Anna Maria Tunnel Alternative Technical Memorandum · PDF filepiston effect of the moving...

FINAL TUNNEL ALTERNATIVE

TECHNICAL MEMORANDUM

Anna Maria Island Bridge Project Development and Environment Study

SR 64 (Manatee Avenue)

From SR 789 (East Bay Drive) to Perico Bay Boulevard Manatee County, Florida

Financial Project ID: 424436-1-21-01

Prepared for:

Florida Department of Transportation District Environmental Management Office

801 North Broadway P.O. Box 1249

Bartow, FL 33831

January 2009

FINAL TUNNEL ALTERNATIVE

TECHNICAL MEMORANDUM

Anna Maria Island Bridge Project Development and Environment Study

SR 64 (Manatee Avenue)

From SR 789 (East Bay Drive) to Perico Bay Boulevard Manatee County, Florida

Financial Project ID: 424436-1-21-01

Prepared for:

District Environmental Management Office 801 North Broadway

P.O. Box 1249 Bartow, FL 33831

Prepared by:

5300 West Cypress Street Suite 200

Tampa, Florida 33607

January 2009

SR 64 – Anna Maria Island Bridge Final Tunnel Alternative Technical Memorandum

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TABLE OF CONTENTS Section Title Page

TABLE OF CONTENTS............................................................................. i

LIST OF FIGURES .................................................................................... ii

1.0 INTRODUCTION ................................................................................... 1-1

2.0 GEOMETRIC DESIGN PARAMETERS ............................................... 2-1 2.1 Typical Section ............................................................................ 2-1 2.2 Horizontal Alignment .................................................................. 2-1 2.3 Vertical Profile............................................................................. 2-3

3.0 MECHANICAL SYSTEMS.................................................................... 3-3 3.1 Ventilation ................................................................................... 3-2

3.1.1 Self-Ventilation:............................................................ 3-2 3.1.2 Longitudinal Ventilation............................................... 3-3 3.1.3 Semi-Transverse Ventilation ........................................ 3-4 3.1.4 Full Transverse Ventilation .......................................... 3-4

3.2 Ventilation Conclusions............................................................... 3-4

4.0 TUNNEL DESIGN ALTERNATIVES................................................... 4-1 4.1 Bored Tunnel ............................................................................... 4-1 4.2 Immersed Tube Tunnel ................................................................ 4-5

4.2.1 Steel Shell ..................................................................... 4-6 4.2.2 Concrete Shell............................................................... 4-7 4.2.3 Immersed Tube Conclusions......................................... 4-7

4.3 Cut and Cover Tunnel.................................................................. 4-9

5.0 CONSTRUCTION COST ESTIMATES................................................. 5-1 5.1 Limitations and Constraints ......................................................... 5-1 5.2 Tunnel Cost Estimating in Practice ............................................. 5-2 5.3 Cost Conclusions ......................................................................... 5-4

6.0 TUNNEL ALTERNATIVE VIABILITY................................................ 6-1

APPENDICES Appendix A: Construction Cost Estimate


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LIST OF FIGURES

Figure Number Title Page

1 Tunnel Alignment .................................................................................... 1-1

2a Typical Section - Option A...................................................................... 2-2 2b Typical Section – Option B ..................................................................... 2-2 3 Crest Profile at Kinney Tunnel in ............................................................ 2-3 4 Cross-Section Through Boat Section....................................................... 2-3

5 Central Fan Longitudinal Ventilation System ......................................... 3-3 6 Jet Fan Longitudinal Ventilation System................................................. 3-3 7 Semi-Transverse Ventilation System....................................................... 3-4 8 Full Transverse Ventilation System......................................................... 3-4

9 Tunnel Boring Machine ........................................................................... 4-1 10a Bored Tunnel Cross Section – Option A ................................................. 4-2 10b Bored Tunnel Cross Section – Option B ................................................. 4-3 11 Bored Tunnel Profile – Option A ............................................................ 4-4 12 Bored Tunnel Right of Way Impacts ....................................................... 4-4 13 Immersed Tube Transportation................................................................ 4-5 14 Immersed Tube Construction................................................................... 4-5 15a Immersed Tube Typical Section – Option A ........................................... 4-6 15b Immersed Tube Typical Section – Option B ........................................... 4-6 16 Graving Basin .......................................................................................... 4-7 17 Immersed Tube Profile ............................................................................ 4-8 18 Immersed Tube Right of Way Impacts .................................................... 4-9 19 Existing Water Depths (feet) ................................................................. 4-10 20a Cut and Cover Typical Section – Option A........................................... 4-11 20b Cut and Cover Typical Section – Option B ........................................... 4-11 21 Cut and Cover Right-of-Way Impacts ................................................... 4-12 22 Cofferdam Construction......................................................................... 4-13


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Section 1.0 INTRODUCTION

This Technical Memorandum documents the results of the analysis performed for the Anna Maria Tunnel Alternative. The tunnel alternative evaluated herein consists of a single tunnel that would be used to provide a transportation link to Anna Maria Island, replacing the existing bridge and roadway causeway system. Refer to Figure 1 for the proposed alignment of the tunnel.

Several design alternatives were considered for the tunnel mechanical systems, structural design and construction methodology. These alternatives are evaluated to establish preliminary sizing of the tunnel cross-section and to aid in the determination of feasibility.

Figure 1: Tunnel Alignment


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Section 2.0 GEOMETRIC DESIGN PARAMETERS

2.1 TYPICAL SECTION

The Anna Maria Tunnel will be maintained as a two-lane facility. Although a design speed of 45 miles per hour (mph) is specified for the bridge alternatives, a 35 mph design speed is used for the tunnel evaluation in order to minimize costs and limit impacts at the tie-in locations. The Federal Highway Administration’s (FHWA's) current philosophy is to maintain the approaching roadway shoulders within the tunnel for safety reasons and also allow for traffic to pass stranded vehicles. This philosophy is a departure from the one foot or two foot shoulders found in most older tunnels. The wider shoulders have a tremendous impact on the cost of the structure. Refer to Figure 2a for typical section ‘A’ of the tunnel. This typical section provides for two 12-foot (ft) lanes, 10-ft shoulders (matching the approach roadway based on 35 mph design speed) and a 12-ft multi-use path on one side. Refer to Figure 2b for typical section ‘B’ of the tunnel. This typical section provides for two 12-ft lanes, 10-ft shoulders (matching the approach roadway based on 35 mph design speed) and 10-ft sidewalks. The standard highway vertical clearance of 16 ft–6 inches (in) is provided. A one foot buffer area is provided for traffic signals, message boards and signing. For this tunnel evaluation, the typical sections for the bridge alternatives (as described above) are used. Consideration should be given, however, to the elimination of the sidewalks in the tunnel for safety and cost reasons. Without sidewalks, 2 ft–7in catwalks for emergency and maintenance access will be required on each side.

2.2 HORIZONTAL ALIGNMENT

The alignment of the tunnel is predominantly straight for economic and constructability reasons. Curvature is limited to the ends of the tunnel to provide the greatest flexibility in construction methodology. The degree of curvature is limited both by the design speed (35mph) and the method of construction. These construction methods are described in detail in Section 4.0 of this document.


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Figure 2a: Typical Section - Option A

Figure 2b: Typical Section – Option B


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2.3 VERTICAL PROFILE

The vertical profiles were developed utilizing maximum grades and K values in accordance with the Florida Department of Transportation (FDOT) criteria. In addition to these criteria, the tunnel ends must be raised high enough so that hurricane surge and waves will not inundate the tunnel. This requirement is accommodated in the design by providing a crest curve in the profile, and surrounding the open end of the tunnel (referred to as the boat

section) with a wall. Refer to Figure 3 for an example of the

crest profile. Refer to Figure 4 for a cross-section of the boat section. Detailed research into this critical wall height elevation will have to be performed if this alternative is further developed. Elevation 15 ft. has been utilized for the conceptual analysis herein.

Figure 4: Cross-Section Through Boat Section

Section 3.0 MECHANICAL SYSTEMS

Figure3: Crest Profile at Kinney Tunnel in Fort Lauderdale, FL


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Tunnel mechanical systems include ventilation, fire protection and drainage systems. Of these three mechanical systems, only ventilation requirements have a significant effect on the overall size of the tunnel cross-section, and are addressed herein

3.1 VENTILATION

A highway requires a mechanical ventilation system for normal operations when the combination of the tunnel length, the roadway grade, the expected traffic flow pattern and the forecasted vehicle emission rate results in a tunnel environment that exceeds accepted air quality standards. The air quality standard that most often determines the need for mechanical ventilation during normal operations is the acceptable level of carbon monoxide (CO) concentration in the airstream.

A mechanical ventilation system is also considered a requirement by the National Fire Protection Association (NFPA) for tunnels whose length exceeds a certain value. Under Standard NFPA-502 (Standard for Road Tunnels, Bridges and other Limited Access Highways, 2004 Edition), provisions for emergency ventilation systems should be made for vehicle tunnels longer than 800 feet (ft). In the case of a tunnel fire, the stated design objectives for an emergency ventilation systems are: to provide a stream of non contaminated air to motorists in a path of egress away from a fire; to produce airflow rates to prevent backlayering of smoke in a path of egress away from a fire; and to limit the air temperature in a path of egress away from a fire to 60 degrees Celsius (140 degrees Fahrenheit) or less. These design objectives for tunnel emergency ventilation systems are intended to enhance the safety of motorists in the tunnel during a fire event by controlling the direction of smoke/heat migration and subsequently removing them from the tunnel environment. This would allow motorists to evacuate the incident tunnel under tenable environmental conditions.

Four types of ventilation systems have been investigated for this project: self-ventilating, longitudinal, semi-transverse and full transverse.

3.1.1 SELF-VENTILATION:

There are instances where the combination of the tunnel length, the roadway grade, the expected traffic flow pattern and the forecasted vehicle emission rate result in a CO concentration that is below the accepted air quality standard - a tunnel of this nature would be classified as being “self-ventilating” during normal traffic operations. In this case, the ventilating force for diluting CO emissions and directing CO concentrations toward the portals is provided by the “piston-action” of moving vehicles. The piston-action of moving vehicles would generate a flow of fresh air into the tunnel at the entrance portal and purge CO-laden airflows from the exit portal. In order for a tunnel to be self-ventilating during normal traffic operations, the tunnel length must be relatively short, the roadway grade must be relatively steep, the traffic flow rate must be relatively high (and unidirectional) and the forecasted vehicle emission rate should be relatively low. The tunnel to Anna Maria Island does not meet many of these requirements. An average maximum self-ventilated length today would be 1500 ft. The overall enclosed length of the tunnel to Anna Maria Island from


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portal to portal would be approximately 6,000 ft, far in excess of the length where self-ventilation is functional.

3.1.2 LONGITUDINAL VENTILATION

A longitudinal ventilation system utilizes either a central fan cluster, shown in Figure 5, or a jet fan system, shown in Figure 6, forcing a longitudinal airflow through the roadway cell supplementing the piston effect of the moving tunnel traffic. Figure 5: Central Fan Longitudinal Ventilation System

The central fan approach utilizes a cluster of fans at either end of the tunnel forcing air into the roadway cell. The fans are reversed in case of a fire to pull the smoke and heat out of the tunnel. The jet fan approach utilizes vane axial fans, which are installed at the tunnel ceiling at selected intervals along the length of the roadway cells. Each blows during normal operations in a single direction. Concerns regarding the ability of a longitudinal ventilation system to effectively and safely handle heat and smoke in a fire have been adequately addressed based on the results of the Memorial Tunnel Fire Ventilation Test Program (MTFVTP).

The longitudinal ventilation approach is the least costly method for ventilating a tunnel requiring forced ventilation. No parallel duct is required for either longitudinal system. The jet fan system has additional advantages in that it requires no ventilation building, air intakes or exhaust. While the longitudinal ventilation system requires increased headroom for the roadway, the absence of air ducts renders this system less costly than other ventilation systems.

The adequacy of a longitudinal ventilation system for the tunnel to Anna Maria Island will have to be investigated fully if this preliminary investigation determines the tunnel’s feasibility for further advancement. The proposed length of tunnel is currently above the limits of the state-of-the-art technology regarding longitudinal tunnel ventilation, and from our preliminary investigations does not appear feasible.

.

Figure 6: Jet Fan Longitudinal Ventilation System


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3.1.3 SEMI-TRANSVERSE VENTILATION

The semi-transverse approach employs one parallel duct for all or a major portion of its length, as shown in Figure 7. Openings between this parallel duct and the roadway permit uniform transfer of air from the duct to the roadway or the reverse, uniform exhausting from the roadway to the duct. A semi-transverse system can be configured for air supply, exhaust or both (where the fan system can reverse airflow at command). Most semi-transverse systems are designed to supply fresh air to dilute emissions uniformly along the tunnel to control CO discharging exhaust through the roadway cell at the portal. In a fire emergency, the flow is reversed so that heat and smoke are removed from the roadway area.

3.1.4 FULL TRANSVERSE VENTILATION

The full transverse approach employs supply and exhaust ducts throughout the length of the tunnel, as shown in Figure 8. Fresh air is supplied to the roadway cell similar to the semi-transverse system. Exhaust, however, is removed primarily through the exhaust duct with a minor portion of the pollutants or smoke exiting through the tunnel portal.

3.2 VENTILATION CONCLUSIONS

Due to the length of the tunnel, a transverse system is assumed to be required for the ventilation of the tunnel. Full transverse systems are more effective in bi-directional tunnels than semi-transverse systems, and this system is assumed in the development of the tunnel cross-sections.

Figure 7: Semi-Transverse Ventilation System

Figure 8: Full Transverse Ventilation System


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Section 4.0 TUNNEL DESIGN ALTERNATIVES

Three different methods can be used to construct a tunnel under a waterway. The purpose of this study is to determine the feasibility of the different construction methodologies, the method that would be the least disruptive, safest, most economical and serviceable, and would result in a tunnel that is an asset to the community.

Figure 9: Tunnel Boring Machine 4.1 BORED TUNNEL

This method of construction would excavate the tunnel in place, that is, to excavate the tunnel with the use of a tunnel boring machine (TBM). Based on the results of the geotechnical study, soft ground tunneling techniques appear to be feasible for this site. The TBM would excavate the earth along the path of the tunnel and, at the same time, install the concrete lining, segment by segment. The TBM is designed to exert sufficient forward pressure to hold back the water and soil pressure while grinding and removing the soil. This type of TBM, as shown in Figure 9, was used for the M30 project in Madrid, Spain.

Refer to Figures 10a and 10b for the cross-sections of the bored tunnel alternative. The circular tunnel for Option A would require a boring about 69 feet in diameter. This option would provide two 12-foot (ft) travel lanes (bi-directional), 10-ft shoulders and a 12-ft multi-use pathway. The circular tunnel for Option B would require a boring about 78 ft. This option would provide two 12-ft travel lanes (bi-directional), 10-ft shoulders and a 10-ft sidewalk on each side. Each concrete liner segment is reinforced and measures


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approximately 4 ft wide and 1.5 ft thick. The thickness of the liner segments are determined by the capacity of the liner to withstand the jacking forces necessary to advance the TBM. It should be noted that this bore diameter exceeds in diameter the largest TBM built or planned in the world.

Figure 10a: Bored Tunnel Cross-Section – Option A


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Figure 10b: Bored Tunnel Cross-Section – Option B The desired cover over a tunnel boring machine in soft ground is typically one bore diameter, or 69 ft for Option A. Reducing the cover to less than the one diameter creates problems with balancing the face pressure. During construction, if the face pressure becomes too high, the face may tend to bubble to the surface causing possible flooding of the TBM and the tunnel. During construction and operation, reduced cover over the tunnel results in a lower factor of safety against buoyancy. A preliminary analysis of the buoyancy of the proposed section indicates that anchors and/or lead ballast will be required to hold the section down. This is typically a problem with very large diameter bores since a large unused vertical space is provided over the roadway, increasing the buoyancy.

The greatest water depth encountered during the investigation is approximately 10 ft deep located approximately 1500 ft offshore of Anna Maria Island. The roadway profile at this point would be approximately 10 ft of water depth + 69 ft of cover + 41 ft from top of bore to roadway level, or 120 ft from sea level down to the roadway profile. The resulting profile, shown in Figure 11 (using 7 percent grades and a 35-mile per hour (mph) design speed), would transition into the causeway nearly one half-mile inland of the shoreline, 240 ft east of East Bay Drive. The Option B tunnel profile transitions to the causeway too close to East Bay Drive and is not considered further. Refer to Figure 12 for the associated right-of-way (ROW) impacts. The bored tunnel alternative is determined to be unfeasible due to the excessive bore diameter, the expense of anchoring the tunnel to resist buoyancy and the excessive profile demands resulting from the depth of the tunnel adjacent to Anna Maria Island.


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Figure 11: Bored Tunnel Profile – Option A

Figure 12: Bored Tunnel Right-of-Way Impacts


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4.2 IMMERSED TUBE TUNNEL

This construction method consists of dredging a trench along the bottom of the waterway and immersing tunnel sections end-to-end within the trench. Prefabricated tunnel sections are floated into place, as shown in Figure 13, ballasted to overcome buoyancy, placed in the excavation, then jacked against the previously placed section. Figure 14 provides some typical means utilized for sinking and positioning of the tunnel segments. The trench is then backfilled by placing the excavated material on top of the tunnel units. There are two options that were evaluated for this method. One is the steel shell section fabricated off-site, possibly in a shipyard, and towed to a deep water site where outfitting would be completed, and the other is the concrete shell section fabricated in a graving basin or dry dock near the site. The immersed tube sections, shown in Figure 15a and Figure 15b, would be rectangular to minimize dredging depths and tunnel approach lengths.

Figure13: Immersed Tube Transportation

Figure 14: Immersed Tube Construction


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Figure 15a: Immersed Tube Typical Section – Option A

Figure 15b: Immersed Tube Typical Section – Option B 4.2.1 STEEL SHELL

A steel shell tunnel section consists of a reinforced concrete lining, which is installed within a continuous steel plate, in this case approximately half-inch thick. The steel shell not only serves as the exterior form for the concrete lining, but also for strength and as a watertight seal. The steel shell tunnel units are initially constructed in a slipway in one or more off-site locations, such as a shipyard or similar facility, launched and then towed to an outfitting area. Because of the shallow draft of the steel shell after launching, the unit can be towed long distances from the shipyard to the outfitting area. The outfitting operation would include completion of the inner concrete lining and road deck and attachment of sinking


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Figure 16: Graving Basin

equipment, such as masts, pontoons, winches and jacks. The necessary balance of weight and buoyancy for the sinking may be maintained by the use of pontoons when the unit has a net negative buoyancy or the unit may remain self buoyant, requiring the addition of ballast concrete around the tunnel section.

4.2.2 CONCRETE SHELL

Unlike the steel shell tunnels, the practice for concrete shell tunnels is to construct all the units in one cycle, if possible, in a graving area or dry dock. A typical graving basin is shown in Figure 16. Such a natural site is not available nearby; therefore, one must be constructed if a concrete tunnel is used. Due to the length of tunnel required, a graving basin of approximately 40 acres would be required to construct all of the concrete shell sections in one cycle. If the graving area is not large enough to construct the entire tunnel units in one cycle, the entire procedure for fabricating the units must be repeated, (i.e., rebuild the floodgate, dewater and maintain pumping to keep a dry basin during the fabrication, fabricate units) flood graving area and remove the floodgate.

Because the graving area must be maintained in a dry condition at all times during fabrication of the tunnel units at an elevation of approximately -34 ft (MSL), a pumping plant of sufficient capacity is required. It is essential that by lowering the ground water by 34 ft, subsidence of the surrounding area does not occur that can cause damage to adjacent existing structures or utilities.

The material within the excavated graving area must be able to support the weight of the tunnel units so settlement that can induce stress in the concrete does not occur. A location for a graving basin near the site has not been identified.

When the concrete units have been completed in the graving dock, they are fitted with end bulkheads and, as appropriate, alignment towers and temporary access shafts, before filling the graving dock with water.

4.2.3 IMMERSED TUBE CONCLUSIONS

Immersed tubes can be constructed with a minimum of 5 ft of permanent cover over the top of the section. The section shown in Figure 15 allows the section to be floated into place, and then ballasted for sinking. The final factor of safety for buoyancy can be accomplished through the use of additional ballast. Refinement of the section to achieve adequate factors of safety without the use of lead ballast should be investigated if this alternative is advanced to preliminary design.


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The greatest water depth encountered during the investigation is approximately 10 ft deep located approximately 1500 ft offshore of Anna Maria Island. The profile at this point would be approximately 10 ft of water depth + 5 ft of cover +10 ft of assumed general scour + 23 ft from top of section to roadway level, or 48 ft from sea level down to the roadway profile. The resulting profile, shown in Figure 17, would emerge approximately 1430 ft inland of the shoreline, 1170 ft east of East Bay Drive, using 7 percent grades and a 35 mph design speed. Due to the resulting distance from the end of the tunnel to East Bay Drive, a lower grade and higher design speed can be considered with this alternative.

Figure 17: Immersed Tube Profile The major disadvantage of this system is that the existing water depth is insufficient to float the immersed tube sections. The outfitted and ready-to-sink sections will carry a draft of approximately 29 ft. Water depths along the proposed alignment vary from 10 ft at its deepest to 2 ft at its shallowest requiring substantial dredging. Turbidity barriers would be required and would be difficult to maintain in place in the swift currents at the site. Also, to maintain the integrity of the existing bridge, the centerline of the tunnel would have to be located 350 to 400 ft from the existing bridge centerline, creating ROW issues at landfall for the construction of the tunnel and the construction of ventilation and security monitoring facilities. Refer to Figure 18 for the associated ROW impacts.


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Figure 18: Immersed Tube Right-of-Way Impacts It is assumed that since a graving basin cannot be located adjacent to the site that the steel shell immersed tube alternative would be utilized. Outfitting of the steel shell would most likely occur at one of the mooring facilities at the Port of Tampa and towed to the final tunnel alignment.

This would require a minimum water depth of approximately 33 ft for the entire journey. Dredging of the channel to the Gulf of Mexico will also have to address relocation of the subaqueous utility cable corridor southeast of the bridge. Water depths around the island are shown in Figure 19.

Based on the excessive dredging requirement for this alternative at and leading to the project site, the immersed tube tunnel alternative is determined to be economically and environmentally unfeasible due to water quality, turbidity, and seagrass impacts.

4.3 CUT AND COVER TUNNEL

The cut and cover tunnel is a conventional reinforced cast-in-place concrete structure, and its construction requires that the work be performed in the dry. That is, it is to be constructed in a dewatered trench kept dry during the construction period. Refer to Figures 20a and 20b for the cut and cover typical sections. The tunnel portals and the approach boat sections, shown in Figure 4, will also be constructed using the cut and cover method.


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Figure 19: Existing Water Depths (feet)

The temporary excavations that are to be dewatered need to be adequately designed to account for significant seepage forces. To counter the upward seepage forces at the bottom of the excavations, various methods are considered. One method is to develop sufficient weight to equalize the uplift forces. This is done by placing concrete of required thickness in a cellular cofferdam that is formed by continuous steel sheet piling. Refer to Figure 21 for pictures of typical cofferdam construction. The cofferdam is dewatered after the concrete is hardened. This method requires that the excavation within the cofferdam be deepened due to the thickness of the mass concrete tremie seal; this is neither practical nor economical. A second method for countering the hydrostatic uplift force is to install anchors into competent rock in order to anchor the concrete tremie slab, which would be poured from face-to-face of the temporary sheet piling or cofferdam at the bottom of the excavation. Seepage forces may also be reduced by increasing the drainage path of the water. This can be accomplished by driving the sheet piling deeper or by driving the piling into an impermeable stratum. A grout blanket can also be implemented to reduce the porosity of the underlying soils.


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Figure 20a: Cut and Cover Typical Section – Option A

Figure 20b: Cut and Cover Typical Section – Option B


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Figure 21: Cut and Cover Right-of-Way Impacts The continuous steel sheet piling forming the cofferdam perimeter would be braced by struts or anchored tie backs to resist the water and earth pressures where necessary for specific locations. Where a large width cofferdam is required in open water, internal bracing is not practical and tie back bracing is not feasible. In such case use of cellular cofferdams becomes necessary. Cofferdams afford the flexibility necessary for temporary excavations. Steel sheeting can be removed and re-installed to build the various phases of construction. In order to maintain stability of the existing bridge during construction of the dry trench, the tunnel alignment would need to be at least 400 ft from the existing bridge. This alignment would result in impacts to mangroves, seagrass beds, and the Neal Preserve. In addition, the archaeological artifacts know to be present on the east end of the existing bridge would be disturbed.

The greatest water depth encountered during the investigation is approximately 10 ft deep located approximately 1500 ft offshore of Anna Maria Island. The profile at this point would be approximately 10 ft of water depth + 5 ft of cover +10 ft of assumed general scour + 20 ft from top of section to roadway level, or 45 ft from sea level down to the roadway profile. The resulting profile is similar to the immersed tube section profile (Figure 17) and would emerge approximately 1430 ft inland of the shoreline, 1170 ft east of East Bay Drive, using 7 percent grades and a 35 mph design speed. Due to the resulting distance from the end


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Figure 22: Cofferdam Construction

of the tunnel to East Bay Drive, a lower grade and higher design speed can be considered with this alternative.

One of the major disadvantages of this system is the extensive amount of excavation that will be required for the construction. Water depths along the proposed alignment vary from 10 ft at its deepest to 2 ft at its shallowest. The width of the excavation is significantly larger than the tunnel section to provide for construction equipment and materials access. The typical dredging depth will be approximately 40 to 50 ft resulting in a large quantity of dredge material. A spoil area of sufficient size would have to be created to handle the dredge material.

To maintain the integrity of the existing bridge, the centerline of the tunnel would have to be located a minimum of 200 ft from the existing bridge centerline, creating ROW issues at landfall for the construction of the tunnel and the construction of ventilation and security monitoring facilities. These issues, however, are substantially less than those for the immersed tube alternative.


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Section 5.0 CONSTRUCTION COST ESTIMATES

5.1 LIMITATIONS AND CONSTRAINTS

Traditional construction cost estimating methods that rely on historical cost data are not well suited for feasibility studies for underground structures. Construction costs vary widely because of subsurface, geographic, and other project-specific parameters. In addition, tunnel construction costs are not generally available in cost databases. Furthermore, the inherently expensive and unknown nature of underground construction often leads to inaccurate cost estimates, which can lead to a significant budget shortfall as the project moves from planning and design to construction. This means that tunnels and other associated underground work cannot be estimated on a "per foot basis" as is customarily done during the early stages of an above-ground transportation project. Labor factors and geologic conditions will often dictate tunnel construction costs. For example, tunnels in Boston have different labor factors than tunnels in Los Angeles, and tunnels in downtown San Francisco through soft-ground will have very different costs than tunnels through weak rock on the west side of San Francisco.

Tunnel construction, is by its very nature, complex, risky, and often fraught with geologic unknowns. In urban areas, a bored tunnel has a distinct advantage of minimizing surface disruptions compared to surface or cut-and-cover configurations. The cut and cover or immersed tube construction approach would be the most suitable for this study.

Tunnel construction progress is also highly dependent on how well the tunneling equipment has been matched to the physical constraints within the tunnel and the subsurface conditions. Major tunneling equipment would need to be specially fabricated for this project. Consequently, since tunnels are linear features requiring linear construction sequencing, small variances in production produce large variations in construction costs as those small production variances are applied against the relatively high hourly costs of labor and equipment. Nearly any variance in equipment performance will affect productivity.

Tunnel construction is therefore highly dependent on production rate of the tunnel crew and the capability of the equipment used, and costs in turn are highly dependent on the physical subsurface conditions encountered.

Geography: The underground work requires specialized labor skills, which is not readily available in Florida so the labor pool would need to be relocated from other parts of the United States or overseas. In addition, underground and related labor unions have varying amounts of influence in different parts of the United States and in different countries. For example, east coast union manning requirements differ from west coast requirements. In addition, regional labor forces· are likely more accustomed to performing work using means and methods that are cost effective locally, but not in other regions. Labor for underground


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work is not readily available in Florida. In addition, the equipment is not readily available and would need to be transported to the area.

Geology: Difficult geologic conditions result in higher construction costs, and conversely favorable geologic conditions can reduce construction costs. Tunnel advance rates in competent rock can easily exceed those in weak soils as the case for Anna Maria under the groundwork table by a factor of 4:1. This means it would take two to three times longer to construct a tunnel at Anna Maria Island than a location with competent rock, since the soils are comprised of sands, silts, clays, and the subordinate limestone. In addition, some geologic conditions are fairly unique to a locale, such as the marine clays of the San Francisco Bay Area ("Bay Mud") or the glacial clays in Boston ("Boston Blue Clay"). The construction of tunneling at Anna Maria would be very similar to construction of the Boston Blue Clay. In these special geologic conditions, the construction engineering and contracting industries have developed specific, long-established approaches to addressing these geotechnical challenges. In addition, tunnel construction is largely a function of excavation considerations and ground support requirements, meaning that settlement of the tunnel sections could vary beyond acceptable tolerances. In general, rock tunnels are harder to excavate, but easier to support, whereas the reverse is true for tunnels in soil. In the case of Anna Maria, tunnel construction will be difficult.

Allocation of Risk: Many owners now recognize the good business sense of allocating risk to the party best suited to manage that risk. On underground projects, risk allocation measures such as Disputes Review Boards, Geotechnical Baseline Reports, and the use of a Differing Site Conditions clause are often used. The efficacy of other risk allocation measures such as Owner Controlled Insurance Programs and Escrow Bid Documentation has yet to be determined. Regardless, the types of risk allocation measures, or the lack thereof, have a profound influence on a contractor's decision whether to bid a project and the amount of contingency placed in a bid for risk.

Market Conditions: When you are dealing with tunnel construction in a location where soil conditions vary and is very poor, the amount of risk the contractor will allocate to the project is not known and therefore the contractor may bid the project high, or may elect to bid other projects with less risk The size and complexity of a project has a large influence on the number of prospective contractors that have the financial capacity and technical experience needed to pursue the work. Large bonding and other financial requirements can be met by either a handful of large contractors or by smaller contractors that form a joint venture. Technical capability comes from personnel experience or by specialty contractors that do certain types of work, (e.g., jet grouting, soil-cement walls, etc.). In either case, financial and technical hurdles can reduce competition, and thus increase bid prices.

5.2 TUNNEL COST ESTIMATING IN PRACTICE

Recent underground projects in the United States have run into budget difficulties, whether real or perceived. Sometimes these problems are related to politics and poor communication with the press and public. In other cases, however, budget problems have been the result of inaccurate cost estimates. Many of these problems can be grouped into one of five categories:


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Location: Tunnel should be only considered for very congested urban location to reduce the cost of utility conflicts and eliminate the impact of traffic maintenance issue. In the case of Anna Maria Island, the tunnel would not have the issue with utility conflicts, vehicle traffic or marine traffic. In the case of the Chesapeake Bay Bridge-Tunnel and the Port of Miami tunnel, the vessel traffic is composed of very large aircraft carriers and cruise ships that would require extraordinary long and high bridge spans. In addition, design of any bridge to withstand ship impact by vessels of this size creates difficult design requirements.

Construction: The construction of a tunnel to replace the Anna Maria Island bridge would be located 300 feet (ft) from the existing bridge. A temporary navigation detour would be required during construction to allow for excavation and dewatering. This would involve dredging a new temporary channel or total closure of the Intracoastal Waterway for portions of the construction duration.

Duration and sequence of construction: The construction of the tunnel would take approximately five years, including the construction of a cofferdam “island” along both sides of the proposed tunnel to allow dewatering and construction equipment as well as to prevent any storms from causing salt water from entering the tunnel during construction. This would impact navigation.

Security and Maintenance: A ventilation system, a fire alarm system, and a dewatering system will be required to meet modern design standards. Trained maintenance mechanics should be available around the clock for operation and maintenance. The tunnel should have a separate electrical system protected at all time from weather emergencies.

Hurricane and Evacuation: It is anticipated that during hurricanes the tunnel will be completely closed to account for the possibility of power loss resulting in loss of lighting and ventilation of the tunnel and loss of positive drainage and dewatering systems.

Scope Creep: As a project develops, the scope can increase due to subsequent third party agreements that are required to keep the project moving forward. In addition, as a project moves into preliminary design and geotechnical explorations are conducted; unforeseen conditions may be revealed which change the scope of work.

Reduction of Contingencies: Cost contingencies related to project definition is typically reduced as the project becomes better defined during the final design phase. However, there are some contingencies endemic to underground construction that remains relatively constant throughout the cost estimating process.

Project Size: There has been a tendency in the past decade to combine smaller contracts into "mega" contracts, with the intention of reducing coordination efforts on the owner's part. But this coordination does not go away; it is merely shifted to the contractor, who will add contingency to his bid based on the complexity and risk inherent in the project.

Project Delivery Method: The Federal Highway Administration (FHWA), along with other agencies, has begun to use design-build and other project delivery methods on new projects. Contractors' inexperience with alternate project delivery methods, or the perception that


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owners are not experienced with alternate delivery methods, also leads to higher contingencies in price.

Escalation and Year of Expenditure (VOE): Large, complex projects often take many years to get through the environmental approval process. As a result, aggressive project implementation schedules sometimes cannot be met, and completion dates are delayed. Cost estimates must be escalated if the VOE is delayed.

Maintenance and Operation Costs: It is anticipated that the annual maintenance and operation cost will be about one percent of the total construction cost, between $3 to $5 million per year, totaling $225 to $375 million over 75 years (not including inflation).

5.3 COST CONCLUSIONS

Concept-level preliminary quantities resulted in an estimated cut-and-cover tunnel construction cost of $412 million (2008 dollars). At this conceptual stage of the project, a range of +25 percent in the actual construction cost may be expected (resulting in a range of $309 million to $515 million). As design progresses, this range may become smaller, but it will never be possible to determine an absolute number for the estimated cost of construction. The detailed preliminary cost estimate is provided in the Appendix.

The base estimate of probable construction cost does not include costs for existing bridge demolition ($773,500), environmental mitigation, utility relocations or right-of-way (ROW) acquisition. Based on this estimate, the tunnel alternative cost far exceeds the cost of any of the bridge replacement/rehabilitation alternatives.

Similar planning is underway by the Chesapeake Bay Bridge and Tunnel District to construct two new parallel tunnels in the next 10 to 15 years at a total cost currently estimated at $688 million.


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Section 6.0 TUNNEL ALTERNATIVE VIABILITY

The Tunnel Alternative was not found to be a viable alternative based on the following factors considered in this analysis:

• Geologic conditions • Geography and availability of skilled labor and equipment • Risk allocation • Environmental impacts • Navigational impacts • Water quality during construction • Constructability • Security of infrastructure • Cost

APPENDICES

Appendix A: Construction Cost Estimate

APPENDIX A CONSTRUCTION COST ESTIMATE

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