed off the tunnel alignment, floated into place over the tunnel alignment and placed into a prepared trench. Placement is facilitated by the addition of ballast to the elements to cause them to be immersed to the pre-determined depth and then joined to the adjacent element(s) already in place.
Limit State – A condition beyond which the tunnel or component ceases to satisfy the provisions for which it was designed.
Load Modifier – A factor accounting for ductility, redundancy and the operational classification of the tunnel.
Mined – Any tunnel construction methodology that involves excavation of the tunnel without opening the excavation to the surface and without the use of a tunnel boring machine, including mechanical excavation, blasting and hand excavation.
Model – An idealization of a structure or structure – ground system for the purpose of analysis.
Owner – Person or agency having jurisdiction over the tunnel.
Regular Service – Condition excluding the presence of special permit vehicles and extreme events.
Rehabilitation – A process in which the resistance or functionality of a tunnel component or connection is either restored or increased.
Resistance Factor – A statistically or experience-based multiplier applied to nominal resistance accounting primarily for variability of material properties, structural dimensions and workmanship, an uncertainty in the prediction of resistance, but also related to the statistics of the loads through the calibration process.
Service Life – The period of time that the tunnel is expected to be in operation.
Tunnel – Road tunnels as defined by the American Association of State Highway and Transportation Officials (AASHTO) Technical Committee for Tunnels (T-20) are enclosed roadways with vehicle access that is restricted to portals regardless of type of the structure or method of construction. The committee further defines road tunnels not to include enclosed roadway created by highway bridges, railroad bridges or other bridges. This definition applies to all types of tunnel structures and tunneling methods such as cut-and-cover tunnels, mined and bored tunnels in rock and soft ground, and immersed tunnels.
Tunnel Boring Machine (TBM) – Machine that excavates a tunnel by drilling out the heading to full size in one operation. Sometimes called a mole, the TBM is typically propelled forward by jacking off the excavation supports emplaced behind it or by gripping the side of the excavation (AASHTO, 2010).
Tunnels shall be designed for specified limit states to achieve the objectives of constructability, safety and serviceability, with due regard to issues of inspectability, maintenance and economy. Additional information regarding tunnel systems, planning, ancillary facilities and appurtenances can be found in Section 2.
Regardless of the type of analysis used, Equation 1.3.2.1-1 shall be satisfied for all specified force effects and combinations thereof.
The specified 150-year design life is appropriate for the design of tunnel geotechnical features and soil-structure-interaction-systems given the high capital costs of rehabilitation and replacement and the likely importance to the transportation network. Internal structures such as roadway slabs and suspended ceilings as well as system components, such as signs, piping and their supports; communication and signal devices and ventilation equipment that are more easily replaced, may have design lives assigned to them by the Owner.
C1.3.1
The limit states specified herein are intended to provide for a buildable, serviceable tunnel capable of safely operating for a specified design life. As defined in Article 1.2, the design life relates to the return period of the transient loads of the strength limit states and hence their nominal magnitude. The design life should not be confused with the service life. As defined in Article 1.2, the service life relates to the eventual demonstrated durability of the tunnel. The service life of a tunnel is not specified in these Specifications, just as the service life of a bridge is not specified in the LRFD Specifications, as the durability of tunnels or bridges is not well quantified.
The resistance of components and connections is determined, in many cases, on the basis of inelastic behavior. In other words, the capacity of tunnel components used to define their nominal resistance at the strength limit states is based upon behavior past first yield of the material. On the contrary, the force effects on the load side of the LRFD equation, Equation 1.3.2.1-1, are determined using elastic analysis but amplified by the specified load factors. This apparent inconsistency is consistent with most modern structural-design codes including the LRFD Specifications. The application of this comparison of loads and resistances for design is a result of incomplete knowledge of inelastic structural action combined with the behavior of the earth surrounding the tunnel that acts in concert with the tunnel structure.
1.3.2 Limit States
1.3.2.1 General Each component and connection shall satisfy
Equation 1.3.2.1-1, for each limit state unless otherwise specified. For service and extreme limit states, resistance factors shall be taken as 1.0 except for bolts. For bolts, the provisions of Article 6.5.5 of the LRFD Specifications shall apply.
ΣηiγiQi ≤ φRn = Rr (1.3.2.1-1)
in which: For loads for which a maximum value of γi is appropriate:
ηi = ηDηRηI ≥ 1.0 (1.3.2.1-2) For loads for which a minimum value of γi is appropriate:
η𝑖 = 1
η𝐷η𝑅η𝐼 ≤ 1.0 (1.3.2.1-3)
where:
C1.3.2.1 Equation 1.3.2.1-1 is the basis of the LRFD
methodology. Ductility, redundancy and operational classification
are considered in the load modifier η. Whereas the ductility and redundancy directly relate to physical strength, operational classification concerns the consequences of the tunnel being out of service. The grouping of these aspects of the load side of Equation 1.3.2.1-1 is therefore, arbitrary. However, it constitutes a first effort at codification. In the absence of more precise information, each effect is estimated as ±5 percent, accumulated geometrically, a clearly subjective approach.
Groundwater is an example of a loading that would be appropriate to apply as a maximum and a minimum. Variation in groundwater elevations are common and due to seasonal changes and tidal influences. Tunnel linings are designed as compression members; therefore, maximum groundwater pressures would produce maximum axial loads and vice versa. As such, both maximum and minimum groundwater loads should be checked.
γI = load factor: a multiplier applied to force effects φ = resistance factor: a multiplier applied to nominal
resistance, as specified herein ηi = load modifier: a factor relating to ductility,
redundancy, and operational classification ηD = a factor relating to ductility as specified in
Article 1.3.3 ηR = a factor relating to redundancy as specified in
Article 1.3.4 ηI = a factor relating to operational classification as
specified in Article 1.3.5 Qi = force effect Rn = nominal resistance Rr = factored resistance: φRn
1.3.2.2 Service Limit State The service limit state shall be taken as restrictions
on stress, deformation and crack width under regular service conditions.
C1.3.2.2 The service limit state provides certain experience-
related provisions that cannot always be derived solely from strength or statistical consideration.
For tunnels, this limit state controls tunnel lining deformation, cracking and leaking.
1.3.2.3 Fatigue and Fracture Limit State The fatigue and fracture limit state shall be taken as
restrictions on stress range as a result of repetitive machinery or ventilation loads at the number of expected stress range cycles.
C1.3.2.3 The fatigue and fracture limit state is intended to
limit crack growth under repetitive loads to prevent fracture during the design life of the component. Additionally, the fatigue limit state is intended to prevent premature failure of anchoring and supporting components subject to machinery or ventilation loads.
1.3.2.4 Strength Limit State Strength limit state shall be taken to ensure that
strength and stability, both local and global, are provided to resist the specified statistically significant load combinations that a tunnel and its component parts are expected to experience in its design life.
C1.3.2.4 The strength limit state considers stability or
yielding of each structural element. If the resistance of any element, including splices and connections, is exceeded, it is assumed that the tunnel resistance has been exceeded. The redistribution of loads that can occur due to structure ground interaction typically allows the tunnel ground supporting structure to support loads in excess of the capacity calculated utilizing linear elastic static analysis. Extensive distress and structural damage may occur under the strength limit state, but overall structural integrity is expected to be maintained.
1.3.2.5 Extreme Event Limit State The extreme event limit state shall be taken to
ensure the structural survival of a tunnel during a major earthquake, flood, tsunami, collision, blast or fire, or when an immersed tunnel is subject to sinking vessel or anchor drag loads possibly during, or in conjunction with, a scour event.
C1.3.2.5 Extreme event limit states are considered to be
unique occurrences whose return period may be significantly greater than the design life of the tunnel.
1.3.3 Ductility The structural system of a tunnel shall be
proportioned and detailed to ensure the development of significant and visible inelastic deformations at the strength and extreme limit states before failure.
For the strength limit state:
ηD ≥ 1.05 for nonductile components and connections
= 1.00 for conventional designs and details complying with these Specifications
For all other limit states:
ηD ≥ 1.00
Internal components of tunnels including structural
elements, equipment supports and their connections shall be designed to exhibit ductile behavior, especially when subject to extreme events such as earthquakes.
Attention shall be given to the ductility of transitions between structural systems of tunnels, specifically at the interfaces between retained cut and cut-and-cover structures, between cut-and-cover structures and mined, bored or immersed tunnels and at the joints between immersed tunnel elements.
C1.3.3 The response of structural components or
connections beyond the elastic limit can be characterized by either brittle or ductile behavior. Brittle behavior is undesirable because it implies a sudden loss of load-carrying capacity immediately when the elastic limit is reached. Ductile behavior is characterized by significant inelastic deformations before any loss of load carrying capacity occurs. The redistribution of load effects provided by ground structure interaction, results in ductile behavior of concrete tunnel linings when proper detailing of the reinforcing is used. Internal components, however, should be designed to exhibit ductile behavior. The provisions of the LRFD Specifications provide guidance and direction for the ductile design of these internal components.
While the LRFD Specifications has a value of ηD less than one in implementation, the value is unused. Traditionally, structural engineers characterize structures as ductile, ηD = 1.00, or non-ductile, ηD = 1.05.
1.3.4 Redundancy The provisions of the LRFD Specifications shall
be used to ensure that the internal components of tunnels and all aspects of cut-and-cover and immersed tunnels incorporate redundancy in their design.
For the strength limit state:
ηR ≥ 1.05 for nonredundant members = 1.00 for conventional levels of redundancy For all other limit states:
ηR ≥ 1.00
1.3.5 Operational Importance
This Article shall apply to the strength and extreme limit states only.
The Owner may declare a tunnel or any structural component, or connection thereof, to be of operational priority. For the strength limit state:
ηI ≥ 1.05 for critical or essential tunnels
= 1.00 for typical and relatively less important
C1.3.4 Tunnel linings for bored and mined tunnels are
considered redundant due to ground/structure interaction and the ability to share load along the length of tunnel.
While the LRFD Specifications includes a value
of ηR less than one, in implementation the value is unused. Traditionally, structural engineers characterize structures as redundant, ηR = 1.00, or non-redundant, ηR = 1.05.
C1.3.5 Such classification should be done by personnel
responsible for the affected transportation network and knowledgeable of its operational needs. The definition of operational priority may differ from Owner to Owner. Guidelines for classifying critical or essential tunnels are as follows:
• Tunnels that are required to be open to all traffic
once inspected after the design event and are usable by emergency vehicles and for security,
defense, economic or secondary life safety purposes immediately after the design event.
• Tunnels that should, as a minimum, be open to emergency vehicles and for security, defense or economic purposes after the design event, and open to all traffic within days after that event.
Owner classified tunnels may use a value of η < 1.0 based on ADTT, available detour length or other rationale to use less stringent criteria.
2.8.6.3 Primary Switchgear 2.8.6.4 Secondary Distribution Systems
2.8.6.4.1 Low Voltage Switchgear/Switchboard 2.8.6.4.2 Panelboards 2.8.6.4.3 Motor Controllers and Control Devices 2.8.6.4.4 Dry-Type Distribution Transformers (600 Volts and Below) 2.8.6.4.5 Power and Convenience Outlets 2.8.6.4.6 Conductors and Cables 2.8.6.4.7 Standby Power Systems
2.8.6.4.7.1 Generator Units 2.8.6.4.7.2 Generator Switchboard
2.8.6.4.8 Uninterruptible Power Supply (UPS) 2.8.6.5 Grounding, Bonding and Lightning Protection
2.8.6.5.1 Systems Grounding 2.8.6.5.2 Equipment Grounding 2.8.6.5.3 Grounding for Personnel Safety 2.8.6.5.4 Grounding Materials 2.8.6.5.5 Lightning Protection
2.8.7 Tunnel Architectural Systems 2.8.7.1 General 2.8.7.2 Egress Design 2.8.7.3 Tunnel Occupant Load Design
2.8.8 Fire Protection 2.8.8.1 Fire Alarm and Detection Systems 2.8.8.2 Tunnel Fire Suppression Systems 2.8.8.3 Structural Fire Protection
2.8.9 Tunnel Security Systems 2.8.10 Corrosion Control Systems
2.8.10.1 Soil and Water Corrosion Control 2.8.10.2 Stray Current Corrosion Control 2.8.10.3 Atmospheric Corrosion Control
2.8.11 Communication and Traffic Control Intelligent Transportation Systems (ITS) 2.8.11.1 Communication Systems 2.8.11.2 Traffic Control and Monitoring ITS
Design provisions for road tunnel structures herein are categorized in terms of the construction methodology used. Minimum requirements are provided for:
This section provides the Engineer with information to determine the overall configuration of the tunnel. The tunnel configuration must accommodate the vehicles permitted to use the tunnel while providing a safe environment for the travelling public during normal and extreme operating conditions.
Minimum requirements for tunnel systems are specified with references to source documents.
2.2. DEFINITONS For definitions not shown, see the LRFD Specifications.
Authority Having Jurisdiction (AHJ) – Office or person charged with enforcing the life-safety code for the tunnel. Bore – Tunnel construction methodology that employs the use of a Tunnel Boring Machine (TBM). Construction Portal – The location of the start of mining or boring operations. This location often adjoins a cut-and-cover section of tunnel that connects to the permanent portal. Cross Passage – Passageway constructed between two adjacent tunnels to provide access between the tunnels. Crown – The highest point of the tunnel lining. Cutter Head – The front end of a mechanical excavator, usually a wheel on a tunnel boring machine that cuts through rock or soft ground. (AASHTO, 2010) Element – Pre-fabricated section of an immersed tunnel that is joined with adjacent sections to construct the tunnel. Face – The ground exposed at the head of the tunnel when tunnel construction is performed by mining or boring. Invert – On a circular tunnel, the invert is approximately the bottom 90 degrees of the arc of the tunnel; on a square-bottom tunnel, it is the bottom of the tunnel. (AASHTO, 2010) Lagging – Wood planking, steel channels or other structural materials spanning the area between ribs. Liner Plates – Pressed steel plates installed between the webs of the ribs to make a tight lagging, or bolted together outside the ribs to make a continuous skin. (AASHTO, 2010) Lining – Structural system constructed in intimate contact with the ground, used to stabilize the tunnel opening and to support hydrostatic loads. Mixed Face – The situation when the tunnel passes through two (or more) materials of markedly different characteristics and both are exposed simultaneously at the face (e.g., rock and soil, or clay and sand). Muck – The ground excavated during tunnel construction. NATM – See sequential excavation method. Permanent Portal – The location where vehicle traffic enters the tunnel during normal operation of the tunnel. Rib – 1. An arched individual frame, usually of steel, used in tunnels to support the excavation; also used to designate the side of a tunnel. 2. An H- or I-beam steel support for a tunnel excavation. (AASHTO, 2010) Segmental Lining – Tunnel lining constructed from segments that make up a ring of support; commonly steel or precast concrete. Segments: Sections that make up a ring of support or lining of a mined or bored tunnel; commonly steel or precast concrete. Sequential Excavation Method – Construction methodology in which the tunnel is mined in specified sequence to control ground movements; also known as the New Austrian or North American Tunneling Method (NATM). Springline – The point where the curved portion of the roof meets the top of the wall. In a circular tunnel, the springlines are at opposite ends of the horizontal center line. (AASHTO, 2010)
2.3. NOTATION 2.3.1. General cd/m2 = candelas per square meter (2.8.5.1.1) (C2.8.5.1.3)
fc = foot candle (2.8.5.2) ft = foot (2.8.5.3) (2.8.6.5.3) (2.8.6.5.4) hr = hour (2.8.3) Lseq = equivalent veiling luminance (2.8.5.1.1) (C2.8.5.1.1) MBtu/hr = Mega (1 million) British thermal units per hour (2.8.3) mph = miles per hour (2.8.3.1) ( 2.8.11.2) MW = Mega Watt (1 million Watts) (2.8.3) ppm = parts per million 2.3.2. Abbreviations ACS: Access control system AI: Analog input AMCA: Air Movement and Control Association International, Inc. ANSI: American National Standard Institute ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning Engineers ASTM: ASTM International, formerly known as American Society for Testing and Materials AWG: American wire gauge CEI: International Commission on Illumination CFD: Computational fluid dynamics CO: Carbon monoxide DC: Direct current DI: Digital input DMS: Dynamic message signs DO: Digital output EIS: Environmental Impact Statement FEIS: Final Environmental Impact Statement EPA: Environmental Protection Agency FHWA: Federal Highway Administration FONSI: Finding of No Significant Impact HMI: Human operator interfaces ICC: International Code Council IDS: Intrusion detection system IEEE: Institute of Electrical and Electronic Engineers IES: Illuminating Engineering Society I/O: Input/output LLF: Light loss factors ITA: International Tunneling Association LED: Light emitting diode LUS: Lane use signals NEMA: National Electrical Manufacturer’s Association NEPA: National Environmental Policy Act NFPA: National Fire Protection Association NIOSH: National Institute for Occupational Safety and HealthNO2 = Nitrogen dioxide
NO Nitric oxide NOx Oxides of nitrogen PIARC: World Road Association PLC: Programmable logic controller PTZ: Pan, tilt, zoom RIO: Remote input/output ROD: Record of decision SCADA: Supervisory control and data acquisition SSSD: Safe sight stopping distance TBM: Tunnel boring machine TSCS: Traffic surveillance and control system TVCS: Tunnel ventilation control system TVS: Tunnel ventilation system TWA: Time weighted average UL: Underwriters Laboratory UPS: Uninterruptable power supply USACE: United States Army Corps of Engineers
The tunnel structure types herein are defined by the methodology used to construct the tunnel. A tunnel type study should be conducted as part of the planning phase of the tunnel to determine the most appropriate construction methodology to be used.
The construction methodology is dictated by: • ground conditions • road typical section • economics • environment and available land surrounding
the portals • risk
Materials for the structural components inside the
tunnel vary and are dictated by economics as well as the construction methodology. Additional information regarding tunnel type studies can be found in AASHTO’s Technical Manual for Design and Construction of Road Tunnels – Civil Elements.
2.4.1. Cut-and-Cover Tunnels
C2.4.1
Cut-and-cover tunnels are defined as those constructed by excavating a trench, constructing a concrete structure to create the tunnel opening in the trench, and covering it with soil. The concrete tunnel structure may be cast-in-place or constructed from precast elements. Detailed design requirements for this type of tunnel are provided in Section 6.
Cut-and-cover construction may be utilized for the entire length of the tunnel or only a portion of the tunnel. Cut-and-cover methodology may be required when constructing a tunnel using mined or bored methodologies. The cut-and-cover portion of the tunnel is utilized to bring the tunnel alignment to the depth required to initiate the mined or bored construction activities.
2.4.2. Mined Tunnels
C2.4.2
Mined tunnels are defined as those constructed utilizing mechanical excavating equipment or blasting without disturbing the ground surface. The choice of mechanical excavating equipment or blasting is dependent on the ground conditions being excavated. The shape of the excavation is customized to the ground conditions and geometric functional requirements of the tunnel. The excavated ground typically requires initial support prior to the construction of a permanent lining. Mined tunnels in competent rock may also be unlined. Detailed design requirements for this type of tunnel are provided in Section 7.
Jacked box tunnels are included in the category of mined tunnels. Jacked box tunnels are prefabricated box structures jacked horizontally through the ground using methods to reduce the friction between the ground and the box. Jacked box tunnels often are used where the tunnel is required to be very shallow and it is not possible to disturb the surface, for example beneath active runways at airports or under railway embankments.
Mined tunnels include those constructed utilizing the sequential excavation method.
Mined tunnels may be constructed in free air or under compressed air.
2.4.3. Bored Tunnels C2.4.3
Bored tunnels are defined as those constructed through the use of specialized equipment known as tunnel boring machines (TBMs) without disturbing the ground surface. The excavation process may be performed in free air or may require a pressurized face depending on ground and groundwater conditions. A bored tunnel may be unlined or lined. Detailed design requirements for this type of tunnel are provided in Section 7.
The TBM consists of a shield to temporarily support the ground, a cutter head for excavating the ground and removing the muck, and supporting equipment.
TBM’s utilized for rock tunneling may include gripper pads to provide forward thrust for the TBM. TBM’s for soft ground and some rock tunnels may include an array of jacks to provide forward thrust for the TBM.
Immersed tunnels are defined as those constructed by immersing a prefabricated element into a prepared trench excavated from the bottom of a waterway. Immersed tunnels are a specialized type of cut-and-cover tunnel. Pre-fabricated tunnel elements are constructed of precast-concrete or a combination of structural steel and concrete, sealed with bulkheads at each end, and floated from the fabrication yard to the construction site. The elements are located over the excavated trench and lowered into place by adding ballast. Once in place the elements are connected to adjoining sections and backfilled. Detailed design requirements for this type of tunnel are provided in Section 8.
Although immersed tunnels are a subset of cut-and-cover tunnels, the portion of the tunnel that is submerged under water is referred to as the immersed tunnel. The tunnel alignment must transition on land from the depth where the tunnel element can be submerged to the depth where a retained cut can be constructed. This transition is constructed utilizing cut-and-cover technology as described in section 2.4.1. This transition is referred to as the cut-and-cover portion of the tunnel.
Immersed tunnel elements must be designed to resist the load effects associated with floating at the fabrication site and transportation to the construction site.
2.4.5. Shafts C2.4.5
Shafts are defined as vertical or steeply inclined excavations that connect the tunnel to the ground surface, and are located between the tunnel portals. Shafts may be excavated from the surface or from underground back to the surface. Shafts are not addressed in this specification.
Shafts are typically used for emergency egress and ventilation when required by the length of the tunnel.
2.5. PLANNING AND ROUTE CONSIDERATIONS
2.5.1. General
C2.5.1
The selection of the type of tunnel shall be based on the geometrical configurations, ground conditions, type of crossing and environmental requirements.
There are multiple components to vehicular transportation systems, including surface roads, bridges, viaducts or tunnels. The selection of a road tunnel over other components for a project (or a portion of a project) is typically determined prior to the design phase. Road tunnels are often selected to shorten travel times and distance, avoid surface traffic congestion or surface disturbance or to cross physical barriers such as mountains, rivers or other water bodies.
The choice for location of tunnels shall be supported by analyses of alternatives, which is typically completed during the planning and National Environmental Policy Act (NEPA) phase of tunnel projects. The Preferred Alternative is typically identified in the final NEPA decision document (e.g., Final Environmental Impact Statement (FEIS)/Record of Decision (ROD), Finding of No Significant Impact (FONSI)) as the final approved location for the alignment.
Tunnel planning is typically subject to NEPA, 42 U.S.C. 4321 – 4347, since most tunnel projects likely involve Federal funding or Federal approval, triggering NEPA. For roadway tunnels involving federal funding, the following NEPA regulations are applicable:
NEPA requires the evaluation of alternatives during the planning phase, and a Preferred Alternative is selected at the conclusion of the NEPA process. The Preferred Alternative is identified in the final NEPA decision document (e.g., FEIS/ROD, FONSI); identifying the final approved location for the alignment. The NEPA document outlines the purpose and need that the proposed project is intended to address, including an evaluation of the effectiveness of the Preferred Alternative in meeting the identified purpose and need
for the project. All of this planning level work is completed early during the planning/NEPA process such that it does not have to be completed separately as part of the tunnel final design phase. There are requirements for a NEPA re-evaluation or supplemental document related to design changes or time elapsed since last NEPA approval, but for the purposes of this design manual, it will be assumed that projects receive NEPA approval, including the general location for the tunnel alignment.
Because so much planning goes into the selection of the Preferred Alternative, the general tunnel alignment is typically determined prior to final design efforts. The NEPA document typically summarizes planning studies carried out prior to NEPA, the scoping process, the development process and screening of alternatives, and the evaluation of alternatives carried forward for detailed study in the NEPA document.
When federal funding is not anticipated for a project, local requirements should be followed for the planning process.
During the design phase, the Engineer shall adhere to the final NEPA decision document regarding compliance with relevant environmental requirements related to the project, including design commitments, mitigation commitments or other commitments that were agreed to be considered or implemented during the design phase of projects. Any proposed changes to the Preferred Alternative shall be evaluated in accordance with 23 CFR Sections 771.129 and 771.130, and shall be approved by the lead federal agency before the agency may proceed with the change.
During the NEPA process, the selection of the approved alignment will have fully evaluated the environmental consequences of the project, including natural, social, and cultural resources, as well as land use, economic, engineering, future planning and cost considerations. The NEPA phase will have also considered and documented public and agency comments related to the tunnel location and alignment.
As the design advances beyond the NEPA phase, attention shall continue to be directed toward providing for a favorable tunnel design that:
• Minimizes environmental effects (natural, social and cultural resources)
needs as well as the need to inspect the facility. Care shall be taken to design and detail components and systems for ease of maintenance and inspection. The following guidelines shall be used to meet this requirement.
• Areas that are defined as confined space during any non-emergency operating mode of the tunnel by any regulatory entity within the jurisdiction within which the tunnel resides should be avoided.
• Access to and egress from spaces such as air plenums above and below the roadway should be provided through pathways that can be used when traffic is present in the tunnel.
• Tunnel systems and systems components, including connections and attachments thereof, shall be accessible for inspection and maintenance and shall not be concealed behind architectural finishes or other tunnel components that are not easily removed.
• Tunnel systems conduits should not be embedded inside structural walls or slabs nor buried in the tunnel backfill or overburden.
• Penetrations of exterior tunnel structural walls and lining for passage of utility systems conduits should be avoided.
• Systems devices (lighting fixtures, signs, signals, smoke alarms, antennae, etc.) should be designed and detailed to withstand the effects of tunnel washing.
• Systems devices should be located in areas where safe access is possible when traffic is present inside the tunnel.
• Drainage systems should be designed and detailed to be inspectable and maintainable. Consideration should be given to site-specific groundwater conditions that could affect the long-term performance of the drainage system.
In some situations, this requirement cannot be met. For example, in mined tunnels with no ventilation plenum under the roadway, drainage systems will have to be buried in the ground beneath the roadway. Creating accessible space for systems constructed below the roadway could be prohibitively expensive. Local code requirements should also be adhered to when locating systems conduit.
If economically feasible, systems devices may be
able to be located over shoulders that can be closed for access to the device for maintenance or replacement. Devices that must be located over traffic lanes will require lane closures for access.
2.7. CLEARANCE AND GEOMETRIC REQUIREMENTS
C2.7
Minimum roadway clearance and geometric requirements shall be taken from the latest edition of AASHTO’s A Policy on Geometric Design of Highways and Streets ((hereafter referred to as the Green Book). Additional guidance is provided in the following articles.
The requirements of this article are intended to provide a safe travel environment inside the tunnel without imposing undue expense on tunnel owners. Information on existing tunnel shoulder and walkways can be found in NCHRP Project 20-68A Scan 09-05, Best Practices for Roadway Tunnel Design, Construction, Maintenance and Operations, April, 2011.
Clearances to obstacles in the tunnel shall be determined from a dynamic vehicle envelope established
Obstacles in the tunnel include the tunnel structure, signs, signals, light fixtures, antennae, ventilation and
from the vehicles anticipated to use the tunnel. The dynamic vehicle envelope shall account for the static envelop plus dynamic behavior such as bouncing, suspension failure, vehicle overhang on curves and lateral movement due to operational characteristics, and driver error. The dynamic envelope shall be applied to all areas where a vehicle can travel, including shoulders and emergency walkways.
firefighting equipment, doors in the open position when opening into the roadway, curbs, walkways, railings, etc.
2.7.1. Vertical Clearances
The minimum vertical clearance for tunnels shall be 16-feet, unless otherwise specified by the Owner.
The minimum vertical clearance for any tunnel shall not be less than that permitted on the facility leading to the tunnel.
2.7.2. Horizontal Clearances
Obstacles shall not encroach on the vehicle dynamic envelop.
The minimum travel lane width shall be 12-feet. The minimum width of roadway in any tunnel shall be the approach roadway travel lane(s) pavement width plus two feet.
2.7.3. Shoulders and Walkways
C2.7.3
When required for safety or when economically feasible, shoulders shall be provided in accordance with the Green Book.
A minimum walkway width of 3’-6” shall be provided outside the shoulders or, when no shoulders are present, outside the roadway. The walkway shall be raised above the roadway by a minimum of six inches.
The requirements of NFPA 502 shall be considered when dimensioning shoulders and walkways.
When the walkway is raised more than six inches above the roadway, consideration should also be given to providing general access to the walkway from the roadway to assist motorists in accessing the walkway during a tunnel evacuation.
2.7.4. Vertical Alignment
C2.7.4
Vertical curves shall be in accordance with the Green Book.
The preferred maximum grade is four percent; however, the absolute maximum grade shall be six percent.
When designing the vertical alignment, consideration should be given to the operational characteristics of the vehicles using the tunnel, the average daily truck traffic and the effect on traffic flow of steep grades.
Consideration should also be given to the demands on the tunnel ventilation system (TVS) generated by vehicles negotiating steep grades and the need to drain the roadway.
2.7.5. Horizontal Alignment
C2.7.5
The tunnel horizontal alignment shall be as short as practical and maintain as much of the alignment on tangent as practical. When required, horizontal curves shall be in accordance with the Green Book except as
Maintaining a tangent through as much of the tunnel as practical will result in the shortest tunnel with the accompanying effect of reducing the cost of the tunnel.
noted below. When designing horizontal curves, the limitations of
the tunneling methodology used to construct the tunnel shall be considered. Consultation with TBM manufacturers shall be included in the preliminary engineering phases of a bored tunnel to ensure that the horizontal alignment can be constructed.
Cut-and-cover and mined tunnels can be built to any roadway geometry, but bored tunnels are restricted to the performance characteristics of the tunnel boring machine used to excavate the tunnel. TBMs are not capable of negotiating a small radius curve. The TBM radius is a function of the diameter of the TBM as well as its overall configuration.
When chorded construction is used for tunnel walls where alignments are curved, chord lengths shall be determined to meet sight distance requirements in accordance with the Green Book.
Sight distance inside the tunnel shall be checked to verify that the design speed is consistent with the curve radii including the fact that the tunnel walls are obstructions to stopping sight distance.
The limitations listed here serve to create the visual impression of curvature to the driver which will be in line with driver expectations inside the tunnel. Additionally, minimizing chord lengths will assist with stopping sight distance.
2.7.6. Tunnel Approaches
The retained cuts at the tunnel approaches shall have as a minimum the same horizontal clearances as the inside of the tunnel.
Vertical alignments shall be established to provide a positive means of protection against flooding when tunnel portals are located in low-lying areas. If it is impractical to establish an alignment that will preclude flooding, then other positive flood protection measures shall be incorporated into the design.
Tunnel approaches shall be equipped with over-height warning devices to alert drivers that are operating a vehicle that is too tall to enter the tunnel. The warning devices shall provide visible means of alerting the driver that include a text warning and flashing yellow lights.
The over-height warning devices shall be located prior to an alternate route and shall provide directions to the alternate route. When it is not practical or useful to locate the over-height warning devices prior to an alternate route, a means to turn over-height vehicles around shall be provided prior to the tunnel entrance.
The over-height detection system alarm points shall be based on AASHTO required vertical clearance within the tunnel. The over-height detection system shall locate receiver/transmitter pairs along the roadway such that the paths between each transmitter-receiver pair shall be parallel such that the beams between the pairs shall define a plane parallel to the detection height.
The over-height detection system shall operate in conjunction with other traffic control components. In the event of an interruption of the beams crossing the roadway in the appropriate sequence, the detector controller shall activate a downstream message, and an audible alarm and strobe light shall warn the driver of the over-height vehicle and provide instructions. An alarm shall also be generated at the remote control room.
Means shall also be provided to stop traffic from entering the approaches to the tunnel as described in NFPA 502. Direct approach roads to the tunnel shall be closed following the activation of a fire alarm in the
tunnel. These approaches shall be closed in such a manner that responding emergency vehicles are not impeded in transit to the fire site. 2.8. SYSTEMS
C2.8
The need for tunnel systems shall be based upon the length, location and alignment of the tunnel. Owners shall undertake studies, including hazard analyses, to determine the appropriate systems required. Not all systems specified herein may be required. When required, systems shall meet the minimum requirements set forth herein.
Systems shall be integrated to be complimentary and be able to be monitored from a remote location(s). A vulnerability study shall be included in the systems integration design to ensure redundancy in critical systems and to avoid failures of multiple systems as a result of the failure of a single system.
Consideration shall be given to the potential of flooding of the tunnel when locating and designing system components. System components shall be designed to be resistant to flooding when not possible to be located outside of areas that could be flooded during and extreme event.
It is recommended that the operation and maintenance staff be included in the design, detailing and integration of the tunnel systems.
Recent events, such as Super Storm Sandy that
occurred on October 31, 2012, should be considered during the design of tunnel systems. Extreme events in coastal areas can cause unanticipated tunnel flooding. Designing resilient, flood resistant systems will result in a quicker recovery from a catastrophic flood event.
2.8.1. Codes and Standards
The latest edition of the following codes and standards are applicable to the design of tunnel systems and shall be used in the absence of specific design criteria developed by the Owner.
− AMCA – Fans and Systems Applications
Handbook
− ANSI/IEEE – ANSI C2 National Electric Code Lighting
− ANSI/IES RP-8 – Standard Practice for Roadway Lighting
− ANSI/IES RP-22 – Standard Practice for Tunnel Lighting
− ASHRAE Handbook of Fundamentals
− CIE 88:2004 – Guide for the Lighting of Road Tunnels and Underpasses
− CIE 193:2010 – Emergency Lighting in Road Tunnels
− ITA – Guidelines for Structural Fire Resistance for Road Tunnels
− NFPA 30 – Flammable and Combustible Liquids Code
− NFPA 37 – Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines
− NFPA 70 – National Electrical Code
− NFPA 72 – National Fire Alarm Code
− NFPA 78 – Safety Code for the Protection of Life and Property Against Lightning
− NFPA 780 – Standard for Installation of Lightning Protection Systems
− NFPA 92B – Standard for Smoke Management Systems in Malls, Atria, and Large Spaces
− NFPA 502 – Standard for Road Tunnels, Bridges, and other Limited Access Highways
− UL 96A – Installation Requirements for Lightning Protection Systems
− UL 1008 – Standard for Safety Transfer of Switch Equipment
2.8.2. Supervisory Control and Data Acquisition
(SCADA) System
C2.8.2
Where such systems are required by NFPA 502, a comprehensive supervisory and control and data acquisition (SCADA) system shall be provided to permit monitoring and controlling of life safety systems and equipment throughout the facility.
The Engineer shall specify the equipment required to be controlled and monitored by the SCADA system specific to each facility based on design guidelines, standards, codes and local agencies. The Engineer shall coordinate the system requirements with the Owner or operating authority. At minimum, all equipment required for safe normal and emergency operation of the tunnel fire life safety systems and equipment should be controlled and monitored by the SCADA system.
The architecture of the SCADA system shall employ a fail-safe redundant backbone (network) topology. The system architecture shall be designed with a redundant ‘hot-standby’ programmable microprocessor based control system, such as a programmable logic controller (PLC), capable of seamless transfer of data upon failure of one of the processors or network connection. These PLCs shall be connected to distributed input/output (I/O) cabinets remote input/output (RIOs) through redundant communications links. The RIO cabinets shall be strategically located throughout the facility to interface with life-safety systems and equipment to minimize hardwire cable/wire runs between field devices and the
SCADA system. The SCADA system shall employ a universal
remote input/output open network protocol, allowing different network manufactured devices the ability to communicate with the between the PLCs and RIO cabinets. Each remote input/output cabinet shall be designed to accommodate the required number of points for the digital input (DI), digital output (DO), analog input (AI) and other data modules as needed to control and monitor connected equipment, with an additional fifty percent (50%) spare for each point type (DI, DO, AI, etc.). The remote I/O cabinet shall be housed in an appropriately rated National Electrical Manufacturer’s Association (NEMA) enclosure suitable for the environment to where it will be installed.
The design of life-safety mechanical and electrical systems and equipment shall incorporate provisions for communication, control, monitoring and indication, to the SCADA system.
The SCADA system shall consist of redundant, reconfiguring communications networks (backbone), servers, and human operator interfaces (HMI) at the head end to facilitate operator interface. The head end communications network shall consist of dual fault tolerant, redundant, reconfiguring fiber optic ring topology with management switches at each node. Servers shall be capable of providing hot standby service as described above.
Operator interfaces shall be provided at a remote control room location and/or locally at the facility as required by the operating agency. Different levels of system access protected by password as defined by the Owner shall be provided. The system shall also be capable of handling and managing data logging and transfer of alarms, alerts and record keeping for historical purposes, control and monitoring of equipment related to the life-safety system.
Record keeping should be in accordance with the Owner’s record retention policy.
2.8.3. Tunnel Ventilation System (TVS)
C2.8.3
The TVS shall be designed to maintain environmental conditions within the tunnel and shall also meet the requirements of NFPA 502 for fire and smoke control.
Most road tunnels require some form of ventilation to mitigate the accumulation of vehicle emissions, and to manage the flow of heat and smoke resulting from a vehicle fire occurring inside the tunnel. Depending on factors such as tunnel length, tunnel geometry, traffic mix and traffic volumes, certain tunnels may be capable of self-ventilating while others require the assistance of mechanical ventilation systems. Self-ventilating tunnels rely solely on the movement of traffic to provide enough air flow through the tunnel to dilute vehicle emissions and disperse them, typically, through the exit portal. Mechanical ventilation systems in road tunnels are typically designed to assist the movement of air
longitudinally through the tunnel; either for the full length of the tunnel or to a location within the tunnel where the vitiated air (or smoke and heat resulting from a fire emergency) can be safely extracted. Mechanical ventilation systems commonly used in road tunnels include transverse and semi-transverse supply and exhaust systems, point extraction (exhaust) systems, jet fan based longitudinal systems and injection fan (saccardo) based longitudinal systems. Detailed information on these systems can be found in both the ASHRAE Applications Manual (Chapter 15) and NFPA 502. The TVS will typically include fans, motors and motor controls, sound attenuators, roadway-to-duct dampers, fan isolation dampers, distribution ducts, turning vanes, air shafts, air plenums, louvers, operating equipment condition monitoring devices and tunnel environmental monitoring devices.
Every road tunnel is unique in its operational and safety related requirements and determining the capacity and type of tunnel ventilation system to be used is dependent on the variety of influencing factors that are unique to that specific tunnel. NFPA 502 identifies the type of influencing factors that should be considered when determining the overall fire protection and life-safety requirements unique to a specific tunnel facility – including the role and performance requirements of the ventilation system during a fire emergency. Once an appropriate tunnel ventilation system is selected to meet the demands of the potential fire emergency in a specific tunnel, operational variations of that same system can be utilized to meet the non-emergency ventilation requirements; such as management of the vehicle emissions under various traffic conditions and equipment outages due to routine maintenance or unexpected failures.
Control and operation of the tunnel ventilation system shall be integrated into a system compatible with the operation plan for the tunnel facility.
The design fire shall be determined based on the types of vehicles and the potential cargos that are anticipated to utilize the tunnel. The following minimum characteristics shall be used to define the design fire:
• Fire heat release rate expressed in MBtu/hr (MW) • Fire Growth Rate as defined by NFPA 92B • Fire Smoke Properties:
o Combustion composition product yield rate o Mass specific extinction coefficient o Mass fuels rate determination o Air fuel ration
NCHRP Synthesis 415 and NFPA 502 shall be referenced for guidance when selecting an appropriate design fire size.
The various tunnel ventilation modes are defined as: • Normal Operations – Periods of time when
vehicular traffic using the tunnel is at average operating speeds (> 30 mph).
• Congested Operations – Periods of time when vehicular traffic using the tunnel is impeded from normal traffic flow, but not at a standstill. Average traffic speeds are between 5mph and 30 mph.
• Standstill Operations – Periods of time when vehicular traffic is moving at a speed less than 5 mph or forced to come to stop due to a traffic event outside or inside the tunnel.
• Emergency Operations – Period of time from the start of a vehicle fire incident to the time the tunnel is purged of all smoke and the incident scene has been cleared by emergency responders.
Operational modes of the ventilation system are often pre-programmed as part of the TVS control system to match predetermined traffic and emergency conditions. Activation of the TVS may be automatic, via an environmental monitoring system and/or time-of-day clock and for emergency operation via an automatic fire detection system. The TVS may be also be activated manually by operational staff located either locally at the tunnel facility or at a remotely located control center. Manual activation of pre-programmed system modes are based on receipt of alarms or alerts from the traffic monitoring system, environmental monitoring system, incident detection system, fire detection system, or by visual assessment via closed circuit television (CCTV).
2.8.3.2. TVS General Performance Requirements
The TVS shall be designed to provide a safe and tenable environment for motorists in the tunnel during all expected operational conditions.
The TVS shall also be designed to mitigate the effects of smoke and heat during an incident involving a fire to facilitate the safe evacuation of motorists and firefighting operations. The design fire shall be determined based on the types of vehicles expected to use the tunnel and the potential cargo to be transported through the tunnel.
The TVS shall be designed to provide the following minimum functions for the operational modes defined above in Section 2.7.3.1:
1. Dilution and removal of vehicle generated
emissions to maintain minimum criteria limits for carbon monoxide (CO), oxides of nitrogen (NOx) and particulate matter to reduce haze as defined in Table 2.7.3.2-1. Air quality monitoring and alarms alerting tunnel operators that the in-tunnel air quality is approaching or exceeding these limits shall be an integral part of the TVS operational control concept.
2. Management of the flow of smoke and heat during a vehicle fire event so as to establish and maintain a safe and tenable egress path to allow motorists to evacuate to a safe area if necessary and to facilitate access to the fire site by emergency responders. Reference NFPA 502 for specific minimum requirements.
3. All tunnel ventilation equipment including ductwork and appurtenances exposed to the heated airstream during a fire shall be designed to meet their performance requirements for a minimum of one hour at a temperature of 482 degrees Fahrenheit or according to the
calculated additional time requirements as per section 2.7.3.3.3, whichever is greater.
4. TVS exhaust and supply air shafts shall be protected at grade to prevent the possibility of dangerous substances or foreign objects to enter the tunnel. In addition, supply air ventilation intakes shall be located to ensure against the recirculation of emissions or smoke from the ventilation exhaust system.
5. When used as a part of the TVS, jet fans shall not be obstructed on the intake or discharge sides by other tunnel fixtures, lighting or signage. Jet fans shall be provided with sound attenuators that reduce fan noise so that acceptable sound levels are met on the tunnel roadway. Refer to NFPA 502 for guidance on acceptable sound levels.
6. The TVS shall be designed with sufficient redundancy to ensure its reliable availability. Reference NFPA 502 for minimum requirements.
Table 2.8.3.2-1 -- Minimum Limits for Carbon Monoxide, Oxides of Nitrogen and Particulate Matter
Pollutant Limiting Value
Carbon Monoxide (CO)
120 ppm up to 15
minutes 65 ppm up to 30 minutes 45 ppm up to 45 minutes 35 ppm up to 60 minutes
Oxides of Nitrogen (NOx)*
Nitric Oxide (NO)
25 ppm
CO Limiting values are based on FHWA/EPA
guidelines. The NO Limiting Value is based on TWA established by the National Institute for Occupational Safety and Health (NIOSH). The NO2 Limiting Value is based on recommendations made by the World Health Organization and the World Road Association (PIARC). K is an extinction coefficient, which reflects the amount a light beam is attenuated over a given distance.
* The main constituents of NOx are NO and NO2 ** K is an extinction coefficient which is a measure of
the amount a light beam is attenuated 2.8.3.3. TVS Design Analysis Calculations
The following minimum calculations are required for the design of the TVS.
2.8.3.3.1. Emissions Ventilation Flow Rate
The emissions ventilation flow rate is defined as the amount of airflow required to maintain criteria limits for CO, NOx, and visibility (haze) generated by vehicles at various speeds. The emission’s ventilation rate shall be determined based upon average projected peak traffic speed and density data. In addition, the average projected traffic mix shall be used in this and other applicable calculations.
The total required emission ventilation rate shall also consider the piston effect (the amount of airflow generated by the movement of vehicles) which shall be based on the vehicle velocities associated with the traffic density data. The United States Environmental Protection Agency’s (EPA) emissions prediction software Mobile 6.2 shall be used to prepare the vehicle emissions factors.
Consideration of any adverse wind conditions at the exit portal(s) is also required when determining emission ventilation flow rates and shall be based on typical winds in the geographical area of the tunnel. A 90th percentile of the winds impinging upon the exit portal shall be used for the tunnel ventilation analysis. The prevailing wind direction with respect to the tunnel axis shall be taken into account in calculating the portal design wind.
The determined emission ventilation flow rates from the tunnel shall be used to perform an emissions dispersion analysis to demonstrate that pollutant concentrations at nearest ambient receptors to the tunnel portals or exhaust ventilation structures are in compliance with applicable local, state and federal air quality regulations.
2.8.3.3.2. Emergency Ventilation Capacity
The amount of airflow required for adequate smoke and heat control during a fire emergency shall be determined using computational fluid dynamics (CFD) modeling. These analyses shall also estimate the smoke
movement and gas temperatures within defined regions both upstream and downstream of the fire. Peak temperature at the surface of the tunnel structure in the region of the fire shall also be predicted.
CFD modeling of fire and smoke movement shall use the design fire established for the tunnel. The design fire size used for determining the emergency ventilation requirements for a specific road tunnel shall be selected based on the complete evaluation of all influencing factors specifically unique to the tunnel.
2.8.3.3.3. Egress Calculations
The spacing of the cross-passages shall be verified as adequate for fire life safety purposes by performance of egress modeling, coupled with the emergency ventilation model. The movement of smoke shall be modeled on a transient basis and shall be compared to the egress time duration. Egress modeling shall be accomplished via computer simulation.
2.8.3.3.4. Fan Sound Attenuation
A noise analysis shall be performed to demonstrate that the TVS operates in compliance with applicable sound criteria at the roadway level and sidewalk level in the TVS fan room. The noise analysis shall include nearby sensitive receptors to ensure compliance with local noise ordinances.
2.8.3.4. Engineering Design Software
2.8.3.4.1. CFD Flow Modeling Software
C2.8.3.4.1
CFD flow modeling software shall be used for the fire and ventilation system airflow modeling. The CFD software shall be able to:
• Predict air temperatures at defined regions,
consider the effects of duct leakage, and • Predict the region of smoke and heat control
within the tunnel roadway.
Commercially available software that satisfy these criteria are: FDS (Fire Dynamics Simulator available from NIST), CFX (available from ANSYS), FLUENT (available from ANSYS), SOLVENT (available from Parsons Brinckerhoff) and STAR-CD (available from CD-adapco).
2.8.3.4.2. Egress Modeling Software
C2.8.3.4.2
Egress modeling software shall have a publicly available validation report with technical information and the following capabilities:
• Simulating various types of tunnel occupants based on gender and age variations, differences in body dimensions and mobility, travel speed and pre-movement time.
• Simulating the tunnel occupant movements as a function of time.
Commercially available software that satisfy these criteria are: SIMULEX, STEPS (available from Mott MacDonald Group, Limited) and LEGION EVAC (available from Legion, Limited).
• Accounting for the effect of crowded conditions on tunnel occupant travel speeds.
• Modeling of the tunnel occupant speeds as a function of vertical travel, both up and down.
• Identifying locations and durations of choke points along the egress paths.
• Providing numerical output as a function of time, the cumulative number of tunnel occupants who have passed each exit or have yet to exit.
2.8.4. Drainage System
C2.8.4
Tunnel drainage systems shall be designed to collect and discharge water that can accumulate in the tunnel due to seepage of the structure, rainfall overflow from approach roadways, snow melt, tunnel washing or firefighting activities. Water shall be collected through inlets into pipes and conveyed via gravity to a sump location that also serves as a pump station.
Collected water may be pre-treated on-site prior to discharge or discharged directly into a sanitary system prior to discharge into natural waterways. All discharged water shall meet local standards for quality prior to discharge into natural waterways.
No inlet structure or portion thereof shall be located in the roadway surface of the travel lanes. Flows shall not extend into the travel lanes. All components of inlets shall be designed to carry the same traffic loading as other components of the tunnel.
Cleanouts shall be provided at intervals that facilitate proper functioning of the system, and at all locations that would be prone to collecting debris that could clog the system.
Tunnel drainage systems shall be designed in accordance with NFPA 502 and capable of capturing and conveying spills of hazardous or flammable liquids with minimum propagation.
Operational protocols such as shutting down discharge pumps in the event of a fuel spill shall be developed to ensure that effluent discharged by pumps meet regulatory requirements.
Tunnel drainage systems can be the recipients of a variety of liquids, including water, washing and firefighting chemicals, motor vehicle fuels and lubricants and liquid cargo of all types.
Owners shall establish the rules for cargo during the planning stage in order to provide guidance to Engineers regarding the requirements for drainage and pump systems
Tunnel operational practices and restrictions on cargo permitted to be transported through the tunnel can assist in limiting the types of liquids required to be conveyed and handled by the tunnel drainage system.
Storm water generated on the immediate approach roadways outside the tunnel portals shall be intercepted to prevent it from entering the tunnel.
The purpose of intercepting storm water at the tunnel portals is to avoid the need for a significantly larger pump station and equipment to handle typical storm flow conditions.
The tunnel drainage systems shall be sized to handle the largest anticipated flows.
Wash water loads typically vary from 150 to 500 gallons per minute. Firefighting loads are determined based on the fire suppression system design flow. Tunnel seepage requirements are determined based on
the preference of the Owner. Typical values are generally less than one gallon/minute/1,000 feet of tunnel.
Operational protocols for discharges from the tunnel drainage systems shall comply with all applicable local and federal environmental regulations.
Miscellaneous drains shall be provided in spaces such as equipment rooms, egress passages, stairwells, etc. and connected to main tunnel drainage system.
Other areas requiring drainage considerations include ventilation ducts and safety or maintenance walks. It is also recommended that any electrical raceways, pull-boxes and hand holes be provided with drainage systems that connect to the tunnel drainage system. This will allow conduits that may fill with water to drain.
For road tunnels in climates where freezing conditions are probable, drainage shall not be permitted to cross over the travel portion of the roadway.
The drainage system components shall be evaluated for the potential of freezing during cold weather and consideration shall be given to protecting the drainage system from freezing.
Locations of reverse curves where super elevation changes direction are locations where drainage can cross over the roadway
The pump station shall be located and sized to provide access for maintenance, repair and replacement of the pumps as well as access for clean out of the sump area. Pumps shall be sized to handle the design flows. Redundant pumps shall be employed. Access to the pump station shall be out of the travel lane whenever possible. If required, access manholes shall be located in the center of the travel lane away from the wheel track area.
The basic components of a roadway tunnel pump station are pumps, discharge piping and operational controls, water level alarms, hydrocarbon detection devices and ventilation.
The pump station shall be designed with settling basins, weirs and screens as required for pre-treatment of collected effluent and to prevent any large solids from entering the pumps.
2.8.5. Lighting
C2.8.5
The lighting design shall comply with the latest applicable manuals listed in 2.8.1.
The tunnel lighting shall be designed to provide adequate daytime illumination in the entrance portion of the tunnels, so that motorists can drive a vehicle safely and efficiently at the posted speed conditions, as well as supporting the eye adaptation of the motorist to lower lighting levels in the interior of the tunnel.
Tunnel lighting consists of lighting for the tunnel approach and roadway, emergency exit pathways and ancillary spaces within the tunnel. This article establishes the basic design requirements for the tunnel roadway lighting system.
2.8.5.1. Tunnel Lighting System
2.8.5.1.1. Design Approach
C2.8.5.1.1
The design methodology of the tunnel lighting system shall be based on ANSI/IES RP-22. Tables 2 & 3 of ANSI/IES RP-22 may be used for preliminary design, but the equivalent veiling luminance (Lseq) method shall be used for final design.
The objective is to utilize the most beneficial luminance level to ensure driver visibility while controlling initial cost.
An equivalent veiling luminance (Lseq) analysis determines the adaptation of the eye of an approaching motorist for that specific day/time/condition. After determining the adaptation of the driver, formulas can then be applied to determine how much lighting is needed inside the tunnel in order see a hazard on the
road from outside the tunnel at a safe sight stopping distance (SSSD). Once inside the tunnel, transition zones are used to reduce the lighting levels at a rate which maintains the visibility of the driver.
Design of tunnel lighting shall consider the following four zones: approach, threshold, transition and interior.
ANSI/IES RP-22 divides the tunnel into separate zones to accommodate the proper adaptation of the human eye. These zones are called ‘approach’, ‘threshold’, ‘transition’ and ‘interior.’ The length of each zone and the lighting requirements for each zone correlate to the curve of eye adaptation. This curve relates speed to visibility by transforming the rate of eye adaptation to SSSD established by AASHTO. The current standards suggest that one SSSD for the threshold, followed by a 10-second transition will provide enough time for a driver to adapt to the tunnel’s interior environment.
The combined length of the threshold and transition zones shall follow the eye adaptation curve defined in ANSI/IES RP-22.
The interior zone immediately follows the combined lengths of the threshold and transition zones. The daytime interior luminance level shall be determined based on the traffic volume as per Table 7 in ANSI/IES RP-22.
Lighting design for tunnels is luminance based design for the roadway. Typical pavements used in the US include R1 which is a concrete pavement and R3 which is asphalt. Pavement types are defined in ANSI/IES RP-8.
The surface reflectances for the tunnel shall be considered in the design of the tunnel. Consideration shall be given for higher reflectance surfaces to benefit from interreflected light within the tunnel. The design shall also consider the maintenance procedures to be used for cleaning these surfaces and how that will affect the maintained lighting values in the tunnel.
Other criteria are given for the walls of the tunnel, and conversions are included in RP-22 in order to use illuminance levels for the wall surfaces.
The nighttime luminance level shall be uniform through the entire length of the tunnel and provide an average luminance level of 2.5 candelas per square meter (cd/m2).
The interior zone illumination levels shall be extended to the exit portal. An exception to this would be bi-directional tunnels or tunnels that may be used as bi-directional under certain operating conditions. For these cases a threshold lighting analysis shall be required.
There is general agreement by authorities that the eye’s natural ability to quickly adjust to an increasing level of illumination precludes the need for providing a higher light level at the exit portal.
In order to allow for visual accommodation approaching and exiting the tunnel, the entrance approach and exit approach lighting shall be no less than one-third of the nighttime tunnel luminance in order to maintain good visibility.
2.8.5.1.2. Design Parameters
C2.8.5.1.2
The design parameters that influence the selection of the lighting design are: (LLF), luminaire efficiency and distribution, reflectance of the interior surfaces; and tunnel geometry. LLF is calculated as follows:
LLF = (LAT) x (VF) x (RSDD) x BO x (LLD) x
(LDD) Eq. 2.7.5.1.2-1
Light loss factors are adjustments that are made in order to adjust the lighting calculations to anticipated field conditions. This overall adjustment is dependent on a series of recoverable and non-recoverable factors which will affect the lighting system’s operation from the time the system is first turned on, through the life of the system. Refer to ANSI/IES RP-22 for additional information and guidance in determining appropriate values for the equation shown.
Where: LAT = Luminaire ambient temperature factor VF = Voltage factor RSSD = Room Surface Dirt Depreciation Factor BO = Burn out factor LLD = Lamp lumen depreciation LDD = Luminaire dirt depreciation factor
Determination of the proper dirt depreciation factor should be through discussions with maintenance personnel and expected environmental conditions of dirt, road spray, and vehicle exhaust.
The single largest light loss factor in tunnels is dirt depreciation. Depending on the operating environment and cleaning cycles used, dirt can reduce the output of the luminaires by over 50 percent.
2.8.5.1.3. Lighting Control System
C2.8.5.1.3
The tunnel lighting control system shall be designed to operate automatically and maintain a predetermined ratio of outdoor luminance to threshold and transition zone luminance for each roadway, under all weather conditions, during daytime operating hours. The system shall prohibit response to sudden and short duration (less than 15 minutes) light level changes and shall respond only to steady and long duration (more than 15 minutes) changes.
The controller shall also consider the ability to monitor multiple luminance sensors located throughout the tunnel, to monitor the maintained luminance level. When any of the luminance meters sense that the lighting level is less than a specified set point, the system shall provide an alert identifying the need for maintenance to the lighting fixtures.
The lighting control system may be integrated into the SCADA system.
A luminance sensor, lighting contactors, and a controller cabinet are the necessary components of a lighting control system. The sensor sends an analog signal to the controller and the corresponding output level is switched on or off using lighting contactors, depending on the frequency received.
The use of lighting controls can greatly improve the ability of the tunnel lighting system to adapt to changing daylight conditions so that when approaching and entering the tunnel, it is easier to ascertain conditions directly beyond the tunnel entrance. Illuminating a tunnel roadway to its highest level of lighting is only necessary during bright and sunny days. Otherwise, considerations should be incorporated into the design to save energy whenever possible. A typical lighting control scheme is depicted in the Table C2.7.5.1.3-1. Table C2.7.5.1.3-1 – Typical Lighting Control Scheme
boards, in turn, shall be connected to switchgear busses normally energized from alternate electrical services. Luminaires shall be circuited that a single circuit failure would not extinguish a large percentage of the normal lighting within the tunnel. To prevent the tunnel from being cast suddenly into complete darkness by simultaneous loss of power from all utility power sources, emergency lighting shall be provided as required by NFPA 502.
2.8.5.1.5. Tunnel Lighting Fixtures
C2.8.5.1.5
Luminaires shall provide the necessary luminance/control while physically staying outside the dynamic traffic envelope. All luminaires within the tunnel shall be watertight and corrosion resistant to protect their interiors from periodic high-pressure wash downs of the tunnel walls and ceiling. All luminaires within the tunnel areas shall be rated for the type of cleaning expected to be used by maintenance personnel. All of the materials used for the lighting fixture construction and wiring within the tunnel shall meet the requirements of NFPA 502. Metallic components of lighting fixtures shall be compatible with the metallic components of supporting hardware to avoid the corrosion problems associated with dissimilar metals in contact with each other.
A tunnel environment requires a high level of corrosion resistance and dust and dirt tolerance. Fixtures and the electrical distribution system are exposed to salt and certain levels of sulfuric acid created from diesel vehicle exhaust. Components also have to be used which tolerate emergency conditions such as fires and if exposed to a fire condition will not cause a hazard within the tunnel like high smoke generation or toxic gases.
2.8.5.2. Tunnel Cross Passages, Egress Stairwells and Ancillary Spaces
Interiors of cross passages and stairwells shall be designed for an average illuminance of 10 footcandles (100 lux). Circuiting for cross passages and egress stairwells shall be designed in accordance with the requirements of the National Electric Code. Electrical equipment rooms shall be designed for an average illuminance of 30 footcandles (fc). Pump stations, battery rooms and storage rooms shall be designed for an average illuminance of 20 fc.
Exits within the tunnel shall be clearly identified by dedicated emergency exit lighting that will help vehicle occupants find the exit when necessary during an emergency. This supplemental lighting provided at the exits shall light the door and surrounding surfaces to a much higher level than the interior of the tunnel, providing the necessary demarcation. This lighting is in addition to the exit markings, strobe lights, directional signs and other indicators.
In order to make the tunnel exits adequately visible, and to familiarize the driver with their position and geometry, the maintained average vertical illuminance at all times on the door and on an area of tunnel wall extending six feet beyond the projected door frame of an emergency exit or cross passage, or outlining the actual opening in the tunnel wall, shall not be less than four times the adjacent wall illuminance or 10 fc. The overall uniformity (average/minimum) of illuminance on this
area shall not be less than 2.0. The light sources used for this lighting shall have a color rendering index of Ra ≥ 60.
Emergency lighting for these areas shall not be less than one fc, shall utilize a selected number of lighting fixtures provided as part of the general illumination, and shall be connected to a uninterruptable power supply (UPS) system, generator or other emergency power source capable of maintaining the lighting levels for a minimum of 60 minutes.
Exit sign lighting fixtures shall be provided to illuminate the designated egress passageways, including the entrance to stairways. Exit sign lighting fixtures shall be wet location, wall or bracket mounted, at a mounting height visible to all occupants within the occupied spaces. All exit sign lighting fixtures shall be connected to the emergency power source.
Additional information regarding signing for tunnel egress can be found in NCHRP 20-59(47) Emergency Exit Signs and Marking Systems for Highway Tunnels.
2.8.5.3. Tunnel Fixed Message Sign Lighting
C2.8.5.3
All ceiling mounted, non-internally illuminated signs located in tunnels shall be externally illuminated using the luminance or illuminance methods in accordance with the following criteria:
Luminance – 96 candelas per square meter minimum Illuminance – 40 lms/ft2 (400 lux) minimum assuming 65 percent maintained reflectance.
The external illumination of tunnel fixed message signs may be provided by light fixtures dedicated to this purpose or illuminated by the general tunnel lighting if the lighting criteria is met by the general tunnel lighting system.
The maximum to minimum uniformity ratio on the sign face shall not exceed 4 to 1. The maximum illuminance gradient produced on the sign face shall be 2 to 1.
Fixtures shall be located so that they do not interfere with sign visibility for drivers of any type of vehicle or impact driver visibility for oncoming traffic in two-way tunnels.
Fixtures used to illuminate fixed message signs shall be connected to the tunnel lighting emergency power source.
2.8.6. Electrical Systems
2.8.6.1. General
Electrical systems shall be designed to provide functionality, durability, ease of maintenance and safety in accordance with the National Electrical Code and National Electrical Safety Code. The electrical installation shall also comply with local building and electrical codes, as well as, tunnel life safety design guidelines. The electrical requirements of other systems, including mechanical, traffic control, fire and surveillance, and SCADA Systems shall be incorporated
into the overall design of the tunnel normal and emergency electrical systems. Close coordination with all other disciplines shall be maintained throughout the design process to confirm that the power needs and redundancy of power supply for each system are provided.
2.8.6.2. Primary Distribution
The electrical system shall perform as a fully redundant system utilizing specific switch-over schemes to minimize the impact of a power interruption. Each end of the tunnel shall be provided with a minimum of one independent electrical service feeder from the local utility. If the tunnel is being fed from only one side, then two independent utility service feeders or one feeder with generator backup shall be provided. Independent feeders are defined as those originating from separate utility substations or buses and meeting reliability requirements of National Electrical Code Article 700. The combination of these feeders shall provide primary distribution throughout the normal operation of the entire tunnel facility. Each incoming service feeder shall have the capability of supplying the entire power requirements for the facility, thus providing the redundancy required to maintain tunnel operations during outages, maintenance or repairs.
The primary distribution system shall be extended from the point of service to the primary switchgear for each service. The primary distribution system shall be designed such that the primary switchgear can be powered from either of two separate utility sources, or single service and backup generator. Necessary cables, transformers, switches, circuit breakers, etc., of each supply shall be separated from each other to maximum practicable extent.
2.8.6.3. Primary Switchgear
The switchgear assembly and all components shall be designed in accordance with the latest applicable standards of ANSI, NEMA and Institute of Electrical and Electronic Engineers (IEEE). The switchgear shall comply with the engineering and operating requirements of the utility supplying the power. The switchgear assembly shall have sufficient short circuit and impulse withstand capability to operate safely, and properly coordinate with the utility company service being provided to protect the electrical equipment being provided. Main, tie and feeder circuit breakers shall be fully rated for continuous service and be sized for the connected loads in accordance with the National Electrical Code.
Provisions shall be included in the design to accommodate the required SCADA functions to permit operation of selected breakers, and remote monitoring and/or alarming of electrical system conditions and breaker positions from a central location
The switchgear shall be installed within an area of the ancillary buildings at the tunnel portals or other suitable locations, or near the utility company service entrance. Spaces containing this equipment shall be positively pressurized and provided with sufficient filtered air, free of moisture, to maintain the ambient temperature below a maximum 104°F.
Primary switching equipment shall be metal-enclosed or metal-clad.
Power transformers, when required for services above 600 volts, shall be UL listed, and step down of the medium voltage service to the required utilization level voltage. Transformers shall be located within interior rooms, where possible, with sufficient filtered or dust-free air, free of moisture, to maintain a maximum ambient temperature of 104°F. Wherever feasible, transformers shall be joined to primary and secondary switchgear in double-ended unit substation configurations.
2.8.6.4. Secondary Distribution Systems
C2.8.6.4
This article specifies minimum requirements for secondary distribution systems, i.e., electrical distribution systems operating at a voltage level below the primary distribution system medium voltage at utilization voltage levels.
These systems include utilization voltage switchgear, motor control centers, distribution, lighting and power panelboards, motor controls, outlets and receptacles, raceway systems and wiring.
2.8.6.4.1. Low Voltage Switchgear/Switchboard
Low Voltage switchgear/switchboards shall consist of circuit breakers positioned in metal-enclosed or metal-clad, free-standing enclosures. In general, the switchgear/switchboard shall be arranged for attachment to a transformer enclosure as a component of a unit substation when possible. Switchgear/Switchboard sections for low voltage service applications shall include main circuit breakers capable of interfacing with the local utility company.
The switchgear/switchboards shall comply with the latest applicable standards of ANSI, NEMA and IEEE and shall include the capability to interface with the SCADA System to provide remote operation of selected breakers, and remote monitoring and alarming of electrical systems conditions and breaker status.
Main and tie circuit breakers in double-ended substation configurations shall be electrically interlocked to prevent both transformers from being paralleled. Additionally, electrical interlocks shall be provided as required to fulfill operational conditions and coordination with the utility company. In addition, mechanical interlocks shall be provided for personnel safety under all conditions of operation and maintenance. Indoor switchgear/switchboards shall be installed within an area of the ancillary building, or other suitable space, set aside for electrical equipment and
accessible to authorized personnel only. Equipment may also be located outside in suitably NEMA rated environmental enclosures.
2.8.6.4.2. Panelboards
Panelboards shall be the bolted circuit breaker type.
2.8.6.4.3. Motor Controllers and Control Devices
Solid state, adjustable speed controllers or reduced voltage starters shall be provided for poly-phase motors and shall be sized to start the equipment motors in accordance with NEMA MG1. Life-safety control features shall be incorporated into starters or controllers for emergency tunnel ventilation fan motors.
Automatic control of emergency tunnel ventilation fan motors shall be provided using programmable microprocessor based control systems for remote control and monitoring. Local control shall be provided from the associated starters or controllers.
2.8.6.4.4. Dry-Type Distribution Transformers (600 volts and Below)
Transformers shall be Underwriters Laboratory (UL) listed. Transformers shall meet NEMA TP-1 standards for energy efficiency.
2.8.6.4.5. Power and Convenience Outlets
Ancillary spaces inside the tunnel such as pump rooms, valve rooms, electrical rooms, cross passages and sprinkler rooms shall be provided with a sufficient number of convenience type receptacles spaced around the perimeter of the interior walls or otherwise located in such a manner as to meet the power requirements of portable equipment.
2.8.6.4.6. Conductors and Cable
All conductors and cables shall be copper except for fiber-optic type cables. Conductors No. 9 American wire gauge (AWG) and smaller may be solid; No. 8 AWG and larger shall be stranded. Minimum size conductors shall be No. 10 AWG for feeders and branch circuits, except those branch circuits connecting convenience receptacles and light switches where No. 12 AWG minimum is acceptable; No. 14 AWG minimum for control, signal, and alarm circuits, and No. 16 AWG minimum for fixture wire.
All wire and cable shall be NFPA 70, thermosetting, heat, moisture and flame retardant, and compliant with the specific requirements of NFPA 502 for construction and smoke emissions under combustion.
Standby or emergency power from engine generators, other type of prime mover or source shall be provided for systems identified by the Owner and/or NFPA 502. The provision of UPS for selected loads that would be adversely affected by even a momentary switching outage is covered in Article 2.7.6.4.9.
The standby or emergency power system shall consist of generator(s) or other type of prime mover or source, switchboards, transfer switches, fuel supply and storage as required, accessories and wiring required to provide standby power to the systems identified.
2.8.6.4.7.1. Generator Units
Generators shall be designed to perform under the environmental conditions to which they will be subjected.
The required operating and capacity characteristics of the generator, based on a risk analysis that includes the consequences of failure of the generator, shall be as determined by the Owner.
Storage tanks shall conform to all local regulations. The system shall conform to NFPA 30, NFPA 37 and NFPA 110. Fuel type shall be such that exhaust emissions comply with Environmental Laws. The cooling system shall be designed to discharge heat outside of the building or enclosure housing the generator. Generators shall be equipped with appropriate silencers. Exhaust shall be released outside the building or enclosure housing the generator.
Automatic transfer switches shall conform to UL
1008.
2.8.6.4.7.2. Generator Switchboard
The switchboard shall be an indoor or outdoor type, metal-enclosed, self-supporting structure. The switchboard shall be compartmentalized in design, with individually mounted devices in the distribution sections. The switchboard shall comply with all applicable provision of UL 891 and NEMA PB-2.
2.8.6.4.8. Uninterruptible Power Supply (UPS)
C2.8.6.4.8
UPS units shall supply uninterruptible power supplies for selected loads. The loads designated to be supplied by the UPS shall be determined by the Owner and/or as required by NFPA 502. Battery protection time shall be determined by the Owner.
Typical loads that are connected to UPS are:
• Traffic surveillance and control system (TSCS) equipment.
• Communications, SCADA, and fire detection and alarm systems.
as described in Article 2.7.6.4.8. The UPS systems shall be designed to operate ‘on line’ such that when normal power fails to the unit, the batteries will provide power for a designated period through the inverter output. If a UPS malfunctions, a static switch shall automatically connect the load to a ‘reserve’ supply while simultaneously opening the inverter-output circuit breaker. A maintenance by-pass shall also be provided to manually transfer the load to the normal supply for routine service or maintenance of the UPS.
The UPS equipment enclosure shall be suitable for the environment in which it is to be installed, recognizing that heat buildup and moisture act to diminish the reliability of a UPS unit.
UPS units whenever feasible, shall be located in spaces which are relatively dry and adequately ventilated with clean air. This criterion shall be weighed against the desirability of locating the UPS unit as close as possible to the load served, so as to reduce to a minimum the length and exposure of direct current (DC) wiring and the UPS output wiring. A provision shall be made for the required monitoring, control and alarm functions to the SCADA system.
2.8.6.5. Grounding, Bonding and Lightning Protection
Electrical equipment shall be grounded to provide safety for personnel and to provide fast, reliable relaying response in accordance with the requirements depicted in the National Electrical Code. Grounding shall be separate but coordinated with corrosion control requirements.
2.8.6.5.1. Systems Grounding
The following grounding requirements shall apply:
1. Three-phase, three-wire, 60 Hertz systems supplied from transformers shall be solidly grounded at the transformer enclosure as per NFPA 70.
2. Three-phase, four-wire systems shall be solidly grounded at their source (transformer secondary, standby generator neutral, or UPS neutral) per NFPA 70.
A connection of the system to earth shall be provided at each substation or switchgear/switchboard section. The connection to earth shall have a sufficiently low resistance to permit prompt operation of the circuit protective devices in the event of a ground fault, and to provide the required safety from shock to personnel who may be in the vicinity of equipment frames, enclosures, conductors or the electrodes themselves. The connection to earth shall consist of a number of individual driven ground rods connected to a grid or mat system. The grid or mat system shall consist of a number of buried conductors forming a network of squares or
rectangles. Buried conductors shall be interconnected at each crossover point by an exothermic welding process. The ground grid or mat shall be connected to ground rods as required. Grids or mats shall be buried in filled trenches or laid on earth and overlaid with at least 18 inches of backfill. Grid or mat locations shall be coordinated with utilities and sewer installations to avoid any direct electrical connection to these systems. The connections to earth shall be designed in such a way that the system resistance measured at the main ground bus in the switchgear room will not exceed two ohms. Grounding systems other than grids or mats that comply with the National Electrical Code may be provided as the location and design require. Medium voltage switchgear shall be grounded in accordance with IEEE 80 for step and touch potential.
Inside spaces with electrical distribution equipment or motors, a minimum 36 in. long by 2 in. copper ground bus shall be installed. These ground buses shall be connected to the main ground grid using copper cables.
2.8.6.5.2. Equipment Grounding
The equipment grounding system shall be interconnected by means of equipment ground conductors to a ground bus in each distribution equipment enclosure such as switchgear, panelboard, motor control center, load center and ground bus. Every feeder, circuit and branch circuit shall contain an equipment ground conductor within the same raceway with the phase and neutral conductors. The ground cable shall be connected to the ground bus located in the distribution equipment. The equipment ground conductor shall be copper, sized to comply with the National Electrical Code. The equipment ground conductor shall be continuous throughout the system, connecting all non-current carrying enclosures, tunnel structural grounds, and equipment and machinery grounds.
Medium voltage cable shields shall be grounded at each splice and cable termination.
The non-current carrying parts of electrical equipment, devices, panelboards and metallic raceways shall be bonded to the local ground bus.
2.8.6.5.3. Grounding for Personnel Safety
Grounding for personnel safety shall be provided to minimize shock hazards as follows:
• In pumping stations, all exposed metallic
structures, ductwork and piping shall be bonded to the local ground bus with a ground conductor.
doors, handrails within reach of electrical equipment (five ft+), and all stairways, shall be bonded to the nearest local ground bus.
• In all manholes, handholes and pull boxes, the ground conductors shall be connected to all exposed, metal surfaces including cable racks, frames and covers, ladders and cable shields (for medium voltage system). In addition, in manholes and handholes, a ground conductor shall be further connected to the ground mat installed at the base of each manhole or handhole or to a local ground electrode for the express purpose of providing for personnel safety. Where the ground grid, mat or local grounding electrode cannot be installed, a bare grounding conducting coil looping around the exterior of the manhole or handhole multiple times may be used as the local ground.
2.8.6.5.4. Grounding Materials
Driven ground rods shall be sectional ASTM 316 stainless steel rods, each ¾ in. by 10 ft long minimum, coupled and driven to the required total depth.
Buried grounding conductors used to form the ground grid, mat, or used to connect the individual ground rods, shall be stranded bare copper cable with a minimum size of #4/0 AWG. Equipment and systems grounding conductors shall be stranded copper covered with green insulation and be of minimum size in accordance with NFPA 70.
Ground bus shall be copper. Wall mounted ground bus shall be mounted on insulators above the floor.
Ground connections of cable to building steel, and cable-to-cable shall be exothermically welded. Exothermically welded connections shall be coated with coal tar epoxy or equivalent waterproofing coating to prevent corrosion of the connection.
Connections of ground cable to equipment ground lugs shall utilize two-bolt lugs.
2.8.6.5.5. Lightning Protection
Lightning protection systems and equipment shall be installed, where required, to provide protection of persons, equipment and facilities against the hazards posed by lightning. The lightning protection system shall comply with the requirements of a UL “Master Label System” as per UL 96A and NFPA 780.
2.8.7. Tunnel Architectural Systems
2.8.7.1. General
C2.8.7.1
The tunnel architectural systems shall be designed to provide functionality, durability, ease of maintenance, driver safety and orientation, and a uniform and pleasing aesthetic quality, while conforming to applicable codes
and standards. The texture, color, and patterns of ceilings, walls
and walkways, and of related elements, shall create a safe interior driving environment for the motorist that assists driver orientation. Only materials that are fire resistant, produce limited smoke generation and do not produce toxic fumes under fire conditions shall be used. Interior finishes in tunnels shall be ASTM E84 Class A (flame spread, 25 or less; smoke developed, 100 or less).
Architectural finishes provide lane definition, assist in legibility of signage through sign placement, and support rapid identification of emergency exits.
Finish materials shall be easy and economical to maintain. Surfaces shall be easy to clean with standard tunnel wash equipment, and should resist corrosion related to entry of water into finishes and connections. Except for items required to be breakaway, materials shall be impact resistant as far as is practical and shall be easily replaceable with matching colors and finishes in the event of damage.
The tunnel finish materials shall be selected with the goal of overall visual integration with other tunnel systems such as lighting, signage, ventilation, fire protection, drainage, egress and communications.
2.8.7.2. Egress Design
C2.8.7.2
Egress procedures and assumptions to be observed in the event of a fire in the tunnel shall be developed and coordinated with other tunnel systems and components. These procedures and assumptions shall be made in compliance with all applicable life-safety codes, agreements and interactions with the Owner, the authority having jurisdiction (AHJ), third parties and first responders.
Additional information regarding signing for tunnel egress can be found in NCHRP 20-59(47) Emergency Exit Signs and Marking Systems for Highway Tunnels.
An overall evacuation plan shall be developed that includes a system of tunnel egress which:
• Protects those who use the tunnel and other
related structures, including motorists, maintenance workers and emergency personnel.
• Creates a tunnel evacuation system with a logical and clearly marked path of egress within all portions of the tunnel to safe refuges within fire-protected areas of the tunnel, adjacent tunnel or safe refuge outside the tunnel.
2.8.7.3. Tunnel Occupant Load Calculations
The Engineer shall calculate tunnel egress requirements based on occupant loads as a multiple of the number of persons anticipated per lane of roadway per maximum length of roadway between exit access doors. Egress requirements shall be in compliance with NFPA 502.
Fire protection shall conform to the requirements of
NFPA 502. A hazard analysis as specified by the Owner shall be performed to determine which of the fire protection systems included herein are appropriate for inclusion in a specific tunnel. When required, systems shall conform to the minimum requirements described below.
An acceptable methodology for a hazard analysis by the Owner is presented in MIL-STD-882E Department of Defense Standard Practice, System Safety. This document is updated periodically with the ‘E’ designating the latest version as of August, 2013.
Consult the AHJ over fire life safety early in the process of determining the fire protection systems required for the tunnel. The systems shall be integrated into a comprehensive tunnel operation and emergency response plan.
Materials selected for use inside the tunnel for any purpose shall not produce toxic fumes or any other toxic by-product when heated.
2.8.8.1. Fire Alarm and Detection Systems
C2.8.8.1
Fire alarm and detection systems shall be provided in accordance with NFPA 502 and NFPA 72 as well as in accordance with all applicable local fire codes.
The fire alarm and detection system may consist of any number of appropriate devices that are integrated into a unified system that can be remotely monitored and can provide notification to tunnel operators, first responders and motorists of an emergency. Typical components of a fire alarm and detection system include automatic heat detectors in the tunnel roadway and ancillary spaces, automatic smoke detectors in ancillary spaces, duct smoke detectors, audible communications, closed circuit television cameras and manual fire alarm pull stations.
Fire detection systems shall be capable of identifying the location of the fire within predetermined zones in order to provide operators with enough information to engage the appropriate emergency response systems.
2.8.8.2. Tunnel Fire Suppression Systems
C2.8.8.2
Tunnel fire suppression systems shall be designed in accordance with NFPA 502 and all applicable NFPA and local standards. Tunnel fire suppression systems shall be designed and integrated with other fire and life-safety systems to produce a comprehensive system that provides functionality, redundancy, durability, ease of maintenance and safety.
Design criteria for required components of the tunnel fire suppression system shall be developed in conjunction with the authorities having jurisdiction incorporating applicable local codes.
Tunnel fire systems can consist of fixed water-based fire-fighting systems such as deluge systems or sprinklers, fixed dry-agent fire-fighting systems, manual fire-fighting systems such as standpipes, fire hydrants and portable fire extinguishers.
Water-based wet-fixed fire-fighting systems shall be protected against freezing.
Fire suppressions systems shall be designed to perform under the environmental conditions expected within the tunnel, cross passages and ancillary spaces.
Water supplies for fire-fighting shall be verified with the local utility. Flow quantities for fire-fighting shall be determined based on the design fire and the fire suppression system sized accordingly.
When standpipes are used, they shall be located within the roadway portion of the tunnel and shall not be embedded in the tunnel structure or the surrounding ground. They shall be protected from vehicle impact and easily accessible.
2.8.8.3. Structural Fire Protection
C2.8.8.3
Structural fire protection for tunnel interior components subjected to the tunnel design fire shall be in accordance with the latest edition of Guidelines for Structural Fire Resistance for Road Tunnels published by the International Tunnel Association (ITA).
The following shall be considered when selecting the protection for a particular application:
• Sacrificial layers and applied protection layers
will occupy space. Space is a premium commodity in underground construction and comes with added cost to the project.
• Applied protection layers usually are applied after the major construction work as a finish. This secondary work element adds time to the construction schedule in addition to the cost of the materials and their installation.
• Protection layers can be integrated into a system of finished architectural panels.
• Specially designed materials typically are more expensive than conventional materials.
• Attachments for applied protection layers must be suitable for the service conditions as well as for the fire conditions. Attachments shall be coordinated with the structural components and can contribute to tunnel leakage. Leakage behind the layers can add weight to the layer which will be transmitted to the supporting structure, as well as to the layer.
• Protection layers will obscure the structure being protected making direct observation and inspection difficult, even if the layers are designed to be easily removed.
• Protection layers shall be capable of surviving vehicle impacts and tunnel maintenance washing.
• The fire protection afforded by specially designed materials and sacrificial layers is immediate, whereas protection layers are not effective until installed.
• Protection layers function to reflect heat away from the structural elements and back into the tunnel environment. This heat reflection shall be accounted for in the design of the tunnel ventilation system.
Structural fire protection can take the form of an applied protection layer, sacrificial layers of the structural system, or materials designed to be fire resistant in the conditions produced by a the design fire.
2.8.9. Tunnel Security Systems
C2.8.9
Tunnel security systems shall be designed to ensure that the roadway, cross passages and ancillary spaces are
Security systems consist of Access Control Systems (ACS), Intrusion Detection Systems (IDS) and a security
secure from intrusion by unauthorized persons while also maintaining safe and accessible egress paths for motorists and tunnel personnel.
system (hardware and software) for monitoring and controlling the ACS and IDS.
When employed, tunnel security systems shall be monitored by the SCADA system.
The requirements for tunnel security including, and beyond, those shown above shall be established through a threat and vulnerability study. Tunnel security shall be integrated into a comprehensive tunnel operation and maintenance program.
Additional information can be found in NCHRP Report 525, Volume 12: Making Transportation Tunnels Safe and Secure.
2.8.10. Corrosion Control Systems
C2.8.10
Corrosion control falls into three main categories: soil and water corrosion control, stray current corrosion control and atmospheric corrosion control.
The purposes of the corrosion control systems are to:
• Avoid premature failure of any component
caused by corrosion during the specified timeframe assigned by the tunnel Owner,
• Minimize annual operating and maintenance costs associated with material deterioration, and
• Provide continuity of operation by minimizing corrosion failures.
2.8.10.1. Soil and Water Corrosion Control
Soil and groundwater characteristics shall be determined and documented during the subsurface investigation. Analysis of data collected from this survey shall be the basis for corrosion control designs. The soil and water samples shall be analyzed for resistivity (or conductivity), pH, chloride and sulfate ion concentrations. The results of the tests shall be used to develop corrosion protection through the use of materials that are resistant to the service conditions, coatings or sealants that protect materials against the service conditions, insulation, electrical continuity, passive cathodic protection and/or impressed current cathodic protection.
All components installed inside the tunnel shall be designed for use in wet conditions.
2.8.10.2. Stray Current Corrosion Control
The design shall minimize the effects of stray currents from facilities that can produce stray currents such as surface transit operations and electrical utility lines. Stray current control shall limit the level of stray currents at the source, under normal operating conditions, rather than trying to mitigate the corresponding effects.
2.8.10.3. Atmospheric Corrosion Control
C2.8.10.3
The atmospheric corrosion conditions, including vehicle exhaust concentrations, shall be derived from a baseline corrosion survey and from local climatological data. Designs and associated coating shall be designed to
Alternating wet and dry conditions can contribute to increased corrosion rates of exposed metal structures and hardware.
significantly decrease atmospheric corrosion rates. 2.8.11. Communication and Traffic Control
Intelligent Transportation Systems (ITS)
Tunnels may be provided with communications elements to allow communications between patrons and tunnel operating personnel, patrons and outside parties, first responders and tunnel operators, and interagency communications between first responders. Tunnels may also be provided with ITS utilizing internal traffic control and monitoring devices for general surveillance of traffic, detection of incidents such as stopped/slowed traffic or fire, and control of traffic flow in response to incidents within the tunnel or direct approach roads.
2.8.11.1. Communication Systems
C2.8.11.1
Tunnel communication systems may consist of the following elements:
• Telephone System: Includes emergency,
maintenance and operations type telephones located throughout the tunnel facility operating over copper or fiber optic wiring.
• Cellular Telephone System: Infrastructure including space, wiring, antennae and power for private cellular telephone companies to extend their service throughout the tunnel facility.
• AM/FM Radio Rebroadcast System: Provides a standard rebroadcast of commercial AM/FM radio frequencies into and throughout the tunnel system, such that the general public is capable of receiving the radio signals through the use of their own automobile radios. The system shall be equipped with an override system allowing for pre-recorded and ad-hoc messages to be broadcast over those frequencies in case of emergency.
• Two-way Radio System: Includes equipment to allow continuous communication between roadway maintenance and operation personnel while traveling through, or working within, the tunnel; or continuous radio communication for the agencies responsible for responding to emergencies within the tunnel or while traveling through the tunnel. The two-way radio system shall be capable of: o Eliminating interference with
communications re-transmitted within the tunnel, including commercial radio rebroadcasts and utility cell phone coverage or with radio communications external to the tunnel.
o Providing for adequate signal strength and signal-to-noise ratio through the tunnel.
the latest standards and adhere to all applicable telecommunication and radio laws and regulations. The tunnel communication system shall have a control system capable of monitoring and delivering communications and be monitored by the SCADA system.
2.8.11.2. Traffic Control and Monitoring ITS
ITS shall be utilized to stop traffic within the tunnel prior to an incident site, such as a fire, on the approach roads upstream of the incident, until it is deemed safe to proceed by the appropriate authorities. These systems are also used downstream of the incident site to expedite the flow of vehicles from the tunnel.
Tunnel traffic control and monitoring systems may
consist of the following elements: • A closed circuit television (CCTV) system for
tunnel and approach roadways shall be provided for general surveillance purposes to enable the remote control room operator to view any part of the tunnel interior, emergency escape routes and approach ramps. Generally cameras will have pan, tilt and zoom (PTZ) capability and be NEMA 4X rated. Dome type cameras shall be used within the tunnel and approaching roadways. Cameras shall be positioned so that if one camera fails, full coverage of the tunnel interior may be obtained by the use of the adjacent cameras on either side. SCADA system interfaces shall allow the nearest camera to an alarm event to be displayed automatically at the remote control room through the use of presets. The alarm event shall be captured through an automatic real-time recording feature for at least two cameras capturing alarm events simultaneously. The remote control room operator shall be able to manually start and stop the recording feature. Each camera image shall have an informational banner with identification, location, date and time in universal time coordinated format. Cameras shall also be provided to monitor the interior of the emergency egress points. At the remote control room, there shall be multiple monitors and recording facilities to assure adequate redundancy in the system. One or more screens shall cycle all the cameras at least once every 60 seconds, while at least one of the other displays a single picture selected by the
C2.8.11.2
Traffic control and monitoring ITS are part of an overall emergency response plan. These components should be integrated into the plan so that traffic operations are controlled in a manner that is complimentary to emergency response and evacuation of the tunnel.
remote control room operator as a “spot” monitor. System shall be scalable and expandable to allow future addition of cameras or monitors.
• Cameras for tunnel and approach roadways shall generally have heavy duty PTZ capability. Color cameras shall be used within the tunnel and approaching roadways. Cameras shall have the following additional attributes: − Solid state design − Vandal resistant enclosure − Automatic focus lenses with auto-iris PTZ,
controllable from the remote control room − Auto-PTZ controllable for preprogrammed
circumstances − No blooming when facing headlights − Low-light black and white mode − Digital “flipping” function
• A rigid mounting of the cameras is essential to provide vibration-free images on the monitors in the remote control room. Where cameras must be mounted on towers, the towers and enclosures must be designed to withstand a 110 mph wind load and still maintain a usable image. If not attached to sign support structures, cameras in the tunnel shall be located on the upper wall near the ceiling, over the maintenance walkway. An alternate location is above the center of the roadway, unless the section of roadway covered by the camera is in a horizontal curve, in which case the camera should be placed on the outside of the curve to maximize the limit of viewing. The exact location of each camera shall be determined through the above criteria tempered by the restrictions to the view induced by the vertical alignment, presence of overpasses, signs or other objects which may obstruct view of the target roadway area. Default PTZ orientation is to face downstream, with traffic flow. The remote control room operator shall also be able to start recording manually and stop recording any time. Every CCTV image shall include an information banner that shall include the camera’s identification, location, date and time in Universal Time Coordinated format. Remote control room operator shall be able to suppress this information when viewing the pictures but not amend or delete it.
• Lane use signals (LUS) shall be located along the tunnel walls and over the roadway at the tunnel portal approaches at regular intervals to indicate the status of each travel lane as either opened or closed, through the use red and green symbols on black background suitable for the full range of ambient lighting conditions where
located. Each LUS head shall be independently controlled to indicate the status of each lane and shall be fully interlocked to prevent any possible conflicting indications, with fault conditions at a signal head to show a blank face. Signal heads shall be double aspect light emitting diode (LED) displays suitable for use with bidirectional traffic. The LUS System shall be monitored and controlled from the remote control room. Traffic stop signals shall be provided to close the tunnel and prevent vehicles from entering, in the event of an emergency.
• Design and operation of the LUS system shall be in conformance to the Manual on Uniform Traffic Control Devices (MUTCD), Chapter 4J, Lane-Use Control Signals, latest edition, and ITE Vehicle Control Signal Heads. Each signal head display shall provide the following indications according to the latest version of the MUTCD: − Steady green arrow: indicating vehicle
traffic may travel in lane − Steady amber ( X ): indicating that a lane
control change is being made − Steady red ( X ): indicating vehicle traffic
shall not travel in or enter lane − Blank
• Interval spacing between signal heads in the tunnel shall be adjusted in areas where sight distance is restricted due to roadway alignment and/or other factors. For example, signal heads shall be more closely spaced on curves than on tangent sections within the tunnel to compensate for sight distance limitations. The specific spacing shall be computed based on horizontal and vertical curvature with the signal located outside the clearance envelope. The LUS display sequence for a lane closure shall use a minimum of two amber “X” indications before two red “X” indications. The spacing of the LUS shall support the MUTCD, and the requisite agency’s maintenance of traffic standards and requirements for setup distance for a “lane closure and buffer zone.” The minimum spacing of LUS shall be designed for the posted speed limit. All LUS system equipment shall be identical at each location within the tunnel and shall be completely interchangeable. The bottom of the LUS head housing shall be mounted above the established Minimum Vertical Clearance Height. LUS location shall be coordinated with the location of other tunnel systems to ensure minimal conflicts. Each LUS within the tunnel shall be visible from at least one CCTV camera. The LUS display elements shall incorporate LED’s with variable output levels to ensure that the
required legibility distances are maintained under all ambient light conditions, as established by the photo sensor control. All LUS shall use identical display technology. The LUS case shall enclose the display elements and withstand a wind loading of 110 mph. Construction and component mounting of the LUS system equipment shall be installed such that no damage is caused by vibration and shock during mounting, and normal operation on mounted support structure. Equipment shall be immune to electro-magnetic impulse signals generated in the tunnel environment due to proximity to other tunnel electrical, mechanical and communications equipment. Manual control of the LUS system shall also be provided at the local field controllers.
• Dynamic message signs (DMS) shall be full matrix type and shall be provided in the tunnel and tunnel approaches at regular intervals above the travel lanes to display instructions and emergency messages to motorists. The signs shall be based on arrays of white LEDs on a black background, visible in bright sunlight and dimmable to suit the full range of ambient lighting conditions. Sign messages shall be programmable by the operators at the remote control room. Signs shall have capability to display traffic management information, including warning and recommended diversions, as well as have inherent advanced fault detection and reporting and conformance to the National Transportation Communications for ITS Protocol (NTCIP) or other industry protocol. If communication with the remote control center is lost and the DMS has no reported errors, the DMS shall display a user-defined graphic/message. The DMS must not display erroneous information due to a fault with the sign or the loss of pixels.
2.8.12. Structural Systems
C2.8.12
Provisions for structural systems for main tunnel members for the various tunnel construction methodologies are specified in Sections 6, 7 and 8.
Structural systems for the support of tunnel ancillary features such as roadway slabs, equipment supports, suspended ceilings, architectural features, light fixtures, signals and signs shall be robust and provide fail safe redundancy against the risk of objects falling into the roadway. The following direction shall be used when designing the connections of ancillary features.
• Non-redundant connections for safety critical attachments shall not be used.
Safety critical attachments occur where the failure of the attachment would result in an ancillary feature falling into the roadway or walkway or cause damage to another critical tunnel element.
• Adhesive anchors in tension shall not be used. • Connections and supports shall be designed for
the load reversal effects caused by the air pressure from passing vehicles.
• Connections and supports shall be designed for dynamic effects as specified in Section 3. The dynamic effect load shall be applied to the supporting structure.
• Connections and supports shall be designed for the temperature they will experience during the design fire.
• Connectors, supporting members and the ancillary feature shall be designed for wet conditions and a highly corrosive environment. See Article 2.7.10.
• Structural connections and attachments shall be detailed in a manner that facilitates visual inspection and access to the connection or attachment for repairs or maintenance. Components of connections and attachments shall be detailed to be easily replaced.
• Connectors and base metal of ancillary features shall be checked for compatibility to ensure that dissimilar metals do not come into contact.
3.4. LOAD FACTORS AND COMBINATIONS 3.5. PERMANENT LOADS
3.5.1. Dead Loads: (DC, DW) 3.5.2. Earth Pressures: (EV, EH)
3.5.2.1. Cut-and-Cover and Immersed Tunnels in Soft Ground 3.5.2.2. Apparent Earth Pressure (AEP) Diagrams for Design of Braced Support of Excavation (SOE)
Walls used as Part of the Permanent Structure 3.5.2.2.1. AEP Diagram for Cohesionless Soil 3.5.2.2.2. AEP Diagrams for Cohesive Soil
3.5.2.2.2.1. Stiff to Hard Cohesive Soil 3.5.2.2.2.2. Medium Stiff to Stiff Cohesive Soil 3.5.2.2.2.3. Soft to Medium Stiff Cohesive Soil
3.5.2.3. Cut-and-Cover and Immersed Tunnels in Rock 3.5.2.4. Mined Soft Ground Tunnels 3.5.2.5. Mined Rock Tunnels
3.7. WATER LOADS: (WA, WAf, ,WAt, WAttsu) 3.8. AIR PRESSURE LOADS: (AP) 3.9. EARTHQUAKE EFFECT: (EQ) 3.10. FORCE EFFECTS DUE TO SUPERIMPOSED DEFORMATIONS: ( TU, TG, SH, CR, SE, PS)
This Section supplements the LRFD Specifications. As such, this Section specifies the minimum requirements for loads and forces, the limits of their application, load factors and load combinations used for the design of the following types of new highway tunnels:
• Cut-and-cover tunnel structures in soil and
rock • Mined tunnel structures in soil and rock • Bored tunnel structures in soil and rock
constructed using Tunnel Boring Machines (TBM)
• Immersed tunnel structures in soil and rock
The loads contained in this Section are applicable to the below grade portions of the tunnel facility only. Loadings for retained cuts, elevated structures, ancillary buildings and other related and support structures shall be taken from design specifications applicable to the structure.
Temporary conditions experienced during tunnel construction shall be analyzed and checked by the Engineer based on assumed construction methodology and sequencing. All assumptions regarding methodology and sequencing shall be shown on the contract documents. Contractors who elect to deviate from the assumptions shown in the drawings are responsible for verifying the adequacy of their design and for making required changes in the design based on their selected means and methods.
Specific design information can be found in the Sections devoted to each of the tunnel construction methodologies:
The construction methodologies used to construct each type of tunnel are significantly different. The ground loads and water loads imposed on the structures are dependent upon the construction methodology and particular design details as wells as ground and groundwater conditions. Also, the estimation of both ground and water loads have varying degrees of uncertainty, depending upon the interaction of subsurface conditions, groundwater conditions, tunnel lining stiffness and construction methodology. The load factors included in this Section were developed to address this uncertainty. Other tunnel specific loads including air pressure, blast loading, fire load, ship sinking and anchor drop, are not dependent upon the construction methodology.
Below grade portions of the facility include ancillary spaces such as mechanical and electrical equipment rooms and cross passages that are built between the roadway portions of the tunnel. Underground spaces constructed in conjunction with support buildings are not included in this specification.
For definitions not shown see the LRFD Specifications. Active Earth Pressure – Lateral pressure resulting from the retention of the soil by a structure or component that is tending to move away from the soil mass. Apparent Earth Pressure – Lateral pressure applied to an excavation supported either by multiple levels of internal bracing or ground anchors, which is sequentially excavated downward from the ground surface. Apparent earth pressure diagrams derived from empirical observations are used to design both the earth retention wall elements and the internal bracing elements or ground anchors. At-Rest Earth Pressure – Lateral pressure existing in the soil before it is disturbed by excavation. At-rest pressure magnitude is a function of the soil formation process and the stress history of the soil subsequent to formation. Conventionally Mined Tunnel – A tunnel mined using controlled blasting methods or some form of mechanical excavation other than a Tunnel Boring Machine (TBM). Design Lane – A notional traffic lane positioned transversely on the tunnel roadway. Design Water Depth – Water depth at mean high water in tidal water bodies, or defined flood stage for interior streams, or maximum pool elevation for streams controlled by dams or other flood control facilities, such as levees. Water depth used to determine design water pressures for sub-aqueous tunnels. Distortion – Change in structural geometry. Dynamic Load Allowance – An increase in the applied static force effects to account for the dynamic interaction between the tunnel and moving vehicles, and for air pressures associated with vehicle movement and the operation of tunnel ventilation systems. Extreme – A maximum or minimum. Final Lining – The permanent tunnel structure, constructed within an excavation which has been supported by temporary (initial) support elements installed concurrently with excavation. Fixtures – Fixtures include lighting, signs, signals, fans and other mechanical equipment, conduits, suspended ceilings, roadway slabs supporting traffic over ventilation ducts and other items attached to the tunnel walls. Lane – The area of roadway receiving one vehicle or one uniform load line. Lithification – The process for the formation of sedimentary rock from compaction or consolidation of soil under the weight of overlying material, often accompanied by the deposition of cementing material in the pore space of the soil. Normally Consolidated Soil – A soil for which the current effective overburden pressure is the same as the maximum pressure that has been experienced. Overconsolidated Soil – A soil that has been under greater overburden pressure than currently exists. Overconsolidated soils generally exhibit greater at-rest earth pressures than normally consolidated soils. Overconsolidation Ratio – Ratio of the maximum preconsolidation pressure to the overburden pressure. Passive Earth Pressure – Lateral pressure resulting from the soil’s resistance to the lateral movement of a structure or component into the soil mass.
Permanent Loads – Loads and forces that are, or are assumed to be, either constant upon completion of construction or varying only over a long time interval. Rock – Material that is formed by the lithification of soil, the cooling and solidification of molten magma, or the alteration of existing rocks under conditions of high pressure and high temperature. Soil – Natural material derived from the decomposition and weathering of rock. Soil that remains in place at its point of formation is called residual soil. Soil that is eroded, transported away from the location of its formation and deposited at a remote location is called transported soil. Squeezing Ground – Ground which generates potentially large pressures on final linings and causes distortion or failure of temporary (initial) support elements from the release of stresses which have developed over time. Squeezing ground generally consists of weak claystones mudstones, shales, overconsolidated clays and soft clays. Sub-aqueous Tunnel – A tunnel constructed beneath a body of water. Swelling Ground – Ground containing minerals that expand upon absorption of water, which can generate pressures on final linings and cause significant distortion or failure of temporary (initial) support elements. Temporary (Initial) Support Elements - Support elements installed concurrently with excavation which stabilize the opening until the final lining can be constructed. Certain types of these elements may perform the dual function of temporary (initial) support and final lining.
3.3. NOTATION 3.3.1. General For notation not shown below, refer to the LRFD Specifications A = cross sectional striking area of a dropped ship’s anchor (ft2) (3.14) AEP = apparent earth pressure for design of excavation support (3.5.2) B = excavated width or diameter of tunnel (ft) (3.5.2.5) DLF = dynamic load factor for dropped ship’s anchor (3.15) de = equivalent diameter of ship’s anchor striking area (ft) (3.15) Er = modulus of elasticity in the longitudinal direction of soil layer overlaying an immersed tunnel (lb/ft2) (3.14) F = triangular dynamic load pulse of a dropped ship’s anchor (3.14) H = total height of cut and cover tunnel (ft); depth of cut-and-cover excavation (ft) (3.5.2.2.1) (3.5.2.2.2) H1 = distance from ground surface to uppermost bracing level or ground anchor for a cut and cover tunnel excavation (ft) (3.5.2.2.2.1) Hn+1 = distance from base of excavation to lowermost bracing level or ground anchor for a cut and cover tunnel excavation (ft) (3.5.2.2.2.1) Hp = rock load (ksf) (3.5.2.5) Ht = excavated height or diameter of tunnel (ft) (3.5.2.5) k = average coefficient of lateral earth pressure (3.5.2.2.1) ma = weight of ship’s anchor in air (lb) (3.14) mw = weight of ship’s anchor reduced by the mass of displaced water (lb) (3.14) Npen = penetration parameter for dropped ship’s anchor (3.14) NS = stability number (3.5.2.2.2) P1 = load in highest level of bracing for internally braced excavations (kips) (3.5.2.2.1) (3.5.2.2.2) P2 = load in the next to highest level of bracing for internally braced excavations (kips) (3.5.2.2.1) (3.5.2.2.2) Pn = load in nth lower level of bracing for internally braced excavations (kips) pa = apparent earth pressure (ksf); maximum ordinate of pressure diagram (ksf) (3.5.2.2) Q = total factored load; tunneling quality index (3.4); (3.5.2.5) Qi = force effects (3.4) T = natural frequency of an immersed tunnel element (sec) Td = minimum duration of impact of a dropped ship’s anchor (sec) (3.14) vi = impact velocity of a dropped ship’s anchor (ft/s) (3.14) x = penetration depth of dropped ship’s anchor (ft) (3.14) γs = unit weight of soil (kcf) (3.5.2.2.1) (3.5.2.2.2) γi = load factor (3.4) γp = load factor (3.4) ηi = load modifier (3.4) 3.3.2. Loads and Load Designation The following permanent and transient loads shall be considered:
• Permanent Loads CR = force effects due to creep DC = dead load of structural components and nonstructural attachments DW = dead load of wearing surfaces and utilities EH = horizontal earth pressure load ES = earth surcharge including foundation surcharges EV = vertical pressure from soil and rock tunnels PI = loads due to piping systems inside the tunnel PS = secondary forces due to post-tensioning SE = effect of settlement of tunnel structure SH = force effects due to shrinkage
• Transient Loads AD = anchor drop AP = air pressure BL = force effect due to blast BR = vehicular braking force CE = vehicular centrifugal force CS = construction loadings EQ = earthquake load FI = force effect due to fire IA = attachment dynamic load allowance IM = vehicular dynamic load allowance LL = vehicular live load LS = live load surcharge PL = pedestrian live load SS = ship sinking TG = force effect due to temperature gradient TU = force effect due to uniform temperature WA = water load WAf = water load due to flooding WAt = transient water load Wtsu = water load due to tsunami
3.3.3. Abbreviations
BEM = boundary element method DEM = discrete element method FDM = finite difference method FEM = finite element method SOE = support of excavation ft = foot/feet ksf = kips per square foot psf = pounds per square foot sec = second
The total factored force effect shall be taken as:
Q = ΣηiγiQ i (3.4-1) Where: ηi = load modifier specified in Article 1.3.2 γI = load factors specified in Tables 3.4-1 and 3.4-2. Qi = force effects from loads specified herein
Components and connections of a tunnel shall satisfy Eq. 1.3.2.1-1 for the applicable combinations of factored extreme force effects as specified at each of the following general limit states:
• Strength T-I – Basic load combination relating
to permanent ground loading conditions that develop over time after the completion of construction, and to the normal vehicular use of the structure. This limit state shall also apply to normal (non-extreme) conditions for internal tunnel components and to initial ground support elements used to support permanent loads in the finished structure. Strength T-I shall be used for the design of fixtures supported by the main tunnel structure, and the connection of the supporting elements attaching those fixtures to the main tunnel structure. Fixtures are defined in Article 3.2.
• Strength T-II – Load combination relating to the temporary ground loads imposed during tunnel excavation. Also related to construction imposed loading on segmental tunnel linings and immersed tunnel segments during fabrication, transportation, handling and erection or placement. Also related to temporary (initial) ground support elements not incorporated into the final structure.
• Extreme Event T-I – Load combination including earthquake.
• Extreme Event T-II – Load combination relating to ship grounding/sinking, anchor drop, flood, tsunami, blast or fire. These loads shall not be combined with earthquake loads. The load combinations presented for Extreme Event T-II are the minimum checks required, since the likelihood of any of these loads occurring simultaneously is remote. The Owner may, at its discretion and after performing a hazard analysis create load combinations that have
C3.4
The load factors specified herein and the resistance factors specified in other sections of these Specifications were calibrated to provide designs with member proportions consistent with the current practice in tunnel design. Load factors that differ from the LRFD Specification do so due to this calibration. Load factors that were not calibrated were carried over from the LRFD Specification. This initial step in the calibration process is considered preliminary and further work is warranted to obtain a comprehensive calibration. Information regarding the process can be found in NCHRP Report 12-89.
These factors are calibrated and not developed from basic principles. Therefore, the Engineer may use pre-LRFD practice to verify/validate the design.
In Table 3.4-1, the variable γp represents load factors for all of the permanent loads, shown in the first column of load factors. This variable reflects that the Strength and Extreme Event limit state load factors for the various permanent loads are not single constants, but they can have two extreme values. Table 3.4-2 provides these two extreme values for the various permanent load factors, maximum and minimum. Permanent loads are always present, but the nature of uncertainty is that the actual loads may be more or less than the nominal specified design values. Therefore, maximum and minimum load factors reflect this uncertainty.
The maximum and minimum values of the load factors do not represent a range of values; rather they represent two values that should be used when developing possible load combinations. All possible combinations should be developed using the two values given. Load combinations using values between the maximum and minimum values should not be used for design.
more than one of these loads applied to the structure simultaneously. In this case, it is recommended that the load factors shown in Table 3.4-1 be used when combining the loads.
• Extreme Event T-III – Load combination relating to flood or tsunami used to check the resistance of the underground construction to the effects of buoyancy.
• Service T-I – Load combination relating to permanent ground loading conditions that develop over time after completion of construction and to the normal vehicular use of the structure. Also related to deflection control of mined and bored tunnel linings, cut and cover tunnel walls, floors and roofs, and crack control when concrete is used in construction of these elements.
• Service T-IA – Load combination relating to service level water loads used to check the resistance of the underground construction to the effects of buoyancy.
• Service T-II – Load combination relating to temporary ground loading conditions that develop during construction. Also related to control during construction of deflections in mined and bored tunnel linings, cut and cover tunnel walls, floors and roofs, and crack control when concrete is used in construction of these elements. Also related to buoyancy resistance of tunnels during construction.
• Fatigue T-I – Fracture and fatigue load combination related to infinite load-induced fatigue life. Fatigue T-I shall be used for the design of fixtures supported by the main tunnel structure, and the connection of the supporting elements attaching those fixtures to the main tunnel structure, that are subject to cyclic loading due to the wind pressure generated by passing vehicles or operation of the tunnel ventilation system. Fixtures are defined in Article 3.2.
The load factors for various loads comprising a load combination shall be taken as specified in Table 3.4-1. All relevant subsets of the load combinations shall be investigated. For each load combination, every load that is indicated to be taken into account and that is germane to the component being designed, including all significant effects due to distortion, shall be multiplied by the appropriate load factor.
The factors shall be selected to produce the total extreme factored force effect. For each load combination, both positive and negative extremes shall be investigated.
In load combinations where one force effect decreases another effect, the minimum value shall be applied to the load reducing the force effect. For permanent force effects, the load factor that produces the more critical combination shall be selected from Table 3.4-2. Where the permanent load increases the stability or load-carrying capacity of a component, the minimum value of the load factor for that permanent load shall also be investigated.
The larger of the two values provided for load factor of TU shall be used for deformations and the smaller values for all other effects. For simplified analysis of underground tunnel concrete structures in the strength limit state, a value of 0.50 for γTU may be used when calculating force effects, but shall be taken in conjunction with the gross moment of inertia of the structural components. When a refined analysis is completed for underground tunnel concrete structures in the strength limit state, a value of 1.0 for γTU shall be used in conjunction with a partially cracked moment of inertia determined by analysis. For underground tunnel concrete structures in the strength limit state, the value of 0.50 for γPS , γCR and γSH may similarly be used when calculating force effects, but shall be taken in conjunction with the gross moment of inertia of the structural components.
The load factor for temperature gradient, γTG, should be considered on a project-specific basis. In lieu of project-specific information to the contrary, γTG, may be taken as: • 0.0 at the strength and extreme event limit states, • 1.0 at the service limit state when live load is not considered, and • 0.50 at the service limit state when live load is
Dead loads shall include the weight of all components of the tunnel lining, ceilings, roadway slabs, appurtenances, mechanical equipment, signs, signals, systems and utilities attached thereto, wearing surface, future overlays and loads from planned or future development.
In the absence of more precise information, the unit weights specified in Table 3.5.1-1 of the LRFD Specifications may be used for dead loads.
C3.5.1 The LRFD Specifications include vertical earth pressure (EV) in the dead load grouping. This has been modified here to include the EV above cut and cover and immersed tunnels. EV is included in the dead load grouping for mined and bored tunnels. The calculation of this load is not always the weight of the earth (soil or rock) above the structure. Soil arching can occur when the tunnel is deep resulting in lower pressures. Rock loads will be a function of the characteristics of the rock. These loads should be determined by experienced geotechnical engineers.
3.5.2. Earth Pressures: (EV, EH)
Except as specified herein, horizontal earth load on cut and cover tunnels and immersed tunnels shall be determined in accordance with the LRFD Specifications.
C3.5.2 For soils retaining cohesive materials, the effects of soil creep should be taken into consideration in estimating the design earth pressures. Evaluation of soil creep is complex and requires duplication in the laboratory of the stress conditions in the field as discussed by Mitchell (1976).
3.5.2.1. Cut and Cover and Immersed Tunnels in Soft Ground
a) Vertical earth pressure (EV) for cut and cover
tunnels and immersed tunnels shall be the pressure resulting from the total thickness of backfill placed directly over the structure. Use moist unit weight above groundwater table and buoyant unit weight below ground water table or surface water level as for cut and cover tunnels. Use buoyant unit weight of backfill for immersed tunnels under the groundwater table or below the surface water level.
b) The earth pressure loads shall be applied taking into account construction stages and sequencing.
c) For immersed tunnels, EV shall include an allowance for siltation above the original design fill level that occurs during the service life of the tunnel. The siltation shall be estimated based upon historical records for the waterway and planned dredging if applicable.
d) The moist unit weight for backfill shall be determined by laboratory testing or recognized engineering values based on specified soil classification for backfill and shall be specified as a maximum permissible value in the construction specifications.
cover and immersed tube tunnels shall be determined in accordance with the LRFD Specifications, except as modified herein.
f) EH is generated by the soil and fill materials and, for immersed tube tunnels, any scour protection located above the fill. The properties of the soil and fill materials shall be well defined. The value of the EH shall be calculated based on the properties of the soil and specified fill materials. At-rest pressures shall be used in the design of tunnels during the permanent condition. The tunnel structure shall also be checked for the design case where the maximum vertical load is applied on the roof of the tunnel and active pressure on the sides of the tunnel.
3.5.2.2. Apparent Earth Pressure (AEP) Diagrams for Design of Braced Support of Excavation (SOE) Walls used as Part of the Permanent Structure
The provisions of this article shall apply only to
construction-stage loadings for braced or anchored SOE walls that are also used as the permanent structural walls for the tunnel in cut-and-cover construction. The provisions of this article only apply to construction stages where multiple levels of anchors or bracing is used. The AEP diagrams provided shall be used to check the walls for the temporary bracing or anchored condition for all stages of excavation. The design of these walls under permanent condition shall be performed separately as presented in the LRFD Specifications.
Except as specified herein, AEP diagrams shall be developed in accordance with the LRFD Specifications.
For top-down constructed structures, a soil-structure interaction analysis that takes into account construction staging, soil/wall stiffness and ground deformation shall be used to estimate lateral earth pressure and ground deformations.
C3.5.2.2 For permanent wall elements, numerical analyses,
as defined in Article 3.5.2.5.c.1, are commonly performed to assess structure and ground displacements, as well as determining load demands for the structural elements. Accordingly numerical analyses should typically be performed for most permanent wall applications. Use of AEP diagrams is considered more appropriate for preliminary design to estimate upper bound loads on the wall and on support braces and anchors.
Anchored walls have the bracing elements external to the excavation, braced walls have the bracing elements internal to the excavation.
AEP diagrams for the temporary braced condition of deep excavations have been developed on the basis of field measurements to approximate the distribution of lateral earth pressure upon SOE walls. The envelope of expected maximum pressures and their distribution along the SOE wall are affected by the excavation process and cannot be determined by conventional lateral earth pressure theory. These diagrams are used for the temporarily braced conditions. For permanent lateral earth loads, refer to the LRFD Specifications.
Construction of cut and cover tunnels in urban and suburban areas usually requires a braced excavation to control deformations and adverse impact on adjacent surficial development. When SOE walls are also used as the permanent walls, they are typically braced in a temporary condition until the permanent structural slabs are constructed. The provisions of this section are intended for checking the SOE walls in this temporary condition.
Bracing of excavations can consist of struts extending across the excavation or ground anchors
AEP for braced excavations in cohesionless soils shall be assumed to be distributed as shown in Figure 3.5.2.2.1-1. The magnitude of the load shall be taken as:
pa = 0.65k γsH (3.5.2.2.1-1) where: k = average of at-rest lateral earth pressure
coefficient and the active lateral earth pressure coefficient for semi-rigid SOE walls (diaphragm (slurry) walls, tangent pile walls or secant pile walls) or the active earth pressure coefficient for flexible walls (sheet piling, soldier piles, etc.).
γs = unit weight of soil (kcf) Use buoyant unit weight of soil below groundwater
and moist unit weight above groundwater table. Hydrostatic water pressure shall be applied in addition to the buoyant weight of soil.
H = total excavation depth (ft)
installed through the SOE wall into the adjacent ground. Use of ground anchors requires the acquisition of temporary construction easements and staging of the construction of the final structure to accommodate de-stressing of the ground anchors.
Support of braced excavations is designed on the basis of AEP diagrams. These diagrams are based on empirical experience and measurement of deformations and member stresses that have been acquired over the years.
For more commentary, please refer to Article C3.11.5.7 of the LRFD Specification
The at-rest and active lateral earth pressure coefficients for cohesionless soils can be estimated using equations included in Articles 3.11.5.2 and 3.11.5.7.1 of the LRFD Specifications, respectively.
In general, if necessary, numerical methods may be used to estimate deformations once the structural members are designed based on the AEP diagrams. The numerical methods shall not be used to evaluate the ground pressures for these structures unless approved by the Owner and verified by prior local experience.
Depending upon available right-of-way, cut and cover tunnels in exurban and rural areas can be constructed within open excavations with properly designed sideslopes.
See Figure 3.5.2.2.1-1. For anchored walls in cohesionless soils, the lateral
loads shall be developed based on the AEP diagrams presented in LRFD Specifications, except for the magnitude of the load (pa). The magnitude of the load (pa) shall be developed based on Equation 3.5.2.2.1-1.
Figure 3.5.2.2.1-1 AEP Distribution for Braced Walls in Cohesionless Soils
3.5.2.2.2. AEP Diagrams for Cohesive Soil
The AEP distribution for cohesive soils shall be determined using the stability number, NS. which shall be defined as:
NS = γsH (3.5.2.2.2-1) Su where:
γs = total unit weight of soil (kcf) H = total excavation depth (ft) Su = undrained shear strength
In the absence of laboratory test data, pa = 0.3γsH should be used for the maximum pressure ordinate when struts or ground anchors are prestressed or locked off at 75 percent of the
The magnitude of the load (pa) included in LRFD
Specifications is applicable for flexible walls only (e.g., sheet piling, soldier piles, etc.). Equation 3.5.2.2.1-1 included in this Section is applicable for semi-rigid walls (e.g., diaphragm walls, tangent or secant pile walls) as well as flexible walls.
C3.5.2.2.2
Laboratory or in-situ testing should be used to
determine representative values of undrained strength for final design.
unfactored design load or less. When struts or ground anchors are prestressed or locked off to 100 percent of the unfactored ground load or greater, a maximum pressure ordinate of pa = 0.4γsH may be used.
The AEP diagrams and earth pressures specified in this section are for braced excavations only. For anchored walls, AEP diagrams and earth pressures included in the LRFD Specifications shall be used.
3.5.2.2.2.1. Stiff to Hard Cohesive Soil
For braced excavations in stiff to hard
cohesive soil (NS ≤ 4), the earth pressure may be determined using Figure 3.5.2.2.2.1-1, with the maximum ordinate of the pressure diagram, pa, computed as:
pa = 0.2 γsH to 0.4 γsH (3.5.2.2.2.1-1)
Figure 3.5.2.2.2.1-1 Apparent Earth Pressure Distribution for Braced Walls in Stiff to Hard Cohesive Soils (FHWA, 1999)
3.5.2.2.2.2. Medium Stiff to Stiff Cohesive Soil
For soils with 4 < NS < 6, use the larger of pa from Eq. 3.5.2.5.2.1-1. 3.5.2.2.2.3. Soft to Medium Stiff Cohesive Soil
For braced excavations in soft to medium stiff cohesive soil (NS ≥ 6), the AEP diagrams and earth pressures included in the LRFD Specifications for anchored walls shall be used.
3.5.2.3. Cut and Cover and Immersed Tunnels in
Rock
a) Lateral rock pressure for design of cut and cover tunnels in rock shall be determined from analysis of tetrahedral or planar failure wedges defined by the vertical excavation line and the top of rock.
b) When data are insufficient to define tetrahedral wedges, lateral rock pressures shall be determined by analyzing planar failure wedges.
c) Analyses will require values for frictional and cohesion components of the shear strength of individual joints. Section 5 of these Specifications provides guidance on determination of these values, as well as representative values of joint friction for particular rock types.
d) Surcharge loads, such as weight of soil overburden and foundation effects shall be added to the wedge analyses.
e) Alternatively, a nominal rock load consistent with local practice for the design of building foundations in rock may be used.
f) For highly fractured and weathered rock, lateral rock pressures may be determined assuming properties of an equivalent soil material.
C3.5.2.3
Tetrahedral wedge geometry can be defined when there are sufficient data on joint dip angle and dip direction to define wedges. See Section 5 of these Specifications for definitions of these terms.
This will result in a conservative evaluation of lateral rock pressure.
Often, there is a gap between the excavated rock surface and the side of a cut and cover tunnel, and the gap is backfilled with soil. When rock excavation is required for an immersed tube tunnel, the excavated trench would be wider than the tunnel element, requiring placement of backfill in the space between the tunnel wall and the excavated rock surface. For these cases, the design also needs to consider the lateral pressure from the soil backfill.
3.5.2.4. Mined Soft Ground Tunnels
a) The EV for mined tunnels in soft ground may be taken as the pressure resulting from the total height of ground directly over the tunnel crown when the height of ground over the tunnel crown is two times the excavated width of the tunnel or less.
b) When the height of ground directly over the tunnel crown is greater than two times the excavated width of the tunnel, the minimum EV shall be the pressure resulting from a height of soil equal to two times the excavated width of the tunnel. The arching action of the
C3.5.2.4
Numerical analyses performed utilizing finite element or finite difference computer software may also be used to determine the earth pressures and resulting load effects for the design of mined soft ground tunnels.
soil shall be evaluated to determine if this minimum pressure should be increased to account for the additional height of soil over the tunnel crown.
c) EH shall be the at rest earth pressure. The EH shall vary linearly between the tunnel crown and tunnel invert.
The at-rest earth pressure immediately after excavation (short term) will be lower than the long term at-rest earth pressure that develops with time. At-rest earth pressure is related to both the material characteristics and the stress history of the geologic stratum in which the tunnel is constructed. Both the short term and long term at-rest pressure shall be checked in combination with lower and upper bounds of groundwater pressures.
3.5.2.5. Mined Rock Tunnels
Rock loads for initial ground support and final lining design may be computed by one of the following methods depending on ground conditions, geometry, size and complexity of the tunnel being designed. The appropriate method shall be selected based on the guidance in the Technical Manual for Design and Construction of Road Tunnels – Civil Elements (AASHTO, 2010).
Where commercially available software is used, the Engineer shall be responsible for the subsequent design.
a) Empirical Methods
1. Rock Load Method as presented in the
U.S. Army Corps of Engineers Manual EM- 1110-2-2901 Tunnels and Shafts in Rock (USACE, 1997). The rock load in ksf is equal to the unit weight of the rock determined by laboratory testing multiplied by the value Hp from Table 3.5.2.5-1. Initial and final support structural elements shall be designed using the final rock load Hp values given in Table 3.5.2.5-1. For rock conditions 4, 5, 6, 7 when above groundwater table, reduce the loads by 50 percent. C = B + Ht (3.5.2.5-1) Where
C3.5.2.5
Detailed information including guidance on the use of the methods described in this section can be found in (AASHTO, 2010). Each method has appropriate applications and limitations.
This method was originally developed by Terzaghi
(1946) and modified by Deere and Miller (1967). This method does not account for ground/structure interaction and the ability of a rock mass to self-support with or without reinforcement. It is based on an assumed rock wedge that is supported by the initial support or final lining. The size of the wedge is a function of the tunnel’s excavated width. This method yields conservative results for loadings on the supporting structure.
Table 3.5.2.5-1 is adapted from the U.S. Corps of Engineers manual EM-1110-2-2901. Refer to Section 5 for additional information.
2. Tunneling Quality Index (Q) developed by Barton, Lien and Lunde (1974), updated by Grimstad and Barton (1993).
3. Rock Mass Rating (RMR) system developed by Bieniawski (1989).
4. Ubiquitous Joint Method For conventional two-lane or three-lane
highway tunnels with a total excavation width in the range of 40 to 55 feet, use a maximum apex height of 15 feet.
An envelope is drawn through the wedge apices and the rock load (ksf) is the total weight of rock beneath the envelope, divided by the opening width.
This load shall be directly applied for the design of the final lining, using appropriate load factors and resistance factors, and using either total unit weight of the rock or buoyant unit weight, depending upon groundwater conditions.
The total unit weight rock load shall be used for design of temporary (initial) support such as rock reinforcement or direct rock support such as steel ribs or lattice girders, without considering water pressure (for cases where excavation drains the rock mass or substantially reduces water pressure on the joints).
b) Analytical Methods
1. Kirsch’s elastic closed form solution.
2. Ground reaction curves as developed by Hoek, Kaiser and Bawden (1995).
This method proposes the use of a Tunneling Quality Index (Q) for the determination of rock mass characteristics and tunnel support requirements. This method is based on field observation and case histories and does not take into account the concept of ground/structure interaction.
In this method, rock wedges developed from
discontinuity information developed by the subsurface investigation program are drawn to scale on a tunnel cross-section. Apparent dip angle and dip directions are plotted, oriented to the azimuth of the tunnel longitudinal axis. This method requires the assumption of a maximum wedge apex height. Apex heights greater than 15 feet would be unusual.
This method is limited to use with simple
geometries and material models and is therefore of limited practical value. This method is considered useful for checking the reasonableness of the design developed using other methodologies.
This method relates internal support pressure to
tunnel wall convergence. This method is based on an assumed elastic behavior of the support, and its ability to resist the deformation of the surrounding rock resulting from the excavation. Although this method accounts for structure ground interaction, it does not fully account for the ability of the rock to support itself with nominal resistance from the supporting structure.
4. Computer software that develops the size and shape of wedges formed in the rock mass surrounding a tunnel excavation based upon the geometry and orientation of the joint sets.
c) Numerical Methods
1. Numerical analysis programs that predict
the behavior of structures and the surrounding ground. Acceptable methods of analysis include finite element method (FEM), finite difference method (FDM), boundary element method (BEM), discrete element method (DEM) and hydrocodes.
This concept is based on the true behavior of rock bolts: to act as reinforcement of the rock arch around the opening.
3.5.3. Surcharge Loads: (ES)
Surcharge loads shall be calculated in accordance with the LRFD Specifications. Surcharge loads shall include loads from existing buildings and other infrastructure foundations, loads from planned or future development, and surcharge from future sedimentation or landslides that could be applied to the tunnel.
The factored soil stress increase on the tunnel wall or lining caused by concentrated surcharge loads or stresses from structures constructed adjacent to or above the tunnel alignment shall be calculated using the unfactored surcharge loads generated by the structure multiplied by the load factor ES.
C3.5.3
Concentrated surcharge loads induced by building and other foundations are typically the result of dead load, live load, wind load and possibly other loads that are associated with load factors other than ES. However, the controlling uncertainty in load prediction for surcharges is the transmission of the surcharge load through the soil to the tunnel below the surface. Hence, ES should be applied to the unfactored concentrated surcharge loads.
When available, construction plans and other information regarding existing buildings should be consulted to establish realistic surcharge load values and to determine the point of application of the loads relative to the position of the tunnel.
3.5.4. Piping Loads: (PI) Loads due to piping for water supply and
firefighting/protection piping shall be determined according to the following:
• Fixed fire suppression (deluge/sprinkler system) –
NFPA 13 • Standpipe – NFPA 14 • Water main serving both type systems – NFPA 24 • ½ - inch of ice accumulation around the entire
perimeter of an exposed pipe in conditions where ice can form on pipes
Tunnel components subject to highway vehicular loads and/or loads associated with specialized vehicles used for tunnel maintenance and operations shall be designed in accordance with the Section 3.6.1 of the LRFD Specifications.
The loading and characteristics of specialized equipment expected to operate inside the tunnel shall be specified by the owner.
Barriers or railing in the roadway portion of tunnels subject to traffic and or bicycle loads shall be designed in accordance with Section 13 of the LRFD Specifications.
C3.6.1.1 When designing for specialized equipment or an Owner-specified special permit design load, consideration should be given to lowering the live-load load factor of the Strength T-I limit state to 1.35 as suggested by the Strength T-II limit state of the LRFD Specifications.
3.6.1.2. Pedestrian Loads (PL)
A pedestrian load of 0.075 ksf shall be applied to components inside the roadway portion of the tunnel that are subject to pedestrian loads.
The pedestrian live load to be applied to components outside the roadway portion of the roadway shall be determined by the planned use for the space.
A minimum live load of 0.1 ksf shall be applied to suspended ceilings over roadways that enclose ventilation ducts large enough for personnel to enter. This load shall be applied to the suspended ceiling in a manner that causes the maximum load effect to the components, the connections and attachments that comprise the suspended ceiling systems.
Ancillary spaces such as staircases, corridors and cross passages adjacent to, or between tunnels, shall be designed for a minimum pedestrian live load of 0.15 ksf.
Spaces containing machinery should be designed for
a live load consistent with the use of the space and local building codes.
Pedestrian railings shall be designed in accordance with Section 13 of the LRFD Specifications.
C3.6.1.2 A detailed discussion of the development of the pedestrian load is given in the AASHTO LRFD Guide Specifications for Design of Pedestrian Bridges (AASHTO, 2009)
The minimum live load stipulated here accounts for
personnel carrying small tools and light equipment into the air duct for the purposes of maintenance and repairs. Tunnel owners may elect to design suspended ceilings for larger loads and should incorporate any special operational aspects of the tunnel into the design loading for the ceilings.
These underground spaces may be used in the event of an emergency evacuation of the tunnel and as such can be congested with motorists exiting the tunnel. Therefore a larger live load is specified for these spaces.
3.6.1.3. Vehicular Dynamic Load Allowance (IM) The static effect of the design truck or tandem, other
than centrifugal or braking force, shall be increased by the percentage specified in Article 3.6.2.1 of the LRFD Specifications.
water lines shall be increased to account for the dynamic effects of starting, stopping and operating the machinery and water lines.
The dynamic effect of mechanical equipment shall be obtained from equipment manufacturers or estimated and confirmed during construction from information provided by the equipment manufacturer.
The dynamic effects from water lines shall be determined as part of the design of the water system in accordance with Article 3.5.4.
Centrifugal forces and braking forces shall be computed in accordance Section 3 of the LRFD Specifications.
3.6.1.6. Live Load Surcharge (LS)
Live load surcharge forces shall be computed in accordance Section 3 of the LRFD Specifications.
3.7. WATER LOADS: (WA, WAf , WAt ,WAtsu)
WA during construction is dependent upon the construction methodology. See Sections 6, 7 and 8 for additional information.
Groundwater elevations shall be determined as part of the subsurface investigation program. Seasonal, flood and tidal variations in groundwater elevations shall be determined as part of the subsurface investigation program.
Water loads applied to the tunnel shall be the actual hydrostatic pressure applied to the tunnel as determined by the ground water elevation or water surface elevation as determined in the subsurface investigation. The hydrostatic pressure shall vary linearly across the height of the tunnel from crown to invert according to the depth.
Water loads for cases where the tunnel is constructed within a confined aquifer shall be based on the groundwater pressures within the aquifer.
Water loads may be reduced if an active drainage system is installed in the tunnel. If an active drainage system is installed, the actual loading applied to the tunnel shall be determined on the basis of groundwater flow rates. Design of the active drainage system shall include provisions for inspection and maintenance of the system.
Tunnels in coastal areas are subject to flooding and tsunamis. Tsunami and flood water surface elevations
A tsunami is in effect a very long period wave,
usually led by the trough. Typical effects of the trough are that it reduces water level in front of sea walls without giving time for the water level behind to fall,
levels shall be determined from historic data and/or modeling. The effects of tsunami and flood water shall be considered in the design and shall not be combined with other extreme loads.
When designing for resistance to buoyancy, only the permanent structural loads shall be used to resist the effects of buoyancy. The weight of items that could be removed from the tunnel such as machinery, lighting fixtures, signs, signals, and architectural finishes shall not be included in the calculation of the resisting forces.
See Article 8.7.1.2.1 for transient water loads for immersed tunnels.
toppling walls towards the sea.
3.8. AIR PRESSURE LOADS: (AP)
AP are developed from two sources: the tunnel ventilation system and vehicles passing through the tunnel.
Air pressures from the ventilation system shall be determined from the design of the tunnel ventilation system and shall be applied to the structure and/or structural system and its connections and attachments to the tunnel in accordance with the load combinations provided.
AP generated by passing vehicles shall be used in the design of attachments to the tunnel and supports for signs, signals, cameras, piping, suspended ceilings and any other feature located inside the roadway portion of the tunnel.
In the absence of more detailed analysis, the air pressure exerted on attachments to the tunnel interior due to passing vehicles may be taken as 10 psf. This force effect shall be applied in both directions and act in a load reversal manner.
The number of cycles for use in the fatigue design of attachments shall be determined based on the number of trucks expected to pass through the tunnel during the anticipated service life of the attachment.
C3.8
Guidance for the design of the tunnel ventilation system can be found in Section 2.
The transient air pressures from the ventilation system and the passing of vehicles can be the source of repetitive loading that requires a check for fatigue. The calculation of air pressures due to passing vehicles is a complex process. Hand methods for calculating air pressures from passing vehicles can be found in the following publication: Subway Environmental Design Handbook, Volume I, Principles and Applications by Associated Engineers, a Joint Venture of Parsons Brinckerhoff Quade & Douglas, Inc., DeLeuw Cather & Company, and Kaiser Engineers, (SEDH), Second Edition, 1976, NTIS PB254788. The maximum pressure acting on attachments to the tunnel interior induced by traffic can be estimated based on the calculation procedures for “Portal Entry” and “Portal Post Entry” in the Subway Environment Design Handbook Volume I. The “Portal Entry” and “Portal Post Entry” calculation procedure predicts the pressure profile in the vicinity of the vehicle entering the tunnel as the vehicle transits the portal and continues down the tunnel. The pressure profile can then be used to determine the maximum pressure transients acting on the tunnel structures. The calculation procedure is demonstrated in Example 3.11 in the handbook.
Alternately, the Subway Environment Simulation (SES) program can be utilized for predicting pressure transients in tunnels. This methodology was utilized to determine the 10 psf pressure value provided in Article 3.8. The data used in the analysis are as follows: Tunnel Data:
o Length = 8,000 feet o Cross-Sectional Area = 510 square feet o Perimeter = 94 feet o Friction Factor = 0.025
Truck Data:
o Length = 60 feet o Cross Sectional Area = 80 square feet o Perimeter = 36 feet o Skim Friction Coefficient = 1.0 o Drag Coefficient = 1.0 o Vehicle Speed = 50 mph
Cases Examined:
o Single truck through the tunnel o Two trucks side by side
The SES program software, reports and background
information can be obtained through the U.S. Department of Transportation, Research and Innovative Technology Administration, Volpe National Transportation Systems Center.
3.9. EARTHQUAKE EFFECTS: (EQ)
See Section 10 for calculation of earthquake effects.
3.10. FORCE EFFECTS DUE TO SUPERIMPOSED DEFORMATIONS: (TU, TG, SH, CR, SE, PS)
3.10.1. Uniform Temperature: (TU)
Tunnel elements subject to uniform temperature change shall be designed in accordance with provisions of Section 3.12 of the LRFD Specifications.
C3.10.1
Examples of tunnel elements that are subject to uniform temperature change included suspended ceilings roadway slabs that enclose ventilation ducts.
3.10.2. Temperature Gradient: (TG)
The temperature gradient between the inside face (face of tunnel lining exposed to the inside of the tunnel) of the tunnel lining and the outside face (face of tunnel lining adjacent to the ground) is a function of the average yearly variation in the outside ambient air temperature at the tunnel site. For normal ranges of tunnel lining thickness (< 30”), the temperature gradient between the inside and outside face of the tunnel lining may be estimated as 4º F per 20º F variation in yearly average outside ambient air temperature. The temperature gradient shall be checked with both the lower and higher temperature on the inside face of the tunnel lining.
Frictional restraint from the foundation and backfill material shall be considered and may influence longitudinal axial forces.
C3.10.2
The temperature gradient described in this Article is the gradient due to temperature differences between the inside and outside face of the tunnel lining due to seasonal changes in ambient air temperature inside the tunnel. This provision shall not be used to check the tunnel lining under fire conditions. For fire loading, see Article 3.12.
The provisions of this section are derived from research performed in the development of the SES program software. Reports and background information can be obtained through the U.S. Department of Transportation, Research & Innovative Technology Administration, Volpe National Transportation Systems Center.
Information regarding local ambient temperature yearly variations should be determined from historical records for the location of the tunnel. If more accurate
information is not available, the maximum and minimum values presented in Figures 3.12.2.2-1 and 3.12.2.2-2 respectively in the LRFD Specifications can be used.
3.10.3. Differential Shrinkage: (SH)
Where appropriate, differential shrinkage strains between concretes of different age and composition, and between concrete and steel, shall be determined with the provisions of Section 5 of the LRFD Specifications.
In the case of concrete immersed tunnels, adequate attention to details and construction methods shall be employed to minimize cracking resulting from shrinkage.
3.10.4. Creep: (CR)
Creep strains for concrete shall be in accordance with the provisions of Section 5 of the LRFD Specifications.
3.10.5. Settlement: (SE)
The effects of settlement of immersed and cut and cover tunnel structures shall be evaluated. Ground and structure settlement due to applied loads, dewatering, excavations, tunneling, pile driving and other construction activities shall be estimated in designs, using generally recognized procedures and methods of analysis.
Differential settlement due to varying ground and loading conditions longitudinally and transversely shall be analyzed and the effects of predicted settlement shall be accounted for.
See Article 8.7.1.1.1 for information regarding settlement of immersed tunnel sections.
When ground treatment is used that could degrade with time (for example ground freezing) the effects of settlement due to the degradation shall be considered regardless of the construction methodology used to construct the tunnel.
C3.10.5
The settlement described herein is the settlement of the tunnel structure and does not address the effects of settlement and ground movement caused by the tunnel excavation on adjacent existing structures and infrastructure. These effects should be evaluated as part of a tunnel project. Methodologies for predicting ground movements due to tunnel excavation can be found in Section 6 & 7 of these specifications.
3.10.6. Secondary Forces from Post-Tensioning: (PS)
The application of post-tensioning forces on a
continuous structure produces reactions at the supports and internal forces that are collectively called secondary forces, which shall be considered where applicable.
C3.10.6
In frame analysis software, secondary forces are generally obtained by subtracting the primary prestress forces from the total prestressing.
3.11. BLAST LOADING: (BL)
Where it has been determined that a tunnel or tunnel
component shall be designed for intentional or unintentional blast effects, the following shall be considered:
• Size of explosive charge • Shape of explosive charge • Type of explosive • Stand-off distance • Location of the charge • Possible modes of delivery and associated
capacities (e.g., maximum charge weight will depend upon the delivery method and can include pedestrians, cars, trucks, buses, etc.)
• Fragmentation associated with vehicle-delivered explosives
charge determine the intensity of the blast effect produced by an explosion. For comparison purposes, all explosive charges are typically converted to their equivalent TNT charge weights.
Explosions inside a tunnel create effects very different from an explosion in the open air. When an explosion occurs in a tunnel, the peak pressure and the impulse associated with the shock front are extremely high and amplified by the confining structure. Because of the close-in effects of the explosion and the amplification of the blast pressures due to reflections within the structure, the distribution of the shock loads on any one surface is non-uniform and extremely complicated.
An explosion in an underground structure produces an air blast that interacts with the tunnel surface and may also induce damage to the surrounding ground. The underground features (tunnel lining and surrounding ground) go through several cycles of expansion and rebound (reflection) due to the shock interactions between the tunnel boundary and the air. The rebound motion is considered to be the most critical with regard to the potential separation of a liner from its confining surrounding grout and/or ground. After or during separation and fragmentation of the liner, the dynamic wave propagates through the surrounding medium, potentially damaging adjacent structures and surface and underground utilities. The disruption of the intimate contact between the tunnel lining and the surrounding medium can cause additional instability in the tunnel lining due to the fact that the tunnel lining and surrounding ground rely on each other for stability. This loss of surrounding ground support can lead to secondary damage due to unanticipated loading on the lining resulting in collapse of the tunnel lining.
Other modes of failure of a tunnel lining due to blast include excessive cracking that lead to excessive water infiltration and flooding for tunnels under the groundwater table; breaching of the tunnel lining can also lead to flooding or the inflow of surrounding earth. The loss of ground surrounding the tunnel due a breach could lead to collapse of the tunnel lining or settlement at the surface.
Post blast analysis of the effects of the explosion should be a component of the investigation into the effects of the blast.
Information on blast loads and their effect on tunnels may be found in S. Choi (2009), FHWA (2003), NCHRP (2006).
3.12. FIRE LOAD: (FI)
Tunnel elements essential to the stability of the opening that are directly exposed to the effects of a vehicle fire in the roadway portion of the tunnel shall be
C3.12
Tunnel elements essential to the stability of the opening include any element in direct contact with the surrounding ground and any element that supports or
designed to resist the heat released by the design fire for a minimum of 1 hour. Analysis of heat transfer to the material that includes material behavior as it absorbs the heat shall be performed.
Alternately, essential tunnel elements can be protected from exposure to heat by sacrificial layers, protective coatings or protective boards. Protective measures shall be designed to provide protection against the heat released by the design fire for a minimum of 1 hour with a maximum temperature at the surface of the protected element of 250º Celsius.
braces an element in direct contact with the ground. Owners may elect to include other components or tunnel elements in this category at their discretion.
Additional information can be obtained from the ITA Guidelines for Structural Fire Resistance of Road Tunnels (Russell, 2004) and NCHRP Synthesis 415 (Maevski, 2011).
3.13. SHIP SINKING: (SS)
The primary sunken ship design case for Extreme Event Limit State T-II shall be assumed to result from a ship of the size approximating those using or expected to use the waterway and which has the most severe load effect on the tunnel. In lieu of more accurate data, the imposed loading of a sunken ship resting on the tunnel may be taken as a uniformly distributed loading over an area not exceeding the full width of the tunnel times a length as measured on the longitudinal axis of the tunnel of 100 ft.
The intensity of an appropriate uniformly distributed loading from a sunken ship may be determined by methods such as those outlined in Chapter 6 of the State-of-the-Art Report, 2nd Edition, International Tunneling Association Immersed and Floating Tunnels Working Group (Pergamon, 1997), where typical values may also be found.
If appropriate, based on marine traffic for Extreme Event T-II, a secondary sunken ship design case shall be assumed to consist of a smaller vessel, such as a ferry or barge, sinking and impacting the tunnel structure with the stem or sternpost in a manner similar to that of a dropped anchor. In lieu of more accurate data for the secondary sunken ship, a static equivalent concentrated load of 225 kips working on an area of 3.3 × 6.6 ft² directly on the tunnel roof may be considered.
3.14. ANCHOR DROP: (AD)
The effect of an anchor impacting the underwater tunnel structure directly or being dragged across the line of the tunnel structure shall be considered. The tunnel structure shall be designed either to resist the full loading imposed by the design anchor system or the fill and scour protection system shall be designed to mitigate the loading, in which case the tunnel structure may be designed for the reduced loading. Rupture of any waterproofing system shall not occur. The design anchor shall be selected as appropriate to shipping using or expected to use the waterway, based on the relevant section of the American Bureau of Shipping Rules.
The intensity of loads from falling anchors may be determined by methods such as those outlined in Chapter 6 of the State-of-the-Art Report, 2nd Edition, International Tunneling Association Immersed and Floating Tunnels Working Group (Pergamon, 1997), where typical values may also be found.
The penetration depth of a falling anchor through tunnel roof protection material shall be estimated. The formulae given in CEB Bulletin d’Information No 187, August 1988, reproduced below, may be used to calculate the anchor penetration depth in granular material.
𝑥 = 0.4𝑁𝑝𝑒𝑛𝑑𝑒 (3.14-1)
𝑁𝑝𝑒𝑛 = 3.28�3.0𝑚𝑤𝐸𝑟𝑑𝑒3
∙ 𝑣𝑖 (3.14-2)
𝑑𝑒 = �4𝐴𝜋
(3.14-3)
𝐴 = 6.5 + 0.005 am (3.14-4)
where
x = penetration depth (ft) penN = penetration parameter
ed = equivalent diameter of anchor striking area (ft)
wm = weight of anchor reduced by the mass of the displaced water (lb)
am =weight of anchor in air (lb)
rE =modulus of elasticity in the longitudinal direction of the layer (lb/ft²)
iv =impact velocity of anchor (ft/s) A =cross-sectional striking area of anchor (ft²)
Figure 3.14-1 Graph of Dynamic Load Factor (DLF) against 𝑻𝒅/ 𝑻
The calculated maximum penetration depth shall not
exceed 90% of the total depth of the protection layer covering the tunnel using the 5% fractal value for rE . The dynamic load factor (DLF) ratio of the static equivalent load on the tunnel roof to the triangular dynamic load pulse diw TvmF /= may be obtained from the Figure 3.14-1 using the minimum duration of impact id vxT /= (where x is calculated with the 95% fractal value for rE ), and the natural period T of the affected element.
3.15. CONSTRUCTION LOADS: (CS)
Recommended construction loading types for the tunnel construction methodologies are provided in the Sections specific to the methodology. The Contractor shall be responsible for verifying that the construction loadings can be supported by the tunnel structural components.
C3.15
The recommended construction loading types are based on typical loads used in the design of tunnel projects. The design for construction loads is highly dependent upon Contractor means and methods.
2. AASHTO. Technical Manual For Design Of Road Tunnels – Civil Elements, 2010
3. Associated Engineers, a Joint Venture of Parsons Brinckerhoff Quade & Douglas, Inc., DeLeuw Cather & Company, and Kaiser Engineers, “Subway Environmental Design Handbook, Volume I, Principles and Applications” (SEDH), Second Edition, 1976, NTIS PB254788
4. International Tunneling Association (ITA) (1988). “Guidelines for Structural Fire Resistance for Road Tunnels”, Working Group Number 6: Maintenance and Repair, May, 2004.
5. American Concrete Institute, “Guide for Determining the Fire Endurance of Concrete Elements (ACI 216R).
6. U.S. Army Corps of Engineers (USACE) (1997). "Engineering and Design, Tunnels and Shafts in Rock" EM 1 1 10-2-2901, May.
7. Terzaghi, K. (1946). "Rock Defects and Loads on Tunnel Supports" Rock Tunneling with Steel Support, R. V. Proctor and T. White, Commercial Shearing Co., Youngstown, OH: 15-99.
8. Deere, D.U., et al. (1967) “Design of Tunnel Liners and Support Systems”, PB 183799. Customer Services Clearing-house, U.S. Department of Commerce, Springfield, Virginia.
9. Barton, N., Lien, R., and Lunde, J. (1974). "Engineering Classification of Rock Masses for the Design of Tunnel Support” Rock Mechanics, Vol. 6, No. 4.
10. Grimstad, E. & Barton, N. (1993).”Updating of the Q-System for NMT. In Kompen, Opsahl & Berg (eds), Proc. Of the International Symposium on Sprayed Concrete – Modern Use of Wet Mix Sprayed Concrete for Underground Support, Fagernes.
11. Bieniawski, Z. T. (1992). “Design Methodology in Rock Engineering. 198 pp. Rotterdam: A.A.
12. Bieniawski, Z. T. (1989). “Engineering rock mass classifications: a complete manual for engineers and geologists in mining, civil and petroleum engineering. 251p., Wiley, New York:
13. International Tunneling Association (ITA) (1997). Immersed and Floating Tunnels State of the Art Report, Working Group No. 11., Tunneling and Underground Space Technology, Pergamon Press, Vol. 12, No.2 (April).
14. The International Federation for Structural Concrete, Information Bulletin No. 187: “Concrete Structures under Impact and Impulsive Loading”, August, 1988.
15. Mitchel, J.K. (1976), Fundamentals of Soil Behavior, J. Wiley and Sons
16. Choi, Sunghoon (2009). “Tunnel Stability Under Explosion”, Parsons Brinckerhoff, Incorporated, New York, New York.
17. McMahon, G.W., “Vulnerability of Transportation Tunnels to Terrorist Attack”, Bridge and Tunnel Vulnerability Workshop, Sponsored by Federal Highway Administration.
18. Russell, R.A. (2004). ITA Guidelines for Structural Fire Resistance of Road Tunnels, International Tunneling and Underground Space Association (ITA), Lausanne, Switzerland.
19. Federal Highway Administration (2003), “Recommendations for Bridge and Tunnel Security”, the Blue Ribbon
Panel on Bridge and Tunnel Security, FHWA.
20. Federal Highway Administration, Reference Manual on Earth Retaining Structures, Publication No. FHWA-NHI-99-025, April 1999.
21. Nation Cooperative Highway Research Program (NCHRP) (2006), “Making Transportation Tunnels Safe and Secure”, NCHRP Report 525, Transportation Research Board, Washington, D.C.
22. Kirsch, G., (1898). „Die theorie der elastizitat und die bedurfnisse der festigkeitslehre.“Veit. Deit. Ing. 42 (28), 797-807.
23. Maevski, I. (2011). Design Fires in Road Tunnels, NCHRP Synthesis 415, National Cooperative Highway Research Program (NCHRP) Synthesis, Washington, DC.
24. AASHTO (2009). LRFD Guide Specifications for Design of Pedestrian Bridges, 2nd Edition, American Association of State Highway and Transportation Officials (AASHTO), Washington, DC.
4.2. DEFINITIONS For definitions not shown, see the LRFD Specifications or the reference document associated with the term.
Aspect Ratio – the nominal length of an individual steel fiber divided by the diameter of the fiber. Dosage – the mass of fibers per unit volume of concrete. Flexural stress – force per unit area at a given distance from the neutral axis, determined using linear elastic assumptions. Lattice Girder— a ground support element consisting of an open lattice of three or four steel bars connected by lacing bars and encapsulated with shotcrete, which is sprayed through the open-work lattice. Modulus of rupture – the peak flexural strength of an unreinforced test beam. Net deflection – the deflection of a beam specimen, measured at mid-span, excluding any extraneous effects due to seating or twisting of the specimen on its supports, or deformation of the support and loading system. Neutral axis – the plane of zero stress in a member subjected to flexural stresses. Peak flexural strength – the maximum flexural tensile stress recorded in a beam test. For a fiber volume fraction of less than approximately 0.5 percent, the peak strength is equal to the modulus of rupture. Residual flexural strength - the flexural tensile stress recorded in a beam test at a stated mid-span deflection. Shotcrete - Sprayed concrete applied as dry mix or wet mix by pneumatic methods. Shotcrete may be either plain shotcrete, applied alone or reinforced with welded wire fabric, glass-reinforced plastic (GRP) fabric or steel fiber-reinforced shotcrete (SFRS), which includes deformed steel fibers distributed throughout the shotcrete. Thin layers of shotcrete (less than 4 in. [100 mm] nominal thickness) placed as surface protection to prevent raveling behaves as membranes. Greater thicknesses should be analyzed as structural members, either alone or compositely with lattice girders. Toughness - The amount of energy absorbed during the cracking process. When a FRC specimen is tested in flexure, toughness is defined as the area under the load-deflection curve up to a certain deflection. Toughness represents the ability of FRC to sustain load after it cracks. Volume fraction – the concentration of fibers in concrete, defined as the volume of fibers per unit volume of concrete.
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-5
The structural design of tunnel components shall conform to the requirements of the LRFD Specifications including all applicable interim changes, except as modified or supplemented herein.
4.4.2. Limit States
The structural behavior of components constructed from concrete, steel, or steel in combination with other materials, and wood shall be investigated for each stage that may be critical during the construction, handling, transportation and erection, as well as during the service life of the structure of which they are a part.
Structural components shall be proportioned to
satisfy the requirements at the strength, extreme event, service and fatigue limit states.
4.4.3. Load Combinations Components and connections shall be designed to
resist load combinations as specified in Section 3 at all stages during the life of the tunnel, including during construction. Load factors shall be as specified in Section 3.
4.5. CONCRETE STRUCTURES
4.5.1. Limit States
4.5.1.1. Service Limit State
The service limit state shall be investigated in accordance with the applicable requirements of Section 5 of the LRFD Specifications with the following modifications:
• The spacing s of mild steel reinforcement bars
in the layer closest to the tension face shall be 0.95 times that calculated in accordance with the LRFD Specifications.
• The provisions governing the control of cracking by distribution of reinforcement are not applicable to structural plain concrete.
• Deformations for structural plain concrete shall be calculated in accordance with the latest edition of the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318).
• Deformations for fiber reinforced shotcrete
C4.5.1.1
In order to better minimize and distribute
cracking in concrete structures, more restrictive criteria than that of the LRFD Specifications is presented as part of this design specification. Cracking is a source of leakage in underground structures. Limiting cracking through more restrictive service requirements combined with dense concrete mixes will tend to minimize leaks associated with cracking.
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-7
shall be calculated in accordance with the LRFD Specifications.
• The provisions governing the stress limits for concrete are not applicable to structural plain concrete or to fiber reinforced concrete.
4.5.1.2. Fatigue Limit State
The fatigue limit state shall be investigated in accordance with the applicable requirements of Section 5 of the LRFD Specifications with the following modifications:
• Fatigue shall be investigated for tunnel
roadway slabs constructed over ventilation ducts and any other concrete element subjected to repetitive loading.
• The fatigue limit state is not applicable to structural plain concrete or fiber reinforced concrete.
4.5.1.3. Strength Limit State
The strength limit state shall be investigated in accordance with the requirements of Section 5 of the LRFD Specifications with the following modifications.
• The provisions for segmental construction contained in Section 5 of the LRFD Specifications are not applicable to segmental concrete tunnel linings or to precast concrete immersed tunnel components.
C4.5.1.3
During tunnel construction, the tunnel excavation process disturbs the in-situ state of stress. An immediate state of equilibrium occurs that will result in initial load effects imposed on the tunnel structure. Strain within the surrounding ground continues to change with time until a final condition of equilibrium is reached which will result in the final load effects imposed on the tunnel structure. The load effects during both the short-term (or temporary) condition and the long-term (or final) are checked using the strength limit state.
In addition to the load effects produced by the
surrounding ground, construction processes may also produce load effects that are temporary, but must be resisted by the structural component. The strength limit state is used to check for construction-process imposed loadings. Examples of this type of load include the handling, transportation and installation loads imposed on precast concrete components such as segmental tunnel linings and concrete immersed tunnel components.
4.5.1.4. Extreme Event Limit State
The tunnel system, including appurtenances inside the tunnel shall be proportioned to resist collapse due to earthquake, tsunami, floods and fires.
The tunnel response due to blast shall be defined by
the Owner.
4.5.2. Reinforced and Prestressed Concrete
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-8
design of tunnel structural components constructed of normal weight or lightweight concrete and reinforced with steel bars, or welded wire reinforcement, and/or prestressed with strands, bars or wires.
4.5.2.1. Material Properties
Refer to Section 5 of the LRFD Specifications for reinforced or prestressed concrete material properties.
C4.5.2.1
Concrete mix designs should be formulated to result in dense concrete to minimize the potential of leakage through the concrete. Dense concrete mixes can be subject to shrinkage cracking. When designing dense concrete mixes, the effects of heat of hydration and the formation of shrinkage cracking should be considered, and the mix formulated to minimize shrinkage cracks. The intent of the mix design should be a durable concrete with low permeability.
4.5.2.2. Design Considerations
Components and connections shall be designed to resist load combinations as specified in Section 3 at all stages during the life of the tunnel, including during construction. Load factors shall be as specified in Section 3.
C4.5.2.2
The assumptions for the service, fatigue, strength and extreme, limit states given in Section 5 of the LRFD Specifications shall be used in the design of reinforced and prestressed concrete tunnel structural components for the force effects due to moment, axial force, shear and torsion.
The provisions for moment redistribution of the
LRFD Specifications may be used for the design of cut and cover and immersed tunnel box sections.
Redistribution of moments by empirical formulas
for cast-in-place concrete linings shall not be used for the design of bored or mined tunnel linings. Modeling redistribution of moments due cracking should be performed using analysis software that can account for the ground structure interaction.
Cast-in-place concrete linings for mined and bored
tunnels and precast segmental concrete linings for bored tunnels shall be designed for the combined effects of moment and axial load in accordance with Section 5 of the LRFD Specifications, with the following exceptions:
• The provisions for additional limits on
reinforcement for compression members specified in the LRFD Specifications shall not apply to tunnel linings for bored and mined tunnels.
Moment redistribution in tunnel linings will occur as the tunnel lining cracks, resulting in varying moments of inertia around the perimeter of the tunnel lining.
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-9
• The provisions for the approximate evaluation of slenderness effects specified in the LRFD Specifications shall not apply to tunnel linings for bored and mined tunnels. The effects of slenderness and stability for tunnel linings for mined and bored tunnels shall be accounted for through a numerical analysis that takes into account the effects of structure / ground interaction, eccentricity of loads, and the secondary effects of deformations of the lining under load.
• Tunnel linings for mined and bored tunnels and the walls, roof and floors of cut and cover and immersed tunnels shall be designed using the LRFD Specifications strength reduction factors for compression members with tie reinforcement.
• The reinforcement provided in cast-in-place concrete linings for bored and mined tunnels, placed parallel to the longitudinal axis of the tunnel, may be considered to provide the same confinement as ties as defined in the LRFD Specifications when the longitudinal reinforcement is placed outside the transverse or circumferential reinforcement.
• The provisions of Section 11 shall be used to determine the load effects associated with seismic events. The load effects determined in this manner shall be used for seismic detailing of reinforcement.
4.5.2.3. Details of Reinforcement
Reinforcement details shall be in accordance with the LRFD Specifications and as specified herein.
• Segments for precast segmental linings shall be
detailed as compression members. • Reinforcement placed parallel to the
longitudinal axis of the tunnel shall be terminated at the edges of the segments or at expansion, contraction and construction joints in cast-in-place linings with hooks or U-shaped bars spliced to the longitudinal bars.
4.5.3. Structural Plain Concrete
The provisions of this article apply to the design of tunnel structural components constructed of normal weight, or lightweight concrete not meeting the minimum reinforcement requirements of Section 5 of the LRFD Specifications or those specified in Article 4.5.4.
The structural design of tunnel components
constructed from structural plain concrete shall conform
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-10
to the requirements of the latest edition of the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318), except as modified or supplemented herein.
Definitions and Notation shall be as per the latest
edition of the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318). 4.5.3.1. Material Properties
• Compressive Strength: The specified compressive strength for concrete (f'c) shall not be less than 3.5 ksi. Otherwise the provisions of Chapter 1 of the latest edition of the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318) shall apply.
• Coefficient of Thermal Expansion: The coefficient of thermal expansion of structural plain concrete should be determined by laboratory tests on the specific mix used. In the absence of more precise data, the thermal coefficient of expansion may be taken as 6.0 10-6/°F for both normal weight and lightweight concrete.
• Shrinkage and Creep: Values for shrinkage and creep shall be in accordance with Section 5 of LRFD Specifications.
• Modulus of Elasticity: The modulus of elasticity of structural plain concrete shall be as per the latest edition of the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318).
• Poisson’s Ratio: Unless determined by physical tests, Poisson’s ratio may be assumed as 0.2. For components expected to be subject to cracking, the effect of Poisson’s ratio may be neglected.
C4.5.3.1
The 3.5 ksi minimum compressive strength is required here to accommodate uncertainties in loadings and the length of time required for the surrounding soil and groundwater to reach its permanent loading condition.
4.5.4. Fiber-Reinforced Concrete (FRC)
The provisions of this article apply to the design of tunnel structural components constructed of normal weight concrete and reinforced with steel fibers cut from cold drawn wire.
The use of FRC shall be limited to the following:
a) Members that are in continuous contact with
the ground on one face in a manner such that ground / structure interaction is ensured.
b) Members for which arch action minimizes flexural tension under all conditions of loading.
C4.5.4
The provisions of this article are based upon research summarized in “Fiber Reinforced Concrete for Precast Tunnel Structures” (Smith, 2011).
To limit flexural tension, and to maintain at least
half the segment thickness in compression, it is recommended to limit the eccentricity of the line of thrust (Mu/Nu) to h/3.
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-11
The use of synthetic fibers as structural reinforcement for concrete tunnel components is prohibited.
Concrete tunnel components are often exposed to extreme heat during fire incidents inside the tunnel. Exposure to high temperatures will cause synthetic fibers to melt which destroys their load carrying capacity. Synthetic fibers may be used to increase the resistance of the concrete to explosive spalling during a fire event.
If lightweight concrete is used in FRC, all design
parameters must be determined through laboratory testing.
The use of lightweight concrete is not recommended for FRC.
4.5.4.1. Material Properties
4.5.4.1.1. Steel Fibers
C4.5.4.1.1
Steel fibers shall be cut from cold drawn wire and shall meet the requirements of ASTM A820 fiber Type 1.
• Anchorage and Bond
Steel fibers shall be deformed to enhance the pull out strength of the fiber.
Deformed fibers are specified to increase the pullout strength of the fibers. The pullout strength combined with the bond strength between the fiber and the concrete accounts for the strength and toughness of the FRC.
Deformed fibers are available in a variety of configurations. The performance of FRC is dependent on the configuration of the deformation and surface qualities of the fiber, combined with the properties of the concrete mix.
Performance of the product selected for inclusion in the FRC shall be verified through testing.
• Tensile Strength
The minimum tensile strength of steel fibers shall be 150 ksi. The tensile strength of the fiber shall be such that that it will pull out of the concrete prior to breaking. The tensile strength and pullout performance of the steel fiber selected for inclusion in the FRC shall be verified by testing prior to production of the FRC.
• Aspect Ratio
The aspect ratio of individual fibers shall be greater than or equal to 65 and less than or equal to 80.
4.5.4.1.2. Fiber Reinforced Concrete
C4.5.4.1.2
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-12
The maximum 28-day compressive strength of concrete mixes used for FRC shall be limited to 8.0 ksi unless full scale testing is employed to verify that greater strengths may be employed without detrimental effect to the final structure. The minimum 28-day compressive strength shall be 4.0 ksi.
Higher concrete strengths may result in less ductile failure, due to more fibers breaking instead of gradually pulling out of the concrete matrix by deforming.
Concrete with a compressive strength of less than 4.0 ksi has not been extensively tested with fibers. Lower strengths could result in fibers pulling out of the concrete matrix prematurely.
• Coefficient of Thermal Expansion
Coefficient of Thermal Expansion shall be in accordance with LRFD Specifications.
• Shrinkage and Creep
Shrinkage and creep shall be in accordance with Section 5 of LRFD Specifications.
• Modulus of Elasticity
Prior to cracking, the modulus of elasticity of FRC may be calculated in accordance with LRFD Specifications. After cracking, the stress-strain response of FRC is nonlinear and shall be determined through laboratory testing of the specific concrete mix used for construction.
• Modulus of Rupture
Unless determined by physical tests, the modulus of rupture of FRC for the concrete strength limits shown above shall be determined in accordance with the LRFD Specifications.
In lieu of testing, the peak flexural strength of fiber-reinforced concrete may conservatively be taken as the modulus of rupture of the plain concrete.
The tensile strength of FRC shall be determined in accordance with Section 5 of LRFD Specifications.
4.5.4.2. Resistance Factors
C4.5.4.2
Resistance factors, φ, shall be taken as: See Smith (2007) for information regarding
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-13
4.5.4.3. Design for Flexure and Axial Force Effects
4.5.4.3.1. Assumptions for Service Limit State
C4.5.4.3.1
The strains in the concrete may be assumed to vary linearly, except in components or regions for which conventional strength of materials is inappropriate, for the design of FRC.
Examples of components for which the assumption of linearly varying strains may not be suitable include deep components such as deep beams, corbels, and brackets as defined in the LRFD Specifications.
4.5.4.3.2. Assumptions for Strength and Extreme Limit
States C4.5.4.3.2
When calculating combined axial and flexural strength, reduce the nominal section thickness by 1” to account for corrosion of the fibers on the tension face.
Factored resistance of FRC components shall be
based on the conditions of equilibrium and strain compatibility, the resistance factors as specified in Article 4.5.4.2, and the following assumptions:
• The maximum usable strain at the extreme concrete
compression fiber shall not be taken greater than 0.003.
• The relationship between concrete compressive
stress and strain, may be considered satisfied by an equivalent rectangular concrete compressive stress block of 0.85 f’c over a zone bounded by the edges of the cross-section and a straight line located parallel to the neutral axis at the distance a = β1c from the extreme compression fiber. The distance c shall be measured perpendicular to the neutral axis. The factor β1 shall be taken as 0.85 for concrete strengths not exceeding 4.0 ksi. For concrete strengths exceeding 4.0 ksi, β1 shall be reduced at a rate of 0.05 for each 1.0 ksi of strength in excess of 4.0 ksi, except that β1 shall not be taken to be less than 0.65.
The stress-strain relationships stated in this section are applicable to FRC meeting the requirements of this specification. Alternative stress-strain relationships can be derived in accordance with ACI 544.8 (Report on Indirect Method to Obtain Stress-Strain Response of Fiber-Reinforced Concrete, subject to approval by the owner.
Strain compatibility equations are typically used to generate interaction diagrams that define the capacity of a member under different flexural and axial load conditions. Load cases are plotted on the same diagram, and the factor of safety determined. The demand and capacity must be determined for a consistent length of tunnel, such as per linear foot.
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-14
• The maximum usable tensile strain shall be set at 0.005, which is estimated to occur at a test specimen deflection at mid-span of 3 mm as per ASTM C1609. The associated stress at the extreme tension fiber is σ3.
• The relationship between tensile stress in the fiber
matrix and strain, may be considered satisfied by an equivalent rectangular concrete tensile stress block of σ3 over a zone bounded by the edges of the cross-section and the neutral axis. Where σ3 = 0.37 f150,3.0 4.5.4.3.2-1 f150,3.0 is described in section 4.5.4.3.3
An equivalent rectangular stress block is
recommended. A more precise stress-strain relationship can be developed, with stress varying from zero at the neutral axis, increasing to a maximum value of σ2 close to the neutral axis, and reducing to σ3 at the extreme tension fiber. However, the added precision and complexity generally provide minimal increase in capacity.
The 0.37 factor converts the flexural stress
calculated assuming linear-elastic behavior to direct tensile stress assuming cracked behavior, with rectangular stress blocks. See Smith (2011) for more information.
4.5.4.3.3. Flexural Members
4.5.4.3.3.1. General
C4.5.4.3.3.1
For components in pure bending, the required fiber type and dosage shall be determined through the use of manufacturer’s data for specified concrete strengths.
Fiber reinforced concrete should not be used for permanent members in pure bending. However, for temporary load cases, such as handling loads on precast members, these provisions can be applied.
The manufacturer’s data should be certified by an
independent testing laboratory.
4.5.4.3.3.2. Factored Flexural Resistance
C4.5.4.3.3.2
The factored flexural resistance shall be taken as:
Mr = φMn (4.5.4.3.3.2-1)
where:
Mn = nominal flexural resistance = f150,3.0 x Sm
φ = resistance factor as specified in Article
4.4.4.2 Sm = Elastic section modulus f150,3.0 = Residual flexural strength, according
to the manufacturer’s data.
Residual flexural strength data is typically provided by manufacturers based on testing in accordance with ASTM C1609. Values are typically provided based on testing of standard specimens at a mid-span deflection of 3 mm. Manufacturers may provide the residual flexural strength directly, or as a percentage of the peak flexural strength.
The manufacturer’s data should be certified by an
independent testing laboratory. The analysis using residual flexural strength should use the full/gross section because, although cracked, the fibers engage throughout the section, to the extreme tension face. Residual flexural strengths reported in accordance with
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-15
ASTM C1609 tests are based on the full/gross section modulus of the test specimen.
4.5.4.3.3.3. Factored Early Age Flexural Resistance
Flexural testing of immature beam samples shall be used to determine the peak flexural strength. If flexural testing is not available, the following relationship may be used to determine the peak flexural strength.
𝑓𝑒,𝑑 = 𝑓𝑒,28 . �𝑓′𝑐,𝑑𝑓′𝑐,28
�23 (4.5.4.3.3.3-1)
where
fe,d = peak flexural strength at age in question fe,28 = peak flexural strength at 28 days f’c,d = compressive strength at age in question f’c,28 = compressive strength at 28 days
Unless otherwise permitted, compression members shall be analyzed with consideration of the effects of:
• Eccentricity • Axial Loads • Variable moments of inertia • Degree of end fixity • Deflections • Duration of loads
4.5.4.3.3.4. Factored Axial Resistance
The factored axial resistance shall be taken as:
Pr = φPn (4.5.4.3.3.4-1)
where:
Pn = (0.80)0.85 𝑓′𝑐𝐴1 (4.5.4.3.3.4-2) φ = resistance factor as specified in Article
4.5.4.2
4.5.4.3.3.5. Factored Axial / Moment Resistance
The factored axial / moment resistance shall account for the presence of both axial and flexural stress using the assumptions set forth in Articles 4.4.4.3.1 and 4.4.4.3.2
4.5.4.3.4. Bearing
4.5.4.3.4.1. General
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-16
b = width of the beam (in) h = height of beam (in)
4.5.4.3.6. Details of Reinforcement
4.5.4.3.6.1. Minimum Dosage Requirements
The minimum average three-dimensional fiber spacing shall be 0.45 times the nominal fiber length. In addition to the minimum fiber dosage provided below, the following fiber spacing relationship shall be satisfied:
𝑠 = �𝜋.𝑑𝑓2.𝑙𝑓
4.𝑣𝑓
3 (4.4.4.3.6.1-1)
where:
s = average distance between fibers (3D) vf = fiber volume fraction = volume of fibers per unit volume of FRC) df = diameter of fiber lf = length of fiber
The minimum fiber dosage shall be 40 lbs/yd3.
4.5.5. Shotcrete
The provisions of this section shall apply to the design of tunnel structural components constructed of reinforced, unreinforced, and fiber-reinforced shotcrete.
Unless otherwise specified herein, design of the
tunnel components constructed using reinforced shotcrete, unreinforced shotcrete and fiber-reinforced shotcrete shall conform to the requirements of Articles 4.5.2, 4.5.3 or 4.5.4 respectively.
Definitions and notation shall be as per the latest
edition of the American Concrete Institute’s Guide for Specifying Underground Shotcrete (ACI 506.5R).
Shotcrete proportions shall be selected that allow
suitable placement procedures using the delivery equipment selected and shall result in finished in-place hardened shotcrete meeting the strength requirements of this specification.
Supplementary cementitious materials like fly ash,
silica fume and slag cement may be used in shotcrete applications to improve workability, durability and chemical resistance.
Steel fibers may be used in shotcrete to improve the
C4.5.5
Applying shotcrete is a process of installing concrete at a high velocity. In general if applied correctly, its behavior and properties are not different from concrete. Shotcrete typically has a compressive strength similar to normal and high-strength concrete, but hardened properties are operator-dependent. Because shotcrete is installed at a high velocity, it will have a higher density than conventional concrete in most cases. The increased density will provide reduced permeability and higher durability. shotcrete mixtures can in fact exceed the compressive strength of most mixtures used for placed walls. The compaction that occurs during application of shotcrete helps achieve improved compressive strength and durability, and low water-cementitious material ratios of shotcrete mixtures produce other benefits, including reduced shrinkage and lower permeability. A well designed shotcrete mix that provides the proper material properties for shotcrete placement is essential to the successful installation of the product.
Proper placement is an important component in achieving good shotcrete results. Defects that occur in shotcrete can be due to poor placement. Proper handling of the material components of the shotcrete at the construction site is also an important component in achieving good shotcrete results.
Synthetic (polypropylene) fibers may be used
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-18
flexural strength, ductility, and toughness. Plastic fibers shall not be used for these purposes.
in shotcrete to improve placement, limit cracking and improve performance during fires. Synthetic fibers are not permitted for strength applications due to the fact that they melt during fire events.
4.5.5.1. Details of Reinforcement
Reinforcement details shall be in accordance with the LRFD Specifications, Article 4.4 of these Specifications and as specified herein:
• The maximum size of reinforcement shall be
No. 5 bars unless it is demonstrated by preconstruction tests that adequate encasement of larger bars will be achieved.
• When No. 5 or smaller bars are used, there shall
be a minimum clearance between parallel reinforcement bars of 2 ½ in. When bars larger than No. 5 are permitted, there shall be a minimum clearance between parallel bars equal to six diameters of the bars used. When two mats are used, the mat closer to the nozzle shall have a minimum spacing equal to 12 bar diameters, and the remaining mat shall have a minimum spacing of six bar diameter.
• The lap splices of reinforcing bars shall utilize the noncontact lap splice method with a minimum clearance of 2 in between bars. The use of contact lap splice necessary for the support of the reinforcement is permitted when approved based on satisfactory preconstruction tests that show that adequate encasement of the bars will be achieved, and provided that the splice is oriented so that a plane through the center of the spliced bars is perpendicular to the surface of the shotcrete.
C4.5.5.1
For additional information with regard to reinforcement details see the latest edition of “ACI 506R Guide to Shotcrete, Reported by ACI Committee 506”, and the “International Building Code”.
4.6. STEEL STRUCTURES
The provisions of this section shall apply to the design of tunnel structural components constructed of steel components, splices and connections for girder and beam structures, frames, trusses and arches and any load carrying steel system, as applicable.
Framed structures shall be analyzed by rational methods which consider the effects of relative stiffness of connected members, relative displacement, rotation of joints and the effects of axial deformations. Consideration shall be given to the variations inelastic properties and stress distribution of complex frameworks resulting from different construction sequences. Any
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-19
limitations on construction operations inherent in the design assumptions shall be noted on the project drawings and specified in the special provisions. Conversely, advantage may be taken of specified construction procedures, or sequences, to effect a more favorable distribution of loads or stresses. 4.6.1. Material Properties
Refer to Section 6 of the LRFD Specifications for structural steel material properties.
4.7. WOOD STRUCTURES
The provisions of this section shall apply to the design of tunnel structural components constructed of wood components, splices and connections for girder and beam structures, frames, trusses and arches and any load carrying wood system, as applicable.
Framed structures shall be analyzed by rational methods which consider the effects of relative stiffness of connected members, relative displacement, rotation of joints, and the effects of axial deformations. Any limitations on construction operations inherent in the design assumptions shall be noted on the project drawings and specified in the special provisions. Conversely, advantage may be taken of specified construction procedures or sequences to effect a more favorable distribution of loads or stresses.
4.7.1. Material Properties
Refer to Section 8 of the LRFD Specifications for wood material properties.
4.8. LATTICE GIRDERS
4.8.1. General
C4.8.1
The provisions of this section shall apply to the design of permanent tunnel structural components constructed using steel lattice girders.
Lattice girders are structural members made of bars with an open lattice. The girder, once encapsulated with shotcrete that has sufficiently cured, can provide initial support for openings and acts together with the shotcrete as an arch. The shotcrete between the girders acts as lagging between these arches.
The structural design of tunnel components constructed from lattice girders shall conform to the requirements of the Article 4.5 of this specification except as modified or supplemented herein:
• All lattice girders shall consist of three or four
primary retaining bars connected by stiffening components.
• The spacing and size of the bars used in a
The main advantage of the lattice girder is its ability in working with shotcrete. Because of the open nature of the lattice girder's construction, shotcrete passes through it, reducing the potential for unconsolidated shotcrete areas behind the girder. The shotcrete can be applied evenly producing an integral lining. On the other hand, some minimum stiffness is required to make sure that the individual bars in the lattice girder will not deform during the shotcrete application process.
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-20
lattice girder shall be designed to allow shotcrete to penetrate into, and behind, the girder with a minimum of shotcrete shadows.
• The maximum centerline spacing of the stiffening components shall be less than three times the height of the girder to assure stability against buckling.
Some important aspects for designing the lattice girders that shall be considered during the design process are:
• Required rigidity, capacity and moment
characteristics. • Avoidance of shotcrete voids (shadowing) for
complete encapsulation. • Required shotcrete lining thickness and cover
to steel.
Lattice girders provide important functions in the tunneling process. They could be used as:
• Temporary ground support once encapsulated with shotcrete.
• Support for steel bar or fabric reinforcement of the shotcrete lining shell.
• Add to the reinforcement for the overall completed lining.
• Provide a template for initial ground support limits.
4.8.1.1. Definitions
Refer to Articles 5.2 and 6.2 of the LRFD Specifications for definitions applicable to this section.
4.8.1.2. Notation
Refer to Articles 5.3 and 6.3 of the LRFD Specifications for notation applicable to this section.
4.8.2. Material Properties
Refer to Articles 4.4 and 4.5 for material properties.
4.8.3. Limit States
4.8.3.1. General
The structural behavior of the components made of steel, or steel in combination with concrete shall be investigated for each stage that may be critical during construction, handling, transportation and erection as well as during the service life of the structure of which they are part.
Structural components shall be proportioned to satisfy the requirements at strength, extreme event, service and fatigue limit states.
4.8.3.2. Service Limit State
The service limit state shall be investigated in accordance with the requirements of Section 5 of the LRFD Specifications unless noted otherwise in this section.
• For composite sections, the service limit state
shall be investigated in accordance with the
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-21
requirements of Section 6 of the LRFD Specifications.
4.8.3.3. Fatigue Limit State
The fatigue limit state does not apply to lattice girders.
4.8.3.4. Strength Limit State
The strength limit state shall be investigated in accordance with the requirements of Sections 5 and 6 of the LRFD Specifications, as applicable.
The strength limit state shall take into account both temporary and final conditions of the load resisting systems (composite or non-composite).
Resistance factors shall be in accordance with the requirements of Sections 5 and 6 of the LRFD Specifications.
4.8.3.5. Extreme Event Limit State
C4.7.3.5
The lattice girder system, which is part of the permanent loads resisting system in the tunnel, shall be proportioned to resist collapse due to earthquake, tsunami, floods and fires.
The tunnel response due to blast shall be defined by the Owner.
The blast referenced here is due to vehicle accidents or intentional explosions intended to do harm to the tunnel.
4.8.4. Design Considerations
Components and connections shall be designed to resist load combinations as specified in Section 3 at all stages during the life of the tunnel, including during construction. Load factors shall be as specified in Section 3.
Unless otherwise specified herein, design shall be in accordance with Sections 5 and 6 of the LRFD Specifications.
4.8.5. Design for Flexural and Axial Force Effects
The assumptions for the service, strength and extreme event limits given in Article 4.4.1 shall be used in the design of lattice girder system.
4.8.6. Shear
The design for shear shall be in accordance with the provisions of Section 6 of the LRFD Specifications.
SECTION 4: STRUCTURAL MATERIALS AND DESIGN CONSIDERATIONS 4-22
The provisions of this section shall apply to the geotechnical considerations for planning, design, evaluation and rehabilitation of road tunnels. Geotechnical considerations that shall be evaluated for tunnels include, but are not limited to:
• Historical, physical and structural geology • Identification and characterization of soil and rock
units • Identification of variation of ground conditions
along tunnel alignment • Identification of geological hazards (e.g., landslides,
rock fall, debris flows, fault zones) • Determination of the groundwater regime and
evaluation of short-term and long-term impacts of the tunnel on it
• Evaluation of soil permeability and rock conductivity for dewatering considerations
• Identification of type and location of hazardous and explosive gases in the ground
• Identification of hazardous materials requiring mitigation or special disposal methods
• Identification and location of underground obstructions and boulders
• Identification and location of karst terrain, caverns, abandoned mines and development of mitigation measures
• Identification of seismicity and seismic hazards • Identification of potentially active faults along the
tunnel alignment or nearby • Location of volcanic centers/hydrothermal activity • Characterization of groundwater chemistry. • Development of ground loads for tunnel design
(short-term and long-term loadings) • Development of hydrostatic pressures for tunnel
design • Evaluation of ground loss, ground deformations and
consolidation settlement caused by tunneling • Evaluation of long-term and time-dependent ground
settlement • Evaluation of impact of tunneling on nearby
structures and utilities • Evaluation of disposal or use of excavated material • Evaluation of the seismic response of the tunnel • Evaluation of soil and rock conditions for
application of ground improvement measures • Evaluation of ground behavior for soft ground
tunnels • Evaluation of rock jointing for stability analysis and
rock reinforcement design • Evaluation of rock abrasion characteristics for
tunnel boring machine (TBM) cutter wear • Ground/structure interaction analysis • Stability analysis for soil and rock slopes for cut-
C5.1
The geotechnical considerations presented in this section include the geotechnical investigation program and the development of geotechnical parameters for planning, design and construction of road tunnels. The information gathered during the investigation program will also be used for geotechnical design analyses, constructability studies and the development of geotechnical reports. The provisions included in this section are based on the guidelines presented in Technical Manual for Design and Construction of Road Tunnels – Civil Elements (AASHTO, 2010), relevant provisions included in AASHTO LRFD Bridge Design Specifications (herein referred to as LRFD Specifications), and the FHWA Geotechnical Engineering Circular No. 5 – Evaluation of Soil and Rock Properties (FHWA, 2002).
Successful planning, design and construction of a highway tunnel require the use of various types of investigative techniques to obtain a broad spectrum of pertinent topographic, geologic, subsurface, hydrogeological and structural information and data. Although most of these techniques and procedures are similar to those applied for roadway and bridge projects, the specific scope, objective and focus of the investigations are considerably different for tunnel projects and can vary significantly with subsurface conditions and tunneling methods.
5.2 DEFINITIONS Bedding— The layering in a sedimentary rock unit. Rock units range from thinly bedded (less than 2 inches between bedding planes) to massively bedded (greater than 10 feet between bedding planes). Bedding Plane— A smooth, continuous plane in a sedimentary rock unit, parallel to the bedding; often a weakness plane. Boulder — A rock fragment with a maximum edge dimension greater than 12 inches, with no upward limit. Boulders may be found disseminated throughout deposits of cohesionless and cohesive soil or may be concentrated in dense layers. Breccia — A zone of fractured rock often associated with a fault. If the breccia zone is porous, it can be a source of significant initial and sustained water inflows into a tunnel excavation. The breccia zone may include soil material filling the pore space between rock fragments. Cobble — A rock fragment ranging in size from 3 inches to 12 inches. Cobbles may be found disseminated throughout deposits of cohesionless and cohesive soil or may be concentrated in dense layers. Core Stone — A rock fragment with a maximum edge dimension greater than 12 inches, with no upward limit. Core stones are masses of rock surrounded by highly weathered to decomposed rock as a result of in-situ weathering. Cohesionless Soil — A soil composed of visible soil particles. Cohesionless soils include non-plastic silt, sand, gravel, cobbles and boulders. Shear strength of cohesionless soils is developed from inter-granular friction. Many natural cohesionless soil deposits have a minor fraction of cohesive soil which adds a cohesion component to shear strength. Cohesive soil — A soil composed principally of sub-microscopic particles. Cohesive soils generally are characterized as clays and plastic silts. Electrical charges on the surface of the plate or rod-shaped soil particles develop a shear strength component that is known as cohesion. Many natural deposits of cohesive soils have minor fractions of cohesionless soil. Fault — A fracture in a rock mass with significant indication of displacement. Fault thickness may range from inches, to hundreds of feet. Gouge — A zone of finely comminuted soil and rock material often associated with a fault. Gouge material can range from clay to sand size, but may include cobble or boulder size material in the same gouge zone. Igneous rock — Rock formed by the cooling and solidification of molten magma. Examples of igneous rocks include basalt, gabbro, granite and rhyolite. Joint — A fracture in the rock mass with minimal indication of displacement. Joints are secondary features that develop after formation of the rock unit. Joints may be formed by tensile or shear forces caused by cooling of magma, or folding or faulting of the rock unit. Lithification — The process for the formation of sedimentary rock from compaction or consolidation of soil under the weight of overlying material and often accompanied by the deposition of cementing material in the pore space of the soil. Metamorphic Rock — Rock formed by the alteration of existing rock under conditions of high temperature and high pressure. Examples of metamorphic rocks include gneiss, phyllite, schist and slate. Parting — A thin layer of one type of sedimentary rock embedded within a sedimentary rock unit of different lithology. Often a weakness plane, such as a shale parting in a sandstone or limestone unit. Rock — Material that is formed by the lithification of soil, the cooling and solidification of molten magma or the alteration of existing rocks under conditions of high pressure and high temperature.
Sedimentary Rock — Rock formed by the lithification of deposits of sand, silt, clay, gravel, cobbles and boulders, or the precipitation of calcium carbonate and magnesium carbonate from water.. Examples of sedimentary rocks include claystone, conglomerate, dolomite, limestone, mudstone, sandstone, shale and siltstone. Shear — A fracture in the rock mass that has an indication of displacement. Shear Zone — An area of concentrated shears. Distance between individual shears in a shear zone can be highly variable. Slickenside — A smooth, polished and often striated surface on a rock joint or in stiff to hard cohesive soils. Soil — Natural material derived from the decomposition and weathering of rock. Soil that remains in place at its point of formation is called residual soil. Soil that is eroded, transported away from the location of its formation and deposited at a remote location is called transported soil.
For notation not shown below, refer to the LRFD Specifications
c = Cohesion (ksf) (C5.5.4) c’ = Effective cohesion (ksf) (5.5.2.2) (C5.5.4) ch = Horizontal coefficient of consolidation (C5.4.5) Dr = Relative density (C5.4.5) D = Disturbance Factor (C5.5.5) E = Young’s modulus (ksf) (C5.4.5) Emax = Small-strain Young’s modulus (ksf) Em = Rock mass deformation modulus (ksf) (C5.5.5) Ei = Elastic modulus (ksf) (C5.5.5) eo = In-situ void ratio(C5.4.5) Gmax = Small-strain shear modulus (ksf) (C5.4.5) (C5.4.6) Ja = Joint alteration number (C5.5.4) Jn = Joint set number (C5.5.4) Jr = Joint roughness number (C5.5.4) Jv = Total number of joints per m3 (C5.5.4)
Jw = Joint water number (C5.5.4) kh = Horizontal hydraulic conductivity (ft/day) (C5.4.5) Ko = Coefficient of at-rest earth pressure (C5.4.5) mv = Volumetric compressibility coefficient (ft2/kip) (C5.4.5) po = Initial mask liftoff pressure (ksf) (C5.4.5) ρtot = Total density (pcf) (C5.4.5) Q = Tunnel Quality Index (C5.5.4) (C5.5.5) RQD = Rock Quality Designation (C5.5.4) SRF = Stress reduction factor (C5.4.5) (C5.5.4) St = Soil Sensitivity (C5.4.5) su = Undrained shear strength (ksf) (C5.4.5) uo = Initial pore water pressure (ksf) (C5.4.5) ν = Poisson’s ratio (C5.5.6) Vs = Shear wave velocity (ft/sec) (5.4.6) σho′ = In-situ horizontal effective stress (ksf) (C5.4.5) σp′ = Preconsolidation stress (ksf) (C5.4.5) σ1 = Major principal stress (ksf) (C5.5.4) σ3 = Minor principal stress (ksf) (C5.5.4) σt = Tensile strength (point load) (ksf) (C5.5.4) σ’ = Effective normal stress (ksf) (C5.5.4) σc = Unconfined compressive strength (ksf) (C5.5.4) φ = Friction angle (degrees) (C5.5.4) φr = Friction angle (degrees) (C5.5.4) 5.3.2 Abbreviations
ATV Acoustic Televiewers BVS Borehole Video System BWI Bit Wear Index CLI Cutter Life Index CPT Cone Penetration Test CPTu Piezocone Penetrometer CD Consolidated drained CU Consolidated undrained DMT Flat Plate Dilatometer DRI Drilling Rate Index
ft Foot/feet GBR Geotechnical Baseline Report GDM Geotechnical Design Memoranda GDR Geotechnical Data Report GSI Geological Strength Index ksf kips per square footNATM New Austrian Tunneling Method OCR Overconsolidation ratio PMT Pressuremeter RMR Rock Mass Rating RQD Rock Quality Designation SASW Spectral analysis of surface waves SCPTu Seismic CPTu SEM Sequential Excavation Method SOE Support of excavation SPT Standard Penetration Test TBM Tunnel boring machine UU Unconsolidated undrained VST Vane Shear Test
5.4.1 General A geotechnical investigation program for a tunnel
project shall use appropriate means and methods to obtain necessary characteristics and properties as the basis for planning, design and construction of the tunnel and the related underground facilities, to identify the potential construction risks, and to establish a realistic cost estimate and schedule. At a minimum, the geotechnical investigation shall be performed to provide information required for evaluation of stability of excavation/tunneling, design of the tunnel structures, impact of excavation/tunneling on nearby utilities and structures, groundwater conditions and the short-term and long-term impact of tunnel construction on these conditions, and identification of potential obstructions and construction risks.
The extent of the investigation program shall be consistent with the project scope (i.e., location, size, budget), the project objectives (i.e., risk tolerance, long-term performance), anticipated site variability and the project constraints (i.e., geometry, constructability, third-party impacts, aesthetics and environmental impact).
C5.4.1 Geotechnical investigation includes all preparatory
office and field work to develop the subsurface investigation and field/laboratory testing program, as well as the subsurface investigation program and other investigations needed to obtain information for planning, design and construction of a tunnel. In general, an investigation program for a highway tunnel project may include the following components:
• Existing information collection and study • Surveys and site reconnaissance • Geologic mapping • Subsurface investigations • Field and laboratory testing • Geophysical testing • Test pits • Environmental studies • Seismicity evaluation • Geospatial data management • Groundwater evaluation
Because the alignment and profile may often deviate from those originally anticipated, phasing of the subsurface investigations may provide an economical and rational approach for adjusting to these deviations.
The elements of the process that shall precede the subsurface investigation and field and laboratory testing program, include a review of local and regional geological publications, a search and review of published and unpublished information at and near the project location, review of available imagery and a detailed field reconnaissance of the tunnel site and its immediate vicinity. The geotechnical investigation program shall be developed by a geotechnical engineer knowledgeable of tunnel design and construction requirements. The geotechnical investigation program shall be appropriate for the project requirements and data needs for planning, design and construction.
The performance of a geotechnical investigation program is part of the process of obtaining information relevant for the design and construction of tunnels. Examples of unpublished information include subsurface data and construction records from the tunnel owner, and from government agencies, utility owners and private owners of nearby structures and facilities. Unpublished information may also be available from individuals with local experience from previous projects in the vicinity of the tunnel, or with tunneling within the same geologic formations.
Special investigation needs based on tunneling methods and anticipated geology along the tunnel profile are summarized in Tables C5.4.1-1 and C.5.4.1-2, respectively.
The guidelines included in this article for the
geotechnical investigation program shall be considered as minimum requirements and shall be supplemented with additional investigation and testing depending on the nature of the project and the subsurface conditions encountered during investigation.
At a minimum, the subsurface investigation component of the geotechnical investigation shall consist of borings, Standard Penetration Tests (SPT), undisturbed sampling in cohesive soils, double or triple tube core barrel coring and packer testing in rock, groundwater measurements and Cone Penetration Tests (CPT) within soil deposits and laboratory testing on
For further information on planning a geotechnical investigation program, refer to Technical Manual for Design and Construction of Road Tunnels – Civil Elements (AASHTO, 2010), the FHWA reference manual on subsurface investigations (FHWA, 2002a), and the AASHTO Bridge Design Specifications (2014),
recovered soil/rock samples, supplemented with other in-situ soil and rock tests as necessary to meet project requirements.
5.4.2 Office Studies
Available materials relating to geological and
subsurface conditions shall be reviewed and evaluated prior to developing a boring program for the project. A list of documents reviewed shall be included in the Geotechnical Data Report (GDR). Potentially relevant actions are included herein.
C5.4.2
Detailed office study of existing reports, geology references, historic maps and evaluation of imagery can maximize the results of the geotechnical investigation program by providing a general overview of the project geologic environment for planning of the subsurface investigation program, as well as indications of potential problem zones that may require detailed investigation. Maps and imagery are also useful in assessing accessibility to possible boring locations, especially in rural or mountainous locations, where construction of access roads or clearing of landing zones for helicopter access for drill rigs may be required.
5.4.2.1 Review of Existing Reports
Existing reports may include geologic reports and maps prepared by the U.S. Geological Survey and the various state geological agencies, county agricultural soil maps published by the U.S. Department of Agriculture and geotechnical and/or construction reports prepared for other projects in the tunnel project vicinity.
C5.4.2.1
Geologic reports discuss the mineralogy of the different rock types in the project area, structural geology and groundwater. County agricultural soil maps, published by the U.S. Department of Agriculture, are on an aerial photographic base, show the distribution of rock types and agricultural soils within the county and contain various engineering soil classification properties, using the AASHTO system, for the different soils. Geotechnical reports prepared for other projects in the tunnel project vicinity may contain extremely valuable information, irrespective of project age. Construction reports, if available, can also provide extremely useful information.
5.4.2.2 Review of Historic Maps
Historic maps may include both planimetric and topographic maps.
C5.4.2.2
Historic maps show terrain conditions and changes in terrain conditions. Locations of former marsh areas and stream areas shown on historic maps can be of assistance in planning effective boring programs, by identifying locations of potentially adverse ground conditions. Historic maps can also provide useful insight about previous land use and site development.
5.4.2.3 Imagery Evaluation
Various types of imagery may include those from various U.S. government and state agencies, county and municipal governments, regional planning agencies and private companies.
C5.4.2.3
Imagery evaluation is important for evaluating possible stability issues at portal locations in both rural and urban locations, where tunnels may pass though significant geologic features, and where inactive landslides may be present. Based on the evaluation using imagery, relocation of portals may be indicated as desirable within overall project location constraints.
Table C5.4.1-1 Special Investigation Needs Related to Tunneling Methods (after AASHTO, 2010; Bickel et al. 1996)
Cut-and-Cover Plan exploration to obtain design parameters and define groundwater conditions for excavation support, assessment of excavation base stability, seepage analyses and dewatering or seepage cut-off systems. Also, define conditions to reliably determine appropriate and cost-effective location to change from cut and cover to mined tunnel or Tunnel Boring Machine (TBM) tunnel construction.
Drill and Blast Obtain data needed to determine blasting patterns and sequencing, estimate stand-up time, determine initial (temporary) support requirements, estimate overbreak and evaluate water inflow.
Rock Tunnel Boring Machine (TBM)
Obtain data on rock hardness to determine cutter ware, cutter costs and penetration rate. Obtain data to estimate stand-up time to determine if an open-type machine will be acceptable or if a fully shielded TBM is necessary. Determine adequacy of rock to support loads from gripper pads. Evaluation of immediate and sustained water inflow.
Roadheader Obtain data on jointing to evaluate if roadheader will be plucking out small joint blocks or must grind rock away, and to estimate overbreak. Obtain data on hardness of rock to predict cutter/pick costs and advance rate. Also, evaluation of immediate and sustained water inflow.
Shielded Soft Ground TBM Obtain data for assessing stand-up time important to face stability and the need for breasting at the face, as well as to determine the requirements for filling tail void. Need to fully characterize and define limits of potential mixed-face conditions. Also, evaluation of immediate and sustained water inflow.
Pressurized-Face (Earth Pressure Balance and Slurry Shield) Tunnel Boring Machines
Obtain reliable estimate of groundwater pressures and of strength and permeability of soil (or rock) to be tunneled. Obtain data to predict size, distribution and amount of boulders or core stones. Need to fully characterize and define limits of potential mixed-face conditions.
Compressed Air Borings must not be drilled on the alignment, and must be carefully grouted to avoid loss of compressed air into old boreholes. Identify strata where air loss may occur. Identify hazardous gases that may migrate due to tunnel air pressure.
Requires comprehensive geotechnical data and analysis to predict behavior, and to classify the ground conditions and ground support systems into multiple categories based on the estimated behavior. Obtain data for design of tunnel supports. Assess potential for ground swelling. Also, evaluate water inflow.
Immersed Tube Obtain soil data to reliably design dredged slopes, to predict rebound of the dredged trench, estimate settlement of the completed immersed tube structure and evaluate liquefaction potential. Also identify and assess potential obstructions and/or rock ledges. Determine appropriate and cost-effective locations to transition to cut and cover or bored/mined tunnels. Obtain data for excavation support for trenching that extends inboard of shoreline.
Jacked Box Tunneling Obtain data to estimate soil skin friction and to determine the method of excavation and support/ground treatment needed at the heading. Also obtain data to design reaction thrust blocks. For launch and receiving pits, obtain data as noted for cut and cover tunnels.
Portal Construction Obtain data to determine appropriate and cost-effective location of portals and to design temporary and final portal structures. For portal excavations, obtain data as noted for cut and cover tunnels.
Shafts Obtain data to identify appropriate and cost-effective locations for the shafts. At each shaft location, obtain data as noted for cut and cover tunnels.
Table C5.4.1-2 Geotechnical Investigation Needs Dictated by Geology (AASHTO, 2010; Bickel et al., 1996)
Hard or Abrasive Rock • Difficult and expensive for TBM or roadheader to excavate. Investigate, obtain samples and conduct lab tests to provide parameters needed to predict rate of advance and cutter costs.
Mixed-Face • Especially difficult for wheel type TBM • Particularly difficult tunneling condition in soil and in rock. Should be
characterized carefully to determine nature and behavior of mixed-face and approximate length of tunnel likely to be affected for each mixed-face condition. Size and concentration of boulders or core stones in mixed-face conditions often are significant.
Karst • Potentially large cavities along joints, especially at intersection of master joint systems; small but sometimes very large and very long caves capable of generating undesirably large sustained inflows of groundwater.
Gypsum/Pyrite • Potential for removal of soluble gypsum or expansion of pyrite because of change of groundwater conditions during and after construction. Evaluate muck disposal issues.
Salt or Potash • Creep characteristics and, in some cases, thermal-mechanical characteristics are very important.
Saprolite • Investigate for relict structure that might affect behavior. • Depth and degree of weathering; important to characterize especially if
tunneling near rock-soil boundary. • Different rock types exhibit vastly differing weathering profiles. • Size and concentration of core stones in saprolitic soils often are significant
parameters. High In-Situ Stress • Could strongly affect stand-up time, support requirements and deformation
patterns, both in soil and rock tunnels. Should evaluate for rock bursts or popping rock in particularly deep tunnels. Conduct in-situ stress testing, such as hydrofracture tests or overcoring, where applicable.
Low In-Situ Stress • Investigate for open joints that dramatically reduce rock mass strength and modulus and increase permeability. Often potential problem for portals in downcut valleys and particularly in topographic “noses” where considerable relief of strain could occur.
Hard Fissured or Slickensided Soil
• Lab tests often overestimate mass physical strength of soil. Large-scale testing and/or exploratory shafts/adits may be appropriate.
Gassy Ground • Test for hazardous, toxic and explosive gasses: methane and hydrogen sulfide as a minimum.
Adverse Geological Features
• Faults Known or suspected active faults. Investigate to determine location and
estimate likely ground motion. Inactive faults are still sources of potentially difficult tunneling conditions
o Faults sometimes act as dams and other times as drainage paths to groundwater.
o Fault gouge sometimes a problem for strength and modulus. High temperature groundwater
• Collect samples for chemical tests. • Sedimentary Formations Frequently highly jointed Concretions could be problem for TBM Presence of weak layers or seams
Table C5.4.1-2 (Cont’d) Geotechnical Investigation Needs Dictated by Geology (AASHTO, 2010; Bickel et al., 1996)
Adverse Geological Features (Continued)
Groundwater o Groundwater is one of the most difficult and costly problems to
control. Must investigate to predict groundwater as reliably as possible.
o Site characterization should investigate for signs of and nature of: • Groundwater pressure • Groundwater flow • Artesian pressure • Multiple aquifers • Higher pressure in deeper aquifer • Groundwater perched on top of impermeable layer in mixed-face
condition • Anomalous or abrupt
o Aggressive groundwater • Soluble sulfates that attack concrete and shotcrete • Pyrites • Acidic
o Short-term and long-term impact of tunnel on groundwater regime requires evaluation.
• Lava or Volcanic Formation Highly variable rock types and conditions Flow tops and flow bottoms frequently are very permeable and difficult
tunneling ground Lava Tubes Vertical borings do not disclose the nature of columnar jointing. Need
inclined borings Potential for significant groundwater flows from columnar jointing
• Boulders (sometimes nests of boulders) frequently rest at base of strata. Cobbles and boulders not always encountered in borings which could be
misleading. Should predict size, number and distribution of boulders on basis of
outcrops and geology Core stones in saprolitic soils are analogous to boulders in glacial or alluvial
soils. • Beach and Fine Sugar Sands Very little cohesion. Need to evaluate stand-up time.
• Glacial deposits Deposits may be very heterogeneous. Boulders frequently associated with glacial deposits. Must actively
investigate for size, number and distribution of boulders. Some glacial deposits are so hard and brittle that they are jointed, and
ground behavior is affected by the jointing, as well as properties of the matrix of the deposit.
• Permafrost and frozen soils Special soil sampling techniques required. Thermal-mechanical properties required.
Table C5.4.1-2 (Cont’d) Geotechnical Investigation Needs Dictated by Geology (AASHTO, 2010; Bickel et al., 1996)
Manmade Features • Contaminated groundwater/soil/rock Check for movement of contaminated plume caused by changes in
groundwater regime as a result of construction. • Existing Obstructions Piles Miscellaneous shoreline structures Previously constructed tunnels Tiebacks extending out into alignment
• Existing Utilities • Age and condition of overlying or adjacent utilities within zone of influence.
5.4.3 Field Reconnaissance and Geologic Mapping
A field reconnaissance shall be performed after
preliminary selection of boring locations to confirm preliminary decisions made on the basis of the office study. The field reconnaissance shall also include detailed study of portal locations and any intermediate construction shaft locations, as well as an assessment of special accessibility requirements for selected borings.
Where outcrops of rock formations in the mining horizon are accessible, a program of geologic mapping shall be planned and executed to supplement the boring program. In areas of substantial relief, the mining horizon may be available for mapping at locations reasonably close to the tunnel alignment that have been identified either by map study or by imagery evaluation.
C5.4.3 The importance of a field reconnaissance in the
early stages of a project cannot be over-emphasized. Investigations during the planning phase of a tunnel could lead to decisions regarding the final configuration of the project. See Section 2 and Appendix A for information regarding tunnel planning.
For a rural project , the field reconnaissance team should be composed of a multi-discipline team that includes expertise in geotechnical engineering, geology, structures, alignments, estimating, and management from both the design consultant and the Owner.
For a tunnel project in rural terrain, the reconnaissance will identify accessibility constraints to the portal locations, both for subsurface investigation and construction purposes.
Access requirements for remote boring locations can also be confirmed, assisting in preparation of specifications for boring access road construction, including acceptable cut slopes, erosion control requirements, stream bridging/fording requirements, stream siltation protection requirements and site restoration requirements. Certain federal and state agencies, e.g., U.S. Forest Service, have very specific requirements relating to temporary road construction and restoration.
If a need for helicopter transportation of drill rigs is identified, specifications for landing zone preparation, maintenance and restoration can be prepared. Also, the feasibility of pumping drilling water from nearby steams vs. requirements for water trucks can also be evaluated during the field reconnaissance.
For an urban project, the field reconnaissance team may be limited to staff with expertise in geotechnical engineering and structures.
For a tunnel project in an urban area, the field reconnaissance can identify surface structures in the portal areas that must be removed or supported. Overhead obstructions and subsurface utilities that may interfere with boring operations can be identified and
alternative locations can be selected for borings. The field reconnaissance shall be documented by a
detailed written report, with location maps and numerous photographs. The report shall be prepared immediately upon return of the field reconnaissance team to the design office.
5.4.4 Geotechnical Borings
Borings shall be used to identify subsurface stratigraphy and to obtain disturbed and undisturbed samples for visual classification and laboratory testing. Borings shall be sufficient in number and depth to establish a reliable subsurface profile at areas of concern (e.g., variations in soil deposits and/or rock formations) and to investigate any geologic hazards (e.g., karstic formations, mined out areas, swelling/collapsible soils, existing fill/waste areas, fault/shear zones, layers susceptible to liquefaction, slope instability potential) that could impact the tunnel design, construction and performance.
Table 5.4.4-1 shall be used as guidance for determining the number and locations of borings. The final number of borings shall be adjusted based on the variability of the anticipated subsurface conditions for the project, as well as the conditions encountered during execution of the investigation program. Additional borings shall be provided to better define localized geologic hazards, such as faults and slope instability, and for assessing potential impacts and protection measures at existing structures and facilities. For economy, the program should be staged to gather the amount of information appropriate to the stage of project development (i.e., planning, preliminary design, final design, etc.).
Borings for tunnels and shafts shall extend to at least 1.5 tunnel/shaft diameters below the proposed tunnel/shaft invert unless there is uncertainty regarding the final profile of the tunnel. Borings for cut-and-cover tunnels and portals shall extend at least 1.5 times the depth of the portal excavation for design of the support of excavation (SOE) system and portal structure foundations. This requirement may be waived if the portal excavation extends into rock. In that case, exploration generally should be advanced below the excavation invert until a minimum continuous core recovery of 85% is recorded with rock quality designation (RQD) greater than 50 over a length of 20 feet.
If there is uncertainty regarding the final profile of the tunnel, the borings should extend at least two to three times the tunnel diameter below the preliminary tunnel invert level.
C5.4.4 For the most part, field classification of soil for a
tunnel project is similar to that for other geotechnical applications, except that special attention must be given to accurately defining and documenting soil grain size characteristics and stratification features. Items of particular importance to tunnel projects are listed below:
• Groundwater levels (general and perched levels),
evidence of ground permeability (loss of drilling fluid, rise or drop in borehole water level; etc.), and evidence of artesian conditions.
• Consistency and strength of cohesive soils • Composition, gradation and density of cohesionless
soils • Presence of lenses and layers of higher permeability
soils • Presence of gravel, cobbles, boulders and core
stones, and potential for nested boulders or concentrated core stones
• Maximum cobble/boulder/core stone size from coring and/or large diameter borings (and based on understanding of local geology) and the unconfined compressive strength, abrasiveness and hardness of cobbles/boulders/core stones (from field index tests or laboratory testing of recovered samples)
• Presence of cemented soils • Presence of contaminated soil or groundwater
In rock, rock mass characteristics (e.g.,
discontinuities) as well as special intact rock properties are of interest for tunnel projects. In addition to general rock lithology and discontinuity descriptions (e.g., predominant joint sets with strike and dip orientations, joint roughness, joint persistence, joint spacing, joint weathering and infilling), other information that should be noted include:
• Presence of faults or shear zones • Presence of intrusive material (volcanic dikes and
sills) • Presence of voids (solution cavities, lava tubes, etc.) • Groundwater levels, artesian pressure, and evidence
of rock permeability (loss of drilling fluid, rise or drop in borehole water level, etc.) In addition to vertical and inclined boreholes,
horizontal boreholes, or a pilot tunnel along the proposed tunnel alignment, can provide a continuous record of ground conditions and information directly
relevant to the tunnel design and excavation. Refer to FHWA (2002 and 2002a) for guidance regarding the planning and conduct of subsurface exploration programs. For further information on critical information to be collected from borings for tunnel projects, refer to Technical Manual for Design and Construction of Road Tunnels – Civil Elements (AASHTO, 2010).
Table 5.4.4-1 Guidelines for Vertical/Inclined Borehole Spacing (after AASHTO, 1988; AASHTO 2010) Structure Type/Ground Conditions Typical Borehole Spacing Cut-and-Cover Tunnels 100 to 300 ft Rock Tunneling
Adverse conditions, closely jointed to sheared and folded rock
Favorable conditions, moderately to widely jointed or massive rock
50 to 200 ft
500 to 1,000 ft Soft Ground Tunneling
Adverse conditions, soft compressible soils or significant groundwater issues. Favorable conditions, moderately dense to very dense or stiff to hard soils
50 to 100 ft
300 to 500 ft Mixed-Face Tunneling
Adverse conditions, irregular and variable rock contact Favorable conditions, relatively uniformly varying rock contact
25 to 50 ft 50 to 75 ft
5.4.5 In-Situ Tests and Sampling of Soil and Rock In-situ tests may be performed to obtain
deformation and strength parameters for soil and rocks for design/analysis purposes. At a minimum, in-situ tests shall be conducted in soils that do not lend themselves to undisturbed sampling as a means to estimate design parameters. When performed, in-situ tests shall be conducted in accordance with the appropriate ASTM or AASHTO standards.
Whenever in-situ tests are performed, several of the in-situ test holes shall be performed adjacent to geotechnical borings to assist in the interpretation of soil classifications at other in-situ test locations, and to provide correlations between in-situ test data and laboratory test results.
A soil/rock sampling program shall be developed to obtain necessary information for planning, design and construction of the tunnel.
C5.4.5 For further information on in-situ testing and
sampling of soils and rocks relevant to highway tunnels and limitations refer to Technical Manual for Design and Construction of Road Tunnels – Civil Elements (AASHTO, 2010)). The in-situ tests usually used for soils and rock are presented in Tables C5.4.5-1 and C5.4.5-2.
When in-situ test results are used to estimate design properties through correlations, such correlations should be well established through long-term wide spread use or thorough detailed measurements that illustrate the accuracy of the correlation.
Often, CPT soundings are performed in soil to obtain in-situ test data to complement the data obtained from geotechnical borings and laboratory soil tests. CPT soundings are an economical means of providing a larger number of exploration locations in comparison to geotechnical borings. In addition, when considered appropriate by the PGE, CPT soundings may be used to replace some of the geotechnical borings, particularly at sites requiring a large number of borings.
5.4.5.1 Soil Sampling
The overburden soil shall be sampled at intervals
not greater than five feet and at changes in strata using Standard Split Spoon Sampler (ASTM D-1586). In addition, undisturbed tube samples from cohesive soil strata shall be collected at intervals not exceeding 15 ft. For bored and mined tunnels, continuous sampling from one diameter above the tunnel crown to one and one-half diameters below the tunnel invert shall be performed.
Continuous rock core generally shall be obtained below the surface of rock, with a minimum core diameter of NX-size (diameter of 2.16 in.).
At the discretion of the PGE, based on his/her evaluation of project geology, the continuous rock coring requirement may be waived for very deep holes. In that situation, coring may begin two to three tunnel diameters (widths) above the crown of the tunnel. The boring may be advanced by rotary drilling methods from the ground surface to the start of coring operations. A post-completion ATV survey may be desirable in this situation if the geologic environment is complex.
Double or triple tube core barrels shall be used to obtain high quality cores representative of the in-situ rock. For deeper holes, coring shall be performed with the use of wire-line drilling equipment to further reduce potential degradation of the recovered core samples. Core runs shall be limited to a maximum length of 10 feet in moderate to good quality rock, and five feet in poor quality rock.
C5.4.5.2 It is desirable to preserve the rock cores retrieved
from the field properly until the construction is completed and disputes/claims are settled. Common practice is to photograph the rock cores in core boxes and possibly scan the core samples for review by designers and contractors.
Drilling through a significant depth of soil and rock above the tunnel horizon using conventional soil sampling techniques and rock coring is a significant expense that may yield limited useful information for deep tunnels. In such cases, an economical approach may be to limit soil sampling and rock coring to zones within and near the tunnel horizon. The limits of sampling and coring should be determined by a geotechnical engineer who is knowledgeable of tunneling and the geologic formations within the project site.
5.4.5.3 Bore Hole Sealing All borings shall be sealed at the completion of the
subsurface investigation, if not intended to be used as monitoring wells. Sealing of the borings shall be done in a manner that will not be detrimental to the anticipated tunnel construction methodology.
Table C5.4.5-1 In-situ Testing Methods Used in Soil (After FHWA, 2002a; AASHTO, 2010)
Method Procedure Applicable Soil Types
Applicable Soil Properties
Limitations / Remarks
Electric Cone Penetrometer (CPT)
A cylindrical probe is hydraulically pushed vertically through the soil measuring the resistance at the conical tip of the probe and along the steel shaft; measurements typically recorded at one to two inch intervals
Silts, sands, clays and peat
Estimation of soil type and detailed stratigraphy Sand: φ′, Dr, σho′
Clay: su, σp′
Excellent tool for providing continuous profile of soil characteristics and can capture thin layers often missed by conventional SPT tests. No soil sample is obtained; The probe may become damaged if testing in gravelly soils is attempted
Piezocone Penetrometer (CPTu)
Same as CPT; additionally, penetration porewater pressures are measured using a transducer and porous filter element
Silts, sands, clays and peat
Same as CPT, with additionally: Sand: uo / water table elevation Clay: σp′, ch, kh
OCR
If the filter element and ports are not completely saturated, the pore pressure response may be misleading; Compression and wear of a mid-face (u1) element will effect readings; Test results not particularly good for estimating deformation characteristics
Seismic CPTu (SCPTu)
Same as CPTu; additionally, shear waves generated at the surface are recorded by a geophone at three ft. intervals throughout the profile for calculation of shear wave velocity
Silts, sands, clays and peat
Same as CPTu, with additionally: Vs, Gmax, Emax, ρtot, eo
Excellent tool for obtaining shear wave velocity profile of the soil which represents maximum soil stiffness, a key parameter for estimating deformation characteristics, at small strains and seismic evaluation of soil response. First arrival times should be used for calculation of shear wave velocity (if first crossover times are used, the error in shear wave velocity will increase with depth)
Flat Plate Dilatometer (DMT)
A flat plate is hydraulically pushed or driven through the soil to a desired depth; at approximately to 12 inch intervals, the pressure required to expand a thin membrane is recorded; Two to three measurements are typically recorded at each depth.
Silts, sands, clays and peat
Estimation of soil type and stratigraphy Total unit weight Sand: φ′, E, Dr, mv
Clays: σp′, Ko, su, mv, E, ch, kh
Membranes may become deformed if over-inflated; Deformed membranes will not provide accurate readings; Leaks in tubing or connections will lead to high readings; Good test for estimating deformation characteristics at small strains
Pre-bored Pressure meter (PMT)
A borehole is drilled and the bottom is carefully prepared for insertion of the equipment; The pressure required to expand the cylindrical membrane to a certain volume or radial strain is recorded.
Clays, silts, and peat; marginal response in some sands and gravels
E, G, mv, su Preparation of the borehole most important step to obtain good results; Provides useful information for calculation of lateral deformation characteristics
Table C5.4.5-1 (Cont’d) In-situ Testing Methods Used in Soil (After FHWA, 2002a; AASHTO. 2010)
Method Procedure Applicable Soil Types
Applicable Soil Properties
Limitations / Remarks
Full Displacement Pressure meter (PMT)
A cylindrical probe with a pressure meter attached behind a conical tip is hydraulically pushed through the soil and paused at select intervals for testing; The pressure required to expand the cylindrical membrane to a certain volume or radial strain is recorded
Clays, silts and peat
E, G, mv, su Disturbance during advancement of the probe will lead to stiffer initial modulus and mask liftoff pressure (po); Provides useful information for calculation of lateral deformation characteristics
Vane Shear Test (VST)
A four-blade vane is hydraulically pushed below the bottom of a borehole, then slowly rotated while the torque required to rotate the vane is recorded for calculation of peak undrained shear strength; The vane is rapidly rotated for 10 turns, and the torque required to fail the soil is recorded for calculation of remolded undrained shear strength
Clays, some silts and peats if undrained conditions can be assumed; not for use in granular soils
su, St, σp′ Disturbance may occur in soft sensitive clays, reducing measured shear strength; Partial drainage may occur in fissured clays and silty materials, leading to errors in calculated strength; Rod friction needs to be accounted for in calculation of strength; Vane diameter and torque wrench capacity need to be properly sized for adequate measurements in various clay deposits
Table C5.4.5-2 Common in situ Test Methods for Rock (after USACE, 1997; AASHTO, 2010)
Parameter Test Method Procedure / Limitations / Remarks
In situ Stress Hydraulic Fracturing
Typically conducted in vertical boreholes. A short segment of the hole is sealed off using a straddle packer. This is followed by the pressurization by pumping in water. The pressure is raised until the rock surrounding the hole fails in tension at a critical pressure. Following breakdown, the shut-in pressure, and the lowest test-interval pressure at which the hydrofracture closes completely under the action of the stress acting normal to the hydrofractures. In a vertical test hole, the hydrofractures are expected to be formed vertical and perpendicular to the minimum horizontal stress. Interpretation of hydrofracturing tests can be difficult at depths less than 150 feet below ground surface.
Overcoring Drills a small diameter borehole and sets an instrument to respond to changes in diameter into it. Rock stresses are determined indirectly from measurements of the dimensional changes of a borehole, occurring when the rock volume surrounding the hole is isolated from the stresses in the host rock. Maintaining correct drill hole orientation is of critical importance.
Flat Jack Test This method involves the use of flat hydraulic jacks, consisting of two plates of steel welded around their edges and a nipple for introducing oil into the intervening space. Flat jack is inserted into the slot, cemented in place and pressurized. When the pins have been returned to the initial separation, the pressure in jack approximates the initial stress normal to the jack. Usually performed in an exploratory tunnel/adit. Disturbance of stress field around exploratory tunnel/adit is a concern.
Modulus of Deformation
Plate Bearing Test
A relatively flat rock surface is sculptured and leveled with mortar to receive circular bearing plates 20 to 40 inches in diameter. The rock surface is loaded and the resulting displacement is monitored. This is easily arranged in an exploratory tunnel/adit. The site should be selected carefully to exclude loose, highly fractured rock.
Borehole Dilatometer Test
A borehole expansion experiment conducted with a rubber sleeve. The expansion of the borehole is measured by the oil or gas flow into the sleeve as the pressure is raised, or by potentiometers or linear variable differential transformers built inside the sleeve. One problem with the borehole dilatometer test is that it affects a relatively small volume of rock and therefore contains an incomplete sample of the fracture system.
Flat Jack Test This method involves the use of flat hydraulic jacks, consisting of two plates of steel welded around their edges and a nipple for introducing oil into the intervening space. Provide measurement points on the face of the rock and deep slot (reference points). Modulus of deformation can be calculated from the measured pin displacements.
Radial jacking test
Loads are applied to the circumference of a tunnel by a series of jacks reacting against circular steel ring members. This test allows the direction of load to be varied according to the plan for pressuring the jacks.
Pressuremeter The pressure required to expand the cylindrical membrane to a certain volume or radial strain is recorded in a borehole. It is applicable for soft rocks.
Table C5.4.5-2 (Cont’d) Common in situ Test Methods for Rock (after USACE, 1997; AASHTO, 2010
Parameter Test Method Procedure / Limitations / Remarks
Dynamic Measurement
The velocity of stress waves is measured in the field. The wave velocity can be measured by swinging a sledgehammer against an outcrop and observing the travel time to a geophone standing on the rock at a distance of up to about 150 ft. The stress loadings sent through the rock by this method are small and transient. Most rock masses depart significantly from the ideal materials, consequently, elastic properties calculated from these equations are often considerably larger than elastic properties calculated from static loading tests, particularly in the case of fractured rocks.
Imaging and Discontinuities
Acoustic Televiewing
ATV produce images of the borehole wall based on the amplitude and travel time of acoustic signals reflected from the borehole wall. A portion of the reflected energy is lost in voids or fractures, producing dark bands on the amplitude log. Travel time measurements allow reconstruction of the borehole shape, making it possible to generate a 3-D representation of a borehole.
Borehole Video Televiewing
The Borehole Video System (BVS) is lowered down boreholes to inspect the geology and structural integrity. The camera view of fractures and voids in boreholes provides information.
Permeability Slug Test Slug tests are applicable to a wide range of geologic settings as well as small-diameter piezometers or observation wells, and in areas of low permeability where it would be difficult to conduct a pumping test. A slug test is performed by injecting or withdrawing a known volume of water or air from a well and measuring the aquifer’s response by the rate at which the water level returns to equilibrium. Permeability values derived relate primarily to the horizontal conductivity. Slug tests have a much smaller zone of infiltration than pumping tests, and thus are only reliable at a much smaller scale.
Packer Test It is conducted by pumping water at a constant pressure into a test section of a borehole and measuring the flow rate. Borehole test sections are sealed off by packers, with the use of one or two packers being the most widely used techniques. The test is rapid and simple to conduct, and by performing tests within intervals along the entire length of a borehole, a permeability profile can be obtained. The limitation of the test is to affect a relatively small volume of the surrounding medium, because frictional losses in the immediate vicinity of the test section are normally extremely large.
Pumping Tests In a pumping test, water is pumped from a well normally at a constant rate over a certain time period, and the drawdown of the water table or piezometric head is measured in the well and in piezometers or observation wells in the vicinity. Since pumping tests involve large volumes of the rock mass, they have the advantage of averaging the effects of the inherent discontinuities. Most classical solutions for pump test data are based on the assumptions that the aquifers are homogeneous and isotropic, and that the flow is governed by Darcy's Law. The major disadvantage is the period of time required to perform a test. Test durations of one week or longer are not unusual when attempting to approach steady-state flow conditions. Additionally, large diameter boreholes or wells are required since the majority of the conditions encountered require the use of a downhole pump.
5.4.6 Geophysical Tests
Geophysical tests shall be used only in combination with information from direct methods of exploration to establish location and identification of the subsurface
C5.4.6 Application of geophysical testing methods and
other relevant information regarding subsurface investigation for highway tunnels are presented in
materials, the profile of the top of bedrock and bedrock quality, the depth to groundwater, the limits of soil layers, the limits of organic deposits, the presence of voids, the location and depth of utilities, the location and depth of existing foundations and other obstructions, etc.
Geophysical tests shall also be used to define the shear wave velocity profile of soil and rock formations, Vs. This parameter, Vs, is an essential parameter for estimating small strain deformation around tunnels and excavations, especially in urban areas. This parameter, Vs, is also essential for seismic site response evaluation and can be used in liquefaction evaluation of soil
The shear wave and pressure wave velocities of the geologic strata required for seismic design shall be evaluated using cross-hole seismic logging, suspension logging, spectral analysis of surface waves (SASW), Multichannel analysis of surface waves (MASW), or similar. For preliminary assessment, dynamic properties based on empirical correlations (e.g., correlations with measured SPT values or laboratory measured undrained shear strength) may be used in the absence of measured data.
Tables C5.4.6-1 and C5.4.6-2. ASTM D6429, Standard Guide for Selecting
Surface Geophysical Methods, provides additional guidance on selection of suitable methods.
Vs is a parameter that is needed to compute the
maximum shear modulus Gmax for analysis of deformations around tunnels and excavation in some geotechnical software that employ finite element and finite difference methods. It is also a parameter needed in seismic site response analysis.
Seismic Refraction Detectors (geophones) are positioned on the ground surface at increasing distance from a seismic impulse source, also at the ground surface. The time required for the seismic impulse to reach each geophone is recorded.
Distance between closest and furthest geophone must be three to four times the depth to be investigated. Reflection from hard layer may prevent identification of deeper layers. Other conditions affecting interpretation: insufficient density contrast between layers; presence of low-density layer; irregular surface topography.
Seismic Reflection Performed for offshore applications from a boat using an energy source and receiver at the water surface. The travel time for the seismic wave to reach the receiver is recorded and analyzed.
The position and direction of the boat must be accurately determined by GPS or other suitable method. Reflection from hard layer may prevent identification of deeper layers.
Electrical Resistivity /Conductivity
Wenner Four Electrode Method is the most commonly used test in the U.S. Four electrodes are placed partially in the soil, in line and equidistant from each other. A low magnitude current is passed between the outer electrodes, and the resulting potential drop is measured at the inner electrodes. A number of traverses are used, and electrode spacing is varied to better define changes in deposits and layering.
Results may be influenced by presence of underground obstructions, such as pipelines, tanks, etc.
Seismic Wave Propagation: Cross-Hole
At least two boreholes are required: a source borehole within which a seismic pulse is generated, and a receiver borehole in which a geophone records generated compression and shear waves. For increased accuracy, additional receiver boreholes are used.
Receivers must be properly oriented and securely in contact with the side of the borehole. Boreholes deeper than about 30 ft should be surveyed using an inclinometer or other device to determine the travel distance between holes.
Up-Hole or Down-Hole
Performed in a single borehole. In up-hole method, a sensor is placed at the ground surface and shear waves are generated at various depths in the borehole. In down-hole method, seismic wave is generated at the surface and one or more sensors are placed at different depths within the hole.
Data limited to area in immediate vicinity of the borehole.
Parallel Seismic Used to determine the depth of existing foundations, an impulse wave is generated at the top of the foundation, and a sensor in an adjacent borehole records arrival of the stress wave at set depth increments.
Requires access to top of foundation.
Ground Penetrating Radar
Repetitive electromagnetic impulses are generated at the ground surface and the travel time of the reflected pulses to return to the transmitter are recorded.
The presence of a clay layer may mask features below that layer.
Gravity A sensitive gravimeter is used at the ground surface to measure variations in the local gravitational field in the earth caused by changes in material density or cavities.
May not identify small changes in density. May be influenced by nearby surface or subsurface features (mountains, solution cavities, buried valleys, etc.) not directly in area of interest.
Magnetics Magnetic surveys can be performed using either ground-based or airborne magnetometers. With ground equipment, measurements of changes in the earth’s magnetic field are taken along an established survey line.
Monitoring locations should not be located near man-made objects that can change the magnitude of the earth’s magnetic field (pipelines, buildings, etc.). Corrections need to be made for diurnal variations in the earth’s magnetic field.
Suspension Logging
Performed in a single borehole. The seismic wave is generated from a source located on a 25-ft long probe. The two receivers are also located on the probe.
Data limited to area in immediate vicinity of the borehole. This method has been used at boreholes with depths greater than 1,000 ft.
5.4.7 Laboratory Tests
5.4.7.1 Soil Tests
Laboratory testing shall be performed to provide basic data with which to classify soils and to provide parameters for design and modeling purposes. When performed, laboratory tests shall be conducted in accordance with AASHTO, ASTM, or Owner-supplied procedures applicable to the design properties needed.
C5.4.7.1
For further information on laboratory soil tests, refer to the FHWA Geotechnical Engineering Circular No. 5 – Evaluation of Soil and Rock Properties (Sabatini et al., 2002).
5.4.7.2 Rock Tests
When performed, laboratory tests shall be conducted in accordance with ASTM or owner-supplied procedures applicable to the design properties needed. Laboratory tests shall be used in conjunction with field tests and field characterization of the rock mass to give estimates of rock mass behavioral characteristics.
TBM performance rates shall be assessed through specialized tests including three drillability and boreability tests, namely, Drilling Rate Index (DRI), Bit Wear Index (BWI), and Cutter Life Index (CLI).
Joint shear strength may be evaluated using laboratory direct shear tests.
C5.4.7.2 Standard rock tests used to evaluate physical
properties of the rock include density and mineralogy tests (thin-section analysis). The mechanical properties of the intact rock core include uniaxial compressive strength, tensile strength, static and dynamic elastic constants, hardness and abrasiveness indices.
5.4.8 Groundwater and Surface Water Investigation
Depth to groundwater shall be recorded at all
boreholes during drilling. Groundwater levels within the limits of the tunnel shall be monitored periodically within groundwater observation wells or piezometers over a prolonged period of time to provide information on seasonal variations in groundwater levels. An appropriate minimum monitoring period shall be at least 12 months, but preferably 24 months, during the project design period. Monitoring shall also continue during
C5.4.8 Groundwater is a major factor for tunnels since it
may not only represent a large percentage of the loading on the final tunnel lining, but also it largely determines ground behavior and stability for soft ground tunnels; the inflow into rock tunnels during construction; the method and equipment selected for tunnel construction; and the long-term performance of the completed structure. Accordingly, for tunnel projects, special attention must be given to defining the groundwater
construction to provide information on the influence of tunneling on groundwater levels.
In addition to groundwater levels, hydraulic conductivity of the soils and rock shall be evaluated through laboratory testing or field tests.
Site specific groundwater chemistry shall also be considered for tunnels, as aggressive groundwater may contribute to a reduction of the lining durability unless special concrete mixtures to resist the attack of aggressive groundwater are used for the final tunnel lining. Groundwater chemistry considered to be aggressive toward concrete durability includes, but is not limited to, factors such as adverse pH, high sulfate content and high chloride content. Although waterproofing membranes should limit the exposure of the final tunnel lining to groundwater, protective measures shall be in place to ensure long-term durability over the design life of the facility.
regime, aquifers, sources of water, any perched or artesian conditions, water quality, temperature, depth to groundwater and the permeability of the various materials that may be encountered during tunneling.
Related considerations include the potential impact of groundwater lowering on settlement of overlying and nearby structures, utilities and other facilities; other influences of dewatering on existing structures (e.g., accelerated deterioration of exposed timber piles); pumping volumes during construction; decontamination/treatment measures for water discharged from pumping; migration of existing soil and groundwater contaminants due to dewatering; the potential impact on water supply aquifers; and seepage into the completed tunnel.
Borehole permeability tests in general provide low cost means for assessing the permeability of soil and rock but may provide information for a limited zone near the test location. Continuous pumping tests, on the other hand, may provide information over an extended area. Further information regarding the details and procedures used for performing and evaluation of these field tests are provided by Mayne et al. (FHWA, 2002a).
Surface water bodies in the immediate vicinity of the tunnel alignment, such as streams, ponds, lakes and reservoirs, shall be identified, and the potential for drainage into the tunnel shall be evaluated.
If there is a hydraulic connection between the surface water body and the tunnel, high initial water inflows and reduced sustained inflows may be encountered that may damage equipment and require the implementation of extensive water inflow control measures.
Although not common, drainage of surface water bodies into tunnels has been reported with high volume initial flows and reduced sustained flows. These incidents have sometimes led to the temporary or permanent drying up of small streams and ponds and to significant delays in tunnel construction.
5.5 SELECTION OF SOIL AND ROCK
PARAMETERS FOR DESIGN
5.5.1 General
The soil and rock design parameters shall be determined using one or more of the following methods:
• In-situ testing during the subsurface investigation,
including consideration of any geophysical testing conducted,
• Laboratory testing, and/or • Back analysis of design parameters based on site
performance data. Local experience, local geologic formation specific
correlations and knowledge of local geology, in addition to broader based experience and relevant published data, shall also be considered in the final selection of design parameters. If published correlations are used in combination with one of the methods above, the applicability of the correlation to the specific geologic formation shall be considered through the use of local
experience, local test results and/or long-term experience.
The focus of design parameter assessment and final selection shall be on the individual geologic strata identified at the project site. The selected design parameters shall be appropriate to the particular limit state under consideration and the corresponding analysis model.
The evaluation of design parameters for rock shall take into consideration that rock mass properties are generally controlled by the discontinuities within the rock mass and not the intact material properties. A combination of laboratory testing of small samples, empirical analysis and field observations shall be employed to determine the design parameters for the rock mass, with greater emphasis placed on visual observations and quantitative descriptions of the rock mass.
5.5.2 Soil Strength
The selection of soil strength parameters for design shall consider the following at a minimum:
• Rate of construction loading relative to the
hydraulic conductivity of the soil (drained versus undrained strengths);
• Loading direction;
• Strain levels; and
• Effect of the construction sequence.
C5.5.2
For additional information, see Article C10.4.6.2.1 of the LRFD Specifications and FHWA Geotechnical Engineering Circular No. 5 – Evaluation of Soil and Rock Properties, 2002.
5.5.2.1 Undrained Shear Strength of Cohesive Soils
Generally, laboratory consolidated undrained (CU) and unconsolidated undrained (UU) testing shall be used to estimate the undrained shear strength, Su, supplemented as needed with values determined from in-situ testing. In-situ testing methods shall be used to compliment laboratory testing in soils that may be susceptible to disturbance during sampling, transportation and laboratory preparation. For relatively thick deposits of cohesive soil, profiles of Su as function of depth shall be obtained so that the stress history and properties can be ascertained.
5.5.2.2 Drained Strength of Cohesive Soils
The drained strength parameters, c’ and φ’, of cohesive soils shall be evaluated by slow consolidated drained direct shear box tests, CD triaxial tests or CU tests with pore pressure measurements. In laboratory tests, the rate of loading shall be sufficiently slow to ensure substantially complete dissipation of excess pore pressure in the drained tests, or in undrained tests, complete equalization of pore pressure throughout specimen.
The drained strength parameters of granular soils shall be evaluated in accordance with the procedures contained in Article 10.4.6.2.4 of the LRFD Specifications.
5.5.3 Soil Deformation
The undrained and drained elastic deformation parameters (e.g., Young’s Modulus, Poisson’s ratio) shall be developed based on the results from in-situ tests (e.g., DMT, PMT) or laboratory tests directly, or based on, empirical correlations with in-situ test measurements (e.g., SPT, CPT).
5.5.4 Rock Shear Strength
The design shear strength parameters for the rock shall be evaluated based on laboratory tests on intact rock specimens, visual assessment or rock mass classification of the recovered cores, and available information from previous studies/investigation on similar geologic materials to account for discontinuities (e.g., joints, bedding, foliation planes), infillings and geologic history.
C5.5.4 In general, the rock shear strength, τ, can be defined
using Mohr-Coulomb envelope as a function of cohesion (c), friction angle (φ), and effective normal stress (σ’) along the rupture surface. The development of rock shear strength parameters is presented in detail in the FHWA Geotechnical Engineering Circular No. 5 – Evaluation of Soil and Rock Properties (Sabatini et al., 2002).
Alternatively, other rock mass classification systems (e.g., Tunneling Quality Index or Q System) can also be used to evaluate rock mass strength parameters. For example, Barton (2002) proposed: c = (RQD/Jn)(1/SRF)(σc/100) (C5.5.4-1) φ = tan-1 [(Jr/Jn) Jw] (C5.5.4-2) where: RQD = Rock Quality Designation Jn = joint set number Jr = joint roughness number Jw = joint water reduction SRF = stress reduction factor σc = unconfined compressive strength Evaluation of these Q System parameters is presented in Table C5.5.4-1
Table C5.5.4-1 (Cont’d) Classification of Individual Parameters for Q System (AASHTO, 2010)
5.5.5 Rock Deformation Modulus
The deformation modulus for rock mass shall be derived from in-situ tests (e.g., borehole dilatometer, borehole jack) or correlations with intact rock modulus and rock mass classification parameters.
C5.5.5
For information on rock deformation modulus, see Tables C5.5.5-1 through C5.5.5-4
𝑅𝑀𝑅22.82�� where 𝐸𝑖 = 7,250 𝑘𝑠𝑖 Nicholson and Bieniawski (1990)
𝐸𝑚 = 14.5 (RMR/10)3 Read et. al. (1999)
Table C5.5.5-2 Estimation of Disturbance Factor, D (AASHTO, 2010)
Description of Rock Mass Suggested Value
Excellent quality controlled blasting or excavation by TBM results in minimal disturbance to the confined rock mass surrounding a tunnel. D = 0
Mechanical or hand excavation in poor quality rock masses (no blasting) results in minimal disturbance to the surrounding rock mass. Where squeezing problems result in significant floor heave, disturbance can be severe unless a temporary invert is placed.
D = 0 D = 0.5 No invert
Very poor quality blasting in a hard rock tunnel results in severe local damage, extending six to nine ft., in the surrounding rock mass. D = 0.8
Small scale blasting in civil engineering slopes results in modest rock mass damage, particularly if controlled blasting is used. However, stress relief results in some disturbance.
D = 0.7 Good blasting D = 1.0 Poor blasting
Very large open pit mine slopes suffer significant disturbance due to heavy production blasting and also due to stress relief from overburden removal. In some softer rocks, excavation can be carried out by ripping and dozing, and the degree of damage to the slope is less.
D = 1.0 Production blasting D = 0.7 Mechanical excavation
The aggressive subsurface environment due to groundwater chemistry/corrosive soils, handling of hazardous minerals within the excavated zone (e.g., asbestiform minerals) and hazardous and explosive gases within the subsurface (e.g., hydrogen sulfide, H2S, and methane, CH4) shall be considered when developing subsurface investigations. If present, necessary measures shall be included in the design of the structures.
The spoils, tunnel muck and dredged material shall be treated, reused or disposed of in accordance with applicable local and national regulations.
All necessary precautions and instrumentation shall be implemented to mitigate the impact of construction of highway tunnels on air quality, noise, vibration, traffic, nearby aboveground and underground structures/ utilities, groundwater, surface water, etc.
5.7 INSTRUMENTATION AND MONITORING
An instrumentation and monitoring program shall be in place for all tunneling projects to:
• Prevent and minimize damage to existing
structures/utilities and the structure under construction by providing data to evaluate the source and magnitude of ground movements and changes to groundwater levels;
• Assess the safety of works by comparison of the observed response of ground and structures with the predicted response and allowable deformations of disturbance levels;
• Develop protective and preventive measures for existing and new structures;
• Select appropriate remedial measures, when required;
• Evaluate critical design assumptions where significant uncertainty exists;
• Determine adequacy of Contractor’s methods, procedures and equipment;
• Monitor effectiveness of protective, remedial and mitigative measures;
• Provide feedback to Contractor on its performance; and
• Provide documentation for assessing damages sustained to adjacent structures allegedly resulting from construction related activities.
Threshold and limiting values regarding vibrations, horizontal and vertical deformations, groundwater drawdown and loads in structural elements, and minimum standards for instrumentation shall be defined. Included with the monitoring plan shall be proposed mitigation measures to be taken in the event threshold
Additional information on geotechnical instrumentation and monitoring is provided by J. Dunnicliff (FHWA, 1998).
and limiting deformations are exceeded. Pre- and post-construction condition surveys of all
critical structures and utilities shall be performed and documented. 5.8 GEOTECHNICAL REPORTS
5.8.1 Geotechnical Data Report
The GDR shall present the factual subsurface data for the project without including an interpretation of these data. All factual geological, geotechnical, groundwater and other data obtained from subsurface investigations shall be included.
Data reduction shall be limited to determination of the properties obtained from individual test samples, while avoiding any recommendations for the geotechnical properties for the soil or rock unit from which the sample was obtained.
The GDR shall contain the following information, at a minimum,:
• Description of the geologic setting; • Descriptions of the site exploration program(s); • Logs of all borings, trenches and other site
investigations; • Groundwater measurements; • Descriptions/discussions of all field and
laboratory test programs; • Results of all field and laboratory testing.
C5.8.1 The GDR avoids making any interpretation of the
data since these interpretations may conflict with the data assessment subsequently presented in the Geotechnical Design Memoranda (GDM) or other geotechnical interpretive or design reports, and the baseline conditions defined in the Geotechnical Baseline Report (GBR). Any such discrepancies may be a source of confusion to contractors and open opportunities for claims of differing site conditions.
For more information on GDR, refer to the Technical Manual for Design and Construction of Road Tunnels – Civil Elements (AASHTO, 2010).
5.8.2 Geotechnical Baseline Report
The GBR shall establish the site specific subsurface conditions to be considered as baseline conditions to develop bids and select means and methods. The GBR shall not be interpreted as a prediction or warranty of the actual site conditions, but rather seen as a contractual instrument for allocating risks and a basis for determining the merits of claims of differing site conditions during construction. The GBR shall be based on factual information presented in the GDR engineering evaluations made in GDM, as well as input from the Owner. The baseline presented in the GBR shall be clear and concise and be such that it can be measured during construction.
C5.8.2 The GBR translates facts, interpretations and
opinions regarding subsurface conditions into clear, unambiguous statements for contractual purposes. Items typically addressed in GBR include:
• Amounts and distribution of different materials
along the selected alignment; • Description, strength, compressibility, grain size
and permeability of the existing materials; • Description, strength and permeability of the
ground mass as a whole; • Groundwater levels and expected groundwater
conditions, including baseline estimates of inflows and pumping rates;
• Anticipated ground behavior and the influence of groundwater, with regard to methods of excavation and installation of ground support;
• Construction impacts on adjacent facilities; and • Potential geotechnical and man-made sources of
potential difficulty or hazard that could impact construction, including the presence of faults, gas, boulders, solution cavities, existing foundation piles and the like.
• Items to be baselined should be limited to those that have significant influence on construction
For further information, refer to the Technical Manual for Design and Construction of Road Tunnels – Civil Elements (AASHTO, 2010) and Geotechnical Baseline Reports for Construction: Suggested Guidelines (ASCE, 2007).
5.9 GEOTECHNICAL DESIGN MEMORANDA
One or more GDM shall be prepared for the project,
based on the project complexity. The number, format and content of the GDM shall be determined by the PGE and the Project Structural Engineer, subject to review and approval by the Project Manager in accordance with Owner requirements.
Separate GDM may be prepared for design of Temporary (Initial) Support and for the design of the final structures of the different tunnel types. Alternatively, a GDM may contain information required for the design of both Temporary (Initial) Support and the final structure.
Each GDM shall contain unfactored ground loads and water loads and ground deformation parameters in the format required by the Project Structural Engineer.
Each GDM shall address construction stages as well as final equilibrium states.
Water loads shall reflect time-dependent variation, including water table elevation lowering during excavation, as applicable, post-completion groundwater level recovery and lining details, i.e., drained or undrained. Water loads for drained linings shall also include a partial clogging condition.
1. American Association of State Highway and Transportation Officials (AASHTO) (2014), LRFD Bridge Design Specifications, 7th Edition
2. AASHTO (1988), “Manual on Subsurface Investigations”, Washington, D.C.
3. AASHTO (2010), Technical Manual for Design and Construction of Road Tunnels – Civil Elements
4. ASCE (2007), Geotechnical Baseline Reports for Construction: Suggested Guidelines
5. Barton, N., Lien, R. and Lunde, J. (1980), “Application of the Q-System in Design Decisions Concerning dimensions and Appropriate Support for Underground Installations,” Int. Conf. on Subsurface Space, Rockstore, Stockholm, 2:553-61
6. Barton, N., (2002), “Some new Q-value correlations to assist in Site Characterization and Tunnel Design”,
8. Bieniawski, Z. T. (1978), “Determining Rock Mass Deformability – Experiences from Case Histories,” Int. J. Rock Mech. Min. Sci. and Geomech. Abstr. 15, 237-247
9. FHWA, (1998), “Geotechnical Instrumentation,” Reference Manual to NHI Training Course No. 13241 – Module 11, Publication No. FHWA HI-98-034
10. FHWA (1999), “Drilled Shafts: Construction Procedures and Design Methods,” by O’Neil, M. W. and Reese,
L. C., Publication No. FHWA-IF-99-025
11. FHWA (2002), Geotechnical Engineering Circular No. 5 – Evaluation of Soil and Rock Properties, FHWA-IF-02-034
13. Grimstad, E. and Barton, N. (1993), “Updating of the Q-System for NMT,” in Kompen, Opsahl & Ber (eds), Proc. of the Int. Symposium on Sprayed Concrete – Modern Use of Wet Mix Sprayed Concrete for Underground Support, Fagernes
14. Hoek, E. and Brown, E. T. (1988), “The Hoek-Brown Failure Criterion – A 1988 Update,” Proceedings, 15th
Canadian Rock Mech. Symposium, Toronto, 3-38
15. Hoek, E. and Diederichs, M. (2006), “Empirical Estimates of Rock Mass Modulus,” Int. J. Rock Mechanics Min. Sci., 43, 203-215
16. Kulhawi, F. H. (1978), “Geomechanical Model for Rock Foundation Settlement, “ J. of the Geotechnical
Engineering Division (ASCE), 104 (GT2), Feb 1978
17. Nicholson, G. A. and Bieniawski, Z. T. (1990), “A Nonlinear Deformation Modulus Based on Rock Mass Classification,” Int. J. Min. and Geological Engineering 8, 181-202
18. Serafin, J. L. and Pereia, J. P. (1983),”Consideration of the Geomechanics Classification of Bieniawski,”
Proceedings, International Symposium on Exploration for Rock Engineering.
19. USACE (1997), EM 1110-2-2901 Tunnels and Shafts in Rock
6.5 CONSTRUCTION OF CUT-AND-COVER TUNNEL STRUCTURES 6.5.1 General 6.5.2 Reinforced Concrete Diaphragm Walls (RCDW) 6.5.3 Soldier Pile and Tremie Concrete (SPTC) Walls 6.5.4 Secant Pile and Tangent Pile Walls 6.5.5 Precast Prestressed Panel Walls 6.5.6 Cast-in-Place Concrete Box Structures 6.5.7 Structural Steel Frames
6.6 LIMIT STATES AND RESISTANCE FACTORS 6.6.1 General 6.6.2 Service Limit State 6.6.3 Strength Limit State 6.6.4 Extreme Event Limit State 6.6.5 Load Factors and Load Combinations 6.6.6 Resistance Factors
6.7 GENERAL DESIGN FEATURES 6.7.1 Ground Movement 6.7.2 Buoyancy
planning, design, evaluation and rehabilitation of cut and cover highway tunnels, and the permanent support of excavation (SOE) systems that are incorporated into the final tunnel structure. The tunnels may be constructed in place or formed of precast sections. Temporary SOE and temporary slopes for open excavations are not included in this Section.
Cut and cover tunnels and their components shall be designed to sustain the most severe combination of loads to which they may be subjected both during construction and after the final stage when the construction is complete and the tunnel is in service.
Effects of erection, bracing, excavation sequence and other temporary loads during the construction shall be considered in the design of cut and cover tunnels and their components.
A structural system study shall be prepared to determine the most suitable structural alternatives for construction of the cut and cover tunnel. This study shall include a determination of the proposed tunnel section, the excavation support system, the tunnel structural system, the construction method (top-down or bottom-up) and the waterproofing system.
When an open cut with stable sloped sides for the earth being excavated is not practical, a SOE system (shoring system) shall be used to stabilize the ground. Permanent SOE systems may be used as part of the final structure if they are designed and detailed accordingly.
Refer to Section 3 for load factors and load combinations that shall be considered during the design of cut-and-cover tunnels.
Refer to Section 4 for material properties and resistance factors that shall be considered during the design of cut and cover tunnels.
Refer to Section 10 for seismic considerations during the design of cut and cover tunnels.
Cut-and-cover tunnel structures shall be designed in accordance with AASHTO LRFD Specifications including all applicable interim changes, except as modified or supplemented here.
C6.1
Cut and cover tunnels are tunnels constructed in
an open excavation or trench, then backfilled with fill material. Shallow depth vehicular tunnels are usually designed as cut and cover tunnels. For invert depths up to 60 feet below grade, this method is often less expensive and more practical than tunneling. Cut and cover vehicular tunnels are also used at the approaches to mined, bored and immersed tunnels.
The ground surrounding a tunnel can act as a supporting mechanism, loading mechanism or both, depending on the nature of the ground, the tunnel size and the method and sequence of constructing the tunnel. Thus, the rock or soil surrounding a tunnel is a construction material.
6.2. DEFINITIONS Reinforced Concrete Diaphragm Wall – Slurry wall designed to span vertically with no structural continuity between panels. Typically reinforced with conventional deformed reinforcing steel bars. Soldier Pile and Tremie Concrete Wall (SPTC) – Slurry wall reinforced with vertical wide-flange steel sections placed at the ends of the panels. Secant Pile Wall – Wall constructed of drilled shafts that intersect the perimeter of adjacent shafts. Slurry Panel – Smallest unit of length of slurry wall constructed at one time. Panels are constructed in an alternate patter; primary panels are constructed first, followed by secondary panels. Tangent Pile Wall – Wall constructed of drilled shafts that touch each other but do not overlap.
6.3.1. General ft2 = square feet (C6.10.2) gals = gallons (C6.10.2) H = The height of vertical wall of a cut-and-cover tunnel from the bottom of the invert slab to top of the roof slab 6.3.2. Abbreviations
ASTM ASTM International (formerly known as the American Society for testing and Materials) AREMA American Railway Engineering and Maintenance-of-Way Association FE Finite Element analysis PTI Post-Tensioning Institute RCDW Reinforced concrete diaphragm wall SOE Support of excavation SPTC Soldier pile and tremie concrete
C6.4.1 Soil and rock parameters to be used in design
should consider possible changes in properties during construction, such as changes in shear strength, unit weight and/or permeability due to ground improvement methods.
6.4.1.1. General A subsurface investigation shall be performed to
obtain information on the ground conditions: • Defining the subsurface profile • Determining soil and rock material properties and
mass characteristics • Identifying geotechnical anomalies, fault zones
and other hazards • Defining hydrogeological conditions • Identifying potential construction risks
Design parameters to be used for excavation support,
and to define an appropriate and cost-effective route and location for cut and cover tunnels shall be obtained through subsurface investigations such as borings, sampling, in-situ testing, geophysical investigation, and laboratory material testing. Refer to Section 5 for geotechnical considerations for cut and cover tunnels and requirements for subsurface investigations.
C6.4.1.1
Ground conditions including geological, geotechnical and hydrological conditions, have a major impact on the planning, design, construction and the cost of a road tunnel.
Surrounding ground acts as a supporting system for cut and cover tunnels, ground properties and groundwater table level and their variations are important parameters to establish for use during the design process.
6.4.1.2. Invert Condition The embedment of both permanent and temporary
supports of excavation shall be analyzed with respect to stability of the base and ground water cut off or dewatering during construction of the structure. The potential for piping and base heave shall be analyzed. Embedment information and appropriate details for the support of excavation (SOE) shall be developed for incorporation into the structural design and contract documents. The need for mitigating measures such as ground improvement shall be investigated. See Section 9 for ground improvement.
If cut slopes are used in lieu of SOE structures, the stability of the cut slopes in soil and/or rock, groundwater conditions and the base of excavation shall be investigated to ensure a stable, dry open excavation.
C6.2.1.2
The Engineer is responsible for investigating the stability of the excavation prior to the construction and backfilling of the permanent structure. Studies should be performed to determine the most economic approach to stabilizing the open excavation.
The Contractor is responsible for the final selection and proper execution of measures to maintain stability of the open excavation during construction when the stability is maintained by temporary means.
6.4.1.3. Envelope Ground The earth and groundwater pressures shall be
calculated in accordance with the provisions of Section 3. The modulus of subgrade reaction may be calculated using conventional soil mechanics used in the design of walls and foundations. Other pertinent ground properties shall be determined in accordance with the provisions of Section 5.
C6.2.1.3
The characteristics of the ground around a cut and cover tunnel determine both how far the structural walls and invert slab can deflect into the surrounding ground, how much pressure the ground and groundwater will exert on the structural wall over time and the effects of loadings from adjacent
Cut and cover tunnels may be designed by applying external loads to a structural model or may be analyzed taking into account the effects of soil structure interaction.
existing infrastructure.
6.4.1.4. Groundwater In addition to subsurface investigations included in
Section 5 for groundwater considerations during the design and construction of cut and cover tunnels, pre-construction exploration shall take into account the information needed to develop a water control plan.
The effect of the hydrostatic pressure shall be considered whenever the groundwater table is detected within the construction envelope or may be reasonably expected to occur within the envelope over time.
Groundwater levels to be used for the design shall be established in accordance with Sections 3 and 5.
Long term variations in the groundwater level shall be considered in establishing the design groundwater elevation. Flood conditions with 500-year return period shall be investigated during the design and shall be evaluated as the extreme load case during the design phase.
Measures shall be taken to control groundwater entering the excavation area during construction, either by creating an impervious SOE wall (e.g., slurry or secant pile walls) or by allowing water to seep through pervious walls, collecting it in sump areas and pumping it out. When pervious SOE walls are used, the potential for soil erosion (piping), unacceptable settlement of nearby structures, roadways and utilities due to consolidation of compressible strata shall be considered in the design. Discharge of water collected in excavations shall be subject to all applicable laws and regulations.
Drawing down the local groundwater table through the use of wells shall consider the potential for settlement of surrounding soils and the effects of settlement on adjacent existing structures and infrastructure.
Design groundwater levels shall be used to determine water pressures and hydrostatic uplift pressures in the cut and cover tunnel structure.
C6.4.1.4 Control of groundwater is required during
construction by cut and cover methods to: • Permit excavation in the dry. • Prevent the migration of groundwater
contaminants, frequently found in urban areas.
• Minimize disturbance, heave or softening of the excavated base and to prevent flow of the materials (piping) through the excavation walls and excavation invert.
• Reduce water pressure acting on the support system during the construction.
Design and execution of protection of the
excavation from flooding during construction is the responsibility of the Contractor.
6.4.2. Materials
6.4.2.1. Concrete Mixes for cast-in-place concrete shall accommodate
placement requirements, which frequently involve pumping over long distances. Set-time and slump shall be considered during design of concrete mixes as they pertain to the means and methods of concrete placement which will vary with the Contractor’s equipment and sequence.
The chemistry of the groundwater shall be considered when designing the concrete mix.
Air entrainment of three to five percent is recommended for the durability of the concrete in moist and cold environments.
Often thick elements are required to support loads or to
C6.4.2.1
Cast-in-place concrete is commonly used to construct cut and cover tunnels due to the ease with which large members can be constructed in restricted work space. The weight of the concrete resists buoyancy; concrete can be detailed to be fairly water- tight, and it is resistant to fire and corrosion. However, other materials such as precast, prestressed concrete, post-tensioned concrete and precast concrete are also used for construction of cut and cover tunnels. The selection of materials is based on construction method and is generally done early in the planning stages.
provide resistance against uplift. Mix designs and specifications appropriate to mass concrete may be required.
Refer also to Section 4 for Concrete material properties.
Refer to AASHTO, 2010 for more details.
6.4.2.2. Structural Steel When used, measures shall be provided to protect
steel members from corrosion and heat from fire events. The design life of the structure shall be considered when developing corrosion protection measures.
Refer also to Section 4 for Structural Steel material properties.
C6.4.2.2
Structural steel framing is not as commonly used now as it was early in the last century due primarily to the difficulties associated with corrosion protection for the structural steel. However, composite designs can be used to reduce the thickness of members where space is constricted. Careful attention to waterproofing and corrosion protection is required for durable facilities.
6.4.2.3. Reinforcing Steel Reinforcing steel shall conform to requirements of
ASTM A615 grade 60 unless corrosive environments call for the use of galvanized, epoxy-coated or stainless steel reinforcement. If reinforcement is to be welded, or if ductility for seismic capacity is required, ASTM A706 shall be specified. Refer also to Section 4 for Reinforcing Steel material properties.
6.4.2.4. Prestressing Steel Prestressing steel shall be bonded and protected against
corrosion, with attention to combating the effects of corrosive compounds in the surrounding soils or groundwater. Bond capacity shall be tested in accordance with ASTM A981. Strand shall conform to ASTM A416, and bars shall conform to ASTM A722. The Engineer shall include relaxation requirements in the project specifications.
Refer also to Section 4 for Prestressing Steel material properties.
C6.4.2.4 Some jurisdictions prohibit the use of prestressed
elements below grade because of concerns about corrosion. However, with appropriate protection, prestressed, precast elements can streamline construction. Prestressed, precast elements have been used to form roofs of shallow buried structures, with thin cast-in-place slabs placed over the top to provide a diaphragm. Prestressed steel is commonly used for permanent rock anchors, with protection against corrosion provided by corrosion-inhibiting compounds (resins and greases), sleeves and grout. Prestressed steel is also often used in the walls of underground storage tanks and in underground parking garages.
6.4.2.5. Shotcrete
Refer to Section 4 for Shotcrete material properties.
C6.4.2.5
Shotcrete is frequently used as a smoothing layer between excavated rock faces, slurry, secant, and tangent pile walls and waterproofing membranes in cut and cover construction. It has also had limited use as a replacement for cast-in-place walls to save the costs and schedule associated with erection and removal of formwork.
When shotcrete is used to replace concrete in the final structure it will be applied in layers with a time lag between the applications. To ensure that the behavior is identical to monolithic concrete, it is
essential to limit the time lag between the applications. It is recommended that a bonding agent be used between layers.
The surface of the shotcrete shall be clean and free of any dust or dirt that might create a debonding between individual layers.
Shotcrete shall not be considered as a barrier to the movement of water over the long term, i.e. shotcrete shall not be used as a waterproofing membrane.
6.5. CONSTRUCTION OF CUT-AND-COVER
TUNNEL STRUCTURES
6.5.1. General Excavations shall be supported by wall systems
laterally supported by temporary bracing and/or temporary or permanent tiebacks, unless the excavation is sloped back to eliminate the need for structural support. These systems shall also minimize ground movements adjacent to the excavation, to protect adjacent utilities, buildings and other infrastructure. Where required, excavation support systems shall be designed to prevent or limit groundwater drawdown outside the excavation limits. SOE systems may be temporary or permanent.
C6.5.1
Cut and cover tunnels are built inside an excavation and covered over with backfill material when the tunnel structure is completed. This method is used when the tunnel profile is not deep, making excavation from the surface both possible and economical.
Cut and cover construction methodologies include ‘top-down’ and ‘bottom-up’ construction. The construction methodology must be considered when developing designs and details.
When the construction area is limited and an open sloped excavation is not possible, a system to support the excavation is necessary to create space for the construction.
Groundwater drawdown can result in unwanted ground settlements that affect adjacent existing infrastructure, migration of contaminates in groundwater and the need for disposal of the groundwater entering the excavation.
Temporary SOE systems shall be designed by the Contractor and removed or abandoned in place, as required by permitting authorities. The drawings shall show the following information:
• Space for the temporary SOE system. • Restrictions on the temporary SOE system. • Design load criteria for the temporary SOE
system. • Limits on the vertical and horizontal spacing of
bracing or anchor elements.
Permanent SOE systems are defined as those that serve both as temporary support and as part of the permanent structure. Since seepage through the permanent excavation support system can occur because the exterior of the support walls cannot be waterproofed, additional mitigation measures, such as an interior cavity wall drainage system or interior impervious architectural facing wall, may be required. Cavity wall drainage systems shall include a collection system and pumps for discharging the collected water.
Permanent support systems can minimize the excavated width of excavation, save schedule and reduce costs.
SOE systems shall be classified as flexible or semi-rigid. Flexible systems shall be considered temporary
Flexible SOE systems include sheet piling and soldier pile and lagging walls.
systems and shall not be incorporated into the permanent structure. The design of these wall systems is not within the scope of these Specifications. Semi-rigid systems may be designed to be incorporated into the permanent structure. When designed to be part of the permanent structure, these wall types shall be designed in accordance with these Specifications.
The design and detailing of the SOE shall consider the sequence of installation and account for the changes in loading conditions and deflections that occur as excavation proceeds, and the anchor or bracing system is installed. If anchors or bracing are not required, the deflection that occurs as excavation proceeds shall be accounted for.
The design shall be checked for an assumed sequence of construction by the Engineer. The assumed sequence of construction shall be shown on the drawings. Constraints that would affect alternate sequences of construction shall be described in the Contract Documents.
When an alternative sequence of construction is allowed by the contract documents and proposed by the Contractor, the Contractor shall verify that the design as shown on the drawings can adequately support the loads imposed by all stages of the construction. If the proposed sequence of construction creates load and deflection effects that cannot be supported by the design shown on the drawings, the Contractor shall develop and construct designs in accordance with this specification that can adequately support the loads and result in deflections that are not detrimental to the structure or surrounding existing infrastructure.
The sequence of construction may be bottom-up, where the excavation is completed first, then the structure is built from the bottom up or it may be constructed from the top-down, where a deck is constructed over the top, for example to allow traffic over the work site, and excavation and construction proceed sequentially downward.
Deflections of wall and other structural systems that are incorporated into the final structure shall be investigated in conjunction with potential settlement of earth behind the walls. Deflections shall be limited to values that do not cause unacceptable movement of existing structures and infrastructure, or cause impacts to the performance and durability of the permanent wall.
6.5.2. Reinforced Concrete Diaphragm Walls (RCDW)
Reinforced concrete diaphragm walls (RCDW) shall
be designed to support vertical, or near vertical faces, of the excavation in areas of soil and decomposed rock below the water table. RCDW’s may be designed to be incorporated into the permanent structure or may be used as temporary SOE. RCDW’s used for temporary SOE are not included in the scope of these Specifications.
The invert slab shall be anchored to the wall to transfer all the force effects from the wall to the base slab and vice versa.
Connection details shall be such that they provide a watertight connection.
Lateral earth pressure and water pressure for reinforced concrete diaphragm walls shall be calculated in accordance with Section 3.
The following are design requirements for RCDW’s:
C6.5.2
Reinforced concrete diaphragm walls are
constructed by excavating a trench to the thickness required for the external structural wall of the cut-and-cover tunnel and to the depth required for geotechnical and structural requirements. The trench is kept open by filling it with dense slurry of bentonite or other naturally occurring clays as it is excavated. For this reason, diaphragm walls are also referred to as slurry walls. Polymer slurries have been used for diaphragm wall construction with variable success. Considering the geometry of the excavated panels and the superior performance of mineral slurries, the use of mineral slurry should generally be specified.
The trench will typically extend for some distance below the bottom of the tunnel structure for
• RCDW’s shall be designed to span vertically between
lateral supporting systems for temporary support conditions encountered during construction.
• RCDW’s shall be designed for the final location of roof, invert and intermediate horizontal structural elements.
• The Engineer shall establish a workable temporary bracing scheme and check the walls for that scheme. The temporary bracing scheme and assumptions associated therewith, including the design loads, shall be included in the contract documents.
• Supplemental reinforcement may be required to serve as internal walers or to distribute forces around inserts or openings.
• Minimum cover protection for the reinforcement steel shall be three in. for the face of the wall permanently exposed to the earth.
• Detailed analyses shall be performed to evaluate wall deflection at each stage of construction and the possible ground movement and adjacent ground settlement due to the wall deflection, and its effect on adjacent existing construction.
• The design shall include removable block-outs for structural connections between the wall and slabs or other permanent structural features.
The design shall account for construction tolerances in
the excavation of the slurry panels and placement of the reinforcement cage. Typical vertical construction tolerance for the excavation of the slurry panels is one percent in the vertical direction. Typical reinforcement steel cage placement tolerances are 1 in. horizontally (not including the vertical slurry panel construction tolerance) and 12 in. vertically.
stability and as needed for water cutoff during construction. Reinforcing steel is lowered into the slurry-filled trench. Concrete is then placed in the trench using tremie pipes, with placement starting from the bottom and proceeding upward, with the concrete displacing the slurry. Once the concrete has cured, excavation for the tunnel construction may begin.
The walls are constructed in panels typically between 5 ft and 20 ft in length. The panel layout is dictated by the geometry of the structure, ground conditions and site restrictions. The panels are constructed in an alternating checker board pattern. Primary panels are constructed first, skipping over adjacent panels. Secondary panels are constructed between the completed primary panels to complete the wall.
For top-down construction, excavation is performed to the depth of the bottom of the roof slab. The tunnel roof slab is constructed on-grade and tied into the diaphragm wall. The tunnel roof braces the diaphragm wall against lateral movement. Depending on the depth of the tunnel, the roof could be the first level of bracing or an intermediate level, with temporary bracing levels above and below it. The excavation then continues downward, with additional bracing or floors constructed as needed. At the bottom of the excavation, the tunnel invert slab is cast and tied into the walls.
For bottom-up construction, as the excavation proceeds, intermediate temporary braces are installed, often at multiple levels. Once the required depth is reached, the construction of the permanent invert slab and any intermediate levels will follow from the bottom to the top. Once the tunnel roof is cast, the trench is backfilled, and the surface restored.
6.5.3. Soldier Pile and Tremie Concrete (SPTC) Walls
Soldier pile and tremie concrete (SPTC) walls shall be
constructed using vertical wide flange or built-up steel members placed at the ends of excavated slurry panels. Concrete is placed between the soldier piles. The soldier piles shall be designed to span vertically to resist the loads transferred by the reinforced concrete spanning horizontally between them.
The following are design requirements for SPTC
walls:
• Steel piles shall be designed to span vertically and to resist all temporary, construction and final loads.
• Assumed excavation and construction sequence for the design of walls shall be explicitly and clearly shown on the contract drawings, including all bracing assumptions and design loads.
• Effect of removal of the internal bracing during the
C6.5.3
SPTC walls are constructed in the same
sequence as reinforced concrete diaphragm walls. However, once the primary panel is excavated, vertical wide flange steel beams or girders are lowered into the panels, one at each end of the primary panels, followed by reinforcing steel cages, if required. Secondary panels are excavated and constructed between the steel members placed in the primary panels. Reinforcing steel when required is placed into the secondary panel. The slurry used to hold the excavated secondary panel open is displaced by structural concrete placed via tremie pipes from the bottom of the panel. SPTC walls provide a relatively watertight wall and significant strength in the vertical direction. They allow greater flexibility in detailing connections to permanent horizontal members than is allowed by the use of rebar couplers in diaphragm walls. Careful attention must be paid to
final phase of the construction of the cut and cover tunnels shall be investigated during the design.
• Minimum cover protection for the reinforcement steel shall be three in. for the face of the wall permanently exposed to the earth.
• Detailed analyses shall be performed to evaluate wall deflection at each stage of construction and the possible ground movement and adjacent ground settlement due to the wall deflection and its effect on adjacent existing construction.
• The design shall include removable block-outs for structural connections between the wall and slabs or other permanent structural features. The design shall account for construction tolerances in
the excavation of the slurry panels and placement of the steel members and reinforcement cage. Typical vertical construction tolerance for the excavation of the slurry panels is one percent in the vertical direction. Typical reinforcement steel cage placement tolerances are 1 in. horizontally (not including the vertical panel construction tolerance) and 12 in. vertically.
waterproofing and corrosion protection of the structural steel members and connections.
6.5.4. Secant Pile and Tangent Pile Walls Secant pile and tangent pile walls shall be constructed
using vertical drilled shafts. The drilled shafts shall be reinforced with deformed reinforcing steel bars or with a structural steel core fabricated from rolled shapes or built-up plate members and backfilled with structural concrete. Reinforcement is commonly placed in alternate secant piles and every tangent pile. If required, steel sections can be placed in every secant pile taking care to avoid interference during installation of secondary piles. Structural concrete shall be used to backfill the shafts after placement of the steel sections. The drilled shafts constructed either adjacent to or touching each other (tangent pile walls) or the drilled shafts shall have some overlap with each other (secant pile walls). The reinforced shafts or the shafts containing steel cores shall be designed to span vertically to resist the loads transferred by the unreinforced concrete shafts between them.
The following are design requirements for secant pile and tangent pile walls:
• Reinforced concrete or steel core piles shall be
designed to span vertically and to resist all temporary, construction and final loads.
• Assumed excavation and construction sequence for the design of walls shall be explicitly and clearly shown on the contract drawings, including all bracing assumptions.
• Effect of removal of the internal bracing during the final phase of the construction of the cut-and-cover tunnels shall be investigated during the design.
• Minimum cover protection for the reinforcement steel or steel core shall be three in.
• Detailed analyses shall be performed to evaluate wall
C6.5.4
Secant pile and tangent pile walls are constructed in a similar manner. Every other pile (primary piles) is constructed first. The remaining piles (secondary piles) are constructed next. Either slurry or steel casings are used to support the excavation when required by ground conditions. Concrete is placed into the shaft utilizing tremie pipes. When used, the slurry is displaced by concrete placed from the bottom of the shaft. Steel casing are extracted as the concrete is placed, keeping a minimum amount of casing within the plastic concrete as the casing is extracted.
Secant pile and tangent pile walls exhibit good strength in the vertical direction. They allow greater flexibility in detailing connections to permanent horizontal members (when steel cores are used) than is allowed by the use of rebar couplers in reinforced concrete piles. Construction tolerances can result in large gaps between piles in both the longitudinal and transverse directions. Often, ground improvement is required after construction of the piles to cut off groundwater flows from between the piles. Careful attention must be paid to waterproofing and corrosion protection of the structural steel members and connections.
deflection at each stage of construction and the possible ground movement and settlement due to the wall deflection and its effect on adjacent existing construction. The design shall account for construction tolerances in
the excavation of the secant or tangent piles and placement of the steel members and reinforcement cage. Typical vertical construction tolerance for the excavation of the tangent or secant is two percent in the vertical direction. Typical reinforcement steel cage placement tolerances are 1 in. horizontally and 12 in. vertically. Typical steel core placement tolerance is six in. vertically.
6.5.5. Precast Prestressed Panel Walls
Precast prestressed panel walls shall be designed and
detailed similarly to conventionally reinforced concrete diaphragm walls.
Precas,t prestressed beams may be used for roof
framing, with the connection possibly formed by pockets in the walls to support the beams.
Detailing of the connection shall be consistent with the structural system and design assumptions.
C6.5.5 Precast, prestressed panel walls with appropriate
details for roof and invert connection in future stages are lowered into the slurry, requiring careful control of the vertical placement. The construction of the wall then follows the same sequence as that described above for reinforced concrete diaphragm walls. This type of the construction will provide the best quality finish wall. These types of wall become less practical for deeper excavations and in locations where utility crossing or obstructions can be expected.
6.5.6. Cast-in-Place Concrete Box Structures
Cast-in-place concrete box structures shall be detailed according to Section 5 of the LRFD Specifications and additional requirements of these Specifications, with the following modifications:
• When the structure lies below the water table, the
minimum amount of temperature and shrinkage reinforcement shall be increased 100 percent for all members of cast-in-place concrete box structures.
• The maximum spacing of the temperature and shrinkage reinforcement shall be 12 in.
C6.5.6
Cast-in-place concrete box structures are the most common system used for cut-and-cover tunnels. Cast-in-place concrete can be easily adjusted to accommodate any geometric irregularities required by function.
For any buried structure, it is desirable to limit cracking. Most damage to these structures is caused by the infiltration of groundwater through the structure over time. The upfront cost of additional temperature and shrinkage reinforcement is minimal compared to the costs of retrofit leak mitigation or management systems. Crack control can be achieved with careful concrete mix design, wet curing, and closely distributed temperature reinforcement. Crack widths on flexural members are frequently limited to 0.08 in., a crack considered to be “self-healing”.
6.5.7. Structural Steel frames
Structural steel frames shall be detailed according to Section 6 of the LRFD Specifications and additional
C6.5.7
Structural steel frames are used in the construction of cut-and-cover tunnels, when there is a
requirements of this specification. Corrosion protection and fireproofing shall be done by
encasing the steel members in concrete or other approved methods.
space restriction and when there is need to take advantage of the high strength-to-weight ratio of steel. Composite construction is also used to make structures more slender.
6.6. LIMIT STATES AND RESISTANCE FACTORS
6.6.1. General Design of cut-and-cover tunnels shall satisfy the
criteria for the service limit states specified in Article 6.6.2, the strength limit state specified in Article 6.6.3 and the extreme event limit states specified in Article 6.6.4. The load combinations of Table 3.4-1 shall be used to investigate the various limit states herein.
6.6.2. Service Limit State
Service T-I: Cut-and-cover tunnels shall be investigated for deflections, crack control, vibration, durability, water tightness and fatigue as developed during the service life of the tunnel.
Service T-IA: Cut-and-cover tunnels shall be
investigated for the effects of buoyancy during the service life of the tunnel, including but not limited to, maximum groundwater elevations and changes in groundwater elevation caused by tides and flooding. The effect of salinity and other inclusion in groundwater that affect the unit weight of groundwater shall be included.
Service T-II: Cut-and-cover tunnels shall be
investigated for the effects of buoyancy that occur during construction. Cut-and-cover tunnels shall be checked for deflection during temporary bracing conditions when vertical wall elements are incorporated into the final structure. Ground movement as a result of deflections of SOE walls that are also permanent structural walls for the structure shall be evaluated. Adjacent structures and infrastructure that could be affected by these ground movements shall be evaluated and protected against potential adverse effects of the ground movement.
C6.6.2
Buoyancy has traditionally been checked as a factor of safety against flotation. However, to include the effects of friction against foundation and vertical wall elements in resisting uplift, a service limit state approach to this check is required.
6.6.3. Strength Limit State
Strength T-I: Cut-and-cover tunnels shall be investigated for strength during normal use, under which the tunnel remains fully serviceable without any damage.
Strength T-II: Cut-and-cover tunnels shall be investigated for strength during construction including, but not limited to, sequence of construction, installation and removal of temporary bracing, backfilling and construction stage loading.
investigated for earthquakes to ensure life safety and survivability of the structure taking progressive collapse into account. Acceptable levels of damage shall be defined by the Owner.
Extreme Event T-II: Cut-and-cover-tunnels shall be investigated for extreme events other than earthquake. Other extreme events such as explosion and fire shall be considered on a project-specific basis and, if relevant, shall be included in Extreme Event II individually but not simultaneously with other events. The Owner may, at its discretion, and after performing a hazard analysis, create load combinations that have more than one of these loads applied to the structure simultaneously. In this case, it is recommended that the load factors shown in Table 3.4-1 be used when combining the loads.
Extreme Event T-III: Cut-and-cover tunnels shall be investigated for a rare event for the simultaneous combination of loads such as flooding. This load combination shall be used for both surface flooding that raises the groundwater elevation and for flooding of the inside of the tunnel.
Under Extreme Event T-III, the following flooding cases shall be investigated:
• Sea level or surface water elevations rise and
produce increased hydrostatic pressures on the tunnel that also reduce the resistance to buoyancy.
• Flooding of the inside of the tunnel occurs that increases the resistance to buoyancy and reduces the net hydrostatic pressure to zero.
damage could include partial failure of tunnel members that result in limited leakage and could include complete loss of service.
Under Extreme Event T-II, it is recommended
that the tunnel sustain no more than light damage and/or minor leaks, while experiencing no significant loss of service during the prosecution of repairs, i.e. partial performance level.
6.6.5. Load Factors and Load Combinations Cut-and-cover tunnels shall be designed for the load
combinations shown in Tables 3.4.-1. When developing the loads to be applied to the
structure, each possible load combination of the loads shall be developed. Engineering judgment may be used to eliminate the combinations that do not govern.
C6.6.5
Section 3 provides guidance on the methods to be used in the computations of the loads applicable to cut-and-cover tunnels. The sequence of construction will impact loading and assumptions. For example, in top-down construction, permanent SOE walls used as part of the final structure will receive heavier bearing loads because the roof is placed and loaded before the base slab is constructed. The permanent SOE walls are also braced as the excavation progresses below the roof slab, resulting in a different lateral soil pressure distribution than would be found in the free-standing walls of a cast-in-place concrete structure constructed using bottom-up construction. The base slab of a top-down constructed tunnel acts as a mat for supporting vertical loads, but it is not available until towards the end of construction of the section, eliminating its use to resist moments from the walls or to act as bracing for the walls. Structures are subjected to a wide range of load combinations during construction. These include the variability of dead and live loads on partially completed configurations, erection and equipment loads, and the full range of environmental loads to which the final structure is subjected, although with differences attributable to return period of events
(storm, earthquake), duration (horizontal pressures due to equipment loading) or intensity (earth pressures changing from active to at-rest). ASCE 37, “Design Loads on Structures during Construction” Chapter 2, discusses the formulation of load combinations and development of load factors for the different cases in a manner that allows the Engineer to formulate reasonable combinations for the different construction stages.
6.6.6. Resistance Factors Refer to Section 4 for resistance factors for each
material used in construction of cut-and-cover tunnel structures.
6.7. GENERAL DESIGN FEATURES
Cut-and-cover tunnels shall be designed with moment resisting joints including ductile detailing in the potential plastic hinge zones for Extreme Event I.
6.7.1. Ground Movement
Ground movement during excavation and construction and its effects on adjacent structures, utilities and other infrastructure shall be carefully considered in all phases of design and construction. Ground movement shall be explicitly addressed in the development of contract documents and carefully monitored in the field once construction begins. Instrumentation to monitor the movements of adjacent existing structures shall be in place during construction. Threshold values and values for total permissible movement of the facility being monitored shall be established.
Detailed analysis and monitoring shall be performed in advance to predict these movements accurately and to adjust the design if necessary.
Numerical modeling shall be performed to estimate ground movements at each stage of the construction.
Mitigation measures shall be designed to limit ground movements to values that can be tolerated by the feature being studied. Existing buildings and facilities shall be evaluated for the soil movement estimated to occur due to excavation activities for construction of cut-and-cover tunnels. This evaluation is not within the scope of this Specification.
C6.7.1
Ground movements during the construction of the cut-and-cover tunnels are more significant compared to the other tunneling methods.
Ground movement can be due to: • Excavations for utility relocations, guide
wall construction, pre-trenching for guide walls and slurry panel installation.
• Deflection of the SOE walls during excavation for the tunnel structure.
• Consolidation due to dewatering during construction and during the service life of the tunnel.
6.7.2. Buoyancy
Cut-and-cover tunnels that are partially or completely below the water table, shall be evaluated for the effect of buoyancy unless a pressure relief system is provided beneath the invert slab. See article 6.11 for requirements for pressure relief systems. Buoyancy forces shall be computed using the density of the groundwater at the site.
C6.7.2
In general, the preferred method for resisting buoyancy is utilizing the dead weight of the structure. In cases where this is not practical, consideration may be given to the utilization of drilled shafts, driven piles and skin friction on RCDW & SPTC slurry panels or on tangent or secant piles. When these
Adequate resistance to flotation shall be provided at all sections for the full hydrostatic uplift pressure on the structure’s foundation. The following requirements shall be met:
• The base slab shall be designed to resist the uplift pressure acting at the bottom of the base slab. The water level shall be determined from the site investigation considering mean high water level and long-term flood events as mandated by the Owner or local jurisdiction. See Sections 3 and 5 for additional information.
• When SOE is incorporated into the permanent structure, the base slab shall be anchored to the excavation support walls to transfer all uplift forces.
• When other means of buoyancy resistance prove to be impractical, permanent tie-down anchors may be used to assist in the resistance to the uplift due to buoyancy. When used, the design of the anchors shall be performed in accordance with: − FHWA-IF-99-015 Geotechnical Engineering
Circular No.4 Ground Anchors and Anchored Systems
− PTI DC35.1-14 Recommendations for Prestressed Rock and Soil Anchors
methods prove impractical, consideration may be given to the use of permanent tie-down anchors to assist in the resistance to buoyancy.
FHWA (1999) indicates that tie-down anchors
can be used to resist hydrostatic uplift. The Owner shall make the determination as to the acceptability of permanent tie-down anchors for resisting buoyancy.
6.7.2.1 Partially Completed Structure
A construction sequence, together with all temporary measures necessary to provide adequate resistance against flotation during all stages of construction, shall be indicated on the contract drawings. Resistance shall consist of the dead load of the completed portion of the structure and any additional resistance provided by the provisions indicated in the construction sequence documents.
Friction between soil and wall elements shall be neglected when calculating resistance during construction.
If the main structure is not symmetrical, rotational effects due to the buoyancy shall be considered.
C6.7.2.1
Contractors generally elect to work in the dry and pump out any water that makes its way into the excavation. Often, loss of power to the pumps results in the flow of the water into the excavation, so that flotation is quickly counteracted by the weight of water above the completed slab.
6.7.2.2 Complete Structure
The complete structure shall be evaluated for the buoyancy effect considering the resistance of the dead weight of the complete structure, plus the weight of the backfill overlaying the cut-and-cover tunnel within the vertical plane through the outer edge of the roof. Provisions shall be specified in the design and/or construction sequence documents to prevent the possibility of the flotation due to the possible rise in water table before all backfill is placed on the structure.
Any component of the tunnel that can potentially be removed for maintenance, repair or rehabilitation shall not be considered when calculating the resistance of the structure to uplift.
C6.7.2.2
Examples of components that should not be included in the calculation of the resistance to buoyancy include:
Some provision for future construction over the in-service tunnel shall be made, so that buoyancy need not be a concern for routine works that may be expected above the finished structure. For example, the top six ft. of backfill may be ignored in the buoyancy analysis.
− Traffic control devices − Ventilation equipment − Electrical equipment − Communication equipment − Finishes
6.7.3 Loading All components of the cut-and-cover tunnel shall be
designed for the applicable loads described in Section 3 and as supplemented in this Article.
Cut-and-cover tunnels shall be designed for any additional loading from existing adjacent buildings when the horizontal distance from the building line, to the nearest face of the tunnel, is less than the depth of the tunnel below the building foundation.
If the foundation of the adjacent buildings are supported by underpinning and/or deep foundations that transfer loads to a depth below the influence line of the tunnel, these additional loads need not be applied to the tunnel.
When the loads from the adjacent structures are not equal on both sides of the tunnel, the most critical unbalanced load case shall be investigated.
As a minimum, the following basic non-seismic loading cases shall be investigated:
6.7.3.1 Symmetrical Loading
The following cases shall be investigated: Case I - Maximum vertical loads including dead loads,
live loads, surcharge loads and hydrostatic pressure with maximum lateral pressure due to soil and rock.
Case II - Minimum vertical load (dead load only), with maximum hydrostatic pressure and maximum lateral pressure due to soil and rock.
Case III - Maximum vertical loads and minimum hydrostatic pressure and lateral pressure due to soil and rock.
6.7.3.2 Asymmetrical Loading
The following cases shall be investigated: Case I – Maximum vertical loads including dead
loads, live loads, surcharge loads and hydrostatic pressure with lateral pressure as follows:
Maximum lateral pressures (at- rest) due to soil loads applied to one side of the structure and active pressure from soil on the opposite side.
Case II – Maximum vertical loads including dead
loads, live loads, surcharge loads with maximum lateral pressure (at-rest) due to soil loads applied to one side of the structure and active pressure from soil on the opposite side.
Case III – For cases where a partial or full-depth
excavation that cannot be braced is anticipated on only one side of the cut-and-cover structure, the cut-and-cover
C6.7.3.2
Where SOE walls are incorporated into the permanent structure, the design of the connections between walls and roof or invert slabs is governed by the unbalanced lateral load combinations.
structure shall be designed for zero lateral earth pressure to the depth of the anticipated excavation on the excavated side of the structure with at-rest earth pressure on the opposite side.
Unbalanced earth pressures that cause sways over
0.005H should be reviewed with respect to the loading conditions, since such cases are unlikely to represent a true loading condition. H is taken as the height of vertical wall from the bottom of the invert slab to top of the roof slab.
6.7.3.3 Construction Condition
Construction sequence and procedures may result in
conditions that are more severe than the general loading conditions mentioned in the previous sections. Stresses in the partially completed structure shall be checked for the appropriate critical conditions at the various stages of construction.
The Engineer shall be responsible to develop a construction scheme and to clearly state assumptions regarding the scheme in the contract documents. If the Contractor uses different methods, the documents shall require that the Contractor provide a complete analysis of all construction stages.
C6.7.3.3
Intermediate stages of the construction may induce higher stresses than the end of construction stage and govern the design of structural members.
6.7.3.4 Distribution of Loads Loads on cut-and-cover tunnels resulting from
existing or future land use at the surface above or adjacent to the tunnels shall be accounted for in the design and distributed according to the following:
Vehicular loads shall be distributed according to Section 4 of the LRFD Specifications.
Train loads shall be distributed according to guidelines provided in AREMA (2014).
6.7.3.5 Superimposed Loads Superimposed loads are defined in Section 3 of the
LRFD Specifications and shall be calculated and applied to cut-and-cover tunnels in load combinations as applicable.
Cut-and-cover construction is often used in urban areas and provisions shall be made for the structures to support urban loadings: future building developments, parks or roadways. Future loadings provided for in the design shall be clearly identified in the contract documents.
6.8 JOINTS
6.8.1 General
Joints in concrete construction shall be categorized as: construction joints, which allow discrete concrete placements in a continuous structure, contraction joints, and expansion joints.
Joints shall be carefully detailed to prevent the infiltration of water yet simultaneously accommodate the
C6.8.1
A main objective in the design of joint spacing is to try to control the locations where the concrete cracks. In cut-and-cover structures, cracks are locations where water will infiltrate into the structure, often causing structural and systems degradation over time.
movements expected over time. All joints shall be provided with waterstops.
Concrete mixes for cut-and-cover structures shall be designed to limit the heat of hydration, using the same provisions that are applied to mass concrete placements. Such provisions may include the substitution of blast furnace slag or fly ash for Portland cement to reduce the heat of hydration, controlled curing in the wet and the control and monitoring of temperature differentials across the curing section.
Provisions to control cracking include heavier longitudinal (temperature and shrinkage) reinforcement and the detailing of joints to control the locations where cracking occurs.
Shrinkage during initial curing is a major cause of concrete cracking.
6.8.2 Construction Joints Construction joints shall be detailed for structural
continuity, with joint surfaces intentionally roughened and saturated with water or coated with bonding agent and with reinforcement extending a lap length into the next concrete placement.
C6.8.2 Construction joints occur between discrete concrete placements.
6.8.3 Contraction Joints
Contraction joints shall be designed to encourage the concrete to crack at the joint location as it shrinks during curing or temperature changes. Contraction joints may be as simple as a saw-cut on the exposed face or the installation of a zip-strip.
6.8.4 Expansion Joints
Expansion joints shall be designed to accommodate expansion of adjacent units and relative movements of dissimilar adjoining structure configurations, such as a tunnel entering a ventilation structure or abutting a tunnel of a different construction.
Provisions for expansion shall be made when a structural element is partially underground and partially above ground, and where the above ground element is attached to the underground element, particular care shall be taken in detailing to accommodate differential thermal movement.
The following are minimum requirements for the expansion joints:
• Reinforcement steel shall not be continuous through
the joint. • Shear forces shall be transferred across the joint,
preferably by a key. • All expansion joints in structures subject to hydrostatic
pressure shall contain waterstops.
6.9 STRUCTURAL ANALYSES
The provisions of Section 4 of the LRFD Specifications shall apply as amended herein.
The analysis of cut-and-cover tunnel structures shall be
performed utilizing general structural or geotechnical engineering software capable of modeling beam elements/shell elements to represent the tunnel structure and spring/plate/solid elements to model the surrounding ground. 6.9.1 Load Distribution and Sharing
Frame analyses shall be performed using rational
elastic methods which consider the effect of relative stiffness of connected members, relative displacement, rotation of the joints and the effects of the axial deformation.
Consideration shall be given to the variations in elastic properties and stress distribution of complex frameworks resulting from different construction sequences.
Any limitation on construction operation inherent in the design assumptions shall be noted on the project documents and specified in the special provisions.
6.9.2 Ground/Structure Interaction
Numerical modeling shall be used to account for ground-structure interaction. The numerical modeling may consist of beam-spring (Winkler springs) models, finite element modeling, or finite difference models.
Three dimensional finite element or finite difference analyses shall be performed where ground conditions or structure geometry vary longitudinally and a two-dimensional model would oversimplify the conditions.
The supporting mechanism of the ground shall be accounted for in analyses that include unsymmetrical loadings. Ground properties for use in analyses that include soil structure interaction shall be determined in accordance with Section 5.
When beam-springs models are used, stiffness of ground springs is computed by multiplying the moduli of subgrade reaction with the tributary area at the corresponding node.
6.9.3 Empirical Methods
Classical force and displacement analysis methods may be used for determining force effects in cut-and-cover structures. Limitations of the method used must be understood by the Engineer, and the design shall account for the limitations. It is recommended that these techniques be used only for preliminary member sizing or as a way to evaluate the reasonableness of the results of numerical analysis.
6.9.4 Frame Analyses Frame analyses shall be performed for all permanent
components of cut-and-cover tunnels. The following loading conditions shall be considered
Waterproofing systems are manufactured by a number of companies. In general, details of system application are left up to the manufacturer, whose responsibility to the facility owner is inherent in the warranty required by the specification. Most manufacturers will not warrant their product unless the seaming, penetration and termination details that they have developed and tested are used in the installation. There is a range of relationships between manufacturers and installers, with some installers explicitly licensed by the manufacturer, while others may only send a technical representative for sufficient time to train the Contractor work force. Membrane Waterproofing:
Several companies manufacture membranes that are delivered in rolls. They may be formed from different plastics, such as PVC or EPDM, or they may be panels of bentonite. Membranes may be detailed to be installed against wet concrete, a “blind side” application. Blind side membranes may be treated to bond or adhere to the wet concrete. These are often applied directly to the excavation cut or shoring wall. Membranes may also be rolled out over cured concrete. Liquid applied waterproofing:
A number of companies manufacture liquid compounds that cure in place to form membranes. These may be painted on or spray applied.
6.10.1 Treatment of Penetrations
Most cut-and-cover structures will have local penetrations for utility entrances. Manufacturers shall provide details to ensure the seal around the penetrations.
6.10.2 Gaskets Gaskets shall be designed to resist the anticipated
hydrostatic pressures through compression of the gasket for the design life of the gasket.
6.10.3 Permissible Leakage
Slurry walls, secant pile walls and tangent pile walls used as the SOE and permanent walls that do not have applied waterproofing, shall be subject to the permissible leakage criteria.
Permissible leakage criteria shall be given in the contract specifications to determine the acceptability of the construction. The tunnel drainage system shall be designed
C6.10.2
Criteria generally include a measured infiltration of volume/ft2/day and a maximum flow at any single point. Typical criteria range from 0.0002 to 0.01
gals/ft2/day, with 0.02 gallon per minute of flow from any single leak. The Owner shall establish the required leakage criteria. Factors to consider when establishing permissible leakage include the long-term management of the incoming water, the selected structural system and associated number of joints, and constituent components of the groundwater including groundwater chemistry and contaminants.
. 6.11 PRESSURE RELIEF SYSTEMS
Pressure relief systems installed to reduce the
hydrostatic pressure shall be sized and detailed to function with minimal maintenance in the in-situ conditions to which they are subjected.
The effect of potential localized lowering of the groundwater table, including settlement of surrounding ground and the impact of that settlement on adjacent existing infrastructure by pressure relief systems, shall be analyzed. Mitigation measures for anticipated settlement shall be designed.
Management of the water collected shall be considered when deciding to build a pressure relief system.
Long-term pumping equipment and a pressure monitoring system shall be provided to ensure the long-term performance of the pressure relief system.
Pressure relief systems constructed in tunnels in cold climates should include measures to prevent freezing. Groundwater that is liberated from these systems that contains mineralization that is likely to precipitate or deposit on pipes shall be conveyed using systems that consider the maintenance and removal of any precipitates that form.
C6.11
Pressures imposed on drained structures may be less than those imposed on structures that are sealed and fully immersed. Drainage systems provide this pressure relief. However, drainage systems must be maintained, because they tend to clog and allow water pressures to build behind the structure. In general, a gravel bedding layer is placed under the invert slab and large diameter perforated pipes at the base of the structural walls, with closely spaced clean-outs, are required. An additional concern in urban areas is the capacity of municipal sewage treatment plants. Many sewerage agencies will not allow groundwater to be pumped into their systems.
2. AASHTO, Technical Manual for Design and Construction of Road Tunnels – Civil Elements, 2010
3. AREMA, Manual for Railway Engineering, 2014
4. Federal Highway Administration (1999), Geotechnical Engineering Circular No. 4, Ground Anchors and Anchored Systems, U.S. Department of Transportation, Office of Bridge Technology, Washington, DC.
5. Post Tensioning Institute (2014), PTI DC35.1-14 Recommendations for Prestressed Rock and Soil Anchors 6. America Society of Civil Engineers, ASCE 37, “Design Loads on Structures during Construction”
7.5 CONSTRUCTION OF MINED AND BORED TUNNEL STRUCTURES 7.5.1 General 7.5.2 Construction of Bored Tunnel Structures 7.5.3 Construction of Bored Tunnel Structures
7.6 LIMIT STATES AND RESISTANCE FACTORS 7.6.1 General 7.6.2 Service Limit State 7.6.3 Strength Limit State 7.6.4 Extreme Limit State 7.6.5 Load Factors and Load Combinations 7.6.6 Resistance Factors
7.7 GENERAL DESIGN FEATURES 7.7.1 General
7.7.1.1 Initial Ground Support 7.7.1.2 Final Lining 7.7.1.3 Cross Passages, Sumps and Ancillary Spaces
The provisions of this Section apply to the planning design, evaluation and rehabilitation of the permanent lining for tunnels constructed by tunnel boring machine (TBM) or mining. The lining types covered by this Section are as follows:
• Cast-in-place concrete (reinforced or structural
plain concrete) • Precast concrete segmental linings (reinforced with
(reinforced with reinforcing steel bars or steel fibers or constructed from structural plain concrete)
• Shotcrete final linings (reinforced with reinforcing steel bars, lattice girders or steel fibers and structural plain shotcrete)
• Pre-fabricated steel liner Bored and mined tunnels and their components shall be
designed to sustain the most severe combination of loads to which they may be subjected both during construction and when the construction is complete, and the tunnel is in service.
Effects of temporary loads during the construction of mined and bored tunnel structures shall be considered in the design.
Refer to Section 3 of the Specifications for load factors and load combinations that shall be considered during the design of mined and bored tunnels.
Refer to Section 4 of the Specifications for material properties and resistance factors that shall be considered during the design of mined and bored tunnels.
Refer to Section 9 of the Specifications for the design of initial ground support elements and ground improvement.
Refer to Section 10 of the Specifications for seismic considerations during the design of mined and bored tunnels.
C7.1 Mined tunnels are constructed utilizing
excavation techniques such as drill and blast, mechanical excavation with road headers or other mechanized equipment. The excavation is performed with an open face and can be performed with or without the use of a shield. The use of a shield is dependent upon the ground conditions.
Bored tunnels in soil or rock are constructed utilizing a tunnel boring machine (TBM).
The ground surrounding a tunnel can act as a supporting mechanism, loading mechanism, or both, depending on the nature of the ground, the tunnel size and the method and sequence of constructing the tunnel. Thus, the rock or soil surrounding a tunnel is considered a construction material and shall be taken into account in the design of tunnel linings
7.2. DEFINITIONS Final Lining – The permanent tunnel structure, constructed within an excavation which has been supported by initial support elements installed concurrently with excavation. Horizon – The cross section of a tunnel, cross passage intersected by a plane perpendicular to the centerline of the tunnel. Lattice Girder— A ground support element consisting of an open lattice of three or four steel bars connected by lacing bars and encapsulated with shotcrete, which is sprayed through the open-work lattice. Overbreak – Quantity of rock actually excavated beyond the perimeter established as the desired tunnel excavated cross section. Tunnel Boring Machine (TBM) – Machine that excavates a tunnel by drilling out the heading to full size in one operation; sometimes called a mole; the TBM is typically propelled forward by jacking off the excavation supports emplaced behind it or by gripping the side of the excavation (AASHTO, 2010).
Sequential Excavation Method – Construction methodology in which the tunnel is mined in specified sequence to control ground movements; Also known as the New Austrian or North American Tunneling Method (NTAM).
Initial Ground Support – Support required to provide stability of the tunnel opening and to maintain the inherent strength of the ground surrounding the tunnel openings, while preventing unnecessary loosening and enhancing the stress redistribution process. This function of support may be enhanced by installation of systematic Tunnel Pre-support and local support where required by ground conditions. It typically consists of reinforced shotcrete, rock reinforcement, pre-support, steel rib or lattice girder sets, or combinations thereof. (AASHTO, 2010)
= the area of the tunnel intersected by a vertical plane (in2) (7.7.2.2) Er = modulus of elasticity of the surrounding ground (ksi) (7.7.5.4)
H = depth to the springline of the tunnel (in) (7.7.2.3) Ie = effective moment of inertia (in4) (7.7.5.3) Ij = movement of inertia of the joint (in4) (7.7.5.3) Ig = gross moment of inertia of the lining section (in4) (7.7.5.3) K = settlement trough width parameter (7.7.2.2) n = number of joints in a precast segmental concrete lining ring (7.7.5.3) R = tunnel excavated radius (in) (7.7.2.3) Sh = horizontal movement of the ground toward the center of the tunnel (in.) (7.7.2.2) Smax = maximum settlement at tunnel centerline (in.) (7.7.2.2) Sy = settlement at a distance y from the tunnel centerline (7.7.2.2) Ux = lateral ground subsurface settlement (in) (7.7.2.3) Uz = vertical ground subsurface settlement (in) (7.7.2.3) Uz=0 = vertical ground surface settlement (in) (7.7.2.3) Vl = ground loss ratio (7.7.2.2) b = the length of the tunnel lining element being modeled (in) (7.7.5.4) kr = radial spring stiffness (kips per inch) (7.7.5.4) kt = tangential spring stiffness (kips per inch) (7.7.5.4) x = lateral distance from the tunnel center line (in) (7.7.2.3) y = horizontal distance from the tunnel centerline (in) (7.7.2.2) z = depth below the ground surface (in) (7.7.2.3) z0 = depth of the tunnel below ground surface at springline (in.) (7.7.2.2) β = limit angle (degrees) (7.7.2.3) ε0 = average ground loss ratio (7.7.2.3) φ = angle of shear resistance of the ground (degrees) (7.7.2.3) θ = the arc subtended by the beam element being modeled (radian) (7.7.5.4) ν = Poisson’s ratio of the ground (7.7.2.3) (7.7.5.4) 7.3.2. Abbreviations
AREMA American Railway Engineering and Maintenance-of-Way Association ASTM ASTM International (formerly known as the American Society for testing and Materials) ESC Excavation support class ft2 square feet gals gallons in inch in2 square inches NFPA: National Fire Protection Association PAS Pneumatically applied shotcrete
7.4.1.1. General Ground conditions, including geological, geotechnical
and hydrogeological conditions, have a major impact on the planning, design, construction and cost of a road tunnel. A subsurface investigation shall be performed to obtain information on the ground conditions to:
• Define the subsurface profile • Determine soft ground and rock material
properties and mass characteristics • Identify geotechnical anomalies, fault zones and
other hazards • Define hydrogeological conditions • Determine the presence of hazardous materials • Identify potential construction risks
Subsurface investigations such as boring, sampling, in-
situ testing, geophysical investigation and laboratory material testing are essential to obtain design parameters for the tunnel lining, and to define the best and most cost-effective route and location for mined or bored tunnels.
Refer to Section 5 for special geotechnical considerations for mined and bored tunnels and requirements for subsurface investigations.
C7.4.1.1
The tunnel lining and the surrounding ground act together to create a stable opening for the tunnel. Detailed information regarding the properties of the surrounding ground is essential for correctly modeling ground/structure interaction. Design loads transferred to the tunnel lining including ground loads, groundwater, surcharge and existing infrastructure loads, are determined through a thorough understanding of the subsurface conditions and surrounding ground properties that is gained through the execution of a well-planned subsurface investigation program.
A carefully planned subsurface investigation program is essential to the successful design and construction of mined or bored tunnels. Minimum requirements for spacing of bore holes and laboratory testing are presented in Section 5. Owners and funding agencies often have supplemental requirements for more comprehensive programs that may also be tied to risk management. Owner and funding agency specific requirements in excess of those contained in these specifications should be considered.
7.4.1.2. Ground Classification For mined and bored tunneling, ground classifications
vary between soft ground and hard rock. As a naturally occurring material, great variation within these two generic classifications can be expected along the tunnel alignment. The ground classification and characteristics often determine the most feasible tunneling methodology. Further information regarding ground classifications can be found in AASHTO (2010).
The characteristics of the ground around a mined and bored tunnel influence both how much the tunnel lining can deflect during construction and service and how much pressure the ground will exert on the structure over time. These properties of the ground determine the nature of the ground/structure interaction that creates the loading in the lining.
Earth pressures shall be calculated in accordance with Section 3.
C7.4.1.2
Ground classification determines the expected behavior during tunneling which in turn determines the most appropriate tunnel construction methodology. Other aspects such as groundwater conditions and the surrounding surface and near surface infrastructure also influence the determination of applicable tunnel construction methodologies.
The length of the tunnel must also be considered in conjunction with ground classification in determining the economically appropriate tunneling methodology since the cost of a TBM may not be economical for a relatively short tunnel length, even if the ground conditions are ideal for the use of a TBM.
7.4.1.3. Groundwater Groundwater conditions shall be determined as part of
the subsurface investigations. The effect of the hydrostatic pressure shall be considered whenever the groundwater table is detected within the tunnel horizon or may be reasonably expected to occur within the tunnel horizon over time.
C7.4.1.3
The presence of groundwater will influence the selection of the tunnel construction methodology, the need for ground improvement and the design of the tunnel lining.
Hydrostatic pressure can create large axial loads in the tunnel lining that can offset the moments
Groundwater levels to be used for the design shall be established in accordance with Sections 3 and 5 and as modified in this Article.
Long-term variations in the groundwater level shall be considered in establishing design groundwater elevation. Flood conditions with 500-year return period shall be investigated during the design and shall be evaluated as extreme load case during the design phase.
In the absence of definitive data, groundwater elevations for the design of non-extreme load combinations shall be taken as five ft above and five ft below observed groundwater elevations in order to account for seasonal and tidal variations.
generated by applied ground and surcharge loads. Therefore, it is important to explore a range of groundwater design values since a lower hydrostatic pressure may produce the governing load effect for the lining design.
The presence of groundwater will also impact the design of the waterproofing system for the tunnel.
Pressure relief systems can be used to relieve groundwater pressures. Pressure relief systems are installed between the initial and permanent ground support systems. See Article 6.11 for additional information.
7.4.1.4. In-Situ Ground Stresses
Load effects on tunnel initial and final linings from the surrounding ground shall be evaluated for the following possible sources of loads:
• Overburden, surcharge or vertical pressure • Horizontal pressure due to vertical pressure • Squeezing due to release of in-situ pressure • Uniform rock loads • Block rock loads • Varying ground conditions around the
perimeter of the tunnel • Varying ground conditions along the tunnel
alignment • Seismic effects
The investigation of the load effects shall be based on ground/structure interaction.
C7.4.1.4
The state of stress in the ground surrounding a tunnel is changed when excavation begins. An understanding of the in-situ state of stress can be gained through a comprehensive subsurface investigation. The change in the state of stress will determine the load effects in the initial ground support and in the final tunnel lining.
Utilizing the change of the state of stress to maximize the load carrying capacity of the surrounding ground is the underlying concept in tunnel design. This concept assumes that the ground and the structure act in concert to create a stable opening with the ground supporting the majority of the load redistribution that occurs during excavation. Thin, flexible structural members are utilized that allow the ground deformations required to mobilize the strength of the surrounding ground. This redistribution of stresses around the tunnel opening results in maximum usage of the load carrying capacity of the ground, while minimizing the bending and shear in the structural members, thereby optimizing the overall design.
The redistribution of stresses also results in movement of the ground. Ground movement needs to be managed in order to mitigate impacts the movement may have on existing facilities and structures. The management of the ground movements is done through the timing of the installation of the ground support and the stiffening of the structural elements used to maintain the stability of the opening. This can lead to increased load on the structural elements. This balancing of the load carrying components of the ground versus the structural elements results in necessary compromises between the most efficient structural element and the management of ground movements.
7.4.2.1. Concrete Mixes for cast-in-place concrete linings shall be
formulated to accommodate placement requirements, which frequently involve pumping over long distances. Set-time and slump shall be considered during design of concrete mixes as they pertain to the means and methods of concrete placement, which will vary with the Contractor’s equipment and sequence. Often, early stripping of formwork for cast-in-place concrete is beneficial to the construction schedule and can provide economy during construction. When high strength is required prior to stripping forms, consideration should be given to the use of high early strength concrete.
The chemistry of the groundwater shall be considered when designing the concrete mix.
Air entrainment of three to five percent is recommended for the durability of the concrete in moist and cold environments.
Refer to Section 4 for Concrete material properties.
C7.4.2.1
Cast-in-place concrete is commonly used to construct tunnel linings for mined tunnels. The geometry of the lining for mined tunnels is established to minimize moment load effects to the greatest extent possible. Cast-in-place concrete can be formed and poured to any geometric shape.
When ground conditions are such that immediate lining of the tunnel excavation is not required, for example, tunnels constructed in competent rock, then cast-in-place concrete can also be used to line bored tunnels. Initial support is installed as the TBM advances, and a cast-in-place final lining is installed against a waterproofing membrane. Initial support can be in the form of reinforcement elements such as rock dowels or bolts, shotcrete (reinforced or plain) or precast concrete segments (reinforced or plain). This construction sequence is known as a two-pass lining system.
Refer to AASHTO 2010 for more details.
7.4.2.2. Structural Steel Refer to Section 4 for Structural Steel material
properties.
C7.4.2.2
Structural steel is used for prefabricated liner sections used for the final lining, as well as for ribs used with lagging to form initial support. Steel liner plates generally would not be used for large diameter tunnels typically constructed for highways.
7.4.2.3. Steel Reinforcing
Normally, reinforcing steel bars shall conform to requirements of ASTM A615 grade 60. However, some corrosive environments may call for the use of galvanized, epoxy-coated or stainless steel reinforcement. If reinforcement is to be welded or if ductility for seismic capacity is required, ASTM A706 should be specified.
Shotcrete lining may be reinforced with welded wire fabric, deformed steel reinforcing bars, lattice girders or steel fibers.
Precast concrete segmental linings may be reinforced with deformed reinforcing steel bars, welded wire fabric and/or steel fibers.
Refer to Section 4 for Reinforcing Steel material properties and resistance factors.
7.4.2.4. Prestressing Steel Prestressing steel shall be bonded and protected against
corrosion, with attention to combating the effects of corrosive compounds in the surrounding soils or groundwater. Bond capacity shall be tested in accordance with ASTM A981. Strands shall conform to ASTM A416, and bars shall conform to ASTM A722. The Engineer shall include relaxation requirements in the project
C7.4.2.4 Some jurisdictions prohibit the use of prestressed
elements below grade because of concerns about corrosion. However, with appropriate protections, prestressed, precast elements can be used to streamline construction. Following are some examples of use of prestressing steel in underground applications. Prestressing steel is commonly used for
specifications. Refer to Section 4 for Prestressing Steel material
properties and resistance factors.
permanent rock anchors, with protection against corrosion provided by corrosion-inhibiting compounds (resins and greases), sleeves and grout.
7.4.2.5. Shotcrete Shotcrete is frequently used as a smoothing layer
applied to irregular rock surfaces prior to the application of waterproofing membranes. Shotcrete may also be used for initial ground support in mined tunnels, alone or in combination with other support elements.
Shotcrete may be used for the final permanent tunnel lining and can be either be structural plain shotcrete or reinforced with reinforcing steel, welded wire fabric, lattice girders or steel fibers. Pneumatically applied shotcrete (PAC) is a variant of conventional shotcrete that can be used for the final permanent tunnel lining.
The surface of the shotcrete shall be clean and free of any dust or dirt that might create a debonding between individual layers.
Shotcrete shall not be considered as a barrier to the movement of water over the long term, i.e. shotcrete shall not be used as a waterproofing membrane.
Refer to Section 4 for Shotcrete material properties and resistance factors.
C7.4.2.5
When shotcrete is used in lieu of concrete in the final structure, it is applied in layers with a time lag between the applications. To ensure that the behavior is equivalent to monolithic concrete, it is essential to limit the time lag between the applications. It is recommended that a bonding agent be used between layers.
7.5. CONSTRUCTION OF MINED AND BORED TUNNEL STRUCTURES
7.5.1. General
Mined tunnels are constructed utilizing traditional excavation techniques such as drill and blast or mechanical excavation. Mined tunnels can be constructed with or without shields, depending on the ground conditions. Mined tunnels have excavated shapes that take advantage of the ground’s natural load carrying capacity, and can result in more efficient use of the excavated cross section than the circular cross section resulting from bored tunnel construction. Mined tunnels can accommodate a wider range of alignment geometry, both vertical and horizontal, than bored tunnels. When the ground is capable of supporting itself for durations long enough to install initial support measures, tunneling can progress without the use of a shield. Initial ground support can take the form of ground improvement or structural supports, and is covered in Section 9 of these specifications.
TBMs are typically utilized for longer tunnels and can be equipped with or without shields. TBM construction results in a circular opening, that may not be the most efficient shape required for internal dimensioning. When the ground is not capable of supporting itself temporarily or when groundwater conditions are such that construction without the use of a shield is not feasible, TBMs are equipped with shields. Because of the length of the excavating mechanism, shield and trailing gear, TBMs are restricted in the alignment geometry that can be constructed along curves both horizontally and vertically.
Design of initial support systems for mined and bored
C7.5.1
TBM’s are sophisticated equipment designed specifically for the ground conditions anticipated to be encountered along the alignment. TBM’s can operate with or without shields, with an open or pressurized face, with conventional excavation equipment, rotating cutterheads or any combination of the above.
tunnels is based on information developed from the subsurface investigation program. Actual conditions encountered during construction will dictate the initial support system used during construction. 7.5.2. Construction of Mined Tunnel Structures
C7.5.2
Construction of mined tunnels is performed through various excavation techniques that all fall under the general category of sequential excavation. Techniques utilized for the construction of mined tunnels include:
• Drill and blast in rock • Roadheader excavation in various materials • Coventional excavation techniques such as
backhoes and rippers The construction of mined tunnels is sequenced in a
manner customized to the ground conditions expected to be encountered. The sequencing includes both the area of the face that is excavated at one time (drift) and the length of excavation performed prior to installation of the initial support (round). The drift can be the entire tunnel face at one time or it can be subdivided into multiple sections. Round lengths will vary as a function of the time limit required to install the initial support. This methodology is also known as the Sequential Excavation Method (SEM).
Ground movements will occur with the excavation of, and are managed by, limiting the size of the drift and the length of the round.
The excavated opening created during each drift excavation is made stable through the use of initial ground support. The initial ground support can consist of ground improvement or structural elements installed prior to excavating the drift, or can be structural elements installed after excavation of the drift, or a combination of support installed before and after excavation. The type and configuration of required initial ground support is dictated by the ground and groundwater conditions and the requirements for limiting ground movements. The type and configuration of the required initial support, along with time restrictions for installation of the support, are referred to as excavation support classes (ESC).
Once the entire tunnel cross section is excavated, the waterproofing system (if required) is installed and the final lining is constructed. Additional stress redistribution will occur over time until a state of equilibrium is reached. Each phase of the excavation and lining construction along with the permanent long-term condition shall be investigated to determine the load effects in the support elements, as well as in the surrounding ground.
Ground conditions will vary within the length of a tunnel. The contract documents shall provide information regarding the ground conditions to be expected along the tunnel length, where each ESC may be required, and the details required to construct both the initial support and the final structural lining.
The number of ESCs should be kept to a minimum to maximize efficiency and economy of construction.
Information regarding initial ground support measures can be found in Section 9.
Refer to AASHTO (2010) for additional information regarding mined tunnels in various ground conditions and the analysis of sequential excavation.
During construction, experienced personnel must
be on site to determine the ESC required as each drift of the tunnel is excavated. Records of the tunneling and installation of ESC should be kept for the entire length of the tunnel. Instrumentation installed and monitored during construction provides real-time data regarding the performance of the ESC. Instrumentation data related to ground movements is used to modify ESCs as required and to ensure that project criteria regarding ground movements are not
exceeded. Overbreak is an unavoidable part of rock tunnel
construction. Overbreak will change the geometry of the tunnel opening. The geometry of the tunnel opening is the fundamental assumption in the design. Changes in the geometry will affect the design and performance of the tunnel structural elements. Increased overbreak will also result in increased quantities for items such as excavation, initial ground support and structural linings.
7.5.3. Construction of Bored Tunnel Structures
This approach is called a one-pass lining. The design of the TBM shall be predicated upon the most difficult ground conditions expected to be encountered.
The Engineer shall identify the performance requirements of the TBM based on the ramifications of ground movement and the ground conditions expected to be encountered. The Contractor shall be responsible for the design, procurement and operation of a TBM that meets the performance criteria set forth by the Engineer.
C7.5.3
The traditional distinction between TBMs – hard rock and soft ground— has become less distinct over the years. Properly designed TBMs are capable of traversing varying ground conditions along a tunnel alignment. They are capable of installing initial ground support as they proceed, which is followed by a final cast-in-place concrete lining. This approach is called a two-pass lining. TBMs can also install a precast segmental tunnel lining designed for the initial and final conditions.
7.6. LIMIT STATE AND RESISTANCE FACTORS
7.6.1. General Design of mined and bored tunnels shall satisfy the
criteria for the service limit states specified in Article 7.6.2, the strength limit state specified in Article 7.6.3 and the extreme limit states specified in Article 7.6.4.
Tunnel structural elements shall be designed for a service life based on consideration of the potential effects of material deterioration, leakage, stray currents, scour, natural and manmade extreme events and other potentially deleterious environmental factors on each of the material components comprising the structure, as well as for load effects experienced as part of the construction process.
7.6.2. Service Limit State
Service T-I: Mined and bored tunnels shall be investigated for deflections, crack control, vibration, durability, water tightness and fatigue as developed during the service life of the tunnel.
Service T-IA: Mined and bored tunnels shall be investigated for the effects of buoyancy during the service life of the tunnel, including but not limited to, maximum groundwater elevations and changes in groundwater elevation caused by tides. The effect of salinity and other inclusion in groundwater that affect the unit weight of groundwater shall be included.
Service T-II: Mined and bored tunnels shall be investigated for the effects of buoyancy that occur during construction when only part of the permanent structure is in
place. Mined and bored tunnels shall also be checked for loadings imposed on the lining as part of the construction process. 7.6.3. Strength Limit State
Strength T-I: Mined and bored tunnels shall be investigated for strength during normal use, under which the tunnel, including the permanent lining and internal components, fixtures and attachments remain fully serviceable without any damage.
Strength T-II: Mined and bored tunnels shall be investigated for strength during construction including, but not limited to, force effects of the thrust of the TBM, handling, storage, shipping and erection of prefabricated tunnel linings, and stripping of cast-in-place and precast concrete tunnel linings.
7.6.4. Extreme Limit State
Extreme Event T-I: Mined and bored tunnel structures shall be investigated for the design earthquake to ensure life safety and survivability of the structure taking progressive collapse into account.
Extreme Event T-II: Mined and bored tunnel structures
shall be investigated for extreme events other than earthquake. Other extreme events, such as explosion and fire, shall be considered on a project-specific basis and, if relevant, shall be included in Extreme Event II individually but not simultaneously with other events. The Owner may, at its discretion and after performing a hazard analysis, create load combinations that have more than one of these loads applied to the structure simultaneously. In this case, it is recommended that the load factors shown in Table 3.4-1 be used when combining the loads.
Extreme Event T-III: Mined and bored tunnel structures shall be investigated for a rare event for the simultaneous combination of loads such as flooding. This load combination shall be used for both surface flooding that raises the groundwater elevation and for flooding of the inside of the tunnel.
Under Extreme Event III, the following flooding cases shall be investigated:
• Sea level or surface water elevations rise that
produce increased hydrostatic pressures on the tunnel that also reduce the resistance to buoyancy.
• Flooding of the inside of the tunnel that increases the resistance to buoyancy and reduces the net hydrostatic pressure to zero.
C7.6.4
Under Extreme Event T-I, acceptable level of damage could include partial failure of tunnel members that result in limited leakage and could include complete loss of service. The Owner shall determine acceptable levels of damage.
Under Extreme Event T-II, it is recommended that the tunnel should sustain no more than light damage and/or minor leaks, while experiencing no significant loss of service during the prosecution of repairs, i.e. partial performance level.
7.6.5. Load Factors and Load Combinations Mined and bored tunnel structures shall be designed
for the load combinations shown in Tables 3.4.-1. When developing the loads to be applied to the
structure, each possible load combination of the loads shall
C7.6.5
Section 3 provides guidance on the methods to be used in the computations of the loads applicable to mined and bored tunnels. The construction methodology (SEM or TBM) will influence the
be developed. Engineering judgment can be used to eliminate the
combinations that will not govern.
loadings. Structures are subjected to a wide range of load
combinations during construction. These include the initial and long-term water and ground loads, variability of dead and live loads on partially completed configurations, erection and equipment loads, and the full range of environmental loads to which the final structure is subjected, although with differences attributable to return period of events (storm, earthquake), duration (horizontal pressures due to equipment loading) or intensity (earth pressures changing from active to at-rest).
7.6.6. Resistance Factors Refer to Sections 4 and 9 for resistance factors for
each material used in construction of mined and bored tunnel structures.
7.7. GENERAL DESIGN FEATURES
7.7.1. General
7.7.1.1. Initial Ground Support
Initial ground support shall be designed for the load effects expected to be experienced by the structure during the initial stages of construction. The initial ground support also shall be designed for load effects generated by the construction methodology.
When cross passages, sumps and ancillary spaces are constructed after the construction of the initial ground support, the load effects generated by creating the openings in the initial ground support shall be accounted for in their design.
Section 3 provides guidance for the determination of loads other than those generated by construction. Section 9 provides guidance relative to the design of initial ground support.
C7.7.1.1 The load effects are generated during excavation
of each drift in the case of mined tunnels, or the initial/short-term load effects generated as the temporary lining exits the TBM in the case of bored tunnels. For mined tunnels, the load on support elements may vary significantly over the sequential excavation process.
7.7.1.2. Final Lining
In addition to their own dead load, final linings shall be designed for the ground and groundwater load effects expected to be experienced by the structure during the life of the tunnel. The final lining also shall be designed for load effects generated by live load, the addition of structural elements or appurtenances supported by the lining, environmental loads, secondary loads and loads imposed by the construction methodology.
Final linings shall be designed assuming that initial ground support elements have no load carrying capacity. Two pass linings shall be designed for long-term (permanent) loading condition only. Single pass linings shall be designed for the initial loading condition or the long-term (permanent) loading condition, whichever governs.
When cross passages, sumps and ancillary spaces are constructed after the construction of the final lining, the
load effects generated by creating openings in the tunnel lining shall be accounted for in the design of the final lining.
Section 3 provides guidance for the determination of loads other than those generated by construction. 7.7.1.3. Cross Passages, Sumps and Ancillary Spaces
Cross passages, sumps and ancillary rooms are generally mined from the completed main tunnel utilizing various techniques. Cross passages, sumps and ancillary rooms shall be designed for the load effects expected from the construction methodology, short-term and long-term ground and groundwater loads and the loading associated with appurtenances and live loads expected during the life of these features.
The Engineer shall indicate the location of required cross passages, sumps and ancillary spaces on the contract drawings.
C7.7.1.3 Cross passages are required by NFPA 502 when
the tunnel length exceeds limits specified. Cross passages are used in conjunction with side by side tunnels.
Sumps are constructed at the low point in the tunnel alignment to collect seepage, wash and firefighting water and spills that occur inside the tunnels. Sumps are equipped with pumps that discharge the collected fluids to the tunnel piping system.
Ancillary spaces encompass a large range of uses, including mechanical and electrical rooms, systems rooms, storage, niches for equipment or ventilation fans, and local widening of the tunnel cross section to provide refuge space for disabled vehicles.
Cross passages, sumps and ancillary spaces, when required, can be constructed before or after the final lining is constructed. They are usually constructed after the initial ground support. The creation of an opening in the ground support and subsequent excavation, creates a redistribution of the stresses in the surrounding ground. Temporary supports inside the main tunnel lining are required to stabilize the tunnel lining by creating redistributed load paths that allow sections of the lining to be removed that are large enough for construction of the cross passage, sump or ancillary space.
7.7.2. Ground Movements
7.7.2.1. General Ground movement occurs primarily due to ground loss
during excavation and due to the change of stress in the ground caused by the excavation process. Ground movement affects the construction process in the following areas:
• The stability of the open excavation at the face of
the tunnel during construction. • Ground movement into the tail void of TBM
tunnels. • Ground movement caused by the excavation of
cross passages, sumps and ancillary spaces. • Surface subsidence due to the excavation process. • Impact of ground movement due to the excavation
process on existing buried utilities, buildings and infrastructure above or adjacent to the tunnel alignment.
• Surface subsidence due to lowering of the
C7.7.2.1
Ground movement during excavation and construction, and its effects on adjacent structures, utilities and other infrastructure must be carefully considered in all phases of design and construction. Ground movement must be explicitly addressed in the development of contract documents and carefully monitored in the field once construction begins. Instrumentation to monitor the movements of adjacent existing structures should be in place and actively monitored prior to construction, during construction and for a period of time after construction to confirm that ground has stabilized. Threshold values and values for total permissible movement of the facility/structure being monitored should be established. The Contractor must be required to halt work should the threshold values be exceeded and mitigation measures should be implemented to keep movement within permissible
The Engineer shall evaluate the potential for ground movement due to the construction methodology assumed during design. The construction methodology assumed during design shall be shown in the contract documents. The Engineer shall identify ground improvement and initial support required along the alignment required to keep ground and structure movements within acceptable limits. The Engineer shall indicate the threshold (point at which remedial action is required to arrest movements) and acceptable limits for ground movement and existing structure and facility movement in the contract documents.
The Contractor shall be responsible for ensuring that the construction methodology employed during construction is consistent with the design assumptions and are executed in a manner that results in movements within the predicted range. Should a construction methodology be used other than that assumed during design and identified in the contract documents, then the Contractors shall be responsible for performing the analysis and design required to identify suitable ground improvement and initial support measures to ensure ground movements are within acceptable limits.
The following sections set forth requirements and limitations of various methodologies for evaluating ground movements during construction. Techniques for evaluating ground movements include empirical, analytical and numerical methods.
values. Existing buildings and facilities shall be
evaluated for the ground movement estimated to occur due to excavation activities for construction of mined and bored tunnels. This evaluation of existing buildings and facilities is not within the scope of this Specification.
The empirical, analytical and numerical approaches generally involve simplifying assumptions regarding the ground conditions, tunnel excavation process and quality of workmanship. These simplifying assumptions include assuming that site soils are either pure sands or pure clays, whereas in nature soils are mixtures of sands, silts, clays and gravels. Empirical and analytical analyses may also require assuming that the ground profile consists entirely of one type of soil, such as sand or clay, and so the most prevalent or dominant soil type is assumed for the entire profile at a particular location. Numerical analyses may involve assuming that the excavation takes place entirely under total stress (undrained), effective stress (drained) or partially drained conditions.
Analytical and numerical analyses include simplifying assumptions related to ground behavior, such as assuming elastic or plastic deformations, ground relaxation, etc. The development of analytical and numerical methods often include comparison to observed behavior on previous projects to assess the ability of the methods to predict performance on future projects. Although the methods vary in the level of detail, they are all approximations used to model the ground behavior.
Additional operations may include groundwater lowering or drainage from the soil, blasting and ground modifications. These operations shall be accounted for when assessing ground movements.
7.7.2.2. Empirical Method
Settlements at the ground surface may be estimated utilizing the following equations derived from those presented by R.J. Mair (1993). The settlement equations shown below are applicable at a specific location along the tunnel alignment. A settlement profile that is parallel to the tunnel should be developed to fully understand the potential ground movements.
The maximum settlement (vertical movement) at the
tunnel centerline is calculated as follows:
𝑆𝑚𝑎𝑥 = 0.4 𝑉𝑙𝐴𝑡𝐾𝑧0
(7.7.2.2-1)
Where:
Smax = maximum settlement at tunnel centerline (in.) Vl = ground loss ratio
C7.7.2.2
Empirical assessments are based on observed field behavior related to simplified ground conditions and are assumed to be applicable to similar ground conditions. The empirical method presented here, Mair (1993), is generally used for initial screening to identify the need for more detailed analyses to assess impacts on adjacent existing features. Empirical assessments can also be used to evaluate the reasonableness of analytical and numerical analyses. The equations provided herein are generally used for soft ground and will produce conservative results for tunnels in rock. These equations assume a ‘green field’ condition. This condition does not take into account the effects of existing loads that may occur along the tunnel alignment from buildings or other infrastructure. Numerical analyses utilizing specialized software can account for these loads.
At = the area of the tunnel intersected by a vertical
plane (in2)
K = settlement trough width parameter z0 = depth of the tunnel below ground surface at springline (in.)
The ground loss ratio Vl is dependent upon the means
and methods along with the quality of workmanship and the ground control exercised during construction. This ratio is expressed as a percentage and can vary from 0.25 percent for good ground control to 1 percent or greater for poor ground control.
The settlement trough width parameter K is dependent on the type of ground and varies from 0.5 in predominantly cohesive soils to 0.25 in predominantly granular soils.
The settlement at a distance y from the tunnel
centerline is calculated as follows:
𝑆𝑦 = 𝑆𝑚𝑎𝑥 ∙ 𝑒𝑥𝑝 � −𝑦2
2(𝐾𝑧0)2� (7.7.2.2-2)
Where:
Sy = settlement at a distance y from the tunnel centerline (in.)
Smax = maximum settlement at tunnel centerline (in.) as calculated by Equation 7.7.2.2-1 y = the distance from the centerline of tunnel (in.)
K = settlement trough width parameter z0 = depth of the tunnel below ground surface at
springline (in.) The horizontal movement of the ground toward the
center of the tunnel at a distance y from the tunnel centerline is calculated as follows:
Sh = 𝑦
𝑧0𝑆𝑦 (7.7.2.2-3)
Sh = horizontal movement of the ground at a
distance y from the tunnel centerline (in.) y = the distance from the centerline of tunnel (in.)
z0 = depth of the tunnel below ground surface at springline (in.)
Sy = settlement at a distance y from the tunnel centerline (in.) calculated by Equation 7.7.2.2-2 Other proven empirical methods for estimating ground
movements may be used at the discretion of the Engineer.
The selection of loss ratio Vl, the ground above
the tunnel, is rarely based purely on cohesive or purely granular soils. The water table elevation will also have an influence on the selection of this value for granular soils. Engineering judgment and experience is required to select an appropriate value for the loss ratio.
7.7.2.3. Analytical Method
Closed form analytical solutions have been developed by Loganathan (2011) that can be used to estimate
C7.7.2.3
The equations provided herein are generally used for soft ground and will produce conservative results
settlements and lateral ground movements at and below the surface of the ground. The equations derived by Loganathan are as follows.
The surface settlement at a distance x from the tunnel centerline is calculated as follows:
for tunnels in rock. These equations assume a ‘green field’ condition. This condition does not take into account the effects of existing loads that may occur along the tunnel alignment from buildings or other infrastructure. Numerical analyses utilizing specialized software can account for these loads.
𝑈𝑧=0 = 𝜀0𝑅2 ∙ 4𝐻(1 − 𝜐)𝐻2 + 𝑥2
∙ 𝑒𝑥𝑝 �−1.38𝑥2
(𝐻𝑐𝑜𝑡𝛽 + 𝑅)2�
(7.7.2.3-1)
Where: Uz=0 = ground surface settlement R = tunnel excavated radius z = depth below the ground surface ν = Poisson’s ratio of the ground H = depth to the springline of the tunnel ε0 = average ground loss ratio x = lateral distance from the tunnel center
line β = limit angle = 4.5 + φ /2 φ = angle of shear resistance of the ground
The subsurface settlement at a distance x from the tunnel centerline and a distance y below the surface is calculated as follows:
𝑈𝑧 = 𝜀0𝑅2 �−𝑧 − 𝐻
𝑥2 + (𝑧 − 𝐻)2 + (3 − 4𝜐)𝑧 + 𝐻
𝑥2 + (𝑧 + 𝐻)2
−2𝑧[𝑥2 − (𝑧 + 𝐻2)][𝑥2 + (𝑧 + 𝐻2)]2
�
∙ 𝑒𝑥𝑝 �− �1.38𝑥2
(𝐻𝑐𝑜𝑡𝛽 + 𝑅)2 +0.69𝑧2
𝐻2 ��
(7.7.2.3-2)
Where: Uz = ground subsurface settlement R = tunnel excavated radius z = depth below the ground surface ν = Poisson’s ratio of the ground H = depth to the springline of the tunnel ε0 = average ground loss ratio x = lateral distance from the tunnel center
line β = limit angle = 4.5 + φ /2 φ = angle of shear resistance of the ground
The lateral ground deformation at a distance x from the tunnel centerline and a distance y below the surface is
Where: Ux = lateral ground subsurface settlement R = tunnel excavated radius z = depth below the ground surface ν = Poisson’s ratio of the ground H = depth to the springline of the tunnel ε0 = average ground loss ratio x = lateral distance from the tunnel center
line β = limit angle = 4.5 + φ /2 φ = angle of shear resistance of the ground
7.7.2.4. Numerical Method
Numerical methods involve the use of finite element or finite difference software to model the response of the ground to the tunnel excavation.
Numerical modeling techniques should be used when other methods indicate that estimated ground movements may exceed tolerable limits for displacement and angular distortion established for the features surrounding the tunnel. When utilizing numerical modeling software to estimate ground movements, existing loads from buildings, highways, railroads and other infrastructure shall be included in the model.
C7.7.2.4
For additional information regarding the use of numerical modeling to estimate ground movements, refer to AASHTO (2010).
7.7.3. Buoyancy
Mined and bored tunnels that are partially or completely below the water table shall be evaluated for the effect of buoyancy. Buoyancy forces shall be computed using the density of the groundwater at the site. Adequate resistance to flotation shall be provided at all sections for the full hydrostatic uplift pressure. When calculating the resistance to buoyancy of the ground above the tunnel, only the ground that is within boundaries created by vertical lines from each springline shall be included.
The effects of groundwater shall be accounted for in combination with ground loads to determine the combined load effects in the tunnel lining.
Buoyancy shall be checked at every stage of completion and only permanent features shall be included in the buoyancy check.
Any component of the tunnel that can potentially be removed for maintenance, repair or rehabilitation shall not be considered when calculating the resistance of the structure to uplift.
Examples of components that should not be included in the calculation of the resistance to buoyancy include:
− Ceiling and roadway structures − Traffic control devices − Ventilation equipment − Electrical equipment − Communication equipment − Finishes
7.7.4 Loading
All components of the mined and bored tunnels shall be designed for the applicable loads described in Section 3 and as supplemented in this Article.
Mined and bored tunnels shall be designed for any additional loading from existing adjacent buildings, when the horizontal distance from the building line to springline of the tunnel is less than the depth of the tunnel below the building foundation.
If the foundation of the adjacent buildings are supported by underpinning and/or deep foundations that transfer loads to a depth below the influence line of the tunnel these additional loads shall not be applied to the tunnel.
When the loads from the adjacent structures are not equal on both sides of the tunnel, the most critical unbalanced load case shall be investigated.
As a minimum, the following non-seismic loading cases shall be investigated:
7.7.4.1 Symmetrical Loading
Case I - Maximum vertical loads including dead loads, live loads, surcharge loads and hydrostatic pressure with maximum lateral pressure due to soil and rock.
Case II - Minimum vertical load (dead load only), with maximum hydrostatic pressure and maximum lateral pressure due to soil and rock.
Case III - Maximum vertical loads and minimum hydrostatic pressure and lateral pressure due to soil and rock.
7.7.4.2 Asymmetrical Loading Case I - Maximum vertical loads including dead loads,
live loads, surcharge loads and hydrostatic pressure with lateral pressure as follows: Maximum lateral pressures (at-rest) due to soil loads, applied to one side of the structure and active pressure from soil on the opposite side, or maximum lateral rock load applied to one side of the structure, zero rock load applied on the opposite side for tunnels in rock.
Case II- Maximum vertical loads including dead loads, live loads, surcharge loads with maximum lateral pressure (at-rest) applied to one side of the structure and
C7.7.4.2
Asymmetrical loading cases should only be used when conditions exist, or are likely to exist, in the future that cause this type of loading.
active pressure from soil on the opposite side, or maximum lateral rock load applied to one side of the structure, zero rock load applied on the opposite side for tunnels in rock. 7.7.4.3 Construction Condition
Construction sequence and procedures may result in
conditions that are more severe than the general loading conditions mentioned in the previous sections. Mined and bored structures shall be designed to resist the load effects generated during construction operations.
The Engineer is responsible to develop a construction scheme and to clearly state assumptions regarding the scheme in the contract documents. When an alternative sequence of construction is allowed by the contract documents and proposed by the Contractor, the Contractor shall provide a complete analysis for all construction stages.
Loads generated by the construction sequence and procedure can include:
− Stripping of forms prior to concrete attaining 28
day compressive strength. − Loads from temporary facilities such as
ventilation, lighting, power cables, conveyor belts, etc.
− Thrusting loads from the TBM. − Loads imposed on segmental linings due to lifting,
stacking, transporting and erecting. − Loads imposed by construction vehicles operating
inside the tunnel. − Loads imposed by backfill or contact grouting.
C7.7.4.3
Intermediate phases of the construction may induce higher stresses than the end of construction stage and govern the design of structural members.
7.7.4.4 Distribution of Loads Loads on mined and bored tunnels resulting from
existing or anticipated future land use at the surface above or adjacent to the tunnels, shall be accounted for in the design and distributed according to the following:
Vehicular loads shall be distributed according to LRFD Specifications.
Train loads shall be distributed according to guidelines provided in AREMA.
7.7.4.5 Superimposed Loads Superimposed loads are defined in the LRFD
Specifications and shall be calculated and applied to mined and bored tunnels in load combinations as applicable.
Mined and bored construction is often used in urban areas and provisions are made for the structures to support urban loadings: future building developments, parks or roadways. Future loadings provided for in the design shall be clearly identified in the contract documents.
The determination of load effects for mined and bored tunnel linings shall take into account ground/structure interaction. Load effects from elements inside the tunnel supported by the tunnel lining shall be included in the ground/structure interaction analysis.
The determination of load effects for tunnel elements not in contact with ground may be analyzed without ground/structure interaction, unless such loads are influenced by ground/structure interaction of other tunnel elements that are in contact with the ground.
Empirical methods may be used for preliminary design.
Final design of tunnel linings shall be performed utilizing general structural or geotechnical engineering software capable of modeling beam elements to represent the tunnel lining and spring or solid elements to model the surrounding ground.
Numerical modeling may be used for the final design of tunnel linings.
Examples of tunnel elements not in contact with the ground include roadway slabs over ventilation ducts, suspended ceilings, equipment attachments, etc.
Commercially available numerical modeling software may not permit the input of factored loads. When utilizing this software, a rational method for incorporating the load factors into the analysis must be developed by the Engineer.
7.7.5.2 Design Tunnel linings shall be designed as compression
members, taking into account the combined interaction of axial and moment load effect.
Tunnel linings shall be designed for the second order effects due to elastic deformation.
Segmental linings shall be designed for the load effects resulting from construction tolerances.
7.7.5.3 Moment of Inertia The gross moment of inertia shall be used when
analyzing cast-in-place and shotcrete tunnel linings. When analyzing a segmental tunnel lining, the
effective moment of inertia shall be used. The effective moment of inertia may be calculated as per Muir Wood (1975) as shown below:
𝐼𝑒 = 𝐼𝑗 + 𝐼𝑔 �4𝑛�2 (7.7.5.3-1)
Where: Ie = effective moment of inertia (in4) Ij = movement of inertia of the joint (in4) Ig = gross moment of inertia of the lining section
(in4) n = number of joints in the lining ring
C7.7.5.3
If approved by the owner, numerical modeling
of the segmental tunnel lining system may be performed in lieu of using Equation 7.7.5.3-1. The numerical model should account for the interaction between adjacent rings and the effect of individual components of the lining system such as bolts, dowels, packers, joint configuration, annual grout and the surrounding ground.
7.7.5.4 Ground/Structure Interaction
Numerical modeling shall be used to account for ground/structure interaction. The numerical modeling may consist of beam/spring models, finite element modeling or finite difference modeling. The spring constants used to represent the static (non-seismic) behavior of the
C7.7.5.4
The equations presented in this article are taken from Engineering and Design, Tunnels and Shafts in Rock, United States Corp of Engineers (USACE) EM 1110-2-2901, 1997. These equations are applicable to soil and rock, because they are based on the theory of
surrounding ground in ground/structure interaction modeling shall be calculated as follows.
The radial spring stiffness may be taken as:
𝑘𝑟 = 𝐸𝑟𝑏𝜃(1+𝜐)
(7.7.5.4-1)
Where: kr = radial spring stiffness (kips per inch) Er = modulus of elasticity of the surrounding
ground (kips per in2)
b = the length of the tunnel lining element being modeled (in)
θ = the arc subtended by the beam element being modeled (radian)
ν = Poisson’s ratio of the surrounding ground The tangential spring stiffness may be taken as:
𝑘𝑡 = 0.5𝑘𝑟(1+𝜐)
(7.7.5.4-1)
Where: kt = tangential spring stiffness (kips per inch) kr = radial spring stiffness calculated in equation
7.7.4.4-1 ν = Poisson’s ratio of the surrounding ground
elasticity. Engineering judgement and experience should be used when determining spring stiffness values for design.
7.8 JOINTS
7.8.1 General
Three types of joints in concrete construction are commonly detailed: construction joints, which allow discrete concrete placements in a continuous structure, contraction joints and expansion joints.
A main objective in the design of joint spacing is to try to control the locations where the concrete cracks. In mined and bored tunnel structures, cracks are locations where water will infiltrate into the structure, often causing structural degradation over time.
All joints in cast-in-place concrete and shotcrete shall be provided with waterstops.
C7.8.1 Provisions to control cracking include heavier
longitudinal (temperature and shrinkage) reinforcement and the detailing of joints to control the locations where cracking occurs. Joints must be carefully detailed to prevent the infiltration of water, yet simultaneously accommodate the movements expected over time.
Shrinkage during initial curing is a major cause of concrete cracking. Concrete mixes for mined and bored tunnel structures should be designed to limit the heat of hydration, using the same provisions that are applied to mass concrete placements. Provisions include the substitution of blast furnace slag or fly ash for Portland cement to reduce the heat of hydration, careful curing in the wet and the control and monitoring of temperature differentials across the curing section.
7.8.2 Construction Joints Construction joints occur between discrete concrete
placements. They shall be detailed for structural continuity, with joint surfaces intentionally roughened and saturated
with water or coated with bonding agent, and with reinforcement extending a lap length into the next concrete placement. 7.8.3 Contraction Joints
Contraction joints are designed to encourage the
concrete to crack at the joint location as it shrinks during curing or temperature changes. Contraction joints may be as simple as a saw-cut on the exposed face or the installation of a zip-strip.
7.8.4 Expansion Joints Expansion joints are designed to accommodate
expansion of adjacent units and relative movements of dissimilar adjoining structure configurations, such as a tunnel entering a ventilation structure or abutting a tunnel of a different construction.
Provisions for expansion shall be made when a structural element is partially underground and partially above ground, and where the above ground element is attached to the underground element, particular care shall be taken in detailing to accommodate differential thermal movement.
The followings are minimum requirements for the expansion joints:
• Reinforcement steel shall not be continuous through
the joint. • Shear forces shall be transferred across the joint,
preferably by a key. • All expansion joints in structures constructed of cast-
in-place concrete and shotcrete subject to hydrostatic pressure shall contain waterstops.
7.8.5 Segment Joints
Segment joints shall be designed to resist the load effects resulting from the loads and loads combinations specified in Section 3.
The design of segment joints for bearing and bursting load effects shall be designed utilizing the actual contact surface area available, taking into account the contact area lost to chamfers, packing and gaskets used to seal the joints.
7.9 WATERPROOFING
Waterproofing for cast-in-place and shotcrete tunnel linings shall be designed to the anticipated hydrostatic pressures. Typical waterproofing systems include:
Waterproofing systems are manufactured by a number of companies. In general, details of system application are left up to the manufacturer, whose responsibility to the facility owner is inherent in the warranty required by the specification. Most manufacturers will not warrant their product unless the seaming, penetration and termination details that they have developed and tested are used in the
installation. There is a range of relationships between manufacturers and installers, with some installers explicitly licensed by the manufacturer, while others may only send a technical representative for sufficient time to train the Contractor work force.
Waterstops are used at joints in cast-in-place concrete linings. Membrane Waterproofing:
Several companies manufacture membranes that are delivered in rolls. They may be formed from different plastics, such as PVC or EPDM, or they may be panels of bentonite. Membranes may be detailed to be installed against wet concrete, a “blind side” application. Blind side membranes may be treated to bond or adhere to the wet concrete. These are often applied directly to the excavation cut or shoring wall. Membranes may also be rolled out over cured concrete. Liquid applied waterproofing:
A number of companies manufacture liquid compounds that cure in place to form membranes. These may be painted on or spray applied.
7.9.1 Treatment of Penetrations
Bored and mined tunnel lining penetrations consist of equipment niches, widening for disabled vehicles or cross passages. The waterproofing of these penetrations shall consist of membrane or liquid applied systems.
7.9.2 Gaskets
Gaskets shall be designed to resist the anticipated hydrostatic pressures through compression of the gasket for the design life of the gasket. Compression of gaskets between rings is accomplished through the thrust of the TBM. Compression of the gasket within a ring is accomplished through tightening of the bolts between gaskets.
7.9.3 Permissible Leakage
Precast segmental concrete linings that do not have applied waterproofing shall be subject to the permissible leakage criteria.
C7.9.3
Criteria generally include a measured infiltration of volume/ft2/day and a maximum flow at any single point. Typical criteria range from 0.0002 to 0.01 gals/ft2/day, with 0.02 gallon per minute of flow from any single leak. The Owner shall establish the required leakage criteria. Factors to consider when establishing permissible leakage include the long-term management of the incoming water, the selected structural system and associated number of joints, constituent components of the groundwater including groundwater chemistry and contaminants.
1. AASHTO, Technical Manual for Design and Construction of Road Tunnels – Civil Elements, 2010
2. AREMA, Manual for Railway Engineering, 2014
3. American Society of Civil Engineers, ASCE 37, “Design Loads on Structures during Construction”
4. Johann Golser, The New Austrian Tunneling Method (NATM), Theoretical Background & Practical Experiences. 2nd Shotcrete conference, Easton, Pennsylvania (USA), 4-8 Oct 1976.
5. Loganathan, Nagen (2011), “An Innovative Method for Assessing Tunneling-Induced Risks to Adjacent Structures”, WSP Parsons Brinckerhoff (http://www.pbworld.com/news/publications.aspx)
6. Muir Wood, A M. (1975). “The circular tunnel in elastic ground. Geotechnique, 1, 1975, 115-127
7. NFPA 502 – Standard for Road Tunnels, Bridges, and other Limited Access Highways
8. Özdemir, Levent (2006). North American Tunneling 2006. Washington, DC: Taylor & Francis. p. 246. ISBN 0-415-40128-3.
9. Rabcewicz, L. V.(1948) “Patentschrift, Oesterreichisches Patent Nr. 165573” (Patent Entry, Austrian Patent Nr. 165573, 1948).
10. Rabcewicz, L.V. (1964). "The New Austrian Tunneling Method," Water Power.
11. Rabcewicz, L. V. and Golser, J., (1973). “Principles of Dimensioning the Supporting System for the New Austrian Tunneling Method,” Water Power, March 1973 Issue.
12. Tunneling: Management by Design, Alan Muir Wood, Taylor & Francis, 2002, ISBN 0203477669
13. U.S. Army Corps of Engineers (USACE) (1997). "Engineering and Design, Tunnels and Shafts in Rock", EM 1110-2-2901, May.
TABLE OF CONTENTS 8.1 SCOPE 8.2 DEFINITIONS 8.3 NOTATION
8.3.1 General 8.3.2 Loads and Load Designations
8.4 DESIGN CONSIDERATIONS 8.4.1 Determination of Ground Properties
8.4.1.1 General 8.4.1.2 Foundation 8.4.1.3 Fill Material 8.4.1.4 Sideslopes
8.4.2 Materials 8.4.3 Support Loss
8.5 CONSTRUCTION 8.5.1 Shipping Channel Traffic 8.5.2 Trench Excavation 8.5.3 Element Length 8.5.4 Fabrication Facility 8.5.5 Transportation 8.5.6 Outfitting 8.5.7 Immersion 8.5.8 Joining and Filling
8.6 LIMIT STATES AND RESISTANCE FACTORS 8.6.1 General 8.6.2 Service Limit States 8.6.3 Strength Limit States 8.6.4 Extreme Limit States 8.6.5 Load Factors and Load Combinations 8.6.6 Resistance Factors
8.7 GENERAL DESIGN FEATURES 8.7.1 Loading
8.7.1.1 Permanent Loads 8.7.1.1.1 Settlement (SE)
8.7.1.2 Transient Loads 8.7.1.2.1 Transient Water Loads (WAp)
This Section supplements the AASHTO LRFD Bridge Design Specification (hereafter referred to as the LRFD Specifications). The provisions of this Section apply to the analysis and design of immersed tunnels constructed of elements of steel, concrete or a combination thereof.
This Section includes provisions for loads that may be encountered during the construction of immersed tunnels. Provisions for permanent loads are contained in Section 3.
This Section also includes provisions for detailing and inspectability.
Temporary works for the fabrication, launching, transportation, immersion and joining of elements are not included. Temporary support of excavation and temporary side slopes for open trench excavation are not covered.
C 8.1
Immersed tunnels are used to cross waterways and typically include a cut-and-cover section at both ends of the tunnel. An immersed tunnel consists of one or more tunnel elements, typically 300 feet or more in length. The elements may be fabricated on or off of the tunnel alignment and designed to be floated, either with or without temporary buoyancy to a prepared bed or excavation at the bottom of the waterway being crossed. The elements may consist of reinforced or prestressed concrete, concrete acting compositely with structural steel or a steel structure with concrete infill. The elements are fabricated in dry docks, diked basins, on submersible barges or on dry land to be launched like a ship, lowered on a ship lift or floated when the fabrication basin is flooded. The ends of the elements are sealed with temporary bulkheads. The tunnel elements are towed to their final location, floated into position, lowered into the prepared bed and joined together. After any additional foundation works have been completed, the sides are backfilled and most often some fill is placed on the roof as protection.
Anchor Release Band – Earthen construction intended to choke the gape of a ship’s anchor, i.e. the space between the hook and the shank, and bring the anchor to the surface.
Element – A section of immersed tunnel between immersion joints.
Engineer – Agency, design firm or person responsible for the design of the tunnel and/or review of design related field submittals.
Fabrication Facility – A dry dock, diked area or factory used to build the immersed tunnel elements.
Fully serviceable – The state of a tunnel under which all traffic normally permitted to travel through the tunnel can do so safely, with all tunnel systems functioning to the extent required for safe travel.
General Fill – Backfill that is placed above the horizontal axis of the element after placement.
Heeling Moment – A moment (a force x distance) about the longitudinal axis in the transverse direction which induces roll (heel). The investigation of heeling moments are an important consideration to an element’s stability.
Immersed Tunnel – A tunnel that is built in sections (elements) in the dry and then floated to the alignment and placed.
Immersion – Lowering the tunnel element into place in the trench.
Immersion Joints – Joints between immersed tube elements.
Inner Reinforcement Cage –The reinforcing steel layer closest to the inside of the tunnel.
Joining – Engaging the jacks and pulling the element being placed to the in place element or existing approach section.
Launching – Sliding the element into the water at the fabrication site, if the site is a ship building facility with a launch.
Locking fill – Backfill that is placed on either side of the element after placement up to the horizontal axis.
Outfitting – Adding internal concrete to the element after float up in the vicinity of the alignment to obtain a larger negative buoyancy than was obtained at float up.
Righting Moment – The lateral moment that can be applied to the element without causing it to capsize or heel.
Terminal Joints – Joints between immersed tube elements and approach structures.
The following permanent, transient and construction loads shall be considered and supplement information given in Section 3:
• Permanent Loads
SE = effect of settlement of tunnel structure • Transient Loads
WAp = transient water loads
8.1.2 Abbreviations ADA Americans with Disabilities Act ft2 square feet gals gallons MSL Mean sea level NFPA 502 National Fire Protection Association 502 Standard for Road Tunnels, Bridges, and other Limited Access Highways USACE United States Army Corps of Engineers
Subsurface investigations and determination of ground properties for design for immersed tunnel structures shall be as specified in Section 5.
8.4.1.2 Foundation A foundation capable of supporting the tunnel
adequately under all design conditions shall be provided. The foundation may be prepared either before or after placing a tunnel element. Where improvements to the underlying soils are required, these improvements shall be prepared prior to preparing the foundation or prior to placing the element, if placed first. Soil improvement may include some form of piles or compaction piles not in direct contact with the underside of an element, or an appropriate type of in-situ ground improvement.
Based upon seismicity, analyses are required to determine the risk of liquefaction of both the underlying soils and of the foundation materials; soil improvement may be required to reduce the risk of liquefaction of the underlying soils to an acceptable level. Foundation materials shall be selected such that they do not liquefy. See Section 10 for seismic design requirements.
Acceptable methods of foundation preparation include:
• Screeded material onto which the tunnel element is
placed. The material shall be capable of transferring the loads from the tunnel element to the underlying foundation materials without causing unacceptable settlement or differential settlement. Settlement analyses shall be performed in accordance with Section 10 of the LRFD Specifications.
• Sand-flow or grouted foundation prepared beneath the
tunnel element while resting on temporary supports. Sand flow shall not be used in seismic areas where the sand bedding may be susceptible to liquefaction. Where a grouted foundation is used, it may be underlain by gravel or stone. The grout shall be formulated such that it neither penetrates the underlying materials nor mixes with the surrounding water. Sand jetting is not recommended.
C8.4.1.2
Immersed tunnels are usually placed within an excavated trench. Some methods of soil improvement may need to be carried out before the trench is excavated to the final depth.
Depending upon the level of seismicity, a screeded
foundation may consist of a well graded material or single-sized stone. The material is placed in the trench, typically through tremie tubes or by clamshell, and then screeded (leveled) to the line and grade required for element placement using a blade or screed box to form a continuous flat surface. An alternative and quicker method is placement of the material by the Dutch Scrading® method, whereby the foundation material (usually stone) is placed using a computer controlled tremie pipe in a zigzag pattern with a slight separation of the rows. Scrading® is used for all widths of tunnels.
For the sand flow method, a sand-water mixture is pumped in through pipes with orifices in a pattern below the tunnel. As the mixture is pumped in, it forms a “pancake” that grows around the orifice in use. The pumping pressure required increases as the diameter of the pancake increases. The pancakes shall be arranged to overlap and to be sequenced such that the entire underside is filled. Sand flow is particularly useful for wide tunnel elements but has also been used for narrow tunnels.
A foundation produced by the grouting and sand flow methods is installed after an element is placed, whereas the screeded foundation is prepared before
Temporary supports shall either be released or designed to fail under the completed weight of the tunnel and the element designed accordingly. If the temporary supports rest on stable concrete pads, any requirements for preloading and all subsequent behavior of the pads shall be determined.
• Supported on discrete supports such as pile bents or
drilled shafts. Deep foundation elements shall be designed in accordance with Section 10 of the AASHTO Specifications.
Foundations, other than discrete supports shall be at
least two ft. thick.
When varying ground conditions occur along the tunnel alignment, or when sharp transitions from soil to rock occur under the tunnel elements, analyses shall be performed to estimate longitudinal and transverse differential settlements within each tunnel element, between adjoining tunnel elements and at the transitions at the ends of the immersed tunnel. These analyses shall be performed in accordance with Section 10 of the LRFD Specifications to determine the magnitude of potential differential settlement. The tunnel elements shall be designed to accommodate the calculated differential settlement.
Settlement of the element while resting on the foundation pads shall be monitored and corrections made to the element elevation in event that movement takes place.
See Section 3 for additional design requirements for foundation settlement.
placing an element. The grouting and sand flow methods may be constructed from inside the tunnel element, whereas the sand jetting method requires external equipment. Sand jetting is no longer used because it produces a less compacted foundation and is less controlled than can be achieved using the sand flow method. Grouting using an underwater thixotropic-type grout is used primarily when severe seismic conditions would make a sand foundation unsuitable.
The thickness of the foundation affects the final
depth of excavation of the trench and height of associated side slopes.
8.4.1.3 Fill Material
Fill materials (backfill) shall consist of locking fill and general fill. The graduation of the materials shall be such that they cannot liquefy during a seismic event.
Locking fill shall consist of appropriately-graded self-compacting material. Locking fill shall be placed in thin lifts equally on each side of the tunnel elements such that lateral displacement of the element during fill placement does not occur. The extent of locking fill shall be sufficient for the element not to be displaced during subsequent placement of general fill and any protection
C8.4.1.3
Filling to existing bed level is sometimes required
for environmental reasons. In most cases, any depression is likely to silt up naturally.
Locking fill typically extends up to about mid-height of an element.
thereof. General fill shall also be placed in lifts to ensure that
displacement of the element does not occur. The gradation of the fill materials shall be designed such that it cannot liquefy in a seismic event. It is common practice to place fill over the top of a tunnel to protect against sinking ships, falling anchors, etc. The required thickness of this fill shall be sized by analysis and is typically at least five feet, depending upon expected vessel size and anchor weight; concrete ballast and concrete protection over the tunnel may be used to reduce the thickness of any needed protective fill. The composition of fill placed over the tunnel shall be such that pressure from sunken ships and falling anchors is cushioned to reduce the risk of damage to the structure below, and shall be confirmed by analysis. Areas of general fill shall not be permitted to contain materials such as rubble rock, masonry and metal without prior agreement from the designer and Owner.
Where fill materials are subject to scour or erosion, for example by waves, currents and/or propeller wash, the uppermost layer of fill (scour protection) shall be designed to prevent scour and erosion.
It may be appropriate to provide anchor release bands against dragging anchors in locations where extreme conditions can arise such that ships could drag anchors that would snag on the tunnel.
Placing fill above the natural bed of the waterway should be avoided unless proven that negative hydrologic or environment consequences will not occur.
Guidance on the suitable gradation of materials
against scour may be obtained from The Rock Manual (CIRIA) and Propeller Induced Scour (Prosser); rock size may be selected using USACE Hydraulic Design Criteria Sheet 712-1 Stone Stability.
Anchor release bands have typically been located 16 ft. clear of the tunnel.
8.4.1.4 Sideslopes
Sideslopes for underwater excavations shall be designed in accordance with Section 11 of the LRFD Specifications. Cut slopes design shall consider potential impact on ground displacements and displacement of existing structures, utilities and facilities within the influence of construction. The design shall ensure that the sides and bottom of any excavation are stable. Designs shall take into account excavation base stability against heave and softening in any cohesive soils during and subsequent to construction. Designs shall take into account the expected external surcharge loads due to existing or proposed structures, traffic and Contractor’s equipment and material stockpiles.
The location of existing features and conditions shall be verified when determining the inclination of sideslopes of the trench excavation for placement of the immersed tunnel elements. Existing features such as utility lines, bridge and other marine structures foundations shall be located to ensure that the sideslope will not affect the exiting feature. Existing riprap scour protection, temporary supports used for construction of existing facilities and features shall be identified, located and accommodated in the design.
Underwater cut slopes shall neither be steeper than 2 Horizontal to 1 Vertical in soil nor steeper than 1
Horizontal to 4 Vertical in rock in areas where utility lines and marine structures are located within the influence of tunnel construction, provided the provisions of Section 11 of the LRFD Specifications are met.
The Contractor shall be responsible for the design of temporary sideslopes excavated for element placement.
The design of underwater slopes shall evaluate the influence of the slope angle on the rate of sediment deposition in the excavated trench and its potential impact to the foundation preparation. 8.4.2 Materials
See Section 4 for materials.
8.4.3 Support Loss
This design consideration is unique to immersed tunnels and can result from construction practices. Support loss shall include loss of support (subsidence) uniformly below the tunnel or to only one side. For the immersed tunnel, a loss of support of 30 ft. over the full width of the tunnel element at any point along the length of the element shall be considered. Uneven support shall be considered also as follows: zero at one side of the element transitioning to full support on the opposite side of the element for a length of 30 feet along the longitudinal axis of the tunnel.
C8.4.3
The value of 30 ft. is empirical, commonly used and takes into account purposely low foundation levels around immersion joints to ensure foundation materials do not get trapped in immersion joints.
8.5 CONSTRUCTION
8.5.1 Shipping Channel Traffic
The design of immersed tunnels shall take into account the maintenance of marine traffic, and establish the criteria for maintenance of marine traffic during construction at the early stages of design.
C8.5.1
The requirements for maintenance of marine traffic may: dictate maximum element lengths, location of immersion joints, construction sequencing, require the dredging of temporary channels to detour marine traffic around the active construction site and time restrictions on construction activities.
Areas required for marine construction operations such as excavation, foundation preparation, immersion and filling shall be clearly marked so that shipping traffic is excluded from such areas.
Typically, a Notice to Mariners must be issued by the responsible authority well in advance of the occupation of such areas. Changes at short notice to the dates of occupation may not be possible.
There will be an impact on shipping movements during dredging, placing the tunnel elements and during fill operations, all of which require specialised equipment. Standby vessels may be required to protect work areas from errant vessels. Proper coordination has made it possible to construct immersed tunnels in even the busiest and in the narrowest of waterways.
appropriate slope or structurally supported, as required, based on ground conditions and site constraints. Structural excavation support systems shall be designed in accordance with recognized design procedures.
The design of temporary structural support of excavation required for trench excavation shall be the responsibility of the Contractor.
The tunnel trench minimum widths shall be shown on the drawings. The width shall be sufficient to ensure working room for divers to safely work and to ensure proper backfilling of elements.
The Contractor shall be responsible for verifying that the bottom width of the excavation is sufficient to ensure the safety of divers and other workers.
Cut slopes shall meet the requirements of Article 8.4.1.4.
8.5.3 Element Length
Design of the element shall be determined based on the length selected. Constructability shall be considered in determining element lengths. Loadings dependent on element length shall be checked for each length used in the design.
The final selection of element lengths shall be the responsibility of the Contractor. The Contractor shall be responsible to perform the design and checks required to ensure that the final structure is adequate for both permanent loads and temporary loads imposed during construction utilizing the selected elements and the Contractor’s proposed means and methods.
C 8.5.3
Many elements have been between 300 and 400 feet long and some have been up to twice that length. The actual length of an element may be governed by the available size of the fabrication facility, the navigable channel along the waterway used to float the elements to the construction site, ability of available tug boats to handle the element, marine traffic, currents, wave height during tow, element shape, vertical profile and horizontal alignment, outfitting facility size and location and placing barge equipment. The length of a tunnel element usually only affects schedule, not methodology.
8.5.4 Fabrication Facility
One or more potential fabrication facilities shall be identified as part of the design. Assumptions relative to the location, size and configuration of the fabrication facility shall be included in the contract documents.
The final selection of the location and configuration of the fabrication facility shall be the responsibility of the Contractor.
C8.5.4
The selection and choice of sites for a tunnel element fabrication facility typically requires consideration of at least the following factors:
• Distance from the tunnel site • Availability over required period of use • Navigational access • Effects on marine traffic • Land access • Overall suitability for construction of steel or
Fabrication facilities have been located both above and below water level. Below water level, a dry dock or temporary casting basin using removable cofferdams might be used. In most instances, the choice is between the use of a purpose built basin or of an existing dry dock. As with shipyards, dry docks are usually committed to ship repair orders well in advance and could be expensive to lease.
Stability shall be checked while a tunnel element is afloat, including all marine operations. Due attention shall be paid to effects of variations in structural dimensions, including results of thermal and hydrostatic effects. Items to consider shall include:
• Sufficient freeboard for marine operations, so that
tunnel elements are relatively unaffected even when waves run over the top. A factor of safety exceeding 1.01 is recommended to guard against sinking due to variations in both the tunnel dimensions and in the densities of tunnel materials and the surrounding water.
• Lateral stability of the element using cross-curves of stability analysis shall have a factor of safety in excess of 1.4 of the area under the righting moment curve against that under the heeling moment curve. See Figure 8.5.5-1.
Figure 8.5.5-1—Right and Heeling Moments
The righting moment is calculated from centroids of weight and displacement as the structure is rotated. The heeling moment is calculated from what is causing the heel to occur, e.g. concrete blocks placed along one edge, wind loads on exposed structure (survey towers, freeboard, etc.). Downflooding angle may not be relevant if the structure is sealed.
Special measures may be required to control tunnel elements in areas with currents or navigation channels and when storms occur.
Navigation lights and sufficient monitoring shall be provided to maintain the elements in safe condition.
Tunnel elements may be constructed in batches or in a sequence that does not permit immediate outfitting and laying. It may be necessary to store elements temporarily afloat for a period that could even be months. The location shall be chosen to satisfy the following criteria:
• Adequate water depth • Bottom suitable for anchoring
C 8.5.5 The distance of the facility from the tunnel
alignment is preferably short, but many elements have been fabricated hundreds or thousands of miles away and towed across open oceans. If built on the alignment of the approaches, towing distances are practically eliminated.
Tunnel elements have been towed on submersible barges or floating on their own. The transportation methods may depend on the route and distance between the fabrication facility and the construction site.
It is recommended that an emergency berth be
identified for tunnel elements, preferably within or close to the placement site.
If individual moorings are used, then each element would require large swing circles. The total area required can be reduced by using moorings at both ends of each element.
• Sheltered location • Away from shipping channels • Sufficient space between elements such that
they cannot collide • Maintenance accessibility
8.5.6 Outfitting
Tunnel elements shall be designed to withstand force effects associated with outfitting performed after being floated. Unequal loadings due to addition of permanent or temporary ballast, structure components or ancillary items shall be analyzed and checked.
External ballast shall not be capable of being scoured away or falling off, e.g. concrete or blocks, or else stone or gravel retained to prevent displacement.
Potential locations for element outfitting shall be identified during the design process.
Determination of the final outfitting location(s) shall be the responsibility of the Contractor. The Contractor shall also be responsible for performing the analysis required to ensure that the outfitting process selected will not damage the element.
C8.5.6
Depending upon how complete a tunnel element is once it is afloat, some outfitting or further construction may be required before leaving the vicinity of the fabrication facility.
One or more outfitting locations often in the vicinity of the final alignment are required to complete at least the primary structure of an incomplete tunnel element, perhaps add some ballast either internally or externally and to add temporary immersion equipment.
8.5.7 Immersion
To lower an element to its final position, either a temporary ballasting system shall be used or the element weight shall be such that the element will itself have sufficient negative buoyancy. If temporary water ballast is used, the water shall be assumed to be fresh water, not saline.
The following checks shall be made for the immersion process: • The total factored resisting force effect immediately
after lowering shall be calculated in accordance with Section 3.7. The buoyancy load shall be calculated based on the mean high tide at the tunnel location.
• The total factored resisting force effect within four hours of lowering shall be calculated in accordance with Section 3.7. The buoyancy load shall be calculated based on the average high wave height at the tunnel location. Stricter requirements may be imposed in busy shipping channels.
• A vertical downward load of not less than 112.5 kips shall be maintained on every temporary bed-level support, if used, until the element is placed on its final foundation.
• When making the above checks, the weight of immersion equipment, temporary connections and temporary ballast shall not be taken into account.
• When making the above checks, the effect of adjacent elements in place shall not be taken into account.
C8.5.7
The checks shown are minimum checks. The purpose of these checks is to ensure the stability, including control over the tunnel element, while it is lowered to its final position.
The value of 112.5 kips has been developed through experience with the construction of immersed tunnels and is not based on research.
The Contractor shall be responsible for making the
checks required by this section. Additional checks that may be required shall be performed by the Contractor.
Tunnel elements shall be installed at an elevation considering an allowance for settlement, such that after completion of the foundation works and all filling, they will be expected to be located within the specified lateral and vertical tolerances.
The Contractor shall be responsible for performing the analysis required to verify that settlement after the lowering and backfilling of an element will not have a detrimental effect on the element.
The tunnel shall be released from any temporary supports onto its foundation prior to placing fill over the tunnel. 8.5.8 Joining and Backfilling
Backfilling shall occur in a manner that does not cause differential loads on opposite sides of elements or immersion joints.
Force effects applied to the elements during joining and backfilling shall be determined and checked under Service II.
For design purposes, it shall be assumed that backfilling will occur simultaneously on both sides of an element with no more than two-ft. differential in height of backfill on the sides of the element.
For design purposes, it shall be assumed that backfilling will occur simultaneously on both sides of immersion joints with no more than two-ft differential in height of backfill on opposite sides of the immersion joint.
If the Contractor elects to backfill in another sequence than stated, the Contractor shall be responsible for performing the analysis and checks required to ensure that the element is not be damaged or displaced laterally during the backfilling operation.
C 8.5.8 A typical joint between elements includes:
• Temporary watertight bulkheads (dam plates); • Temporary watertight access doors in the
bulkheads; • Permanent joint seals or gaskets; • Temporary dewatering equipment including any
pumps and piping; • Temporary location devices (to guide and adjust the
element horizontally and vertically into place relative to the preceding element) and may include wedges, jacks and shims;
• Provisions for transferring shear across a joint (horizontally and vertically).
8.6 LIMIT STATES AND RESISTANCE FACTORS
8.6.1 General
Design of immersed tunnel elements shall satisfy the criteria for the service limit states specified in Article 8.6.2, the strength limit state specified in Article 8.6.3 and the extreme event limit states specified in Article 8.6.4.
Immersed tunnel elements shall be designed for a service life based on consideration of the potential effects of material deterioration, leakage, stray currents, scour, natural and manmade extreme events and other potentially deleterious environmental factors on each of the material components comprising the structure.
investigated for deflections, crack control, vibration, durability, water tightness and fatigue as developed during the service life of the tunnel.
Service T-II: Immersed tunnel elements shall be investigated for the effects of external environmental forces and element floating stability that can arise during construction, including but not limited to during launching, transportation, immersion, placing, joining and backfilling. 8.6.3 Strength Limit States
Strength T-I: Immersed tunnel elements shall be investigated for the conditions during normal use, under which the tunnel remains fully serviceable without damage.
Strength T-II: Immersed tunnel elements shall be
investigated for load effects imposed on immersed tunnel elements during construction including, but not limited to, launching, flotation, transportation, outfitting, immersion, joining and backfilling.
8.6.4 Extreme Limit States
Extreme Event T-I: Immersed tunnel elements shall be investigated for earthquake to ensure life safety and survivability of the structure taking progressive collapse into account. Acceptable levels of damage may include partial failure of tunnel members that results in limited leakage or complete loss of service. The Owner shall determine acceptable levels of damage.
Extreme Event T-II: Immersed tunnel elements shall be investigated for the design earthquake, ship sinking or anchor impact individually, not combined. Other extreme events such as tsunami, explosion and fire shall be considered on a project-specific basis and, if relevant, shall be included in Extreme Event II individually but not simultaneously with other events.
Extreme Event T-III: Immersed tunnel elements shall
be investigated for a rare event for the simultaneous combination of loads such as flooding. This load combination shall be used for both surface flooding that raises the ground water elevation or the level of open water, and for flooding of the inside of the tunnel. The following flooding cases shall be investigated under this limit state:
• Sea level or surface water elevations rise that
produce increased hydrostatic pressures on the tunnel and reduce the resistance to buoyancy, and
• Flooding of the inside of the tunnel that increases the resistance to buoyancy and reduces the hydrostatic pressure to zero.
C8.6.4
Under Extreme Event T-II, it is suggested that the
tunnel sustain no more than light damage and/or minor leaks, while experiencing no significant loss of service during the prosecution of repairs, i.e. partial performance level.
Immersed tunnel elements shall be designed for the load combinations shown in Tables 3.4.-1.
8.6.6 Resistance Factors
See Section 4 for material resistance factors. Resistance factors for buoyancy shall be as follows: Immediately after immersion: 0.975 6 hours after immersion: 0.960 Permanent condition: 0.900 If the element is submerged in a public waterway, the
resistance factor for buoyancy may be increased to 0.98 for short periods after immersion only under controlled conditions.
8.7 GENERAL FEATURES OF DESIGN
8.7.1 Loading
This article supplements Section 3 with information unique to immersed tunnels.
8.7.1.1 Permanent Loads
The following provides design information for permanent loads unique to immersed tunnels. Refer also to Section 3 for permanent loads.
8.7.1.1.1 Settlement (SE)
Structures shall be designed to accommodate both short-term and long-term settlements. Where uneven settlements on a foundation may occur, structures shall be designed to accommodate the resulting stresses. The effects of placing fill on or around an element or on adjacent elements shall be taken into account; lateral movements can occur in soils that are non-uniform laterally and where the soil surface is sloping. Anticipated variations in soil properties shall be taken into account in analyses. Ground and structure settlement due to applied loads, dewatering, excavations, tunneling, pile driving and other construction activities shall be estimated in designs, using generally recognized procedures and methods of analysis. Analyses shall be performed to estimate foundation rebound due to excavation of the overburden soils and the recompression of the foundation soils due to placement of the tunnel and backfill material. Settlement analyses shall consider compression of the foundation course placed beneath the tunnel elements. Analyses shall also be performed to estimate the longitudinal and transverse differential settlement within
C8.7.1.1.1
Settlement may occur at the fabrication facility as well as at bed level after immersion, for example due to soft soils, in which case their effects are investigated each time the pressure on the soils changes. For an element on temporary supports after immersion, one temporary support may settle more than another. Fixing an immersion joint (removing a degree of freedom using shear keys, for example) also changes subsequent behavior. Short-term settlements may occur after placing an element due to compaction of the foundation layer.
Selection of a more favorable fabrication site, improvement of poor soils, proper preparation of the foundation and planned placement of fill can reduce settlement effects.
For a typical immersed tunnel, the effective weight of the structure is less than the soil it is replacing.
each tunnel element, differential settlement between adjoining tunnel elements and differential settlement at the transitions at the ends of the immersed tunnel. Measures shall be taken to prevent sharp transitions from soil to rock foundations, such that the tunnel structure is reinforced to accommodate the stresses arising or to mitigate the effects of sharp transitions. Alternatively, the tunnel structure shall be reinforced to accommodate the stresses arising or shall be designed to mitigate the effects of sharp transitions.
8.7.1.2 Transient Loads
The following provides design information for transient loads unique to immersed tunnels. Refer also to Section 3 for transient loads.
8.7.1.2.1 Transient Water Loads (WAp) This load represents the effects of variations in water
level above and below MSL (Mean Sea Level). At least two water levels shall be considered: For Strength I, the water level shall have an annual probability of being exceeded of 0.2 or greater, and for Extreme Event limit states, water levels with a probability of being exceeded once during the design life. The effects of global warming and rise or fall of the land mass shall be taken into account. Where pore water pressure relief is not provided from beneath an immersed tunnel, then passing ships, seismic events and the troughs of tsunamis and large waves can result in pore water pressure beneath the tunnel tending to lift the tunnel. Both maximum and minimum hydrostatic loads shall be used for structural calculations as appropriate to the member being designed. The design shall take into account the fact that the specific gravity of water may vary according to depth, prevailing weather conditions and season. The effect of suspended material shall be taken into account in determining the specific gravity of water.
In the absence of better data, global warming may be assumed to result in a two ft rise in MSL.
C8.7.1.2.1
Such variations are caused by tides, storms, tsunamis, large waves, etc.
8.7.1.3 Construction Loads The strength and stability of tunnel elements, and all
components, shall be ensured at every stage of construction, especially when afloat.
The determination of the magnitude of construction loads and the analysis of immersed tunnel elements for construction loading shall be the responsibility of the Contractor.
8.7.1.3.1 Launching (LA)
Loads occurring during launching or flooding of a dry facility shall be analyzed to ensure that no damage will occur to the element.
C8.7.1.3.1
Where elements are ballasted down during the flooding of a dry facility, for reasons of control, it is advisable to float one end up before the other when
During towing and mooring, sufficient freeboard shall exist so that tunnel elements are relatively unaffected even when waves run over the top. When elements are constructed on or transported by barge, they shall be designed for possible static and dynamic loads including those occurring under storm conditions according to barge location.
When a storm warning is issued, or forecast wave heights are expected to exceed operational limits, all marine operations shall be ceased temporarily; marine plant and floating tunnel elements shall be sent to their designated storm moorings or shelters. It is recommended that an emergency berth be identified for tunnel elements, preferably within or close to the placement site. Special measures may be required to control tunnel elements in areas with currents or navigation channels.
Temporary prestressing to hold segment joints closed during transportation shall be sufficient to ensure that tensile stresses in joints do not occur.
C8.7.1.3.2
Were joints to open, slamming could damage joints
when they close and joint seals could be permanently damaged.
8.7.1.3.3 Immersion (IN)
Attachments on an element that are used to lower elements during immersion shall be designed using a load factor of 2.0. Typically, four such attachments are used. The design load for the attachments shall assume that the lowering equipment fails or breaks at any one attachment. The design load shall also consider possible dynamic loads cause by wind and wave action resulting in differential motion between the element and the lay barge, etc., from which it is suspended.
C8.7.1.3.3
Where immersion is carried out by a jack-up or other platform resting on ground, or by a shore-mounted crane, wave action on the element may cause significant loads on lowering equipment.
8.7.1.3.4 Joining (JO)
Loads arising during the joining process shall be considered during the design.
As part of the joining process, temporary equipment may be used to assist in aligning an element to the previously placed element (horn beams at the inboard end, cables attached to the remote end, wedges in the joint, etc.). To assist in obtaining an initial seal of gaskets in the immersion joint, some form of jacks or other pulling equipment, either attached to the elements or the bulkheads, may be used to pull the elements together.
8.7.2 Joints
All immersed tunnel joints shall be watertight throughout the design life, and shall accommodate expected movements caused by differences in temperature, creep, settlement, earthquake motions, method of construction, etc. Displacements in any direction shall be limited so that the waterproof limits of a joint are not exceeded.
Joint shear capability shall take into account the influence of normal forces and bending moments on the shear capacity of the section. The design shall take account of shear forces generated where the faces of the joints are not normal to the tunnel axis.
Joints shall be ductile, in addition to accommodating longitudinal movements. Tension ties may be required to limit movement so that joints do not leak or break open, especially during a seismic event.
The axial compression of tunnel elements and bulkheads due to depth of immersion, as well as temperature effects, shall be taken into account in determining joint dimensions at installation. The effects of higher or lower internal air pressure in sealed tunnel elements due to variations in temperature, etc., due to compression, expansion and immersion shall be taken into account.
The design of primary flexible seals at tunnel joints
shall be designed to take into account:
• Maximum deviations of the supporting frames relative to their design location
• Maximum deviation of the planes of the frames • Relaxation of the seal • A capacity of at least a further 0.4 in. of compression • The minimum compression shall be 0.4 in. greater
than the compression required to maintain a seal
A secondary seal capable of carrying the full water pressure shall be fitted across each immersion joint. For flexible immersion joints, the seal shall be capable of being inspected, maintained and if necessary replaced. The seal shall be capable of absorbing the long-term stresses and movements of the joint. Secondary seals shall be protected against damage from within the tunnel. The protection shall have a fire resistance similar to adjacent surfaces inside the tunnel. All joints within the tunnel shall be finished to present a smooth surface, e.g. using cover plates.
Metal hardware in joints shall have a design life adequate to fulfill its purpose throughout the design life of the joint, especially nuts and bolts for primary and secondary seals, taking into account any protection thereto.
In the event that an initial seal is not obtained after immersion and joining, the joint shall be designed so that a backup method of obtaining an initial seal is available.
8.7.3 Scour Protection
Scour protection shall be placed over the tunnel backfill in areas where wave and current scour can occur and where scour from ship propeller wash is likely to
occur. Calculations shall define the extent of protection
needed and the necessary grading of the material. The material shall be durable for at least the design life of the tunnel, or else regularly inspected and replaced on an as-needed basis. The method of placing this material shall ensure that larger sized stones do not penetrate the general backfill and shall not cause damage to waterproofing of the tunnel (if used). The protection layer shall not be placed by bottom dumping.
8.8 STRUCTURAL ANALYSIS
Structural analysis shall be as specified in Section 4 of the LRFD Specifications, where a number of permissible analysis methods are described. Members shall not be in pure tension. Nominal cover to reinforcement shall not be less than that required to meet fire resistance requirements.
The modeling shall be based on elastic behavior of the structure as specified in Article 4.5.2.1 of the LRFD Specifications.
Analysis and design shall take axial loads and secondary effects into account. The LRFD Specifications Article 4.5.3.2 terms this type of analysis as “large deflection theory”.
For two or three dimensional finite element analyses, the loads on exterior surfaces of members shall be applied taking into account load eccentricity and that member surfaces may have dimensions that differ from member centerline dimensions.
For serviceability analyses, reinforcement shall be
designed for crack widths less than or equal to 0.008 in. at two-inch depth of cover.
C8.8
Classical force and displacement methods are often used in the structural analysis of concrete immersed tunnel elements. Other methods (as described in this section) may be used, but will rarely yield results that vary significantly from those obtained using classical methods.
For steel immersed tunnels analyzed using the same structural model, the efficiency of any curvature of the steel members will not be fully utilized. Most general purpose structural analysis programs have routines based on these principles for dimensional models.
It may be appropriate to use two or three dimensional finite element analyses to model the tunnel in both the transverse and longitudinal directions, especially for steel tunnels. Finite element models identify load sharing, account for secondary effects and identify load paths. Steel immersed tunnel elements are complex assemblies of plates that might be curved, and have stiffeners and diaphragms. Simplifying these systems to the point where classical methods of analysis can be used may undermine the efficient use of materials that can result from complex load paths.
Some designers design for crack widths less than or equal to 0.006 in. notwithstanding the considerable increase in reinforcement required.
8.9 WATERPROOFING
8.9.1 Waterproofing Systems
External waterproofing for concrete tunnel elements shall be considered. Notwithstanding the provision of a membrane, the underlying structural concrete shall be designed to be watertight. The materials of a waterproofing system shall have a proven resistance to the specific corrosive qualities of the surrounding waters and soils. The materials of the system shall be flexible and strong enough to span any cracks that may develop during the life of the structure. The waterproofing shall preferably envelop the element where exposed to soil and water.
For steel tunnels, the outer steel membrane may act as waterproofing membrane.
C8.9.1
Waterproofing, particularly if it is adhered to the concrete, may reduce potential leaks and the amount of leak repairs required, should leaks occur. Some segmental tunnels have omitted waterproofing.
For steel waterproofing membranes used on concrete or steel elements, either an appropriate corrosion protection system shall be used to ensure that the minimum design thickness is maintained during the life of the facility, or an added sacrificial thickness shall be provided. Nonstructural steel membranes shall be no less than 1/4 in. thick and shall be watertight. Steel plates shall be joined using continuous butt welds. All welds shall be inspected and tested for soundness and tested for watertightness.
Bituminous membranes shall not be used. The waterproofing system shall preferably adhere at
every point to the surfaces to which it is applied so that, if perforated at any one location, water cannot travel under it to another. The areas of free water flow between a non-adhering membrane and the underlying concrete in case of leakage shall be limited to no more than 100 ft². Depending upon the type of waterproofing used, protection on the sides and top of the tunnel elements may be required to ensure that it remains undamaged during all operations up to final placement and during subsequent backfilling operations. 8.9.2 Water Infiltration
Groundwater infiltration shall be limited to 0.002 gal/ft²/day.
No dripping or visible leakage from a single location shall be permitted.
C8.9.2
The allowable water infiltration value is based on criteria obtained from the International Tunneling and Underground Space Association (ITA), Singapore’s Land Transport Authority (LTA), Singapore’s Public Utilities Board (PUB), Hong Kong’s Mass Transit Rail Corporation (MTRC), and the German Cities Committee, as well as criteria used by various projects in the United States and others abroad for both highway and transit tunnels.
SECTION 9 –INITIAL GROUND SUPPORT ELEMENTS AND GROUND IMPROVEMENT
TABLE OF CONTENTS
9.1 SCOPE 9.2 DEFINITIONS 9.3 NOTATION 9.4 GENERAL
9.4.1 Criteria for Initial Ground Support Design 9.4.1.1 Soft Ground and Mixed Face Conditions 9.4.1.2 Rock Conditions
9.4.2 Initial Support Types Appropriate for Different Ground Conditions 9.4.2.1 Soft Ground and Mixed Face Conditions 9.4.2.2 Rock Conditions
9.5 INITIAL GROUND SUPPORT ELEMENTS 9.5.1 Soil and Rock Reinforcement Elements
9.5.1.1 Steel Reinforcement Elements 9.5.1.2 Glass Reinforced Plastic (GRP) Elements 9.5.1.3 Friction Rock Stabilizers
9.5.2 Lattice Girders 9.5.3 Shotcrete 9.5.4 Steel Ribs and Lagging
9.6 LIMIT STATES AND RESISTANCE FACTORS 9.6.1 General 9.6.2 Service Limit State 9.6.3 Strength Limit State 9.6.4 Extreme Limit State 9.6.5 Resistance Factors
9.7 INITIAL SUPPORT DESIGN 9.7.1 Rock Reinforcement
9.7.1.1 Pattern Rock Reinforcement 9.7.1.1.1 Selection of Element Length and Spacing 9.7.1.1.2 Pattern Design
9.7.1.2 Spot Rock Reinforcement 9.7.1.3 Deformation and Excavation Sequence Analysis
9.8 GROUND IMPROVEMENT 9.9 PORTALS, CROSS PASSAGES AND ANCILLARY STRUCTURES 9.10 REFERENCES APPENDIX 9-A: RECOMMENDATIONS FOR SOFTWARE USE FOR DESIGN AND PERFORMANCE EVALUATION OF ROCK REINFORCEMENT, SHOTCRETE AND DIRECT ROCK SUPPORT FOR UNDERGROUND OPENINGS IN ROCK
SECTION 9: INITIAL GROUND SUPPORT ELEMENTS AND GROUND IMPROVEMENT 9-2
This Section provides guidance for the selection and specifications for the design of initial support elements for conventionally mined and Tunnel Boring Machine-mined (TBM) tunnels in soft ground, mixed-face, and rock conditions. The specifications in this Section are for construction that is considered temporary in that it is not accounted for as part of the permanent load carrying elements of the final tunnel structure. In general, the elements covered under this section are considered sacrificial. This Section also discusses various ground improvement methods that may be used in support of tunnel excavation operations.
The design of initial ground support for mined and bored tunnels may be performed by the Engineer. Alternately, a performance specification may be developed by the Engineer whereby the Contractor selects means and methods for initial support systems. However, since actual reaches of specific ground conditions will likely vary from what is included in the contract documents, during construction the ground condition is identified at the site and the appropriate initial ground support is installed.
This Section does not address support of excavation for open-cut construction.
C9.1 Contract documents typically include designation of
anticipated reaches of individual ground conditions. The initial ground support is designed for these specific ground conditions.
SECTION 9: INITIAL GROUND SUPPORT ELEMENTS AND GROUND IMPROVEMENT 9-3
9.2 DEFINITIONS Face Bolt— Reinforcement placed to support and limit inward movement of face between rounds. Typically Glass Reinforced Plastic (GRP) to prevent damage to drilling and cutting tools. Friction Rock Stabilizer— A rock reinforcement element which develops load resistance from frictional contact with the drill hole surface; functions effectively as a dowel. Heading— The leading edge of the tunnel excavation. One-pass precast concrete segmental lining— Full perimeter precast concrete segmental lining with connectors and gaskets installed concurrently with mining by TBM. This lining functions as the permanent tunnel structure as well as the temporary support; a single shell lining. Pattern Rock Reinforcement— Rock reinforcement installed on a fixed longitudinal and transverse spacing and intended to function as a unit with the ground, to mobilize the self-support of the ground through arch action or beam building. Permanent Support— A type of support with a service life equal to the service life of the tunnel, which can be relied upon as part of the final tunnel structure; forms part of a “double shell lining”; permanent support requires protection against corrosion. Rock/Ground Anchor— A tensioned rock reinforcement element with a much larger load capacity than a rock bolt; often installed for permanent use, with special corrosion protection. Rock Bolt— A tensioned rock reinforcement element. Rock Dowel— An untensioned rock reinforcement element. Rock Quality Designation (RQD)— A measure of rock mass integrity based on the condition of rock core samples. Rock Reinforcement— Steel or composite element installed in a drilled hole and anchored or bonded to develop tension and shear resistance to ground deformations and displacements across joints or other weakness planes. Rock Surface Protection— Material installed to prevent fallout or raveling of ground exposed in the tunnel excavation perimeter between rock reinforcement elements. Rock surface protection may include chain link fencing, welded wire fabric, Glass Reinforced Plastic (GRP) fabric and thin shotcrete membranes. Steel channel sections or mine straps (thin flexible sheet steel members with pre-punched holes) are often used with rock surface protection. Self-Drilling Anchor— A reinforcement element with a sacrificial bit for installation in drill holes that may collapse or otherwise be blocked before conventional reinforcement elements can be installed. Spile— A pre-reinforcement element generally installed around the perimeter of the tunnel and extending beyond the temporary excavation face; used to increase ground stand-up time to facilitate installation of initial support. Spot Rock Reinforcement— Rock reinforcement installed intermittently as required or directed to supplement pattern rock reinforcement or to support isolated rock wedges. Stand-up time— Amount of time that an unsupported heading or excavation face (length and span) will stand before the ground begins to move into the excavation. Stand-up time varies from indefinite for strong, intact, self-supporting rock to minutes in weak ground. Steel Rib— A steel shape expanded to full perimeter contact with the rock in tunnels mined by TBM or erected tightly against the rock with timber blocking in conventional mined excavations.
SECTION 9: INITIAL GROUND SUPPORT ELEMENTS AND GROUND IMPROVEMENT 9-4
Two-pass precast concrete segmental lining— Full perimeter precast concrete segmental lining installed concurrently with mining by TBM. A “secondary lining” of cast-in-place concrete or shotcrete is constructed inside the segmental lining. The two distinct systems form a “double shell lining”.
SECTION 9: INITIAL GROUND SUPPORT ELEMENTS AND GROUND IMPROVEMENT 9-5
9.3.1 General A = Area of sliding surface (ft2) (9.7.1.1) B = Load bearing capacity of bolt (dowel) (ksf) (9.7.1.1) c = Cohesion of sliding surface (ksf) (9.7.1.1) F = Safety factor (9.7.1.1) ko = Horizontal stress ratio (A.2.2) l = Center to center spacing between ribs (ft) (9.7.3) M = Bending moment (kip-ft); Distance between supports (ft) (9.7.3) N = Number of bolts (dowels) (9.7.1.1) w = Ground load with appropriate load factor applied (kip/ft) (9.7.3) W = Weight of wedge (kip) (9.7.1.1) ϕ = Friction angle of sliding surface (9.7.1.1) 9.3.2 Abbreviations ACI American Concrete Institute DEM Distinct Element Method ft Foot/feet GBR Geotechnical baseline report GDM Geotechnical design memoranda GDR Geotechnical data report GRP Glass reinforced plastic ksf kips per square foot RMR Geomechanics classification RQD Rock quality designation SDA Self drilling anchors SEM Sequential excavation method SFRS Steel fiber reinforced shotcrete TBM Tunnel boring machine
SECTION 9: INITIAL GROUND SUPPORT ELEMENTS AND GROUND IMPROVEMENT 9-6
9.4.1.1 Soft Ground and Mixed Face Conditions The Engineer shall prepare Geotechnical Design
Memoranda (GDM) containing information required for the design of initial ground support. This information shall be developed on the basis of conventional soil mechanics theory, based on results of subsurface investigations and field and laboratory testing contained in the Geotechnical Data Report (GDR) prepared for the project.
The GDM may also contain recommendations for groundwater control, any requirements for pre-reinforcement of the tunnel perimeter or the tunnel face for tunnels mined using the Sequential Excavation Method (SEM), and recommendations for employment of various ground improvement methods in support of tunnel excavation, either by SEM or by Tunnel Boring Machine (TBM). SEM excavation in rock may include use of controlled blasting methods, excavation by roadheader and excavation by various types of mechanical rock excavation equipment, such as impact hammers, as appropriate to ground conditions observed in the excavations.
See Section 5 of these Specifications for recommended GDR content and for guidance for GDM format and content.
.
9.4.1.2 Rock Conditions
The information in this article applies to various rock classes defined by the Engineer in the Geotechnical Baseline Report (GBR), which generally shall be prepared in parallel with the GDM. Occasionally, special water control measures may be required. Requirements for such measures will be discussed in the GDM.
Generally, four rock classes shall be anticipated for
most tunnel design projects. For special situations, a fifth class may be developed. For tunnel projects with uniform rock conditions, no more than two rock classes may be required. The rock classes shall be identified by the Engineer in the GBR. Each class also may be readily identified during tunnel excavation by direct visual observation by a trained geotechnical engineer or engineering geologist prior to the installation of support elements.
Each of the rock classes shall be defined as follows: Rock Class I: Fresh intact rock with a joint/fracture
spacing in excess of six ft. Joints/fractures are very widely spaced, with very widely spaced clusters of very closely to closely spaced fractures. Slickensided fractures are very
C9.4.1.2 Information regarding recommended software
for the design of initial support for rock can be found in Appendix A.
The classes are differentiated on the basis of
joint (fracture) spacing and the degree of decomposition and weathering of the rock mass. The behavior of each class in response to excavation and the development of load on the initial support and the permanent tunnel lining are determined principally by these factors.
Each rock class can be differentiated on the basis of information contained in the boring logs and the results of field and laboratory testing. This information can be found in the project Geotechnical Data Report (GDR).
Rock Class I: This material generally would
exhibit a Rock Quality Designation (RQD) greater than 85. It would be classified as Rock Condition 1 to 3 in Table 5.6.4-1. Standup time (time before
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rare. Mineralization along joints/fractures is rare and few infilled joints/fractures are observed. Joint surfaces range from planar, rough and irregular, undulating, smooth; or undulating, rough or irregular.
Rock Class II: Typically slightly weathered rock with a
joint spacing from two to six ft. Shear/fault planes, joint/fracture weathering and alteration products may be present in this rock class. Open, infilled and slickensided fractures are easily observed. Observed shear/fault planes can contain clay or disintegrated rock between rock surfaces, with a thickness of alteration products generally less than six in. The rock mass contains distinct sub-domains of lower quality rock characterized by clusters of very closely to closely spaced fractures and persistent infilled fractures.
Rock Class III: Typically, moderately weathered rock
unsupported rock begins to fall or ravel around the excavation perimeter) in mined tunnels is generally greater than 24 hours in this rock class for excavations less than about 25 ft in width.
For tunnel excavations greater than 25 ft in width, generally characteristic of road tunnels, sequential excavation of multiple drifts is recommended. Round lengths up to 12 ft are feasible for mined excavations using controlled blasting or roadheader excavation.
This material generally can be supported by spot rock reinforcement or pattern rock reinforcement, with longitudinal and transverse spacing between rock reinforcement elements of five to six ft in both conventionally mined tunnels and TBM-mined tunnels.
Pattern rock reinforcement often is used in TBM-mined tunnels in conditions where spot reinforcement might be used in conventionally mined tunnels. The reason is that wedges that normally would be dislodged by vibration or gas pressures when explosive excavation is used may remain in place for an indeterminate period before falling without warning along the length of the tunnel. Occasional use of rock surface protection between rock reinforcement elements is recommended to prevent fallout of small blocks or raveling of the excavation perimeter. Rock surface protection does not need to be installed concurrently with installation of rock reinforcement.
Rock Class II: This material generally would exhibit an RQD ranging between 75 and 85. It would be classified as Rock Condition 3 or 4 in Table 5.6.4-1. Standup time in mined tunnels is generally greater than 8 hours but less than 24 hours in this rock class for excavations less than about 25 ft in width.
For tunnel excavations greater than 25 ft in width, generally characteristic of highway tunnels, sequential excavation of multiple drifts is recommended. Round lengths up to 8 to 10 ft are feasible for mined excavations using controlled blasting or roadheader excavation.
This material generally can be supported by pattern rock reinforcement, with longitudinal and transverse spacing between rock reinforcement elements of four to five ft in both conventionally mined tunnels and TBM-mined tunnels. Use of rock surface protection between rock reinforcement elements is recommended to prevent fallout of small blocks or raveling of the excavation perimeter. Rock surface protection may need to be installed concurrently with installation of rock reinforcement.
Rock Class III: This material generally would
exhibit an RQD ranging between 50 and 75. It would
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with a joint/fracture spacing of less than two ft, or multiple and random joint/fracture sets with smooth or slickensided surfaces, irrespective of joint/fracture spacing, or multiple zones of brecciated and heavily fractured rock with clay or disintegrated between rock surfaces or one or more shear/fault planes with a filling thickness greater than six in.
Rock Class IV: Typically a shear zone or fault with
multiple zones of brecciated and heavily fractured rock with clay or disintegrated rock between rock surfaces with conditions varying markedly over short distances. Water control can be a significant issue since water may be trapped in a shear zone. Use of various ground improvement methods installed around the excavation perimeter and in the excavation face may be necessary.
be classified as Rock Condition 4 or 5 in Table 5.6.4-1 of this Specification. Standup time (time before unsupported rock begins to fall or ravel around the excavation perimeter) in mined tunnels is generally less than 8 hours in this rock class for excavations less than about 25 ft in width, unless pre-reinforcement, such as spiles, is installed ahead of the face.
For tunnel excavations greater than 25 ft in width, generally characteristic of highway tunnels, sequential excavation of multiple drifts is recommended. Round lengths no greater than four ft are recommended for mined excavations using controlled blasting or roadheader excavation.
This material generally can be supported by pattern rock reinforcement, with longitudinal and transverse spacing between rock reinforcement elements of 3 to 4 ft. Use of Self Drilling Anchors (SDA) may be recommended if there are concerns with drill hole stability.
Use of rock surface protection between rock reinforcement elements is recommended to prevent fallout of small blocks or raveling of the excavation perimeter. Rock surface protection generally may need to be installed concurrently with installation of rock reinforcement.
Alternatively, this material can be supported by steel ribs using steel, timber or concrete lagging and by lattice girders with shotcrete. Supplemental installation of rock reinforcement or SDAs may be necessary if an alternative support methodology is used.
Rock Class IV: This material generally would
exhibit an RQD ranging between 10 and 50. It would be classified as Rock Condition 5 or 6 in Table 5.6.4-1 of this Specification. Stand-up time in mined tunnels is generally less than 4 hours in this rock class for excavations less about 25 ft in width, unless pre-reinforcement such as spiles is installed ahead of the face.
For tunnel excavations greater than 25 ft in width, generally characteristic of highway tunnels, sequential excavation of multiple drifts is recommended. Widths of individual drifts should be limited to no more than 10 ft. Round lengths no greater than four ft are recommended for mined excavations. Because of the nature of the material, excavation is most efficiently performed with conventional mechanical earth excavation equipment or by roadheader.
This material is excavated using the SEM and supported principally with lattice girders and shotcrete. SDAs can be used with the lattice girders and shotcrete. Use of pre-reinforcement such as spiles around the tunnel perimeter and the use of
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various ground improvement methods to stabilize the face may be necessary. Use of steel ribs with steel or timber lagging is an alternative to lattice girders and shotcrete. Use of spiles and ground improvement methods may be necessary with this support alternative also. Tight timber lagging or immediate application of shotcrete over steel lagging generally would be required.
9.4.2 Initial Support Types Appropriate for Different Ground Conditions
9.4.2.1 Soft Ground and Mixed-Face Conditions For tunnels excavated by TBM, use precast concrete
segmental linings. These linings may be either a one-pass precast concrete segmental lining or a two-pass precast concrete segmental lining with a cast-in-place concrete final lining. See definitions in Article 9.2.
For tunnels excavated by SEM, use combinations of
lattice girders, shotcrete, bolts, dowels or Self Drilling Anchors (SDA). See definitions in Article 9.2.
C9.4.2.1 Historically, one-pass systems using cast iron,
ductile iron or fabricated steel segments have been used. Two-pass systems using steel ribs with full perimeter timber lagging as initial support and a cast-in-place concrete final lining also have been used.
For tunnels excavated by TBM, using either one-pass or two-pass precast concrete segments, the segments generally are designed for the worst anticipated combined ground and water loads along the tunnel alignment. It generally is not considered economical to have multiple segment designs, principally for construction logistics reasons (having the right segment type, at the right place, at the right time).
For tunnels excavated by SEM, the Engineer has the option of designing different cast-in-place concrete or shotcrete final linings for defined reaches of tunnel. Generally, the internal geometry of the tunnel remains constant, but lining thickness and reinforcement bar size and spacing can be adjusted for different loading conditions.
9.4.2.2 Rock Conditions
For tunnels excavated using conventional mining methods, including SEM, use bolts, dowels, friction rock stabilizers, SDA, steel ribs and lagging or lattice girders and shotcrete. See definitions in Article 9.2.
C9.4.2.2 For tunnels excavated by TBM, one-pass precast
concrete segmental linings are frequently used. Two-pass systems using precast concrete segments, steel ribs and lagging and rock reinforcement as initial support with a final lining of cast-in-place concrete are still commonly used in TBM-mined tunnels in rock.
For TBM-mined tunnels using precast concrete segments for support, segments generally of a single design are used over the full length of the tunnel, for the same reasons as in soft ground and mixed-face tunnels. The most unfavorable loading conditions will govern the design.
9.5 INITIAL GROUND SUPPORT ELEMENTS
C9.5
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Various ground support elements may be used on tunneling projects. They include the following basic types:
• Soil and rock reinforcement elements • Lattice girders • Shotcrete • Steel ribs and lagging Representative element material properties are
contained in Section 4. Properties are current as of the date of publication of these specifications. Always confirm current values of material properties for use on a specific project.
This article lists and discusses the elements most commonly used for initial support.
9.5.1 Soil and Rock Reinforcement Elements Tensioned soil and rock reinforcement elements shall
be defined as bolts. Untensioned soil and rock reinforcement elements shall be defined as dowels. High capacity, tensioned soil and rock reinforcement elements shall be defined as ground anchors.
Selection of products shall be based on required loadings developed by the Engineer. Manufacturer data provides capacity information for individual products.
9.5.1.1 Steel Reinforcement Elements Steel reinforcement elements shall include solid and
hollow bars. Hollow bars used with a sacrificial drill bit shall be defined as SDA.
Construction specifications shall include provisions for pull testing of a minimum percentage of installed rock reinforcement elements (both pre-production and a selected number of production elements) and requirements for replacement of elements that fail the test.
C9.5.1.1 Various types of anchorages and corrosion
protection methods are used with steel reinforcement elements. Anchorage methods include mechanical anchorage, resin grout anchorage and cement grout anchorage. Corrosion protection methods include epoxy coating, galvanization, plastic sleeves, cement grout and resin grout.
Mechanical anchorage or resin anchorage of rock bolts is almost immediate. Fast set resin cartridges are used to develop the anchorage. Cement grout anchorage requires time to develop, which is a problem in certain ground conditions.
Conversely, successful installation of rock reinforcement elements through a full column of resin cartridges requires careful coordination of hole diameter, cartridge diameter, and reinforcement element diameter and proper rotation and thrust of the reinforcement element to rupture the resin cartridge, mix the resin and catalyst, and achieve effective encapsulation of the reinforcement element by the resin.
9.5.1.2 Glass Reinforced Plastic (GRP) Elements
Glass reinforced plastic (GRP) reinforcement elements shall include solid and hollow bars, similar to steel reinforcement. Hollow GRP bars may be used as SDA.
C9.5.1.2 GRP elements use the same range of anchorage
types as steel reinforcement elements. However, the reinforcement element material is itself corrosion resistant, so less care is necessary to achieve an installation resistant to long-term corrosion. GRP elements can be excavated easily by TBM or
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roadheader, without damaging the excavation equipment and often are used for temporary, sacrificial installations.
9.5.1.3 Friction Rock Stabilizers Two basic types of friction rock stabilizers, the Swellex
type and the Split Set type, may be used.
C9.5.1.3 These friction rock stabilizers originally were
proprietary, but the patents have expired and elements of both types are now available from multiple manufacturers. The Swellex type is a hollow steel tube that is inserted in the drill hole and inflated under high water pressure (approximately 30,000 psi) forcing the steel against the rock. The Split Set type is a split steel tube of slightly larger diameter than the drill hole that is forced into the hole.
Friction rock stabilizers have limitations on capacity and require greater displacement to develop their anchor loads than grouted or mechanical elements.
9.5.2 Lattice Girders Lattice girders in various grades of steel and in GRP
may be used.
C9.5.2 Lattice girders are used as reinforcement for
shotcrete. Lattice girders have minimal load carrying capacity and are dependent upon the time-dependent composite action of lattice girders and shotcrete.
9.5.3 Shotcrete Two spraying methods may be used, dry mix and wet
mix. The shotcrete mix may be either plain shotcrete or shotcrete reinforced with steel fibers at various dosage levels (Steel Fiber Reinforced Shotcrete, SFRS) or polypropylene fibers. Plain shotcrete may be reinforced with welded wire fabric or reinforcing bars. GRP fabric and reinforcing bars may also be used as reinforcement for specific applications. Cement may be partially replaced with fly ash or silica fume. Accelerators and other additives may be included in the mix.
Certification of nozzlemen shall be required by the project construction specifications. The American Concrete Institute (ACI) provides requirements for certification of nozzlemen.
C9.5.3
Quality of shotcrete is operator dependent, even
when a robotic (remote) applicator is used. Attention to detail during shotcrete application is essential for a quality product.
9.5.4 Steel Ribs and Lagging
Steel ribs shall consist of conventional rolled shapes. They may be used with timber or steel lagging. Steel lagging may be steel channel sections, steel liner plate bolted to the web of the rib, specially fabricated steel lagging or mats composed of steel bars or heavy steel wire. Ribs may be installed with appurtenances such as steel tie rods and collar braces.
When blocking, lagging and other rock surface protection materials are installed outside the exterior flange of the rib, the rib may be included within the final lining cross-section and relied upon for support.
C9.5.4
Blocking, lagging and other rock surface
protection materials are normally installed outside the exterior flange of the rib for tunnels excavated in rock by controlled blasting methods, roadheader or
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When full or partial lagging is installed between the
flanges of the rib, the rib shall not be relied upon as part of the final lining cross-section.
various types of mechanical excavators. For TBM tunnels, the circular steel rib section
normally is expanded into full perimeter contact with the tunnel excavation surface for soft ground, mixed-face and rock tunnels.
Timber lagging generally is installed between the rib flanges in soft ground and mixed-face TBM tunneling. Full perimeter lagging, close lagging above springline and skeleton lagging below springline, or skeleton lagging around the perimeter are used in soft ground and mixed-face tunneling, depending upon soil conditions.
Steel bar or heavy steel wire lagging used in some TBM-mined tunnels in rock is installed outside the exterior flange while the rib is expanded. Please note that the actual function of this lagging is to provide protection for the labor force during application of shotcrete through and against the lagging, and for the installation of rock reinforcement elements though openings in the lagging mat if additional support is required.
Close control is necessary to prevent miners from installing timber lagging inside the bar/wire lagging during the excavation phase if the rib is designed to be included within the final lining cross-section. Otherwise, timber lagging and deformed bar/wire lagging may have to be removed prior to placement of tunnel reinforcement and concrete. Potentially hazardous rock falls may occur during this lagging removal operation.
9.6 LIMIT STATES AND RESISTANCE FACTORS
9.6.1 General
Initial ground support elements shall satisfy the criteria for the service limit states specified in Article 9.6.2, the strength limit state specified in Article 9.6.3 and the extreme event limit states specified in Article 9.6.4. When steel ribs are incorporated into the final tunnel lining, they shall also meet the requirements of Section 7.
9.6.2 Service Limit State
Service T-II: Initial ground support elements shall be investigated for deflections to ensure that items not intended to be part of the final tunnel structure do not encroach into the envelope of the final tunnel lining.
9.6.3 Strength Limit State Strength T-II: Initial ground support elements shall be
investigated for load effects imposed during construction including, but not limited to, ground loads, water loads, TBM jacking loads and loads from temporary appurtenances attached to the element.
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Refer to Section 10 for design earthquake for construction conditions and temporary or initial works.
9.6.5 Resistance Factors
The following minimum resistance factors shall be used when designing initial ground support elements except when manufacturer’s data requires a lower resistance factor. In which case, the manufacturer’s data shall be used.
Axial Tension: Steel rock reinforcement elements 0.5 GRP rock reinforcement elements 0.6 Friction rock stabilizers 0.5 Shear: Steel rock reinforcement elements 0.4 GRP rock reinforcement elements 0.4
Friction rock stabilizers shall not be used to resist
shear.
9.7 INITIAL SUPPORT DESIGN This section specifies design procedures for different
ground support elements.
9.7.1 Rock Reinforcement
C9.7.1
Current practice in design of rock reinforcement for tunnels uses conventional limit equilibrium methods with a factor of safety for the design and allowable working stresses for the reinforcement elements.
In rock reinforcement design, the safety factor is
analogous to the LRFD load factor but with a single load factor applied to total load, and the allowable working stress divided by the ultimate strength is analogous to the resistance factor. This article has been developed including load factors and resistance factors in this manner. For references cited in this article, load factors can substituted for factors of safety when calculating design loads utilizing procedures contained therein.
9.7.1.1 Pattern Rock Reinforcement
9.7.1.1.1 Selection of Element Length and Spacing C9.7.1.1.1
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Select element length and longitudinal and transverse
spacing of elements using criteria in Table 9.7.1-1 or by the application of the Geomechanics Classification (RMR) or the NGI Q-System. These systems are described in Section 3.
Table 9.7.1-1 may be used to select rock reinforcement element length and spacing for conceptual or preliminary design purposes with the element length and spacing adjusted later, on the basis of analyses done during further design stages.
Table 9.6.1-1 is based on precedent experience.
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Table 9.7.1.1-1 Minimum Length and Maximum spacing for Rock Reinforcement (USACE, 1980)
Parameter Empirical Rules
Notes
Minimum Length Greatest of:
a. Two times the bolt spacing
b. Three times the width of the critical and potentially unstable rock blocks*
c. For elements above the spring line:
1. Spans less than 20 ft – ½ span
2. Spans from 60 ft to 100 ft – ¼ span
3. Spans 20 ft to 60 ft – interpolate between 10 ft and 15 ft lengths, respectively
d. For elements below the springline
1. For openings less than 60 ft high – use lengths determined in c. above
2. For openings greater than 60 ft high – 1/5 the height
Maximum Spacing Least of:
a. ½ the bolt length
b. 1 ½ the width of the critical and potentially unstable blocks*
c. 6 ft Greater spacing than six ft would make attachment of surface treatment such as chain link fabric difficult
Minimum Spacing 3 to 4 ft
* Where the joint spacing is close and the span is relatively large, the superposition of the two bolting patterns may be appropriate; e.g., long heavy bolts on wide centers to support the span, and shorter and thinner bolts on closer centers to stabilize the surface against raveling due to close jointing as outlined by Reed (1970).
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Design the rock reinforcement pattern with a minimum load factor of 1.5 using one of the following methods:
Ubiquitous Joint Method: Based on results of
geotechnical investigation, plot capable wedges on the transverse section of the tunnel, using the apparent dip of the wedges. Draw an envelope through the wedge apices. Design rock reinforcement for rock contained within the envelope on the basis of the suspension effect or the reinforced rock arch concept per Bischoff, J.A. and Smart, J.D., 1977. See Figures 9.7.1.1-1 and 9.7.1.1-2.
Figure 9.7.1.1-1 Gravity Wedge Analysis to Determine Anchor Loads and Orientations (USACE, 1997)
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Figure 9.6.1-2 Reinforced Roof Arch (Bischoff and Smart, 1977)
Rigid Block Method: Based on results of geotechnical investigation use rigid block analytical software to identify all possible wedges and the unsupported load factor of each wedge. Use software to design pattern rock reinforcement to satisfy Equation 1.3.2.1-1.
Rock Load Method: Based on results of geotechnical
investigation, evaluate the rock load using Table 3.5.2.5-1. Design rock reinforcement on the basis of the reinforced rock arch concept per Bischoff and Smart.
9.7.1.2 Spot Rock Reinforcement
Design rock reinforcement with a minimum load factor of 1.5 using the following method:
(1) Estimate the wedge weight based on field observation. (2) Design rock reinforcement on the basis of the
suspension effect. Use a minimum of two bolts per
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wedge. Spot rock reinforcement also can be designed using rigid block analysis.
9.7.1.3 Deformation and Excavation Sequence Analysis
Analyze ground deformation during excavation for openings supported by rock reinforcement using Distinct Element Code software for analysis. Adjust rock reinforcement element length and spacing as necessary to maintain deformation within acceptable limits.
C9.7.1.3
Deformation and excavation sequence analysis is a critical step in the design of rock reinforcement for tunnels to confirm that the excavation will remain stable during all stages of excavation, minimizing potential hazards to overlying development.
Use of two-dimensional analysis generally is sufficient. Three-dimensional analysis should be used as necessary for low-cover situation, adverse ground conditions and for intersection evaluation.
9.7.2 Shotcrete
9.7.2.1 Shotcrete Membranes
Shotcrete applications with a total applied thickness of less than four to six in. may be designed on an empirical basis. Alternatively, membranes may be analyzed for general shear, adhesion, flexure and punching shear as load carrying mechanisms.
9.7.2.2 Structural Shotcrete Shotcrete applications with a total application
thickness greater than six in. shall be designed as reinforced concrete, plain concrete or fiber reinforced concrete using the loads contained in the GDM, the load factors and load combinations in Section 3 and the resistance factors in Section 4. Reinforcement shall consist of lattice girders, reinforcing steel, steel fiber, welded wire fabric, GRP reinforcing bars or GRP fabric.
Tunnel excavation in rock, soil and mixed face by SEM using shotcrete shall be modeled using three-dimensional modeling software. Time-dependent strength gain of shotcrete shall be considered in the modeling process.
9.7.3 Steel Ribs and Lagging Steel ribs shall be designed using the load factors and
load combinations in Section 3 and the resistance factors in Article 9.6.5.
Bending moment for the design of lagging shall be calculated as follows:
𝑀 = 𝑤𝑙2
12 (Eq. 9.7.3-1)
Where: w = the ground load with the appropriate load factor
specified in Section 3 l = the center-to-center spacing between ribs
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9.7.4 Precast Concrete Segments Precast concrete segments shall be designed for the
loads in the GDM, in accordance with the requirements of Section 7.
9.7.5 Spiles
The Engineer shall determine the need for spiles. Spile type, spacing, length and minimum embedment shall be specified in the contract documents. Spiles shall consist of the following types of elements:
1. Reinforcing bar or threadbar spiles, with or
without embedment in cement or resin grout. 2. SDA, installed with or without concurrent
grouting while drilling. 3. Groutable pipe spiles.
9.8 GROUND IMPROVEMENT
The Engineer shall identify locations along the tunnel alignment that will need ground improvement. The GDM will identify the required station limits of ground improvement, the appropriate method(s) of ground improvement at each location and the required dimensions of the improved ground mass.
and fracture grouting 4. Ground freezing Where feasible, ground improvement shall be
performed from the ground surface above the tunnel, to reduce interference with the excavation cycle and associated cost increases from delays in excavation.
Ground improvement is specified based on the material properties and ground behavior required to limit movements of the ground due to excavation. These properties shall be determined through numerical analyses and tolerable movement of adjacent infrastructure and considerations of face stability. Once the desirable properties are identified, the ground improvement performance may be specified.
Ground improvement performance is highly dependent on Contractor means and methods, so the ground improvement shall be specified as a performance requirement. If the performance requirement cannot be satisfied, structural underpinning of the affected infrastructure shall be required or, where face stability is an issue, alternative means to stabilize the face shall be specified.
C9.8
Additional information on ground improvement can be found in “Ground Improvement Methods”, FHWA-NHI-04-001, Federal Highway Administration (FHWA) (2004) Washington, D.C.
The Strategic Highway Research Program hosts a website that provides state of the art information on ground improvement techniques and other geotechnical topics. The website address is: www.geotechtools.org.
Initial support for portals, cross passages, and other ancillary structures shall be designed in accordance with the requirements of this Section. Perimeter reinforcement in the form of spiles or use of ground improvement methods may be required to support these excavations. The need for such measures shall be identified by the Engineer, and the required measures shall be described in a GDM.
C9.9 Cross passages and other ancillary structures such as pump stations, electrical and mechanical rooms, etc. are included in the tunnel project.
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1. AASHTO. Technical Manual For Design Of Road Tunnels – Civil Elements, 2010
2. Bischoff, J.A and Smart, J.D. “A Method Of Computing A Rock Reinforcement System Which Is ly Equivalent To An Internal Support System”, Design Methods In Rock Mechanics, Sixteenth Symposium On Rock Mechanics, ASCE, 1977
3. FHWA, Training Course in Geotechnical and Foundation Engineering – Rock Slopes, 1998
APPENDIX 9-A: RECOMMENDATIONS FOR SOFTWARE USE FOR DESIGN AND PERFORMANCE EVALUATION OF ROCK REINFORCEMENT, SHOTCRETE AND DIRECT ROCK
SUPPORT FOR UNDERGROUND OPENINGS IN ROCK A1.0 Introduction This Appendix provides recommendations for the use of software for the design and performance evaluation of initial support systems consisting of rock reinforcement, shotcrete or direct rock support, operating alone or in combination. These recommendations are based on experience with the use of UNWEDGE and UDEC. A comment regarding the use of 3DEC for more complex situations is included. This appendix does not constitute the endorsement of any particular software, nor implies that other software that are available or may be introduced in the future are in anyway less reliable than the software described herein. A2.0 General Description of Software Packages A2.1 Rigid Block Software Rigid Block software evaluates support requirements for 3-dimensional, rigid block wedges. Wedge geometry and size are based on the following parameters:
• Discontinuity dip angle and dip direction. • Geometry of the underground opening, azimuth of longitudinal centerline and grade of opening. • Joint strength properties used in the software include: − Friction − Cohesion
Wedge size may be determined exclusively by excavation geometry, dip and dip direction, or can be scaled by using criteria of apex height and length along the longitudinal axis. The software also includes the capability to evaluate the effects of horizontal stress ratio, ko, and joint water pressure on support requirements. Rigid Block software generally can evaluate the performance of several types of rock reinforcement elements including:
• Grouted deformed rebar or threadbar (Dywidag bar and similar) • Cable bolts • Swellex bolts and equivalent • Split Set bolts and equivalent • Expansion shell rock bolts
Rigid Block software can evaluate the performance of shotcrete based on perimeter shear. Rigid Block software also generates required confining pressure on the wedge face exposed in the opening perimeter, which can be used as input for the design of direct rock support (steel ribs). Dip, dip direction, apex height, and trace length can be varied to assess support requirements based on variations in wedge geometry. A2.2 Two-Dimensional Distinct Element Method (DEM) Software Two-dimensional DEM software is used to model the behavior of jointed rock masses with or without initial support. It does not analyze a specific -dimensional wedge, as does Rigid Block software, but can model in two dimensions variations in joint spacing, apparent dip and persistence. Thus, it is a valuable analytical tool. Two-
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dimensional DEM software can also evaluate deformations of the rock mass and overlying structures, as well as stress distributions and stress flow around the openings. This is very helpful when the excavation is at shallow depth below overlying streets or structures. Input to DEM software includes the following:
• Joint apparent dip and a range of variation for this parameter • Joint spacing and a range of variation for this parameter • Joint aperture and a range of variation for this parameter • Joint persistence and a range of variation for this parameter • Joint friction • Joint cohesion • Joint dilation • Joint normal and shear stiffness • Stiffness of rock blocks bounded by joints. • ko • Support element characteristics, similar to UNWEDGE, but adding deformation characteristics
Using the data on dip, dip direction, spacing and persistence, the software develops a 2-dimensional, randomly generated model representative of possible variation in the field. It then evaluates the behavior of the support system, the rock mass and the foundations of overlying structures, including deformations of the rock, overlying foundations, and support elements and stress distributions within the rock mass and the support elements. A2.3 Three-dimensional Distinct Element Method (3-D DEM) Software Use of 3-D DEM software may be limited to critical structures or very complex geometries because of its sophistication and the expertise required for developing the input geometry and subsequent analysis time. However, with continued software and hardware development, these issues will become less important. A3.0 Recommendations
A3.1 Use of Rigid Block Software Rigid Block software may be used for the design of rock reinforcement. The Engineer will have to develop parameters determining wedge size, the joint strength properties and water pressure distribution along the joint surface at various stages of construction based on project-specific conditions. During the initial excavation stage, only the support provided by rock reinforcement elements or direct rock support elements should be considered. Time of application and consequent strength gain of shotcrete during sequential excavation operations cannot be predicted. Experience indicates that specification requirements or approved excavation and support installation sequences, particularly shotcrete application, may be violated during construction, either occasionally or systematically. Shotcrete should be considered only as an enhancement to short-term factors of safety. Shotcrete can be considered for evaluation of long-term excavation stability and factors of safety as follows:
• composite action with rock reinforcement and direct rock support after excavations excavated sequentially have been opened to their final dimensions
• water pressures on joint surfaces have been modified from the initial excavation condition as a result of groundwater recovery due to the presence of a relatively impermeable membrane (shotcrete) around the excavation perimeter.
SECTION 9: INITIAL GROUND SUPPORT ELEMENTS AND GROUND IMPROVEMENT 9-24
Selection of the factor of safety for the initial stage, depending exclusively on the supporting capacity of rock reinforcement and direct support, should be selected on the basis of project-specific requirements. As guidance, factors of safety should be in the range of 1.3 to 1.5. This evaluation can be done outside the LRFD framework. When dip and dip direction are varied within Rigid Block software to develop families of wedges, a lower factor of safety can be used for wedges which the Engineer feels, based on experience and rock mapping data, have a lower probability of occurrence. However, in no case should the initial stage factor of safety be less than 1.3. Factor of safety for long-term excavation stability, prior to placement of the final lining, should be in the range of 1.5 to 2.0. This evaluation can be done outside the LRFD framework. The factor of safety will be very dependent upon the assumptions used for hydrostatic pressure. Hydrostatic pressures operating on joints will reduce the frictional component of joint shear strength. This is the reason for the higher range of factor of safety. However, the higher hydrostatic pressure is appropriate to assume when the 28-day strength of shotcrete is used. Rigid Block software can be used to evaluate support requirements related to sequential excavation of large underground openings. An example of this would be the design of initial support for an opening with a nominal width of 60 ft. Top heading excavation will be accomplished using a central drift and two side slashes with a nominal width of 20 ft each. Completion of excavation to invert will be by two or more benches. If excavation is performed by excavating the two side slashes in sequence before excavating the center drift, wedge size in the crown of the slashes will be determined exclusively by slash geometry, and the wedges in each slash should be similar, although there may be differences based on dip and dip direction of the various joints. When the center drift is excavated, a larger wedge, daylighting in one or both of the side slashes may develop. The rock reinforcement determined in the analysis of the side slashes must be modified to reflect the end-of-excavation stage. If the excavation is performed by excavating the center drift first, followed by excavating the two side slashes in sequence, a large wedge spanning across the combined width of the center drift and slash may develop. The rock reinforcement pattern may have to be modified to reflect this situation. Finally, excavation of the second slash may result in wedges that require further modification of the rock reinforcement pattern. The rock reinforcement pattern should be designed to support the largest wedge that develops from the analyses. Both excavation sequences: side slashes, center drift and center drift, side slashes should be evaluated, because the Contractor could select either approach. A similar approach should be used in evaluating benching The length of the rock reinforcement elements should be checked for constructability. If the reinforcement elements are to be installed without couplers (and couplers should be avoided for a number of reasons), there must be sufficient room to install them, bearing in mind the length of the drill and drill boom. Current hydraulic rock drills have a length of approximately 33 – 43 in. To allow for the length of the drill, the chuck which grips the rock reinforcement element, and hydraulic hoses at the top of the drill, use a clear distance from the drill hole collar to any interfering rock surface not less than the element length plus five ft. If the element length used in the Rigid Block analysis doesn’t meet this criterion, either adjust drift size, within constraints permitted by geologic conditions, or reduce element length and spacing between elements to provide an equal factor of safety. A3.2 Use of Two-Dimensional Distinct Element Method (DEM) Software Two-dimensional DEM software should be used to evaluate the performance of initial support systems designed by the use of Rigid Block software, using the methodology described above. A minimum of three iterations of variations of joint patterns should be considered. A greater number of joint pattern variations may be analyzed.
SECTION 9: INITIAL GROUND SUPPORT ELEMENTS AND GROUND IMPROVEMENT 9-25
Initially, the analysis for each joint pattern should be run without the presence of the initial support to observe progressive raveling/stopping of the rock blocks without support. This is a useful aid for understanding the mechanisms that would develop without the support and to communicate the problem to clients and owners. For each joint pattern selected for analysis, the sequential excavation of the underground opening should then be simulated using 2-D distinct element analysis, with output after the execution of each construction sequence of:
• Rock mass deformations and stresses, • Stress trajectories, • Deformations and stresses of support elements, and • Deformations of foundations of overlying structures
The results of the analyses for each joint pattern should be reviewed by the Engineer for reasonableness. Some of the joint patterns randomly generated by two-dimensional DEM software, which indicate instability or failure of the support system, may have a low probability of occurrence, based on prior experience. Design based on such unlikely joint patterns is not appropriate. The project specifications should include provisions for regular geologic mapping of the excavations to indicate if potentially adverse conditions may be developing. They should also include provisions for installation of additional support to control any such adverse conditions. A3.3 Use of Three-dimensional Distinct Element Method (3-D DEM) Software 3-D DEM software can be used for analysis of the performance of initial support systems designed by the use of Rigid Block software, when the project geotechnical engineer/underground engineer recommends that its use is warranted.
10.6. SEISMIC LOADS AND LIMIT STATES 10.6.1. Load Combinations and Load Factors 10.6.2. Resistance Factors
10.7. SEISMIC SITE CHARACTERIZATION AND DYNAMIC SOIL/ROCK PROPERTIES 10.7.1. General 10.7.2. Seismic Site Classes 10.7.3. Dynamic Soil/Rock Properties
10.8. SEISMIC DESIGN 10.8.1. General 10.8.2. Seismic Loading Effects 10.8.3. Method of Analysis and Assessment – Ground Shaking Effects
10.8.3.1. Transverse Ovaling/Racking Deformation Effects 10.8.3.1.1. Simplified Method for Ovaling Response of Circular Tunnels 10.8.3.1.2. Simplified Method for racking Response of Rectangular Tunnels 10.8.3.1.3. Numerical Modeling Approach
10.8.3.2. Longitudinal Axial and Curvature Deformation Effects 10.8.4. Method of Analysis and Assessment – Ground Failure Effects
10.8.4.1. Liquefaction and Liquefaction-induced Ground Deformations 10.8.4.1.1. Evaluation of Liquefaction Potential 10.8.4.1.2. Post-liquefaction Settlements 10.8.4.1.3. Liquefaction-induced Lateral Spreading
10.8.4.2. Seismic Slope Instability and Landslides 10.8.4.3. Active Fault Crossing Displacement Effects
10.9. SEISMIC DESIGN OF TEMPORARY STRUCTURES 10.10. INTERFACES WITH CROSS-PASSAGES AND OTHER STRUCTURES 10.11. SEISMIC JOINTS 10.12. NON-STRUCTURAL COMPONENTS AND EQUIPMENT 10.13. REFERENCES
This Section supplements the seismic provisions of
the AASHTO LRFD Bridge Design Specifications (hereafter referred to as the LRFD Specifications) and the AASHTO Guide Specification for LRFD Seismic Bridge Design (hereafter referred to as the Guide Specifications). As such, this Section applies to the seismic design, evaluation, and rehabilitation of highway tunnels. During a seismic event, underground structures deform as the ground is deformed by seismic waves and shall be designed to accommodate the seismic deformations imposed by the ground (Wang, 1993 Hashash et. al 2001, and AASHTO 2010).
A performance based evaluation shall be adopted for underground structures, underground shaking and ground failure. For analysis of performance of underground shaking, a pseudo-static or dynamic seismic soil-structure interaction analysis using ground deformation approach (as opposed to the inertial force approach) shall be followed as described in this section.
To evaluate the ground deformations imposed by potential ground failures (e.g., liquefaction, landslide) and for analysis of tunnel performance underground failure, provisions included herein as supplemented by those included in the LRFD Specifications shall be used.
Seismic analysis and design for non-structural components, equipment attachments and support and ancillary structures (e.g., ventilation, control, and administrative buildings) shall be designed in accordance with local building codes.
C10.1
During earthquakes, surface structures experience
inertial loads depending on the shaking intensity of the underlying ground and the vibratory characteristics of the structure itself. Underground structures, on the other hand, behave differently than above ground structures during earthquakes due to the surrounding soil/rock medium.
The main factors influencing the tunnel seismic performance can be summarized as: (1) seismic hazard, (2) geologic conditions and (3) tunnel design, construction and condition. Seismic hazard refers to ground shaking effects and ground failure. Based on tunnel performance during earthquakes, the damaging effects of ground failure are greater than the ground shaking effects. Unfavorable geologic conditions including, but not limited to, soft soils, shear zones, transitions from soil to rock and surrounding geologic units with major contrast in stiffness may result in stress concentrations on the lining or differential displacements during earthquakes. Elements of tunnel design, construction and condition (e.g., tunnel lining and support system, junctions of tunnels with other structures, degree of cracking and deterioration of concrete/steel materials for existing tunnels) may influence tunnel seismic behavior as well.
For other definitions not included herein, refer to Sections 3 and 10 of the LRFD Specifications and the Guide Specifications, respectively. Compressibility Ratio – A term defining the relative compression stiffness of the ground and a circular structure in the ground.
Contractor – Entity responsible for the construction of the tunnel and associated construction engineering.
Design – Proportioning and detailing the components and connections of a tunnel.
Ductility – The ability of a material to sustain loads beyond the elastic range. Fiber Reinforced Concrete sustains loads after cracking and is therefore ductile. Unreinforced concrete rapidly loses strength after the initial crack and is therefore brittle, rather than ductile.
Embedment Depth Ratio – Ratio of soil cover thickness (from ground surface to top of the underground structure) to height/diameter of the tunnel.
Engineer – Agency, design firm or person responsible for the design of the tunnel and/or review of design related field submittals.
Flexibility Ratio – A term defining the relative ovaling stiffness of the ground and a circular structure in the ground. For a rectangular structure, this term defines the relative racking stiffness of the ground and the rectangular structure in the ground.
Owner – Person or agency having jurisdiction over the tunnel.
Permanent Ground Displacements – Ground displacements associated with failure of the ground as a result of seismic events, such as displacements as a result of liquefaction, lateral spreading and landslides.
Transient Ground Displacements – Ground displacements induced by passage of seismic waves (e.g., ovaling/racking, axial, and curvature deformations).
10.3 NOTATION For notations not shown refer to Sections 3 and 10 of the LRFD Specifications and the Guide Specifications. C = Compressibility Ratio of Tunnel (dim.) (C10.8.3.1.1) CP = Apparent Propagation Velocity of P-waves (ft/s) (10.5.2.6) CR = Apparent Propagation Velocity of R-waves (ft/s) Cs = Apparent Propagation Velocity of S-waves (ft/s) (10.5.2.6) E = Young’s Modulus (ksf) (10.7.3) F, Frec = Flexibility Ratio of Tunnel (dim.) (C10.8.3.1.1) Fy = Site factor for long period range of the acceleration spectrum (C10.5.2.2) G = Shear Modulus (ksf) (10.7.3) PGA = Peak Ground Acceleration (g) (10.8.3.1.1) (C10.8.4.2) PGD = Permanent Ground Displacements (in. or ft) (10.8.2) PGV = Peak Ground Velocity (in./s) (C10.5.2.2) (10.8.3.1.1) QV = Loads due to Vertical Seismic Motions (kips) (10.8.3.1.2) S1 = Horizontal spectral acceleration at one second (C10.5.2.2) TGD = Transient Ground Displacements (in. or ft) (10.8.2) ∆s = Racking Deformation (in.) (10.8.3.1.2) φr = Resistance factor for geomaterials (10.6.2) v = Poisson’s ratio (dim.) (10.7.3)
10.4 SEISMIC PERFORMANCE AND SCREENING REQUIREMENTS
10.4.1. Seismic Performance Criteria and Design
Earthquake Levels A two-level target performance criterion shall be
adopted for seismic design and analysis of tunnels in consultation with the Owner. The structure shall provide a high level of assurance for protection of life safety during and after a Safety Evaluation Earthquake (SEE). The structure shall also provide a high level of assurance of continued operation during and after a Functionality Evaluation Earthquake (FEE). The associated requirements for these performance levels are as follows:
SEE: Such an event has a small probability of
exceedance during the design service life of the facility. Following the SEE, some interruption in service is permitted. When subjected to the SEE, it is acceptable that the structures behave in an inelastic manner. There shall be no collapse and no catastrophic inundation with danger to life. Any structural damage shall be controlled and limited to the elements that are repairable. The structure shall be designed with adequate strength and ductility to survive loads and deformations imposed on the structure during the SEE, thereby preventing structural collapse and maintaining life safety.
FEE: Such an event has a lower return period
than the SEE. There shall be minimal interruption in service during or after the FEE. When subjected to the FEE, the structure shall be designed to respond in an elastic manner. There shall be no collapse, and only minimal damage to structural elements is permitted. Such damage shall be minor and repairable. The structure shall remain fully operational immediately after the earthquake, allowing a few hours for inspection. For each target performance level described above,
design earthquake return periods shall be established to evaluate ground motion parameters required for analyses and seismic loads as described in Article 10.5.1. If the Owner determines that the tunnel is not a critical structure, single-level performance criterion may be used. For these non-critical structures, the target performance shall be established by the Owner.
C10.4.1
Most tunnels are considered as critical
infrastructure and relatively more difficult to repair, when compared to regular bridges or other above-ground structures. Hence, multi-level target performance criteria are adopted for major tunnel projects.
Refer to discussion in Article C10.5.1 for selection of design earthquakes corresponding to the target performance levels adopted by these Specifications.
10.4.2. Structural Design Check Strain limits for the tunnel lining (e.g., concrete,
steel reinforcement) shall be established for each design earthquake level to achieve the target performance criteria described in Article 10.4.1. The resulting strains
C10.4.2
In lieu of better information, the strain limits in Table C.10.4.2 may be applied for mined/bored tunnel and cut-and-cover tunnel linings. When selecting strain limits for underground structures, factors such as
from the seismic demands combined with the static demands shall be checked against these strain limits to evaluate the structural integrity of the tunnel.
The special components within the tunnel structural
lining (such as gaskets at the segmental lining joints and the joint itself) shall be designed to accommodate stresses and deformations associated with the design earthquake levels adopted for the tunnel consistent with the established target performance criteria.
concrete confinement, ease of access and repairability at critical locations (e.g., near longitudinal and radial joints at mined/bored tunnel lining) should be considered.
10.4.3. Screening Criteria The level of seismic design and analysis effort
required for each tunnel shall be established once all information regarding all the potential seismic hazards, structural complexities, subsurface conditions and performance requirements are collected.
For preliminary assessment of the ground shaking
impacts or screening, empirical procedures presented in Article 10.8 or simplified numerical models may be used. The underlying assumptions/limitations for these empirical procedures are discussed in Article C10.8.
C10.4.3
For screening analysis of ground failures (e.g., liquefaction and liquefaction-induced ground movements), the established procedures along the lines of Idriss and Boulanger (2008) and MCEER Technical Report MCEER-98-005 (Youd, 1998) may be used.
earthquakes shall be selected based on the risk acceptable to the Owner during the design life of the structure. A minimum design life of 100 years shall be used to evaluate the design earthquake return period unless otherwise specified by the Owner.
When selecting design earthquakes for seismic
design and analysis, the target performance criteria specified as part of Article 10.4.1 shall be considered.
C10.5.1
The collapse of transportation tunnels could have
catastrophic effects as well as profound social and economic impacts.
For the SEE level event, infrastructure owners have
used return periods varying from 1,000 years to 2,500 years (majority 2,500 years; NCHRP Project 20-68A, 2011) or a combination of a design earthquake from a probabilistic hazard analysis along with a deterministic hazard (e.g., California High Speed Train Project). A design earthquake with 2,500 year return period corresponds to 4 percent probability of exceedance in 100 years, approximately.
To avoid lengthy down time and to minimize costly
repairs, a more frequent seismic event is selected for
FEE level analysis. In high seismic areas (e.g., western United States), FEE event with 100-yr return period (corresponding to approximately 65 percent of probability of exceedance in 100 years) is generally defined. In areas where earthquake occurrence is much less frequent (e.g., eastern United States) or when the consequence of disruption to the operation of the system is grave, an earthquake event with a return period greater than 100 years (up to 500-yr return period or an event corresponding to 20 percent probability of exceedance in 100 years) is selected for FEE level analysis.
10.5.2. Ground Motion Hazard Analysis
Once the design earthquakes are established, the
design ground motion parameters at the project site shall be determined through ground motion hazard analysis to evaluate seismic loads (e.g., force and displacement demands) using either one of the following methods:
(1) Using existing hazard analysis results published by United States Geological Survey (USGS) or other credible agencies; or
(2) Project-specific and site-specific seismic hazard evaluation.
Site-specific seismic hazard analysis, if used, shall
be performed per Article 3.10.2.2 of the LRFD Specifications, except for the following:
(1) The earthquake return periods specified in Article 10.5.1 of this Specification shall be used.
(2) The acceleration response spectrum from the site-specific hazard analysis shall be no less than two-thirds of the design response spectrum developed using the site factors given in Article 3.10.3.2 of the LRFD Specification.
C10.5.2 Information used for seismic source
characterization can often be obtained from the publications of the USGS and various state agencies. If there is significant lag time between development and publication, the published hazard results may not incorporate recent developments on local or regional seismicity. There are also other situations where published hazard results may be inadequate and require site-specific seismic hazard evaluation. These situations may include: (1) the design earthquake levels (i.e., return period) are those different than assumed in the published results, (2) for sites located within six miles of an active surface or shallow fault where near-field effect is considered important and (3) the published hazard results fail to incorporate major developments on local or regional seismicity.
When using existing hazard analysis results
published by USGS or other credible agencies, latest available version of the hazard information should be incorporated, unless there is any site-specific reason to use the previous versions.
10.5.2.1. Earthquake Magnitude and Distance Earthquake magnitude shall be determined for the
design earthquakes established for design and analysis. If probabilistic seismic hazard analysis is performed, the magnitude of the dominant earthquakes from disaggregation shall be used. In a number of areas in the U.S. the hazard can be dominated by more than one source (example Seattle, or Midcontinent). The use of conditional mean spectra is encouraged to develop realistic hazard demand.
C10.5.2.1
Earthquake magnitude and distance are required when deriving design spectra-matching ground motion time histories or when performing permanent ground deformation analysis (e.g., liquefaction). Dominant earthquake magnitudes and distances, which contribute principally to the probabilistic design response spectra at a site, as determined from national ground motion maps, can be obtained from disaggregation information on the USGS website: http://geohazards.cr.usgs.gov.
10.5.2.2. Peak Ground Motion Parameters Various types of ground motion parameters
including, but not limited to, peak horizontal ground acceleration (PGA) and peak ground velocity (PGV),
C10.5.2.2
The Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments (NCHRP Report 611, 2008) provides a discussion on
may be required for seismic analyses, depending on the analysis method used. PGA shall be established based on (1) published hazard results by USGS or other credible agencies; (2) attenuation relationships if deterministic seismic hazard evaluation is performed; or (3) disaggregation analysis if site-specific probabilistic seismic hazard evaluation is performed. PGV shall be evaluated using empirical correlations with ground motion parameters or directly from the velocity time-histories established for the project.
estimation of PGV based on various ground motion parameters.
For preliminary evaluations, values of PGV may be
evaluated using the following correlation between PGV and design horizontal spectral acceleration at one second (S1, in units of g) as given by Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments (NCHRP Report 611, 2008):
PGV (in/sec) = 55 Fv S1 (10.5.2.2-1)
where Fv is the Site Factor for long period range of the acceleration spectrum given in Table 3.10.3.2-3 of Section 3 of the LRFD Specifications.
Alternatively, PGV can also be evaluated based on
the site-specific seismic response analysis.
10.5.2.3. Attenuation of Peak Ground Motion Parameters
Ground motion parameters shall be derived at the
elevation of the tunnel. Thus, the peak ground motion parameters shall be adjusted to reflect attenuation of ground motion with depth according to Table 10.5.2.3-1, unless detailed site-specific analysis is performed to evaluate attenuation with depth.
Table 10.5.2.3-1 Ground Motion Attenuation with Depth (modified after AASHTO, 2010)
Tunnel Depth (ft)
Ratio of Ground Motion at Tunnel Depth to Motion at
Ground Surface ≤ 20 0.95 – 1.0
20 – 50 0.75 – 0.95
50 – 100 0.5 – 0.75
≥ 100 0.5
C10.5.2.3 The values given in Table 10.5.2.3-1 correspond to
conservative estimates of ground motion attenuation with depth (AASHTO, 2010). For depths between the limits of each range, corresponding ground motion attenuation ratio can be estimated by linear interpolation.
10.5.2.4. Design Response Spectra
The design response spectra shall not be used directly in the design of tunnels, except for deriving other ground motion parameters or developing ground motion time histories, or for the design of permanent nonstructural components (including architectural
components) and their attachments and the attachments for permanent equipment (including mechanical/electrical systems). The design response spectra shall be established based on the guidelines included in Article 3.10.4.1 of the LRFD Specifications.
10.5.2.5. Ground Motion Time Histories
The developed time histories shall match the target
design response spectra and have characteristics that are representative of the seismic environment of the site and the local site conditions. Response-spectrum-compatible time histories shall be used as developed from representative recorded motions. Analytical techniques used for spectrum matching shall be demonstrated to be capable of achieving seismologically realistic time series that are similar to the time series of the initial time histories selected for spectrum matching.
Where recorded time histories are used, they shall be scaled to the approximate level of the design response spectrum in the period range of significance. Each time history shall be modified to be response-spectrum-compatible using the time-domain procedure. At least three response-spectrum-compatible time histories shall be used for each component of motion (horizontal, longitudinal and vertical) in representing the design earthquakes. The design actions shall be taken as the maximum response calculated for the three ground motions in each principal direction. If a minimum of seven time histories are used for each component of motion, the design actions may be taken as the mean response calculated for each principal direction.
For near-field sites (D < 6 miles), the recorded horizontal components of motion that are selected shall represent a near-field condition and shall be transformed into principal components before making them response-spectrum-compatible. The major principal component shall then be used to represent motion in the fault-normal direction and the minor principal component shall be used to represent motion in the fault-parallel direction.
C10.5.2.5
For additional information on ground motion time histories, see Article C4.7.4.3.4b of the LRFD Specifications.
10.5.2.6. Spatially Varying Ground Motion Effects The effect of spatial variations of ground motions
on long tunnel structures resulting from the effects of wave passage (i.e., different arrivals of seismic waves at different parts of the structure) and local soil overburden shall be considered. The wave-passage effect may be accounted for by assuming a time lag of the ground-motion time histories between any two locations along the tunnel alignment (i.e., estimated by dividing the distance between the two locations by the horizontal wave travelling velocity along the tunnel alignment).
The horizontal propagation shear wave velocity,
C10.5.2.6 The horizontal propagation shear wave velocity,
CS, in general, reflects the seismic shear wave propagation through the deeper rocks rather than that of the shallower soils where the tunnel is located. In general, this velocity value varies from about 1.25 to 2.5 miles per second; (O’Rourke and Liu, 1999). Similarly, the pressure wave propagation velocities, CP, generally vary between 2.5 and 5 miles per second.
In addition to wave passage effects and local site
conditions, the near-field effects, incoherence effects, cross-correlation between orthogonal components of
CS, and the pressure wave propagation velocities, CP, shall be established based on consultation with experienced geologists/seismologists. In absence of site specific data Cs and Cp may be assumed to be 1.6 miles per second and 3.1 miles per second, respectively (AASHTO, 2010).
ground motion at different locations along the tunnel alignment may also need to be considered. For additional guidance on evaluation of the spatial variation of ground motions, refer to Article C4.7.4.3.4b of the LRFD Specifications, Seismic Soil-Foundation-Structure Interaction by the Caltrans Seismic Advisory Board Adhoc Committee (CSABAC, 1999) and LRFD Seismic Analysis and Design of Transportation Geotechnical Features and Structural Foundations (FHWA, 2011).
10.6 SEISMIC LOADS AND LIMIT STATES Seismic loads on underground structures shall be
evaluated using pseudo-static or dynamic soil-structure interaction analyses using ground displacement approach, either following simplified methods or using numerical modeling as described in Article 10.8, except for the retaining structures, buildings, miscellaneous permanent structures, non-structural components and equipment attachments. Seismic effects on the underground structures shall be investigated for Extreme Event I limit state per the LRFD Specifications. The resulting seismic demands shall be combined with demands from non-seismic loading case.
10.6.1. Load Combinations and Load Factors The seismic and non-seismic loads shall be
combined using the load combination and load factors specified in Article 3.4 of this Specification for Extreme Event I. The load factor for live load in Extreme Event I shall be determined on a project-specific basis.
Where seismic loads (i.e., deformations) are determined using ground-motion time histories as input for evaluation of impact due to ground shaking, the ground motions for any one instant shall be input simultaneously in two horizontal directions and the vertical direction.
C10.6.1
For further discussion on live load factor for Extreme Event I, please see commentary in Article C3.4.1 of the LRFD Specifications.
10.6.2. Resistance Factors Values of resistance factors for the geomaterials
(i.e., soil, rock) shall be taken as φr = 1.0 for seismic evaluations. Resistance factors for the structural materials shall be selected as per the guidance included in previous Sections of this Specification.
10.7 SEISMIC SITE CHARACTERIZATION AND DYNAMIC SOIL/ROCK PROPERTIES
Subsurface conditions (e.g., soil stratigraphy, water
level, dynamic soil/rock parameters) shall be evaluated for seismic design and evaluations. Subsurface investigations, including borings and laboratory tests, shall be conducted to provide pertinent and sufficient information for seismic design and analysis of tunnels. Subsurface investigations shall be carried out per the guidance in Section 5 of these Specifications.
Subsurface exploration shall also be carried out to determine the potential presence of geotechnical/geological seismic hazards that may affect the performance of the tunnels under the design seismic event.
Laboratory tests shall be performed to determine soil type, strength and deformation characteristics of soil and rock. For tunnels in high seismicity areas, it may be appropriate to conduct special dynamic or cyclic tests and establish the liquefaction potential or stiffness and material damping properties of soil at some sites.
The groundwater level shall be determined. Seasonal groundwater fluctuations shall also be evaluated.
Soil parameters that may be required for seismic
design and analysis include: • Initial dynamic shear modulus at small strains or shear wave velocity, • Shear modulus reduction and equivalent viscous damping characteristics as a function of shear strain, • Cyclic shear strength of soils (peak and residual) and • Liquefaction resistance of soils.
For additional information on laboratory testing,
see Article C6.2.2 of the Guide Specifications.
10.7.2. Seismic Site Classes Evaluation of seismic site classes may be required
if ground motion parameters are to be developed using design response spectrum. If required, seismic site classes shall be evaluated per Article 3.10.3.1 of the LRFD Specifications.
10.7.3. Dynamic Soil/Rock Properties
Soil/rock stiffness parameters (e.g., Young’s modulus, E; shear modulus, G; Poisson’s ratio, ν) under dynamic loading conditions shall be defined. The values corresponding to small shear strain levels (i.e., less than 10-4 percent) as well as the effect of shear strain level on these parameters shall be established. The shear wave propagation velocity of the subsurface soil/rock layers shall also be determined to allow estimating transient ground shearing strains.
If a non-linear effective-stress modeling technique is adopted in site response analysis, soil parameters required to characterize the selected non-linear constitutive model shall be validated through experimental testing data or case history studies.
In addition to dynamic soil/rock properties, other parameters required for evaluation of ground failure shall also be defined (e.g., fines content for liquefaction susceptibility analysis).
C10.7.3
Previous studies including those from laboratory tests have shown that the shear modulus values are dependent on the shear strain levels. At low shear strain levels (i.e., less than 10-4 percent), the shear modulus values can be reliably estimated from the field-measured shear wave velocities, such as using the seismic cone, downhole, cross-hole, P-S logging and SASW (spectral analysis of surface waves) techniques. As the shear strain increases, the shear modulus degradation effect becomes significant. The shear strain level is also a function of the ground shaking intensity. As the ground motion intensity increases, the shearing strain increases, resulting in reduced equivalent shear modulus.
Typical relationships between the shear modulus degradation and the shear strain level are given by the Electric Power Research Institute (ERPI) report Guidelines for Determining Design Basis Ground Motions, Volume I: Method and Guidelines for Estimating Earthquake Ground Motion in Eastern North America (ERPI, 1993), Darendeli (2001) and Menq (2003).
corresponds to the values measured at the very small strain level. The effective shear wave velocity and the corresponding strain-compatible soil modulus during earthquake shaking should be reduced for strain compatibility before engineering design purposes.
10.8 SEISMIC DESIGN
10.8.1. General Seismic design and analysis of the tunnel structures
shall be performed using pseudo-static or dynamic soil-structure interaction analyses following ground deformation approach, unless otherwise noted. The internal forces and strains from the seismic evaluations shall be considered incremental and shall be combined with those from other loading conditions using the Extreme Event I loading combination.
10.8.2. Seismic Loading Effects In addition to static loads, tunnels shall be designed
to accommodate the effects resulting from two types of seismic loading:
(1) Ground shaking (i.e., transient ground
displacement, TGD); and (2) Ground failure (i.e., permanent ground
displacement, PGD). The combined effects of bending moment and
thrust force on the tunnel lining shall be evaluated as well as the impact of resulting shear forces/displacements on the structural integrity of the tunnels.
The general methodology for evaluating the effects of TGD and PGD shall be in accordance with those outlined in Articles 10.8.3 and 10.8.4, respectively.
C10.8.2
Ground shaking refers to the vibration of the ground produced by seismic waves propagating through the crust of the earth. During ground shaking, the underground structures undergo three primary modes of deformation: ovaling/racking, axial and curvature deformations (Figure C10.8.2-1).
(a) Ovaling Deformation of Circular Cross-section
(b) Racking Deformation of Rectangular Cross-section
(d) Curvature (Bending) Deformation Along Tunnel Figure C10.8.2-1: Ovaling/Racking and Axial/Curvature Deformations (AASHTO, 2010)
The ovaling/racking deformation is caused
primarily by seismic waves propagating perpendicular to the tunnel longitudinal axis, causing deformations in the plane of the transverse tunnel cross section. Vertically propagating shear waves are generally considered the most critical type of waves for this mode of deformation. For circular tunnel sections, the resulting effects are cycles of additional stress concentrations with alternating compressive and tensile stresses in the tunnel lining. Several critical modes may result (Owen and Scholl, 1981):
• Compressive dynamic stresses added to the compressive static stresses may exceed the compressive capacity of the lining locally.
• Tensile dynamic stresses subtracted from the compressive static stresses reduce the lining’s bending moment capacity, and sometimes the resulting stresses may be tensile.
During earthquakes, a rectangular box structure in soil or rock will experience transverse racking deformations (sideways motion) due to the shear distortions of the ground, in a manner similar to the ovaling of a circular tunnel. The racking effect on the rectangular structure is similar to that of an unbalanced loading condition. For rigid frame box structures, the most critical mode of potential damage due to the racking effect is the distress at the top and bottom joints.
The axial and curvature deformations are induced by components of seismic waves that propagate along the longitudinal axis and/or by spatially varying ground motions resulting from local soil/site effects. To accommodate the axial and curvature deformations imposed by the surrounding ground, the lining will develop axial and bending strains.
Ground failure broadly includes various types of ground instability such as fault rupture, tectonic uplift and subsidence, landslides, liquefaction (including liquefaction-induced lateral spreading, settlement, floatation, etc.). Each of these PGDs may be potentially catastrophic to the tunnel although the damages may be localized. To avoid such damage, some sort of ground improvement is generally required unless the design approach is to accept displacement, localize the damage and provide means to facilitate repairs.
10.8.3. Method of Analysis and Assessment – Ground Shaking Effects
When subject to TGD, the tunnels shall be
evaluated for at least three primary modes of deformation that occur during seismic shaking: ovaling/racking, axial and curvature deformations. The effects of soil-structure interaction shall be taken into account in the analysis and design.
ovaling/racking response of tunnel structures shall be based on either: (1) simplified analytical methods or (2) numerical modeling approach, depending on the degree of complexity of the soil-structure system, subsurface conditions, the seismic hazard level and the importance of the structure. Transverse cross-sections analyzed for ovaling/racking deformations shall include, but are not limited to, locations where:
• The free-field relative displacements between the top and the bottom of the tunnel are large (significant ovaling/racking deformation);
• The tunnel is embedded partially in soil and partially in rock, or at an interface between two strata with a stark contrast in stiffness; and
• The structural configuration or framing system is irregular.
All critical cross-sections along the alignment shall be analyzed for ovaling/racking deformations.
C10.8.3.1
The simplified analytical methods have been developed based on ideal conditions and assumptions as follows:
• The tunnel is of completely circular shape for ovaling
response or rectangular shape for racking response. • • The material surrounding the tunnel is uniform and
isotropic. • The tunnel is very deep, away from the surface so
there are no reflection/refraction of seismic waves from the ground surface.
• Only one single tunnel is considered (i.e., there is no interaction from other tunnel or structure in proximity). The actual soil-structure system encountered in the
field for underground structures may require the use of numerical methods. The numerical modeling approach should be considered in cases where the simplified methods are less applicable, more uncertain or inconclusive, or where the site is located in a severe seismic environment or where case history data indicate relatively higher seismic vulnerability for similar type of structures, or the subsurface profile consists of highly variable soil/rock conditions.
10.8.3.1.1. Simplified Method for Ovaling Response of Circular Tunnels
When the simplified method is used for evaluation
of ovaling effects on circular tunnels, the analysis shall be based on the maximum free-field shear strains in the ground caused by the governing vertically propagating shear waves of the design earthquake event. The free field ground strains shall be estimated based on correlations with peak ground motion parameters (PGV, PGA) or from site-specific seismic site response analysis.
The analysis shall take into account the soil-structure interaction effect to provide safe as well as realistic design. The interface condition (i.e., full-slip versus no-slip) at the interface between the exterior wall of the structure and the surrounding soils shall be conservatively assumed to attain maximum structural response in terms of bending moments and thrust/hoop forces.
The simplified method for ovaling response of deep circular tunnels located in a relatively homogenous soil or rock outlined in AASHTO Technical Manual For Design Of Road Tunnels – Civil Elements (AASHTO, 2010), may be used where appropriate.
For evaluation of maximum free-field shear strains in the ground, equivalent-linear (e.g., SHAKE; Schnabel et al., 1972) or non-linear seismic site response analysis programs may also be used. When a seismic site response analysis is performed, the design maximum free-field shear strain shall be based on the maximum shear strain value computed for the full vertical profile of the tunnel (i.e., from crown to invert).
C10.8.3.1.1
Using the average maximum ground shear strain value (i.e., differential shear displacement between crown and invert divided by the height of the tunnel) may lead to underestimation of the structure response, especially for relatively shallow tunnels (i.e., Embedment Ratio, the ratio of soil overburden to the height of the tunnel, less than 2.0).
To account for soil-structure interaction effects, two relative stiffness parameters are defined in AASHTO (2010). The flexibility ratio (F) represents the relative distortion stiffness between the surrounding ground and the lining and tends to govern the bending response (distortion) of the lining. The compressibility ratio (C) represents the relative ring compression stiffness between the surrounding ground and the tunnel lining and tends to dominate the thrust/hoop forces in the lining. When F<1.0, the lining is considered stiffer than the ground, and it tends to resist the ground and therefore deforms less than that which occurs in the free-field. On the other hand, when F>1, the lining is expected to deform more than the free-field. As the flexibility ratio continues to increase, the lining deflects more and more than the free-field and may reach an upper limit as the flexibility ratio becomes infinitely large. This upper limit deflection is equal to the deformations displayed by a perforated ground (i.e., an excavated conduit in the ground with no lining stiffness).
During an earthquake, in general, slip at interface between the exterior wall of the structure and the surrounding soils is a possibility only for tunnels in soft soils, or when seismic intensity is severe. For most tunnels, the condition at the interface is between full-slip and no-slip. In computing the forces and deformations in the lining, it is prudent to investigate both cases and the more critical one should be used in design and analysis.
10.8.3.1.2. Simplified Method for Racking Response of Rectangular Tunnels
Racking deformations shall be defined as the
differential sideways movement between the top and bottom elevations of rectangular tunnel structures, shown as ∆s in Figure 10.8.3.1.2-1. The resulting structural internal forces (bending moment, thrust and shear) or material strains in the tunnel lining associated with the seismic racking deformation, ∆s, shall be derived by imposing the differential deformation on the structure in a simple structural frame analysis.
Figure 10.8.3.1.2-1: Soil Deformation Profile and Racking Deformation of a Box Structure (AASHTO, 2010)
Loads due to vertical seismic motions, Qv, on
rectangular structures shall be accounted for by applying a vertical pseudo-static loading equivalent to the product of the vertical seismic coefficient and the combined dead and design overburden loads used in static design. This vertical pseudo-static loading shall be applied by considering both up and down direction of the motions; whichever results in a more critical load case shall govern.
The closed-form solutions accounting for soil-
structure interaction presented in AASHTO Technical Manual For Design Of Road Tunnels – Civil Elements (AASHTO, 2010) may be used where appropriate. Seismic demands due to racking deformations and vertical seismic motions shall then be combined with non-seismic loads using the appropriate loading combination for Extreme Event I.
10.8.3.1.3. Numerical Modeling Approach
When numerical modeling approach is required, the
ovaling/racking deformations shall be evaluated based on the static response deformation spring-beam method or two-dimensional finite element or finite difference continuum method of analysis, capable of capturing soil-structure interaction effects.
In using the continuum method of analysis, the following considerations shall be included in developing the model for the tunnel cross-section subjected to ovaling/racking deformation:
• As a minimum, analyze the structure, surrounding ground and seismically-induced deflections as a two dimensional soil-structure model.
• Include in the model, if relevant, the internal decks and walls to assess their effects on stress concentration and tunnel deformation.
• Model the effects of the liner joints, particularly where the joints are not properly restrained against opening and closing.
• Accurately model the soil stratigraphy and
C10.8.3.1.3
The static response deformation spring-beam method uses the structural beam and soil spring model. In this method, the displacement of the ground at the tunnel location is first calculated and then applied to the tunnel by imposing the displacement via the surrounding ground springs. The calculated ground displacement profile is specified at the support end of the ground springs, forcing the ground springs to displace the tunnel structure. It is to be noted that the specified ground displacement in this case is not the free-field ground displacement. Rather, it is the amplified ground displacement surrounding the excavated boundary of the tunnel sectional area (i.e., ground displacement assuming there is a void of the size of the tunnel excavation in the ground). The design ground deformations applied at the support end of the ground springs should be derived from free-field shear deformation profiles using the seismic site response analysis. The stiffness of the ground springs for the static response deformation spring-beam methods can be derived from the strain-compatible dynamic
dynamic soil properties relative to the geotechnical profile and cross-section.
• Apply the free-field deformations due to the propagation of shear wave based on seismic site response analyses. In general, the deformation analysis can be performed using pseudo-static method in which displacements are statically applied to the soil-structure system.
• Evaluate the loads and deformations not only in the liner segments themselves but also at the joints.
The use of elastic models and evaluation of stresses are acceptable if the structural response is within the elastic range. If the response is beyond the yield state into inelastic range, a non-linear inelastic model shall be used and the resulting strains shall be evaluated.
subgrade moduli, which represent the pressure per unit deflection. Thus, the dynamic subgrade modulus should be multiplied by the tributary area of the soil/rock represented by an individual spring to estimate the spring stiffness (i.e., force required for unit deflection).
There are three types of two-dimensional continuum models that have been used in the engineering practice:
(1) Pseudo-static Seismic Coefficient Deformation Method: The ground deformations are generated (induced) by seismic coefficients and distributed in the finite element/finite difference domain that is being analyzed. The seismic coefficients can be derived from a separate one-dimensional, free-field seismic site response analysis. The pseudo-static seismic coefficient deformation method is suitable for underground structures buried at shallow depths. The general procedure in using this method is outlined below: • Perform one-dimensional free-field
seismic site response analysis (e.g., using SHAKE, DMOD or DEEPSOIL). From the results of the analysis derive the maximum ground acceleration profile expressed as a function of depth from the ground surface.
• Develop the two-dimensional finite element/finite difference continuum model incorporating the entire excavation and soil-structure system, making sure the lateral extent of the domain (i.e., the horizontal distance to the side boundaries) is sufficiently far to avoid boundary effects. The geologic medium (e.g., soil) is modeled as continuum solid elements and the structure can be modeled either as continuum solid elements or frame elements. The side boundary conditions should be in such a manner that all horizontal displacements at the side boundaries are free to move and vertical displacements are prevented (i.e., fixed boundary condition in the vertical direction and free boundary condition in the horizontal direction). These side boundary conditions are considered adequate for a site with reasonably leveled ground surface subject to lateral shearing displacements due to horizontal excitations.
• The strain-compatible shear moduli of the soil strata computed from the one-dimensional site response analysis should be used in the two-dimensional continuum model.
(expressed as a function of depth from the ground surface) derived from the one-dimensional site response analysis is applied to the entire soil-structure system in the horizontal direction in a pseudo-static manner.
• The analysis is executed with the tunnel structure in place using the prescribed horizontal maximum acceleration profile and the strain-compatible shear moduli in the soil mass. It should be noted that this pseudo-static seismic coefficient approach is not a dynamic analysis and therefore does not involve displacement, velocity or acceleration histories. Instead, it imposes ground shearing displacements throughout the entire soil-structure system (i.e., the two-dimensional continuum model) by applying pseudo-static horizontal shearing stresses in the ground. The pseudo-static horizontal shearing stresses increase with depth and are computed by analysis as the product of the total soil overburden pressures (representing the soil mass) and the horizontal seismic coefficients. The seismic coefficients represent the peak horizontal acceleration profile derived from the one-dimensional free-field site response analysis. The lateral extent of the domain in the two-dimension analysis system should be sufficiently far to avoid boundary effects. In this manner, the displacement profiles at the two side boundaries are expected to be very similar to that derived from the one-dimensional free-field site response analysis. However, in the focus area near the tunnel construction, the displacement distribution will be different from that of the free field, reflecting the effects of soil-structure interaction (i.e., presence of the tunnel structure) as well as the effect that portion of the earth mass is removed for constructing the tunnel (i.e., a void in the ground).
(2) Pseudo-dynamic Time History Analysis: The procedure employed in pseudo-dynamic analysis is similar to that for the pseudo-static seismic coefficient deformation method, except that the derivation of the ground displacements and the manner in which the displacements are imposed to the two-dimension continuum system are different. The pseudo-dynamic analysis consists of stepping the soil-structure system statically through displacement time-history simulations of free-field displacements obtained by a seismic site
response analysis performed using vertically propagating shear waves (e.g., SHAKE DMOD, or DEEPSOIL analyses). Under the pseudo-dynamic loading, the transverse section of a tunnel structure will be subject to these induced ground distortions. If warranted, the inelastic behavior of the tunnel structure can also be accounted for and incorporated into the model.
(3) Dynamic Time History Analysis: Generally, the inertia of a tunnel is small compared to that of the surrounding geologic medium. Therefore, it is reasonable to perform the tunnel deformation analysis using pseudo-static or pseudo-dynamic analysis in which displacements or displacement time histories are statically applied to the soil-structure system. The dynamic time history analysis can be used to further refine the analysis when necessary, particularly when some portion(s) of the tunnel structure can respond dynamically under earthquake loading, i.e., in the case where the inertial effect of the tunnel structure is considered to be significant. In a dynamic time history analysis, the entire soil-structure system is subject to dynamic excitations using ground motion time histories as input at the base of the soil-structure system.
The ground motion time histories used for this purpose should be developed to match the target design response spectra and have characteristics that are representative of the seismic environment of the site and the site conditions, or they can be developed from 1-D seismic site response analysis (e.g. SHAKE, DMOD, DEEPSOIL) and taken at the depth corresponding to the depth of the soil-structure interaction numerical model. Special energy absorbing boundaries should be incorporated into the model to allow radiation of the seismic energy rather than trapping it.
10.8.3.2. Longitudinal Axial and Curvature
Deformation Effects
When evaluating the longitudinal response of the tunnel structures, either the simplified analytical method or relatively more complex numerical modeling approach, shall be used depending on the degree of the complexity of the soil structure system, the seismic hazard level and the importance of the structures. The soil-structure interaction effects shall be taken into account.
The simplified analytical method to evaluate free-field strains due to axial and curvature deformations outlined in AASHTO, 2010 may be used where appropriate.
If a very stiff tunnel is embedded in a soft soil deposit, significant soil-structure interaction effects exist, and the free-field deformation procedure may lead to an overly conservative design. In this case, a simplified beam-on-elastic-foundation procedure shall be used to account for the soil-structure interaction effects as outlined in AASHTO, 2010.
Numerical modeling approach for evaluation of longitudinal response shall be used for cases where tunnels encounter abrupt changes in structural stiffness, or run through highly variable subsurface conditions where the effect of spatially varying ground motions, due to local site effects, becomes significant. These conditions include, but are not limited to, the following:
• When a rectangular tunnel section is connected to a station end wall or a rigid, massive structure such as ventilation building.
• At the junctions of two tunnels or at the tunnel/cross-passage interface.
• When a tunnel transverses two distinct geological media with sharp contrast in stiffness, for example, a tunnel passing through a soil/rock interface.
• When a tunnel is locally restrained from movements by any means (i.e., hard spots).
Numerical analysis for the evaluation of
longitudinal response of a tunnel structure is typically performed by a three-dimensional pseudo-dynamic time history analysis in order to capture the two primary modes of deformation: axial compression/extension and curvature deformations. Since the inertia of a tunnel is small compared to that of the surrounding geologic medium, the analysis is generally performed by using the pseudo-dynamic approach in which free-field displacement time histories are statically applied to soil springs connected to the model of the tunnel (to account for the soil-structure interaction effect). The general procedure for the pseudo-dynamic time history analysis in the longitudinal direction involves the following steps:
• The free-field deformations of the ground at the tunnel elevation are first determined by performing dynamic site-response analyses. For the longitudinal analysis, the three-dimensional effects of ground motions as well as the local site effect including its spatially varying effect along the tunnel alignment should be considered. The effect of wave travelling/phase shift should also be included in the analysis.
• Based on results from the site response analyses, the free-field ground displacement time histories are developed along the tunnel axis. The free-field displacement time histories at each point along the tunnel axis can be defined at the mid-height and mid-width of the tunnel, can be further defined in terms of three time-history displacements representing ground motions in the longitudinal, transverse and vertical directions.
• A three-dimensional finite element/difference structural model is then developed along the tunnel axis. In this model, the tunnel is discretized spatially along the tunnel axis, while the surrounding soil/ground is represented by discrete springs. If inelastic structural behavior is expected, non-linear inelastic structural elements should be used to represent the tunnel structure in the model. Similar to the ground motions, the soil/ground springs are also developed in the longitudinal, transverse horizontal and transverse vertical directions. The properties of the springs shall be consistent with those used in the site response analysis described above. If non-linear, the behavior of the soil/ground should be reflected in the springs. As a minimum, the
ultimate frictional (drag) resistance (i.e., the maximum frictional force) between the tunnel and the surrounding soil/ground should be accounted for in deriving the longitudinal springs to allow slippage mechanism, should it occur.
• The computed design displacement time-histories described above are then applied, in a statically stepping manner, at the support ends of the soil/ground springs to represent the soil-tunnel interaction. The resulting sectional forces and displacements in the structural elements (as well as in the tunnel joints if applicable) are the seismic demands under the axial/curvature deformation effect.
10.8.4. Method of Analysis and Assessment – Ground Failure Effects
Stability of the ground surrounding the tunnel
structures, including natural and backfill soils located within a zone that may influence the performance of the structures during and after earthquakes, shall be considered in the seismic design and analysis. This assessment shall consider the potential for ground failure from fault rupture, tectonic uplift/subsidence, liquefaction, seismically-induced settlement, lateral spreading, slope instability (landslide), and increases in lateral earth pressures.
C10.8.4
In general, it is not feasible to design a tunnel
structure to withstand large ground displacements. The proper design measures in dealing with the unstable ground conditions may consist of: (1) Ground stabilization, (2) Removal and replacement of the problem soils and (3) Reroute or deep burial to bypass the problem zone.
With regard to the fault displacements, the best strategy is to avoid any potential crossing of active faults. If it is not possible, then the general design philosophy is to accept and accommodate the displacements by either employing an oversized excavation, perhaps backfilled with compressible or collapsible material, or using a ductile lining to minimize the instability potential of the lining.
If liquefiable soil deposits or unstable soil masses that are susceptible to landsliding are identified along the tunnel alignment, more detailed evaluations may be required to assess whether liquefaction or landsliding would be expected to occur during the design earthquake and to assess the impacts on the tunnel.
10.8.4.1. Liquefaction and Liquefaction-induced Ground
Deformations
The effects of liquefaction and liquefaction-induced ground deformations shall be evaluated at relevant locations along the tunnel alignment. These effects include: (1) uplift, buoyancy and floatation of the underground structures, (2) large lateral displacements and (3) post-liquefaction settlement and deformations (total and differential settlements).
C10.8.4.1
In general, the effects of liquefaction will depend on the amount of soil that liquefies and the location of the liquefied soil with respect to the tunnel. On sloping ground, lateral flow, spreading and slope instability can occur on relatively thin layers of liquefiable soils, whereas the effects of thin liquefied layers at sites with level ground surface may be insignificant.
If liquefaction is estimated to occur adjacent to a tunnel lining or wall, a potential consequence can be yielding of the lining or wall due to the increased lateral earth pressures in the liquefied zone. The pressure exerted by a liquefied soil may be as large as the total overburden pressure.
An initial screening study followed by more refined analyses and evaluation of the impact on the tunnel structure shall be conducted. A site-specific seismic site response analysis or the empirical procedures, as applicable, shall be used for liquefaction evaluation.
For soil layers deeper than 65 ft below ground surface, only site-specific seismic site response analyses shall be performed to estimate earthquake-induced shear stresses.
If earthquake-induced shear stresses are evaluated using the empirical procedures, the site-adjusted peak ground acceleration (PGA) should be used (see Article 10.5.2).
The estimated earthquake-induced shear stresses from either the empirical procedures or a seismic site response analysis, shall then be compared with the liquefaction resistance calculated based on SPT/CPT data.
The earthquake magnitude shall be determined as presented in Article 10.5.2.
For soil layers in which the initial liquefaction (triggering) is to occur based on the analyses discussed above, a liquefaction impact analysis based primarily on a deformation approach shall be performed.
If the liquefaction impact analyses indicate a potential for rendering the structures unsuitable for their purpose owing to movement, appropriate mitigation measures shall be incorporated.
The empirical liquefaction evaluation procedures discussed herein are generally applicable for liquefaction analysis up to the depth of about 65 to 80 ft below the ground surface or mud line.
For fine-grained soils up to the depth of 65 ft below
the ground surface or mud line, the evaluation criteria for liquefaction (or for significant strength reduction potential) may be in accordance with those discussed in the MCEER Technical Report (MCEER-98-005) and Idriss and Boulanger (2008). Appropriate laboratory shear strength testing data (e.g., tri-axial cyclic shear strength tests) can also be used to supplement this evaluation.
For granular soils up to the depth of 65 ft below the
ground surface or mud line, empirical procedures based on CPT (cone penetrometer test) or SPT (standard penetration test) data may be used for liquefaction potential evaluations (Idriss and Boulanger, 2008; NCEER, 1997).
Care should be taken when interpreting CPT cone
resistance of thin sand layers sandwiched between silt or clay layers with lower penetration resistance (Youd et al., 2001, and Idriss and Boulanger, 2008).
10.8.4.1.2. Post-liquefaction Settlements Post-liquefaction settlements occurring as the
excess pore water pressure generated during the earthquake gradually dissipates after the shaking ends, shall be estimated by multiplying the post-liquefaction volumetric strain by the thickness of the liquefiable layer based on the average cyclic shear stress induced by the earthquake and the penetration resistance of the soil. The post-liquefaction settlements calculated using the procedure described above are the total post-liquefaction settlements. The tunnels shall be designed to accommodate not only the total settlements but also the differential settlements. The minimum differential settlement to be used in the design shall be one-half of the total settlement.
If soil layers susceptible to liquefaction are identified, the potential of lateral spreading and impacts on the tunnel structure shall be evaluated.
C10.8.4.1.3
Analysis of impact of liquefaction-induced lateral spreading is necessary if the limits of potentially liquefiable zones encroach upon the limits of the tunnel and if the tunnel is embedded in soil deposits that are potentially liquefiable. If the potentially liquefiable layers are present only above the tunnel crown level at relatively more shallow depths, any strength loss or permanent ground deformations (e.g. flow failure) will not impact the integrity of the tunnel structure.
10.8.4.2. Seismic Slope Instability and Landslides
The potential for seismically induced landslides
and slope instability shall be evaluated along the tunnel alignment. If quasi-static slope stability analysis is used, the capacity-to-demand ratio shall not be less than 1.0 for the FEE and SEE. If the computed capacity-to demand-ratio from the quasi-slope stability analysis is less than 1.0, an impact study shall be performed based on earthquake-induced slope movements, using the Newmark Time-History Analysis (Newmark, 1965). The impact of the potential slope movements on the affected structures shall be assessed. If the impact assessments yield unacceptable performance of the structures, appropriate mitigation measures shall be incorporated.
C10.8.4.2 A capacity-to-demand ratio of 1.0 for the seismic
slope stability analysis is used as a screening step for evaluating the need for a more rigorous seismic displacement analysis. A capacity-to-demand ratio of 1.0 or greater from a seismic slope stability analysis typically indicates acceptable seismic displacements. However, further evaluation of seismic displacements and the resulting impact on the tunnel structure is warranted if a capacity-to-demand ratio less than 1.0 is obtained from seismic slope stability analysis.
When quasi-static seismic stability analysis is performed for permanent structures, the horizontal seismic coefficient should not be less than 50 percent of the peak ground acceleration, PGA (expressed as a percent of the gravitational acceleration constant), at the site location of interest. The PGA should account for the site effect (i.e., due to the presence of overburden soil) and should be developed as outlined in Article 10.5.2. A detailed discussion of seismic slope stability evaluation methods is given in Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments (NCHRP Report 611, 2008).
10.8.4.3. Active Fault Crossing Displacement Effects
The evaluation of the effects of active fault
crossing displacements on the integrity of the tunnel includes characterization of the free-field displacement (i.e., displacement in the absence of the tunnel) where fault crosses the tunnel and assessment of the effects of the characterized displacements on the tunnel. The analysis shall be performed using analytical procedures.
C10.8.4.3
A detailed discussion of evaluation of the effects of active fault crossing displacements is given in AASHTO Technical Manual For Design Of Road Tunnels – Civil Elements (AASHTO, 2010), including the analytical models for assessment of the seismic demands associated with fault rupture. For a discussion of estimating fault rupture displacement demands, refer to LRFD Seismic Analysis and Design of Transportation Geotechnical Features and Structural Foundations (FHWA, 2011).
10.9 SEISMIC DESIGN OF TEMPORARY STRUCTURES
Temporary structures, such as excavation support
systems and the underground structures under temporary conditions, shall be designed to resist seismic loads. The design earthquake for the temporary structures shall be established in consultation with the Owner considering the regional seismicity, duration of temporary conditions and the target performance levels for these structures. As a minimum requirement, the tunnel structures shall be designed to avoid collapse and any damage shall be repairable. In any case, the design seismic loads for the temporary structures shall not be less than 50 percent of the FEE ground motion intensity.
In locations where adjacent buildings and their foundations create an interaction configuration in conjunction with temporary ground support structures
C10.9 Depending on the regional seismicity, duration of the temporary conditions and established target performance criteria, design seismic loads equivalent to 50 to 100 percent of FEE levels have been used for the temporary structures by various infrastructure owners.
that would significantly influence the seismic response of the adjacent buildings themselves, the combined group of temporary ground support and building structural configurations shall also be analyzed as a single permanent structure.
10.10 INTERFACES WITH CROSS-PASSAGES
AND OTHER STRUCTURES
For interfaces located in soils and mixed-ground conditions, potential stress concentrations shall be evaluated and reinforcement enhancements shall be incorporated in design, as necessary. Alternatively, expansion joints may be considered to accommodate potential differential movements.
C10.11
A local three-dimensional continuum model may be used to evaluate lining stress concentration and interaction of the tunnel and the cross-passages.
10.11 SEISMIC JOINTS If the results of the longitudinal tunnel response
analysis indicate that seismic demands are above the limits for the tunnel structure, seismic joints or other alternatives shall be incorporated to reduce the seismic demands. If seismic joints are found to be necessary, the seismic displacement demands for the seismic joint shall be evaluated based on 3-D numerical models capable of modeling the longitudinal response of the tunnel together with seismic joints.
C10.10
Seismic joints are special flexible tunnel connections that are capable of dissipating several inches of large relative movements generated by the ground during seismic events (e.g., 12 in. of lateral movement, North Point Tunnel in San Francisco; FHWA 2004) by allowing the adjacent sections of the tunnel to move or rotate relative to each other and thereby reduce the seismic demands on the rest of the tunnel.
10.12 NON-STRUCTURAL COMPONENTS AND EQUIPMENT
Permanent nonstructural components (including
architectural components), their attachments and the attachments for permanent equipment (including mechanical/electrical systems) supported by a structure shall be designed in accordance with the International Building Code (IBC). The components will be considered to have the same Seismic Use Group Category as that of the structure that they occur or to which they are attached. The horizontal acceleration response spectra determined based on the seismic site classes shall be used for the seismic design of the permanent nonstructural components, equipment and their attachments.
The inertial forces exerted by these components shall also be considered while designing the tunnel lining.
3. AASHTO. Guide Specification for LRFD Seismic Bridge Design, 2012
4. ASCE. Guidelines for the Seismic Design of Oil and Gas Pipeline Systems, Committee on Gas and Liquid Fueld Lifelines of the Council on Lifeline Earthquake Engineering, American Society of Civil Engineers, 1984
5. CSABAC. Seismic Soil-Foundation-Structure Interaction, final report. Caltrans Seismic Advisory Board Ad Hoc Committee on Soil-Foundation-Structure Interaction (CSABAC), California Department of Transportation, Sacramento, CA, 1999.
6. Darendeli, M.B. Development of a New Family of Normalized Modulus Reduction and Material Damping Curves, Ph.D. Dissertation, University of Texas at Austin, 2001
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12. Menq, F.Y. Dynamic Properties of Sandy and Gravelly Soils, Ph.D. Dissertation, University of Texas at Austin, 2003
13. NCHRP Project 20-68A. Scan 09-05 Best Practices for Roadway Tunnel Design, Construction, Maintenance, Inspection, and Operations, 2011
14. NCHRP Report 611. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments, Transportation Research Board, 2008
15. O’Rourke, M.J., Liu, X. Response of Buried Pipelines due to Earthquake Effects, MCEER Monograph No. 3, 1999
16. Owen, G. N., and Scholl, R. E., Earthquake Engineering of Large Underground Structures, prepared for the Feral Highway Administration, FHWA/RD-80/195, 1981.
17. Wang, J.-W. Seismic Design of Tunnels – A Simple State-of-the-Art Design Approach, Parsons Brinckerhoff Monograph No.7, 1993
18. Youd, T.L. Screening Guide for Rapid Assessment of Liquefaction Hazard at Highway Bridge Site¸ Technical Report, MCEER-98-005, 1998.
19. Youd T.L., Idriss, I.M., Andrus, R.D., Arango, I., Castro, G., Christian, J.T., Dobry, R., Finn, W., D.L. Harder, L.F., Jr., Hynes, M.E., Ishihara, K., Koester, J.P., Liao, S.S.C., Marcuson, W. F., III, Martin, G.R. Mitchell, J.K., Moriwaki, Y., Power, M.S., Robertson, P.K., Seed, R.B., and Stokoe, K.H., II, “Liquefaction resistance of soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils,” Journal of Geotechnical Geoenvironmental Engineering, ASCE, 127, No. 10, pp. 817-833, 2001
A.2 ABBREVIATIONS AHJ: Authority having jurisdiction APE: Area of potential effects EIS: Environmental Impact Statement FEIS: Final Environmental Impact Statement EPA: Environmental Protection Agency FHWA: Federal Highway Administration FONSI: Finding of no significant impact MOA: Memorandums of agreement NEPA: National Environmental Policy Act NFPA: National Fire Protection Association NIOSH: National Institute for Occupational Safety and Health PA: Programmatic agreements PIARC: World road association PST: Portable sediment tanks PST: Portable Sediment Tank ROD: Record of decision SHPO: State Historic Preservation Office TBM: Tunnel boring machine USACE: United States Army Corps of Engineers
Tunnel construction has the most dramatic impact on the natural environment where the tunnel interfaces with the surface. These interfaces often involve large open excavations. The impact of the large open excavations required for tunnel construction must be considered during the environmental planning phase of the project.
During dredging for immersed tunnels, consideration shall be given to investigating certain excavation methods for the possibility of limiting suspended solids, such as the use of sealed buckets. During the National Environmental Policy Act (NEPA) phase, existing fauna and flora and other ecological issues shall be investigated to determine whether environmentally and ecologically adverse consequences are likely for a specific project, as well as an assessment on fish migration and spawning periods, and mitigation measures shall be identified.
Depending on the tunnel alignment, road tunnels can in many ways lessen environmental impacts as compared to surface roadways and bridges. Tunneling can help to reduce traffic congestion, improve regional air quality, reduce noise and improve aesthetics. Tunnels also provide opportunities for improved economic potential from land development opportunities at the surface. On the contrary, immersed tunnels have impacts related to underwater bed levels and dredging activities. Dredging generates bottom disturbance and creates turbidity in the water.
As the project advances into final design, ensure compliance with the approved NEPA document, continue to evaluate and assess environmental impacts for consistency with the NEPA document, continue to avoid and minimize impacts as much as possible during design, implement environmental commitments and mitigations during the design phase, as applicable, and obtain all environmental permits or approvals required as part of a specific project. Any proposed substantive changes to the Preferred Alternative shall be evaluated in accordance with 23 CFR Sections 771.129 and 771.130, and shall be approved by the lead federal agency before the agency may proceed with the change.
Environmental effects that are typically evaluated during NEPA/planning and into the design phase include ecological (such as the effects on natural resources and on the components, structures and functioning of affected ecosystems), water quality, aesthetic, historic, cultural, communities, businesses, economic, noise, dust or health, whether direct, indirect or cumulative. Effects may also include those resulting from actions which may have both beneficial and detrimental effects, even if on balance the agency believes that the effect will be beneficial.
Numerous federal, state and local permits and approvals are typically required and obtained during the design and construction phases for various aspects of projects.
A.3.2 Environmental Permits
CA.3.2
Coordination with relevant federal, state and local regulatory and resource agencies shall continue throughout final design and construction. All permits and approvals shall continue to be identified throughout the design and construction phases of projects, and applied for and obtained at appropriate times to adhere to the project design and construction schedules.
Permits and approvals are typically obtained as the project design and limits of disturbance are further refined. This includes implementing avoidance and minimization design measures and finalizing construction staging and access areas. Permits include approvals for all media including water, air, land, cultural resources, threatened and endangered species, waste management/hazardous materials handling, transportation and disposal, among others, as applicable to a specific project.
In addition to federal permits and approvals, state and local agencies often require permits and approvals for certain activities.
As such, the design team shall be familiar with all previously made commitments or mitigations so that appropriate measures are included as part of the project’s design plans and construction contract. Previously obtained permits shall also be reviewed for
In an effort to avoid and/or minimize potential effects during construction of a project, environmental commitments and mitigation measures are typically identified during the NEPA phase that the construction contractors will be required to follow.
special permit conditions or mitigation requirements that are to be considered and implemented during design.
A.3.3 Avoidance/Minimization of Environmental Resources
CA.3.3
During the design phase for any tunnel project, avoid and minimize environmental impacts as the design and limits of disturbance are refined. Different design strategies shall be considered and implemented during design to achieve effective avoidance or minimization of resource impacts. Typical tunnel design strategies that shall be considered for avoidance or minimization include horizontal/vertical alignment shifts, retaining walls, steep slopes and other design techniques, such as the use of sealed clam shell dredge buckets for reducing turbidity. Additionally, different construction techniques shall be considered during the design phase as a means to avoid and minimize the disturbance to environmental resources. Access to the project site and construction staging areas shall also be considered.
Construction-related impacts can often be minimized by appropriate design stage decisions and by implementation of construction techniques and construction access.
Best management practices shall also be considered to minimize environmental impacts. During design, appropriate measures for controlling sediment and runoff during construction shall be incorporated into design plans. A variety of measures shall be considered during final design to avoid and minimize storm water-related water quality problems caused by earth disturbance. Appropriate controls shall be selected for addressing unique characteristics and problems posed by specific sites.
Soil and erosion control best management practices shall be in accordance with federal/state/local standards and specifications for soil erosion and sediment control, as applicable to the project. The potential for water quality effects shall be minimized through adherence to sediment and erosion control plans, which include best management practices such as silt fence, super silt fence, sediment basins, sediment traps, earth dikes, diversion fence, stone and gabion outlet structures, outfall protection and other methods to capture potential sediments from exposed soils. In addition to treatment of sediment laden runoff resulting from surface construction activities, Filter Bags, Portable Sediment Tanks (PSTs) or other acceptable filtration devices shall be considered during the design phase to filter discharge pumped from subsurface activities such as tunnel boring and cut-and-cover construction.
Consideration during design shall be given to necessary dewatering operations. Necessary dewatering permits shall also be obtained at the appropriate time, either during design or construction, and dewatering shall be performed according to permit specifications/requirements. Pretreatment techniques shall be addressed, such as the need for oil/water separation, grit chambers and/or chemical treatments that may be required prior to discharge. Filtration methods, sedimentation basins, controlled pumping rates and
State and local regulations and permitting requirements often dictate the method of disposal.
protection of catch basins with filter fabric and/or hay bales shall be addressed in the design to limit discharge of suspended solids into storm drains or surface waters. Contaminated material may need to be placed in confined disposal facilities.
A.3.4 Floodplains
Flood elevations and required freeboard shall be taken as established by the owner with regard to the anticipated service life of the tunnel accounting for weather events, climate change and sea-level rise.
Construction occurring within the FEMA designated 100-year floodplain shall comply with FEMA approved local floodplain construction requirements, and be in accordance with the requirements of Executive Order 11988. If, after compliance with the requirements of Executive Order 11988 and U.S. DOT Order 5650.2, new construction of structures or facilities are to be located in a floodplain, accepted flood proofing and other flood protection measures shall be applied.
A.3.5 Wetlands/Waterways
CA.3.5
During the design phase, wetlands and waterways shall be delineated and flagged in accordance with the U.S. Army Corps of Engineers (USACE) Wetland Delineation Manual or appropriate regional supplement. If these resources were previously delineated during an earlier phase (e.g., NEPA or planning phase), verify with the regulatory agencies that the delineation remains valid and that all resources remain jurisdictional. The extent of the field delineation shall also be evaluated to confirm that the wetlands/waterways delineation study area encompassed the entire limit of disturbance established during the design phase. If not, a supplemental field delineation may be required.
Waters of the US, including wetlands, are regulated under Section 401 and 404 of the Clean Water Act. Executive Order 11990 of the Federal Register (FR) (42 FR 26961E.O. 11990, May 1977), entitled Protection of Wetlands, was enacted to avoid to the extent possible the long- and short-term adverse impacts associated with the destruction or modification of wetlands, to avoid direct or indirect support of new construction in wetlands wherever there is a practicable alternative and to ensure that proposed construction incorporates all possible measures to limit harm to the wetlands. Many states also regulate these resources.
Impacts to wetlands/waterways will vary depending on a specific project and may include impacts at the surface, near portals or portal approaches, or impacts may occur from dredging operations associated with the construction of immersed tunnels.
Installation of an immersed tunnel requires agency coordination and procurement of a Section 404 permit and other related disposal site permits.
Immersed tunnels are often selected over bridges for water crossings for various reasons such as shorter portal approaches and shorter overall length as compared to bridges. Tunnels can also be used to avoid the potential hazards to navigation that can be created by bridges. Tunnels can also often be constructed in soils that would otherwise create challenges to a bridge structure. Despite some of these advantages, trench excavation in any waterway is an environmentally sensitive issue. Immersed tunnels often have negative impacts related to underwater bed levels and dredging activities. Dredging generates bottom disturbance and creates turbidity in the water, which can affect aquatic ecology, fish/aquatic species habitat, fish migration and spawning periods.
Once the environmental conditions have been set by
the planning and permitting process, extreme care shall
be taken to meet these permit conditions, including incorporation of design elements and clear specifications to the Contractor.
During design, scheduling of construction activities, use of environmentally friendly construction techniques, equipment and innovative methods of dealing with contaminants shall be considered.
Trench excavation is often complicated by contaminated materials, tides, storms and construction restrictions in waterways due to environmental concerns.
Wetland/waterway avoidance or minimization design strategies committed to during earlier phases shall be adhered to, and any additional design techniques for further avoidance/minimization shall be considered and implemented where practicable and to the greatest extent practicable. For example, for an immersed tunnel, dredging methods and equipment shall be designed to limit the dispersal of fine materials in the water. Turbidity or silt curtains or other measures shall be used where appropriate. Methods, materials and mitigation measures shall be specified to avoid or reduce the impacts of excavation, filling and other operations on the aquatic environment.
Where the discharge of fill material into waters of the U.S., including wetlands, are unavoidable, Section 404 of the Clean Water Act, and Section 10 of the Rivers and Harbors Act of 1899 permits shall be obtained from the U.S. Army Corps of Engineers (USACE). Similar state and local permits may also be required. Measures shall also be taken to mitigate these impacts in accordance with Section 404 and Section 10.
Mitigation measures employed to compensate for unavoidable project effects to waters of the U.S., including wetlands, shall follow federal and state regulations and guidelines, as well as other recommendations from federal and state resource agencies.
A.3.6 Navigable Waterways
CA.3.6
Navigable waters within the project study area shall be identified, and a Section 10 permit shall be obtained, if applicable.
The USACE regulates structures that are located in, under or over navigable waters of the U.S. under Section 10 of the Rivers and Harbors Act of 1899.
Navigable waters of the U.S. are those “waters that are subject to the ebb and flow of the tide and/or are presently used, or have been used in the past, or may be susceptible for use to transport interstate or foreign commerce” (33 C.F.R Part 329.4). Coordination with the USACE and U.S. Coast Guard is recommended to determine the navigability of certain waterways.
Projects requiring a Section 10 permit may range from a simple tunnel bore crossing of a small stream considered to be navigable to an immersed tunnel project within a large waterway or harbor.
All impacts on navigation in all navigable waterways shall be considered and addressed, and often require extensive permitting.
During design, groundwater levels shall be evaluated to determine the need and extent for dewatering operations. When groundwater levels are higher than the base level of the tunnel, excavations require dewatering. Dewatering systems vary depending on soil permeability. Precautions shall be taken on dewatering to avoid impacts such as lowering the water table outside the excavation, which could cause settlement of adjacent structures, loss of soil, impact on vegetation, drying of existing water supply wells and potential movement of contaminated plumes, if present. Consider the use of impermeable excavation support walls that extend down to a firm, reasonably impermeable stratum, which will reduce or cut-off water flow. Other design techniques may include impervious retaining walls, such as steel interlocking sheeting, secant pile walls or concrete slurry walls, for use in deeper less pervious layers to reduce groundwater inflow during construction and limit draw-down of the existing groundwater table. Other dewatering techniques to be considered depending on a project include sumps and pumps within the excavation to draw down the water.
Necessary dewatering permits shall be obtained, and dewatering must be performed according to permit specifications/requirements.
A.3.8 Cultural Resources
CA.3.8
Historic and archaeological resources shall be assessed in accordance with the regulations (36 CFR Part 800) implementing Section 106 of the National Historic Preservation Act of 1966, and the area of potential effects (APE) for the project shall be determined by the lead federal agency in consultation with the State Historic Preservation Office (SHPO). Be aware of any Programmatic Agreements (PA) or Memorandums of Agreement (MOA) executed during planning, and adhere to the commitments at the appropriate times during design and construction.
Compliance with Section 106 of the National Historic Preservation Act of 1966 is typically initiated during the NEPA process; however, impacts to cultural resources must continue to be evaluated during the design phase to ensure continued compliance. PA’s or MOA’s are also typically executed during the NEPA planning phase; however, they are typically implemented during the design and construction phases. Continued coordination with the SHPO and continued assessment is typically required as the project advances into the design and construction phases.
During the design phase, cultural resources shall continue to be evaluated to ensure that the design complies with the original Section 106 assessment of affects. Additionally, if a Section 106 PA or MOA was executed for the project, the Engineers shall review the PA or MOA for stipulations related to cultural resource processes for the completion of the project and for mitigation requirements in accordance with Section 106 of the National Historic Preservation Act of 1966 (as amended), as applicable.
During the design phase, it is typical that a Phase IB archeology identification survey of limits of disturbance of the subsurface alignment be conducted, along with any additional Phase II archeological evaluation studies of archeological sites identified, and Phase III archeological data recovery efforts for National Register-eligible sites that cannot be avoided. The Section 106 PA or MOA typically outlines these work efforts.
The design team and lead agency shall continue consultation during design with the SHPO, with input from other consulting parties, for identifying additional potential historic properties, identifying and minimizing and/or mitigating unanticipated adverse effects, and providing project plans and soliciting comments on design-related issues of the built project components.
Adhere to commitments related to continued coordination with Section 106 Consulting Parties.
Typically, the PA or MOA will stipulate requirements for continued coordination efforts during design with
references to type of coordination necessary and at what design milestones.
A.3.9 Environmental Justice
CA.3.9
Federal projects shall comply with Executive Order 12898, “Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations,” as implemented through the U.S. Department of Transportation Order (US DOT Order) 5610.2(a) to Address Environmental Justice in Minority Populations and Low-Income Populations. Disproportionate impacts to Environmental Justice populations must continue to be evaluated during the design phase to ensure continued compliance, which may include continued public outreach to Environmental Justice communities. Refer to FHWA’s Guidance on Environmental Justice and NEPA, December 16, 2011.
Compliance with Executive Order 12898 is typically completed during the NEPA process.
A.3.10 Section 4(f) Resources
CA.3.10
The proposed use of land from any significant publicly-owned public park, recreation area, wildlife and/or waterfowl refuge or any significant historic site shall not be approved as part of a federally-funded or approved transportation project unless:
• It is determined that there is no feasible and prudent avoidance alternative to the use of land from the property, and the action includes all possible planning to minimize harm to the property resulting from such use (23 CFR 774.3(a)), or
• It is determined that the use of the Section 4(f) properties, including any measures to minimize harm (such as avoidance, minimization, mitigation or enhancements measures) committed to by the applicant, would have a de minimis impact on the property (23 CFR 774.3(b)).
The provisions of this article are mandated by Section 4(f) of the U.S. Department of Transportation Act of 1966, 49 USC § 303(c). Refer to FHWA’s Section 4(f) Policy Paper, July 20, 2012.
Impacts to Section 4(f) resources shall continue to be evaluated during the design phase to ensure the design and impacts to Section 4(f) resources are consistent with the final Section 4(f) Evaluation. If design changes are proposed that affect Section 4(f) resources, additional impact assessment, evaluation and coordination with agencies having jurisdiction over the Section 4(f) resources will likely be required.
Compliance with Section 4(f) is typically completed during the NEPA process.
A.3.11 Environmental Compliance
CA.3.11
All environmental commitments made throughout the NEPA and permitting processes shall be met at the appropriate time, either in design, construction or post construction, depending on the commitment, and agency coordination. Review of project elements shall continue through all phases of the project, as appropriate. During
Typically, Environmental Management/Compliance Plans are prepared to ensure that environmental commitments are adhered to and identify the coordination necessary for limiting potential impacts to the environment, protected resources and communities within and adjacent to the project area.
APPENDIX A: PLANNING AND ROUTE CONSIDERATIONS A-10
design, the design team shall coordinate with the NEPA/planning team to obtain all environmental commitments agreed to prior to the design phase. Engineers shall request a copy of the Environmental Management/Compliance Plan, or similar document, if one was prepared for the project. Whether outlined/documented in an Environmental Management/Compliance Plan or not, the design team shall obtain relevant environmental information related to the design, such as:
• Environmental requirements of the project that require compliance to federal, state and local regulatory permit conditions and the procedures defined to meet them.
• Environmental commitments and mitigation measures stipulated within the final NEPA decision document, Section 106 PA or MOA, if applicable, regulatory agency permits/approvals and any other commitment document, to ensure that these requirements are identified in the Contract documents
• Responsibilities and actions required to maintain compliance with environmental requirements during design and construction.
• Necessary procedures for communication, documentation and review of environmental compliance activities for the construction contract.
• Protected resources within the project area and the types of mitigation measures needed to protect them.
• Ensure that the contract documents include requirements for Contractors to provide all means and methods to avoid or minimize impacts to the environment and general public.
A.3.12 Public and Agency Involvement
CA.3.12
The public shall be involved throughout the NEPA process.
Public and agency involvement is integral to the overall project efforts throughout the planning and design phases of projects. Public involvement early during the planning and NEPA phase allows for consideration of public comment related to the range of alternatives, the tunnel alignment/location and other concerns such as environmental impacts.
Sec. 1506.6 of NEPA requires the lead federal agency to involve the public throughout the NEPA process. The lead agency for federal highway projects is typically FHWA. Specifically, the FHWA policy 23 CFR Section 771.105(c) states, “Public involvement and a systematic interdisciplinary approach are essential parts of the development process for proposed actions.” The lead agency has the responsibility to ensure compliance with NEPA and prepare the environmental document. The lead agency is also responsible (during NEPA) for identifying and inviting cooperating and participating agencies. Cooperating agencies as defined in 23 CFR Section 771.109(3), are federal agencies that are
APPENDIX A: PLANNING AND ROUTE CONSIDERATIONS A-11
requested to participate during the NEPA phase because they have jurisdiction by law or special expertise. Cooperating agencies may also include state or local agencies and tribal governments. There may be multiple cooperating agencies depending on the issues associated with the project. Participating agencies may include federal, state, Indian tribal and local government entities with an interest in the project.
Effective public and agency involvement typically assists in a meaningful and productive process that results in minimal project delays. It is important to learn the viewpoints and opinions of stakeholders in transportation projects as it is the stakeholders that tend to be the users or those affected by the construction or traffic related to projects.
Although NEPA requires public involvement throughout the NEPA process, the Engineer shall satisfy any public and agency involvement requirements or commitments for a specific project that extend into the design phase. Engineers shall request a copy of the Public/Agency Involvement Plan, if one exists, to understand and support in its implementation for planned activities during the design phase. If not already outlined prior to the design phase, the lead federal or state agency shall consider development of an appropriate Public/Agency Involvement Plan during the design phase to obtain needed input and most effectively respond to problem situations or agency/public concerns during design.
Refer to the NEPA decision document for information on cooperating and participating agencies, and for any specific public and agency involvement commitments that require implementation during later phases of the project, including the design phase.
Public and agency involvement during design will vary depending on the project, level of controversy, permits required (some permits require the opportunity for a public hearing), lead federal or state agency and associated public/agency involvement policies/guidelines, and commitments related to continued outreach efforts, among other considerations. For some projects, a Public/Agency Involvement Plan is prepared that outlines the plan to engage the public and agencies throughout the life of the project, from planning, through design and construction.
A.3.13 Economics
CA.3.13
During the design phase, adhere to the final NEPA decision document. Any proposed substantive changes to the Preferred Alternative shall be evaluated in accordance with 23 CFR Sections 771.129 and 771.130 and shall be approved by the lead federal agency before the agency may proceed with the change. If a reevaluation or supplemental document is required, the economics shall also be reassessed. As the project moves into final design, construction duration and budget projections, as well as right-of-way requirements, shall continue to be refined and evaluated. Any newly acquired private property shall be transferred to transportation use and become part of the public right-of-way, thus removing the property from the local tax base.
During the NEPA phase, the characteristics of the existing economy within the project study corridor and the likely effects associated with the project’s Preferred Alternative on the economy is considered and documented. In addition to general economic impacts, the NEPA document typically also discusses the fiscal impact of the project on local property taxes. An analysis of tax revenue changes as a result of the proposed project is typically conducted and documented in the NEPA document. For example, the analysis would be based on the estimated right-of-way needs associated with construction of the Preferred Alternative.
During the design phase, tunnel construction materials shall be selected with due consideration of projected future costs. During design, the effects of trends in labor and material cost fluctuations shall be projected to the future tunnel construction year. The cost of future expenditures during the projected service life of the tunnel shall be considered. Regional factors, such as availability of skilled labor, material, fabrication, location, shipping and erection constraints, shall also be considered.
APPENDIX A: PLANNING AND ROUTE CONSIDERATIONS A-12
During the design phase, the option for accepting alternative contract plans may be considered and bid competitively. If this option is selected, the alternative plan designs shall be equally safe, serviceable and aesthetically pleasing.
Cost comparisons of structural alternatives shall be based on long-range and life-cycle considerations, including inspection, maintenance, repair and/or replacement. Maintenance costs after initial construction shall be considered when evaluating costs.
A.3.14 Aesthetics
CA.3.14
During the design phase, adhere to the final NEPA decision document as it relates to visual and aesthetics.
During the NEPA phase, the proposed aesthetics at portals and ventilation shafts are typically very conceptual. NEPA requires the assessment and consideration of aesthetics, but it is typically more related to the tunnel portal and shaft locations and features as opposed to specific aesthetic elements and treatments, which are typically not known during the planning phase. There are typically commitments or requirements to engage the public or other stakeholders during the design phase to present alternative aesthetic options and treatments and obtain public comments.
There are multiple engineering and environmental factors that are considered and assessed in selecting the locations of portals and ventilation shafts. Among the considerations, portals and ventilation shafts shall be located such that they satisfy environmental and air quality requirements, as well as the geometrical configuration of the tunnel.
For road tunnel projects, the most visible permanent elements of tunnels are the portals and ventilation structures.
As design progresses beyond the NEPA phase, review commitments related to aesthetics identified in the final NEPA decision document or other commitment documents or agreements.
Tunnel portal and ventilation shaft locations are identified during NEPA, but the details of the aesthetic elements are not typically known at this early phase. There are often commitments made during the early planning phases to seek community and stakeholder input regarding the aesthetic elements and treatments of tunnel portals to address visual impacts.
The Engineers shall consider an aesthetically pleasing portal that fits appropriately within its surroundings and shall implement context sensitive design techniques, particularly in historic-eligible areas or within community settings. The project may have prior commitments to seek input from certain agencies, stakeholders, communities or Section 106 Consulting Parties related to the aesthetics design of portals or ventilation shafts.
Not only is the geographic location of the portals and ventilation structures important to consider, but the materials and vertical and horizontal positioning is also important.
Other design elements or mitigation measures to consider, if required, may include the potential use of structured screening, architectural and landscape treatments to reduce effects to neighboring properties, pedestrian lighting, decorative paving materials, incorporation of public art, among other design considerations.
A.4 GEOLOGIC FEATURES AND SUBSURFACE CONDITIONS
APPENDIX A: PLANNING AND ROUTE CONSIDERATIONS A-13
An appropriate subsurface investigation including soil borings, rock cores, laboratory testing of ground samples, geophysical studies, geological mapping, survey and site reconnaissance, groundwater monitoring and environmental studies shall be conducted in accordance with Section 5 of this specification.
A phased subsurface investigation shall be conducted to obtain the information necessary and appropriate for each design phase of the project that also reflects the selected procurement delivery method.
Subsurface investigations performed during the planning phase of a project should be planned to gather information that generally identifies the ground conditions along proposed alignments. This can assist in determining the appropriate tunneling methodology and preferred alignment locations. Subsurface information gathered early in the process can be helpful to avoid large changes after preferred alignments are identified.
A.4.2 Ground Conditions
CA.4.2
Ground conditions for tunneling shall be defined as soft ground or rock or mixed-face.
The tunneling methodology selected for a project shall be suited to anticipated range and type of ground and groundwater conditions along the length of the tunnel.
The ground and groundwater conditions dictate the appropriate tunneling methodology and may have an impact on the feasibility of the tunnel. Ground conditions typically transition from one type to the next, for example, soft ground can overlay rock. There may also be transition zones where soft ground transitions to rock without a defined boundary. It is ideal to be able to tunnel with the full face in a single ground condition for the entire length of the tunnel, but this is rarely the case.
CA.5 EXISTING INFRASTRUCTURE
CA.5
During the NEPA process, impacts to any existing infrastructure shall be considered and assessed based on the preliminary tunnel design. The choice for location and profile of the tunnel shall have already been supported by analyses of alternatives, which is typically completed during the planning and NEPA phase of tunnel projects.
During design, review the NEPA document related to anticipated impacts to existing infrastructure, as well as any design-related commitments or other mitigation measures. As the design phase advances beyond the NEPA phase, attention shall continue to be directed toward providing for favorable tunnel design that minimizes adverse impacts to other existing infrastructure while maintaining safety. Existing infrastructure to consider avoiding and minimizing impacts to during design, construction and post construction may include, but are not limited to, roadway facilities and traffic, public transit systems (e.g., buses, light rail, heavy rail), pedestrian and bicycle facilities (e.g., sidewalks, bike lanes, trails), public parking, water and power lines, railroads, bridges, other tunnels and portals, communication systems, residential, commercial and public institutional buildings (e.g., schools, post offices) and utilities, among others.
Inevitably, impacts to existing infrastructure will likely result from any tunnel project’s Preferred Alternative.
Any design related commitments shall be included in the design plans and specifications. For example, a specific sidewalk design width may have been a commitment made during the NEPA phase that would require implementation of that specific design feature during the design phase. Appropriate specifications shall
APPENDIX A: PLANNING AND ROUTE CONSIDERATIONS A-14
also be included in the contract documents for any construction phase commitments that shall be completed during construction. For example, there are often commitments or mitigation measures to ensure that neighborhood streets and sidewalks be returned to existing conditions upon completion of construction. The design team shall include these types of construction specifications in the contract documents during the design phase, even though implementation of the commitment is not fulfilled until the construction phase.
Be aware of and obtain any necessary permits related to infrastructure impacts. Even if a permit is not required, there are typically requirements or commitments to coordinate infrastructure impacts during the design and construction phases. Coordination and permit procurements shall be completed at the appropriate times during design, construction and post construction, including operation and maintenance activities.
Some permit requirements extend beyond the construction phase of a project. An example is the planting of vegetation that must be monitored until the vegetation has established itself.
A.6 CONSTRUCTABILITY CONSIDERATIONS
CA.6
The design phase of tunnel projects shall identify and design based on anticipated construction methods, activities and sequencing that are reasonably expected to be employed and undertaken during the construction phase. The NEPA document typically includes commitments or mitigation measures to be considered or implemented during the design and construction phases to mitigate or minimize negative environmental effects. During design, these commitments or mitigation measures previously agreed to, shall be reviewed and implemented, as appropriate.
The vertical profile of the tunnel shall be set with respect to the geologic conditions and existing underground infrastructure as well as surface infrastructure above the tunnel. Ground subsidence and vibrations during construction shall be monitored and controlled. Staging areas shall be identified and made secure. Construction noise shall be managed and muck removed and disposed of. During design, the design team shall consider construction means and methods for avoiding and minimizing impacts from construction activities. For example, consideration of using a barge in certain areas for the storage and transport of material rather than creating excessive traffic along haul routes. Proper planning and restrictions on activities shall be performed in order to meet local ordinances and accommodate existing adjacent land use.
Though the location and type (i.e., bore, immersed, cut-and-cover) of tunnels are typically identified in the planning and NEPA phase, construction details are not finalized during these early phases, and therefore require much greater attention to detail during the design phase. For the majority of their length and depending on the construction method, tunnels have no impact on the surface. Cut-and-cover construction would be very disruptive at the surface during construction, while tunnel boring is only disruptive at the surface portal locations.
During design, construction specifications shall be developed such that construction contractors comply with applicable environmental regulations and obtain necessary permits for the duration of construction. Construction of a project shall follow applicable federal, state and local laws for building and safety, as well as local noise ordinances, as appropriate.
The areas that are typically most affected by construction activities generally comprise the areas immediately bordering the construction activity. However, in some cases, effects from construction activities extend beyond the immediate area surrounding construction sites. For example, the traffic effects of delivering or transporting material off site includes a bigger study area than the study area for the noise effects
APPENDIX A: PLANNING AND ROUTE CONSIDERATIONS A-15
of constructing a tunnel portal. A fabrication location site shall be identified for
immersed tunnels, as required. Dredging methods and equipment shall be designed to limit the dispersal of fine materials in the water. Turbidity or silt curtains or other measures shall be used where appropriate. Methods, materials and mitigation measures shall be used to avoid or reduce the impacts of excavation, filling and other operations on the aquatic environment. The Engineer shall identify an acceptable and approved containment disposal site for the muck removed from the excavated trench for construction of an immersed tunnel.
Construction of an immersed tunnel consists of excavating an open trench in the bed of a water body. Tunnel elements are fabricated off site, transported to the tunnel location, lowered into the excavated trench, connected together and backfilled.
During design, the Construction Environmental
Protection Plan shall be reviewed and adhered to as applicable to the design phase. Specific environmental requirements and controls shall be tailored to the construction contract(s) and included in contractor specifications.
Prior to construction, and often during the design phase, Construction Environmental Protection Plans or a similar type of plan, are prepared. The purpose of these plans is to identify means, methods and coordination necessary to limit potential impacts to the environment, protected resources and communities within and adjacent to a project. These reports typically identify:
• Project specific environmental requirements required to comply with federal, state and local regulatory permit conditions and the procedures defined to meet them.
• Define environmental commitments and mitigation measures stipulated within the NEPA document to ensure that these requirements are identified in the Contract documents.
• Define responsibilities and actions required to maintain compliance with environmental requirements during design and construction, and to effectively respond to problem situations or agency/public concerns.
• Establish necessary procedures for communication, documentation and review of environmental compliance activities for the construction contract.
• Describe protected resources within the project area and the types of mitigation measures needed to protect them.
• Ensure that Contractors submit all documents required in the Contractor documents as they pertain to the Contractor’s work, and ensure that Contractors provide all means and methods to avoid or minimize impacts to the environment and general public in compliance with the construction contract documents.
A.6.1 Construction Methodology
CA.6.1
A number of construction methods may be used to build tunnels depending on geological and environmental conditions, cost, schedule, alignment, tunnel size and length and other factors. Specific construction methods and activities may be refined during design and as the construction delivery methods are identified (Design-Build or Design-Bid-Build, or
Additional information regarding tunnel construction methodology can be found in AASHTO’s Technical Manual for Design and Construction of Road Tunnels – Civil Elements.
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other delivery methods). Typical tunneling construction methods that shall be
considered and evaluated during design and depending on specific project circumstances (i.e., under land, under water) include cut-and-cover excavation, bored or mined tunnels or immersed tunnels. Depending on project site conditions, construction methods shall be evaluated and determined, and may include drilling and blasting, tunnel bore machines (TBM), sequential excavation method, jacked tunnel methods, among others. Different methods may work for different construction activities – whether it is the actual tunnel portion or an associated tunnel portal.
The selected construction method will have an effect on the types of environmental impacts expected during construction.
The types of equipment typically used for construction include various earth-moving machines (excavators, graders, bulldozers, loaders, etc.), cranes, pile drivers, augers, drills, compaction rollers and tampers, concrete trucks, pumping equipment, generators/compressors, various types of trucks (flat bed, dumps, trailers, etc.) and mechanical excavating equipment such as TBM’s and roadheaders.
Actual construction methods and materials will vary depending in part on how the construction contractors choose to implement their work to be most cost effective, within the requirements set forth in bid, contract and construction documents, as well as to comply with mitigation requirements.
A.6.2 Noise and Vibration
CA.6.2
Tunnel construction and operation shall comply with applicable regulations, contract specification requirements and noise and vibration limits, as applicable. Noise and vibration concerns for most highway tunnel projects include impacts from construction operations and from future traffic at approaches to the completed tunnel.
During the planning, NEPA and preliminary engineering phase of major tunnel projects, a noise assessment is conducted in accordance with NEPA, 23 CFR 772, Procedures for Abatement of Highway Traffic Noise and Construction Noise, as amended July 13, 2010, and other relevant federal and state guidance.
During the design phase, key noise and vibration sources and receptors shall be considered and assessed. A more detailed noise and vibration impact assessment may be warranted during the design phase that considers appropriate control measures that demonstrate compliance with state or local noise criteria or other project-specific criteria. The design team shall work with the Construction Management Team, if one exists, to develop noise and vibration control strategies for integration into the design specifications. Vibration analysis should be conducted on a case-by-case basis as deemed appropriate and included in the noise analysis or in a standalone vibration analysis report.
Road tunnel vibration concerns are typically related to construction operations, such as drilling and blasting, and not long-term operation effects. There are no federal requirements directed specifically to highway traffic induced vibration. All studies that highway agencies have done to assess the impact of traffic-induced vibrations have shown that both measured and predicted vibration levels are less than any known criteria for structural damage to buildings (FHWA, Highway Traffic Noise: Analysis and Abatement Guidance, Appendix G, December 2011).
To minimize potential noise and vibration impacts due to construction activities, the design team shall consider, evaluate and incorporate performance controls into the contract specifications. This includes specifications related to equipment (e.g., noise and vibration suppression devices or other abatement measures such as enclosures and barriers for the protection of sensitive receptors).
The location of noise and vibration monitoring sites are identified and preliminary predicted long-term operational and short-term construction effects are assessed and documented in the NEPA document and detailed technical reports. Mitigation measures are typically investigated to determine their effectiveness in reducing or eliminating noise effects. The NEPA document will likely include some recommended measures to mitigate or reduce any potential noise and vibration effects, however, approved control measures are considered and incorporated into the project during final design.
The design team shall also consider the need for development of a Noise and Vibration Control Plan, which is typically comprised of a Noise Monitoring Plan, Noise Abatement Plan, Vibration Monitoring Plan and a Vibration Abatement Plan. During the design
The Noise and Vibration Control Plan is typically implemented prior to construction to avoid and minimize noise and vibration impacts. It is not uncommon that the contract specifications require the Contractor to develop this plan.
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phase, the need for and the responsible party for preparing these plans shall be determined, and these documents shall be prepared prior to construction.
A.6.2.1 Noise Controls
CA.6.2.1
A Noise Monitoring Plan and a Noise Abatement Plan, when required, shall identify and implement mitigation measures to control noise levels during construction.
The Noise Monitoring Plan typically includes: • Requirements for testing equipment to
demonstrate compliance with noise limits and procedures for reporting compliance,
• Source Limits and Performance Standards to meet noise level thresholds for daytime, evening and nighttime hours at adjacent sensitive land uses,
• Monitoring and reporting procedures, including receptors locations, noise monitoring locations, type of noise measurement devices, response procedures to be taken for any exceedance of specified noise limit and compliance response and resolution procedures.
The Noise Abatement Plan shall include consideration of noise reduction methods such as:
• Temporary noise barriers at laydown approaches
• Routing of trucks and placing equipment farther from noise-sensitive receptors
• Alternative construction methods, with special low noise emission level equipment, and quieter demolition or deconstruction methods
• Concrete crushers or pavement saws for concrete deck removal, demolitions or similar construction activity
• Alternative piling techniques such as bored or augured piling, rather than impact piling
• Use of local power grid to reduce the use of generators
• Attach intake and exhaust mufflers, shields or shrouds to equipment
• Line noise-deadening materials (rubber) to inside of hoppers, conveyor transfer points or chutes
• Noise barriers, screens or enclosures to reduce the noise from activities such as jackhammers, spoil being loaded into trucks, concrete trucks mixing concrete
• Restrict hours of operation whenever possible; fit jackhammers, air compressors, generators, light plant and cranes with silencer, and
• Clad crane with timber paneling, and possibly locate ventilation fans, dewatering pumps, air compressors and generators in the tunnel.
A.6.2.2 Vibration Controls
CA.6.2.2
The Vibration Monitoring Plan and Vibration Abatement Plan shall identify and implement a proactive
The Vibration Monitoring Plan typically includes requirements or stipulations for:
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approach to reduce vibration levels and the possibility of community complaints during construction. Measurements shall be taken to establish the potential impact of construction activities, such as drill and blast excavation, on structures.
• Performing pre-construction surveys of all historic, non-historic and potentially fragile buildings to identify appropriate vibration thresholds at each site, as applicable
• Monitoring construction vibration levels at all vibration-sensitive structures within the influence area of the project during construction at applicable daytime, evening and nighttime periods. Monitoring is typically conducted during highly disruptive construction activities, such as blasting, pile driving and drilling, particularly if situated adjacent to a sensitive receptor.
• Process for performing vibration measurements upon receipt of a vibration complaint, at the complainant’s location during activities representative of the offending operation.
Depending on the particular project construction methodology and sensitive structures and infrastructure, the Vibration Abatement Plan shall, as a minimum, include consideration of the following types of mitigation measures:
• Construction mitigation measures for each structure based on the preconstruction surveys.
• Use of deep saw-cuts and/or concrete cutters to minimize the transmission of vibrations from pavement-breaking operations to foundations of nearby structures.
• Use of drilled or vibratory methods rather than impact pile drivers where feasible for installation of retaining walls and other foundation elements.
• Re-Route of truck traffic and heavy equipment to avoid impacts to sensitive receptors.
• Secure street decking over cut-and-cover excavations.
• Eliminate bumps in temporary roadways and decking.
• Schedule work to limit night time impacts in residential areas and limit duration of vibration impacts.
• Heightened attention and controls when working in Historic Districts and near historic structures. It may be necessary to implement special vibration protection measures in areas near historic resources and particularly older fragile buildings.
A.6.3 Muck Removal and Disposal
CA.6.3
During design, consideration shall be given to the removal and disposal of muck from the tunnel. The contract specifications shall outline the procedures or require that the Contractor develop a Muck Handling Plan that outlines proper procedures in accordance with applicable federal, state and local regulations. Similarly, the process and responsibility for procuring necessary
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permits shall also be outlined in the contract documents. The contract specifications or the Muck Handling
Plan shall outline the muck sampling and testing procedures prior to disposal and/or re-use as fill material as well as the disposal procedures. As the design advances, potential disposal and/or recycling facilities in the project region shall be identified for both non-hazardous and hazardous materials.
The material disposal procedures will vary based upon the results of the testing.
During construction, the Contractors shall determine quantities to be excavated and disposed of, as well as amounts intended to be stored temporarily on site and location of storage. Off-site storage and transport of materials to disposal sites require the identification of intended transport means. During design, various options for on/off-site storage and transport routes to disposal sites shall be identified. Without storage capacity for the muck on-site, all muck brought out of the tunnel shall immediately be loaded onto surface trucks or rail cars for disposal. The amount of trucks needed shall be estimated, and a traffic analysis shall be performed to minimize impact on other traffic in the construction area. Consider sensitive land uses and community/stakeholder input when identifying truck haul routes and minimizing impacts to surrounding areas related to the storage and transport of materials. Potential noise concerns to surrounding communities or other sensitive noise receptors shall be considered and addressed during construction. Contract specifications and the Muck Handling Plan shall address means for controlling dust and noise, such as covering conveyors and hoppers, lining hoppers with rubber and enclosing trucks.
In general, the machinery typically used to move spoils include large cranes as well as vertical conveyors to enable loading into trucks and, in some cases, barges.
For immersed tunnel construction, identify an acceptable and approved containment disposal site for the muck removed from the excavated trench. Contaminated materials shall be disposed of in special spoil containment facilities, while uncontaminated materials, if suitable, can be considered for backfill. Plan for early identification and approval of a suitable disposal facility as most existing spoil disposal facilities are too small to accommodate the quality and quantity of wet material excavated for immersed tunnel construction.
Tightened environmental restrictions often present challenges related to the disposal of material. Many of the water bodies such as harbors or causeways have contaminated sediments requiring special handling.
Transport of excavated material from trenches within water bodies typically use methods such as hydraulic dredging, which essentially pumps the material to the containment facility.
While this article does not address hazardous contaminant, during the design phase, proper investigations and studies shall be conducted to identify the potential for hazardous contaminants within the construction zone. At a minimum, the contract specifications shall outline the responsibility for providing relevant environmental compliance documents and managing contaminated materials.
A.6.4 Staging Areas
CA.6.4
Identify and set up staging areas where construction machinery and other equipment and materials would be delivered, stored and operated. Staging areas may also accommodate assembly, launching and removal of
Tunnel project construction typically requires disturbance on above-ground sites for the temporary stockpiling of spoils (muck) from the tunnels and for construction materials, machinery and workers to enter
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TBM’s, means/methods for ventilating the tunnel, storage of excavated soil and rock, electrical service equipment or substation and maintenance, truck loading/unloading and rebar cage assembly, among other uses.
and exit the areas being excavated. Depending on available land uses within a particular project study, very few vacant parcels are often available within close proximity to the proposed alignment that could be used for staging areas, particularly in urban environments.
Construction staging areas, also referred to as “laydown areas,” are sites that are used for the storage of materials and equipment and other construction-related activities.
Construction staging areas shall be considered and identified during the design phase, if not earlier. Their locations shall be selected based on a variety of considerations and factors, including but not limited to, size, proximity to construction work areas, existing and surrounding land uses, proximity to sensitive habitats, communities and sensitive noise/air/dust receptors, among others.
Immersed tunnel construction requires large staging areas for the fabrication of the tunnel elements. These staging areas must have water access in order to launch the elements for transportation to the tunnel site.
During design, identify staging areas of adequate size and proximity to the alignment to minimize construction traffic through the project study corridor and to provide adequate space and access for construction activities.
Staging areas are typically fenced and are often lit for security.
During design, these staging areas shall be identified and leases and permits obtained for proposed use and impacts. For any staging area that is ultimately used for construction of the project, the Contractor shall be required as part of contract specifications to comply with applicable local zoning laws and other applicable federal, state and local rules and regulations, and obtain necessary permits and approvals.
While the Contractors may or may not choose to use the identified sites, they are likely candidates and provide a reasonable scenario to assess potential environmental and community effects (during the planning/NEPA and design phase) that may occur from the activities and operations of construction staging areas.
A.6.5 Traffic and Public Transportation
CA.6.5
During design, consideration shall be given to traffic, public transportation and pedestrian management during the construction phase for any tunnel project. This shall include planning and design strategies for avoiding, minimizing or mitigating impacts to public roadways, sidewalks, bike lanes and the maintenance of access to residences, businesses and public services throughout the project area.
Construction-related effects often include increased traffic because of street or lane closures and restricted access to businesses or residences, as well as local area transit affects from such closures and restrictions. These disruptions may include bus stop closures, provision of temporary bus stops, scheduled delays and bus route detours.
During design, consideration shall be given to the development and implementation of a Maintenance of Traffic Plan and Transportation Management Plan for construction. This requirement and process may be included in the Contract Documents to provide specific guidance on traffic, public transportation and pedestrian management within the construction zones, haul routes and construction staging areas.
Typical mitigation measures that may be implemented by the Contractors, but shall be considered during design and incorporated into design plans and contract documents, as applicable, (and based upon the Maintenance of Traffic Plans and Transportation Management Plans) may include:
• Hold meetings to inform the public on proposed
bus route changes prior to the initiation of bus
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revenue service. • Develop and implement a plan to mitigate
impacts to bus stops and routes during construction.
• Develop and implement plans for maintaining access and circulation with other transit facilities.
• Advance public notice to motorists of the nature, extent and duration of lane closings and detours.
• Place detour signage in strategic locations, and use appropriate warning signs.
• Encourage construction during off-peak hours whenever feasible.
• Minimize disruption of access to residences and businesses; maintain open at least one entrance to a property where multiple entrances exist.
• Coordinate with other projects in the area that have potential to impact roadways and create cumulative effects.
• Establish parking policies for construction workers that will help minimize impacts to residences and businesses. Encourage contractors, inspectors and other personnel to use transit, if available, and dissuade the use of private vehicles.
• Install signage and barriers for protecting and guiding pedestrians.
• Relocate bus stops at construction sites to minimize the impacts on surface transit passengers.
• Remove curbside parking where necessary at construction zones to provide maximum road width for traffic lanes.
• Relocate loading and unloading areas as needed to minimize the impact on businesses in the areas.
• Maintain or relocate pedestrian sidewalks within construction areas.
• Provide physical separation between construction zone and sidewalks. Separations may consist of concrete barriers, wood fencing or protective mesh fencing.
• Place time restrictions on certain construction activities such as trucking of tunnel spoil, delivery of materials, etc.
A.6.6 Safety and Security
CA.6.6
During design, safety and security considerations related to the design, construction and operation of any tunnel project shall be considered and implemented. All tunnel projects shall address in the design and contract documents current safety and security systems and procedures to protect tunnel users and workers, as well as nearby communities.
Depending on the lead federal and/or state agencies, Safety requirements come from state and federal
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the design team shall identify and adhere to system safety, fire and life safety and security design criteria for the governing agency(ies).
authorities. The unique safety aspects of the underground environment should be taken into consideration when determining the safety systems to be employed in a tunnel project.
Potential impacts shall be assessed during design for identifying whether or not adequate provisions for safe and secure operations can be made, if the project is expected to alter the patterns of auto, transit or pedestrian accidents and what design features are included to minimize these accidents.
During design, a Safety Plan shall be developed that establishes mechanisms for identifying and addressing hazards associated with tunnel operations, maintenance and inspections and provides a means of ensuring that the proposed system is implemented with thorough evaluation of the potential effect on safety. The Safety Plan shall provide a framework for ensuring user and employee safety.
During construction of a tunnel, the safety of construction workers shall be considered and implemented by the construction team. Careful consideration shall be given to the actual worksites so that workers are protected. Workers shall be protected from falls, moving equipment and electrocution. Tunnels shall be adequately stabilized to reduce the risk of instability in the exposed ground or falling materials. Appropriate lighting shall be required within tunnels for safety. Sufficient oxygen shall penetrate the tunnel while undesirable or harmful gases or byproducts are maintained at acceptable levels and proper ventilation is maintained. The tunnel shall be protected against drainage/leakage of water into the tunnel. Emergency plans shall be in place for tunnel fires, including appropriate escape/rescue chambers. The contract specifications shall require strict compliance with all safety measures and local, state and federal regulations.
Safety during construction is the responsibility of the Contractor; however, design and contract documents must be prepared with safety as a key consideration.
Contractors shall install fencing and shielding at all construction sites to reduce hazards and the vulnerability to trespassing and vandalism and to protect adjacent walkways and streets. Contractors shall be required to adhere to applicable federal and state safety protocols.
The safety and security of construction workers and the general public is a key element of construction activities. Introduction of on-site construction equipment, including heavy industrial cranes and trucks hauling excavated material from access shafts on local roads, can create potential safety hazards for pedestrians and motorists. Numerous construction workers operating or working in concert with equipment at the various construction staging area locations can create increased opportunities for safety and security breaches. The construction sites and related equipment can be potentially vulnerable to safety and security violations, particularly during times of construction equipment shutdown and construction site closure.
Projects shall incorporate appropriate firelife safety requirements into all aspects of the project design and construction. The fire-life issues related to road tunnels shall consider alignments, tunnel cross section, emergency exits, ventilation provisions, geometrical configuration, right-of-way, separation of roadways and pedestrians, cross passages and costs, among other considerations.
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Recommended AASHTO LRFD Tunnel Design and Construction Specifications
Recommended Construction Specification Sections
The following specification section headings represent work associated with tunneling and related activities. They are primarily civil sections. Other sections that would be required for a completed project including systems, mechanical, electrical, plumbing, architectural, ventilation, utilities, maintenance and protection of traffic, traffic control, etc. are not included in this listing. The sections are organized according to the Construction Specifications Institute format and numbering system, although section numbers have not been assigned. Typical specifications for civil projects (for example, Structural Steel, Cast-in-Place Concrete, Excavation and Backfill) are readily available from other sources and can be tailored to specific projects as needed.
• ADJACENT STRUCTURES CONSTRUCTION SURVEYS • ENVIRONMENTAL ASSESSMENT • CHEMICAL SAMPLING AND ANALYSIS • HAZARDOUS MATERIALS ASSESSMENT • SUBSURFACE INVESTIGATION • SHOTCRETE • SHOTCRETE FOR INITIAL SUPPORT • SHOTCRETE FINAL LINING • PNEUMATICALLY APPLIED CONCRETE (PAC) LINING • PRECAST CONCRETE SEGMENTAL LINING • INJECTION GROUTING • MASS CONCRETE • DAMPPROOFING AND WATERPROOFING • SHEET WATERPROOFING • FLEXIBLE MEMBRANE WATERPROOFING • GEOTECHNICAL INSTRUMENTATION AND MONITORING • OFF-GASSING MITIGATION • ROCK EXCAVATION IN OPEN EXCAVATION • GROUNDWATER CONTROL • GROUND IMPROVEMENT • UNDERPINNING • PROTECTION OF EXISTING FACILITIES, STRUCTURES AND UTILITIES • TEMPORARY SUPPORT OF EXCAVATION • PERMANENT GROUND ANCHORS • SLURRY WALLS • SOLDIER PILE TREMIE CONCRETE WALLS • SECANT/TANGENT PILE WALLS • SHAFT EXCAVATION AND SUPPORT • TUNNEL EXCAVATION BY DRILLING AND BLASTING • SEQUENTIAL EXCAVATION • CONTROLLED BLASTING • TUNNEL EXCAVATION BY TUNNEL BORING MACHINE • EARTH PRESSURE BALANCE TUNNEL BORING MACHINE • SLURRY FACE TUNNEL BORING MACHINE • MINED TUNNELING • COMPRESSED AIR WORKING • TUNNEL GROUTING • MINED TUNNEL EXCAVATION FOR CROSS PASSAGES • REMOVAL OF TUNNEL OBSTRUCTIONS
APPENDIX B: B-3
February, 2016
• MUCK HANDLING AND DISPOSAL • ROCK REINFORCEMENT AND TEMPORARY SUPPORT • CAST-IN-PLACE CONCRETE TUNNEL LINING • PRECAST CONCRETE TUNNEL LINING • VAPOR MITIGATION CONTROL • FUGITIVE DUST CONTROL
