Vibration Mitigation for the Regional Connector Transit Corridor
Authors
Michael J. Lehnen, PE Assoc DBIAMott MacDonald444 S Flower St, Suite 2200Los Angeles, CA [email protected]
Hugh Saurenman, PhD, PEATS Consulting215 N. Marengo Avenue, Suite 100Pasadena, CA [email protected]
William Baker, MS, PEMott MacDonald181 Metro Drive, Suite 510San Jose, CA [email protected]
Andrew WongATS Consulting215 N. Marengo Avenue, Suite 100Pasadena, CA [email protected]
Number of Words4416
ABSTRACTRegional Connector Transit CorridorThe Los Angeles Metro’s Regional Connector Transit Corridor (RCTC) light rail subway project extends from the Metro Gold Line Little Tokyo/Arts District Station to the 7th Street/Metro Center Station in downtown Los Angeles, allowing passengers to transfer to the Blue, Expo, Red and Purple Lines, bypassing Union Station. The 1.9-mile alignment will serve Little Tokyo, the Arts District, Civic Center, The Historic Core, Broadway, Grand Ave, Bunker Hill, Flower St and the Financial District, and will also provide a one-seat ride for travel across Los Angeles County.
The RCTC subway alignment will pass through the heart of downtown Los Angeles, and mitigating vibration and groundborne noise from traveling LRT trains is a requirement that must be addressed by Regional Connector Constructors (RCC), the Skanska / Traylor Brothers JV design-build contractor. To address the issue RCC, through its final design firm MM and subconsultant ATS, performed detailed vibration testing and computer modeling to determine the most effective means of achieving the project goals. The initial testing demonstrated that the project criteria could be met with a 10-12 Hz floating slab track (FST) system. This paper will outline the vibration testing procedures and results, and the approach used to design the floating slab track (FST) system.
The Los Angeles County Metropolitan Transportation Authority’s (Metro) Regional Connector Transit Corridor (RCTC) Project extends and connects Metro’s existing Light Rail Transit (LRT) system from the Gold Line Little Tokyo/Arts District Station to the Blue Line 7th Street/Metro Center Station in downtown Los Angeles. The 1.9-mile underground alignment will serve Little Tokyo, the Arts District, Civic Center, The Historic Core, Broadway, Grand Ave, Bunker Hill, Flower St and the Financial District. The project alignment is shown as the dashed black line below:
RCTC Project Alignment
The RCTC project will also provide a one-seat ride for travel across Los Angeles County. Passengers will be able to travel from Azusa to Long Beach and from East Los Angeles to Santa Monica without transferring lines.
This new Metro Rail extension will offer an alternative transportation option to congested roadways and provide significant environmental benefits, economic development, and employment opportunities throughout Los Angeles County.
Regional Connector Constructors (RCC) is a joint venture of Skanska USA Civil West California District Inc. (Skanska) and Traylor Bros, Inc. (Traylor) who have been awarded design-build (DB) contract C0980by Metro for the design and construction of the RCTC Project.
The design team is led by Mott MacDonald ( MM). MM is the lead designer responsible for overall design management, and detailed final design of the alignment, trackwork, utilities, bored tunnels and cross-passages, cut and cover tunnels and stations, geotechnical, tunnel ventilation, station and tunnel HVAC and plumbing, communications, traction power and overhead contact system (OCS). ATS is a subconsultant to MM, responsible for analysis and design work associated with permanent noise and vibration requirements.
During the preliminary engineering phase of the project, Metro and its consultants Connector Partnership Joint Venture (CPJV), with CPJV subconsultant Wilson Ihrig and Associates (WIA), undertook preliminary testing and analyses to determine the location of noise and vibration sensitive receptors along the RCTC project alignment.
This work informed Metro’s Environmental Impact Report/Statement (EIR/S), and also formed the basis for final trackwork design requirements to be included in the Design/Build contract.
The end result of the preliminary engineering phase work concluded that vibration mitigation would be required in two places along the project alignment where sensitive receptors are located:
A Floating Slab Track (FST) system would be required to mitigate groundborne vibration in the Bunker Hill neighborhood.
An Isolated Slab Track (IST) system would be required in the Little Tokyo neighborhood.
For the FST system, the contract documents indicated that a 5 to 12 Hz FST system with resilient fasteners would be the minimum mitigation necessary to meet project requirements. However, comparable measures could be developed and submitted for Metro approval, as final analysis and determination of mitigative trackforms were to the responsibility of the DB contractor.
DESIGN/BUILD CONTRACT REQUIREMENTS
The scope of work for the DB contract included the following requirements related to noise and vibration analyses and reporting, and FST trackwork final design:
Field investigations and noise and vibration testing in certain sensitive receptors along the RCTC project alignment.
Analysis and reporting of findings resulting from the field investigations.
Final design of mitigative trackforms.
Mock-up testing program for the FST system.
Operational phase monitoring.
Each of these topics are discussed in the following sections.
Impact Criteria and Target Groundborne Noise LevelsA key feature of the RCTC project was the presence of very sensitive musical performance and recording spaces almost directly above the proposed locations for the two bored tunnels and one of the stations(2nd/Hope Station). Before the design-build process was initiated, Metro had spent considerable effort negotiating with the stakeholders to develop mutually-agreed upon criteria for acceptable groundborne vibration and groundborne noise. Most of the focus was on groundborne noise that could interfere with musical performances and the recording of those performances. The basis of the acceptability thresholds for the majority of the spaces defined by the building managers was the limits provided for in the FTA manual Transit Noise and Vibration Impact Assessment for a “General” impact assessment. For twospaces considered particularly sensitive, the acceptability limits were based on monitoring of ambient noise that was performed during nighttime hours when background noise was at a minimum.
Table 1 shows the measured noise levels for the 31.5, 63, and 125 Hz octave bands for one of the special spaces. The Metro Specification states that, “the criteria shall be satisfied if the measured noise levels are no greater than 1 decibel higher than the criteria for each respective hall.” The levels in the specification are the sound levels exceed for 1%, 10%, 50%, and 90% of the measurement periods. This is a very strict set of criteria and it is difficult to accurately identify target noise levels that will ensure that none of the LNN level will be increased by more than 1 decibel due to light rail transit operations.
The approach ATS used was to use the measured levels provided by Metro in Table 1 to consider how much the vibration levels in each octave band varied. We then picked a “target level” that would make it highly unlikely that the background levels would increase by more than a decibel. The total time that trains will generate groundborne noise was estimated based on a design train speed of 30 mph and a maximum train length of 270 ft. At 30 mph, it will take a 3-car train 6.2 seconds to pass a specific location. Assuming a maximum of 24 trains per hour in each direction during peak periods, train groundborne noise will be near maximum levels for 589 seconds per hour, about 8% of the time.
The limited duration of the train events means that train noise is expected to have the greatest effect on L10 and L1, some effect on L50, and little effect on L90. Therefore, to minimize the potential for train operations to cause more than a 1 decibel increase in any of the LNN levels, the target selected for the maximum groundborne noise level from train operations was 5 dB below L10 level in Table 1. As seen in Table 1, the target levels for this space were close to or lower than the L90 level. Mitigation was specified that was sufficient to reduce the predicted groundborne noise levels to below the target levels.
To illustrate just how strict these criteria are, also shown in Table 1 are the thresholds of human hearing as defined in ISO 226:2003. Of course, the ISO 226:2003 curves are approximate thresholds for those with typical hearing who have not been compromised by exposure to extremely loud noise or the normal degradation of hearing associated with aging. Another factor is that those who are involved with performing and recording of music will sometimes have much better hearing than the average and may be able to discern sounds that are well below the curve in ISO 226:2003.
Threshold of Hearing 65 36 22Sound pressure measurements obtained with the “slow” sound level meter setting (1-second time constant).
Note: Threshold of hearing based on curves in ISO 226:2003.
Vibration Prediction Approach
Because of the sensitivity of the performance spaces and the level of concern expressed by the stakeholders, efforts were made to ensure that the predictions would be as accurate as possible. The general approach used was to follow the procedures for a detailed vibration assessment that are presented in the FTA Guidance Manual. This prediction procedure was originally developed by Nelson and Saurenman, and is based on empirical measurements. The first measurement is to determine how vibration will propagate through the soil from the subway tunnel to the buildings and the second measurement is to characterize the vibration forces that will be generated by the light rail trains operating in the tunnels.
Figure 1 illustrates the procedure to characterize the propagation of vibration from a subway tunnel through the soil to the foundation of adjacent buildings. A borehole drilling rig was used to drill boreholes to the future depth of the tunnel. Then the soil penetration test (SPT) drop-hammer on the rig was used to excite vibration waves in the soil. The exciting forces of the impacts were measured with a load cell attached to the bottom of the drill string and the resulting vibration waves were measured with accelerometers located at the ground surface and inside the buildings of concern. This test provides the Point Source Transfer Mobility (PSTM) that is used to estimate the Line Source Transfer Mobility (LSTM).The procedure for using a series of PSTMs to estimate an LSTM is defined in the FTA Guidance Manual.
Detailed testing was performed to provide sufficient support for alternative designs. To obtain the most value from each borehole test, up to 15 accelerometers were used in each test and measurements were performed at three depths: 10 ft. above the future top of rail, at the same depth as the future top of rail, and 10 ft. lower than the future top of rail. Tests were performed at four boreholes with 10 to 15 accelerometer positions for each borehole. Accelerometers were located at the ground surface to obtain a measure of how vibration attenuates with distance and up to 10 accelerometers were located at various spaces inside the buildings. The indoor accelerometers were positioned to characterize each of the sensitive spaces defined by the Metro specifications as well as in adjacent spaces and in lower levels used for parking.
Figure 1. Procedure for Measuring Line Source Transfer Mobility
Several approaches were used to define the most accurate, but still conservative, estimates of the appropriate PSTMs for each of the sensitive spaces. In several cases the measurement inside a performance space was not valid because the signals from the SPT drop-hammer were lower than the background vibration. In one sense this is good news because if the signal from the SPT drop-hammer is not transmitted into the performance space, it is likely that vibration from future train operations will be substantially reduced before it reaches the most sensitive spaces. However, it complicates the process of developing accurate estimates of the future groundborne noise and vibration.
Figure 2 illustrates a situation in which the measurement inside a key performance space was not valid, but a measurement at a parking level closer to the borehole was valid. In cases such as this, the appropriate PSTM was estimated based on the closest measurement that had valid coherence. In Figure 2, L1, L2 and L3 were measurements inside the performance space and P6 and P7 were measurements in parking levels that were closer to the borehole. Based on the data shown in Figure 2, there were structural features that reduced the amplitude of outdoor vibration that reached the performance space. The conclusion is that basing the predictions for the performance space on the measurements in parking levels 6 and 7 will provide a conservative estimate of the future levels of groundborne vibration inside the performance space. Note that the pronounced peak at 125 Hz for positions L2 and L3 inside the performance space is not an indication of a floor resonance that was excited by the pulses from the drop hammer. Rather, the peaks were caused by the background vibration inside the performance space.
Figure 2. Performance Space PSTM and COH with Low Coherence
Figure 3 shows the PSTM and coherence measurements for the performance space that is closest to the future subway tunnel. In this case L1 represent the measurement inside the sensitive space and L2 represents a measurement inside an adjacent space. The coherence is not perfect but reasonable at frequencies greater than about 20 Hz and the L1 coherence is generally greater than the L2 coherence. The sections of the lines that are grayed out indicate data that was not considered suitable for estimating the true PSTM. In a case such as this, to ensure that the predictions are on the conservative side and that errors will tend to over predict future vibration levels, the maximum of the curves from locations considered representative were used rather than an average of the curves.
Figure 3. PSTM and COH for Most Sensitive Performance and Recording Space
Final predictions showed that groundborne vibration is not anticipated to exceed the project criteria in any of the sensitive spaces identified in the project specifications. However, the predicted groundborne noise levels exceed the target limits in several of the sensitive spaces. The groundborne noise predictions for the sensitive space that will be closest to the subway tunnel are shown in Table 2 along with the predicted levels with two different floating slab track (FST) options as mitigation. The two options examined were asteel coil-spring design that would achieve a resonance frequency of 6 Hz and a natural-rubber spring system that would achieve a resonance frequency of 10 to 12 Hz. Both systems are modeled as single degree of freedom mass-spring-damper systems. In both cases the predicted groundborne noise is mitigated sufficiently to meet the target noise limits for the space. Based on discussions with RCC regarding constructability and cost-effectiveness, the 10 to 12 Hz FST was the selected design.
The assumed effectiveness of the FST system was verified using a 3D finite element model. The FEM model showed results similar to those predicted by the SDOF model although there were some significant differences caused by various resonances in the slab and more complex vibratory motion than rigid-body vertical motion. However, the predicted insertion loss of the FST was still sufficient to eliminate all of the predicted groundborne noise impacts.
Based on recommendations resulting from the refined noise and vibration analyses, the focus of the final design effort was to develop a constructible, cost-effective FST system configuration capable of meeting the 10-12 Hz natural frequency requirement.
Initial discussions between RCC, MM and ATS centered around the ability to meet the 10-12 Hz requirement with a traditional FST system, in lieu of the coil spring system.
The team worked together closely during this initial design development period, with the resultant outcome being a Cast-In-Place (CIP) concrete FST system. The configuration provided the necessary mass to meet the 10-12 Hz frequency requirement, and through extensive coordination with RCC also resulted in a cost effective and constructible solution given the constraints of constructing the FST system in the underground bored tunnel and station structures. One notable challenge was to shape the floating slab to fit within the tight tunnel constraints while having enough mass to meet the frequency criteria. The station slab was simpler because there is more room to work. This and other design considerations are discussed below.
The CIP approach is unusual for the industry. Generally, the pre-cast method is used to expedite placement of FST segments in the confined space of tunnels. To our knowledge the use of CIP FST systems in bored tunnels is not “typical” or is limited to short sections, special trackwork and transition slabs. Building the entire tunnel FST using the CIP approach is precedence-setting and we needed to work closely with the builder to ensure constructability.
The design effort made use of historical examples of CIP FST. For example, it is used in underground box structures where there is special trackwork in the following locations:
Buffalo LRT Project (1981)BART SFO Extension (1998)LA Metro CBD to N. Hollywood - Wilshire/Alvarado Double Crossover (1987)Sheppard Subway Toronto (1998)
We also have knowledge of CIP FST for an At-Grade installation:
SF MUNI (1995)
Our first-hand experience with Pre-Cast floating slab track at grade was beneficial to our design of the elastomeric support pads and side pads:
The FST system is comprised of discrete panels consisting of a permanent sheet steel formwork, steel reinforcement and CIP concrete supported on natural rubber pads. There are also rubber side and end pad assemblies to secure the individual panels against each other, and against concrete curbs along the alignment. That, along with the track components - direct fixation track fasteners and 115RE rail - makes up the FST system.
Shown in light blue lines in Figure 5 below is the location of the FST system along the RCTC project alignment. Due to the aforementioned analysis performed during the PE phase of the project to determine the location of sensitive receptors, the extent of FST mitigation required was determined to be through the 2nd/Hope Street Station (approximately 305 Route Feet) and approximately 1050 Route Feet southward into the bored tunnels.
Figure 4 – Limits of FST (shown in light blue lines)
The bored tunnel cross section provides a limited space to work between top of rail and invert making it a challenge to design a slab of enough mass/unit length to meet the vibration criteria. The width of about 10’ was determined by the available space near the tunnel invert. Starting with a cross-sectional area of approximately 10.5 ft^2 the team was challenged to work on the sectional shape dimensions to meet the target Slab Mass per LF of at least 1550 lbs/LF to meet the 10-12 Hz criteria. The elastomeric support pad rubber formulation and dimensions and placement configuration were designed to be compatible with the target mass. The support pad dimensions are 12” diameter, 3” thick with 1” diameter center hole and the spacing configuration is 4’ lateral X 4’ longitudinal. The support pads were designed for total deflection on the order of 16% of height, under full dead and live load. Side and separation pads are of dimensions 12”x6”x2”.
To achieve the required target mass, a variety of different floating slab shapes and configurations were evaluated.
We were able to develop a section shape that met the required mass without having to implement unusual measures. The slab depth design was finalized at approximately 1’3” (1.25’). Based on RCC’s constructability input we kept the top of slab flat. The direct fixation (DF) track fasteners will be installed directly on the top of the slab, therefore eliminating the traditional second pour plinths. Anchor inserts will be embedded in the CIP slab to support the track fasteners. We ended up with sufficient cross sectional area that resulted in a slab mass of approximately 1900 lb/LF when calculated along with the mass of the track components. Conversely, the constraint on the possibility of having a slab that was too heavy is that it would overwhelm the support capabilities of the elastomeric support pads.
We specified that the formwork “pans” could be made of flat or profiled sheet steel. Profiled sheet steel would be a way to use a thinner gauge. The weight of pans can be about 2000 to 2500 lbs as determined by in-tunnel equipment handling capabilities.
To determine the gauge of the sheet steel for the formwork pans we looked at what has been used on similar previous FST projects. The gauge needs to be of suitable rigidity for withstanding the rigors of handling in the tunnel construction environment with no deformation and with no bulges when the concrete is poured into the formwork. The weight of formwork pans and nested bundles cannot be prohibitive for handling. Generally, No. 6 and No. 10-gauge sheet steel has been used on previous similar applications, with the 6 gauge (0.1943 in. or +3/16” thick) typical for bottom of form, and 10 gauge (0.1345 in. or +1/8” thick) typical for the sides of the formwork. Based on input from RCC, number 6-gauge sheet steel was included in the final design.
The builder and form fabricator are tasked with final configuration of the sheet steel formwork to suit their fabrication capabilities and means and methods for construction. The formwork pans are to be galvanized and can be shaped to “nest” so they can be bundled for transport into the tunnel. The form sections should be of limited length for handling in “nested” bundles and long enough to make reasonable slab section with forms joined to make a slab of designed length. The length of sheet steel formwork “forms, pans or panels” can be about 10 to 12’ - determined by ability to handle the forms in the tight quarters of the tunnel. Once in place, the panels can be joined to make a slab of design length, in this case 36’ long for the tunnel FST slabs and 40’ long for the station FST slabs. The method of jointing and sealing the form sections is left to RCC. Where the FST is in a curve in the bored tunnels the form section ends may be angled, overlapped or otherwise joined/sealed to make the 36’ long FST slab sections. The top of tangent slabs are profiled with a 1/8”/ft. cross slope from track centerline to facilitate surface drainage.Where slabs are in a curve, the track and top of concrete is super-elevated making cross-drainage toward the low side of the slab. The track profile in the station is flat/level and tunnel track profile is on a slope creating longitudinal drainage.
Design collaboration with our Stations, Systems and Mechanical design teams determined the placement of blockouts for deluge piping, Insulated Joints, track circuits and power bonding.
Contained in the C0980 contract is a requirement for the DB contractor to “prepare, implement and report the results of a pre-construction field testing program to measure the natural frequency characteristics of the floating slab track system”.
Preparation of a field testing plan to describe the proposed testing location, methods and operationsImplementation of the actual field testing programPreparation of a report to document the results of the field testing programDemonstration that the measured natural frequency of the mock-up loaded with the truck mass and vehicle static load is within +/- 5% of the design natural frequency
Details of the mock-up testing program are beyond the scope of this paper; however, at the time of writing the field testing plan has been prepared and approved by Metro, and RCC is in the process of developing the specifics of implementing the mock-up testing program.
MONITORING REQUIREMENTS
To ensure compliance with the agreed criteria, ongoing monitoring of train vibration is required. Permanent vibration monitors will be installed in the tunnel that are capable of continuously measuring 1/3 octave band levels over the 6.3 Hz to 200 Hz bands. The data from the monitors will be collected in real time and continuously evaluated. A baseline measurement of pre-revenue levels will be collected, and this will be compared to levels during operations.
CONCLUSIONS
Due to the sensitivity of various properties along the RCTC project alignment, stringent noise and vibration criteria were included in the C0980 DB contract.
To complement the work that was performed during the PE phase of the project, the C0980 contract required additional field testing to refine noise and vibration parameters and inform the trackwork design.
Through the collaborative efforts of Metro, local stakeholders, CPJV/WIA, RCC and MM/ATS, the field testing was very successful and allowed for development of a robust trackform design to meet the project’s stringent criteria.
Early collaboration between MM/ATS and RCC was beneficial to come up with a buildable, cost-effective design that met the vibration mitigation criteria.
Although the original floating slab systems built in the 70’s were cast in place, over the past two or three decades, it has been more common to use precast systems with the precast panels. Recent LRT projects typically use Pre-cast FST sections in tunnels the CIP method in station boxes or for large, irregularly shaped floating slabs where special trackwork units are placed. We developed a viable design to use CIP for both bored tunnel and station applications.
Design of appropriate vibration mitigation to protect high-profile sensitive receivers is always a challenge. The approach used in this project included early collaboration between the Designer and Builder, which was important in developing a buildable design that meets the vibration mitigation criteria. To date, the process has been very successful, with all parties working together toward a common goal.
ACKNOWLEDGEMENTSAuthors wish to acknowledge the Los Angeles County Metropolitan TransportationAuthority, Connector Partnership Joint Venture (CPJV), Wilson Ihrig and Associates (WIA) and Regional Connector Constructors (RCC) for allowing us to present the information contained in this paper.
Team The Regional Connector Constructors (RCC) is a joint venture of Skanska USA Civil West California District Inc. (Skanska) and Traylor Bros, Inc. (Traylor) who have been awarded design-build (DB) contract by Metro
The design team is led by Mott MacDonald (MM)
Noise and vibration analysis are conducted by ATS Consulting
FST Mock-up test Prepare a test plan Construct and test mock up of FST design Show that the natural frequency meets contract requirements
Revenue Service monitoring Sensors shall be placed in the tunnel Continuous monitoring of vibration in 6.3-200 Hz bands Real time analysis Levels compared to pre-revenue baseline
AREMA 2016 Annual Conference & Exposition
Conclusion The DB team developed a construcable and cost effective CIP FST configuration
CIP is less common than pre-cast FST The DB team was able to create a design that
Meets the mitigation requirements for sensitive receivers
