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Technical Manual for Design and Construction of Road Tunnels - Civil Elements
Chapter 5 - Cut and Cover Tunnels
This chapter presents the construction methodology and excavation support systems for cut-and-cover road tunnels and describes the structural design in accordance with the AASHTO LRFD Bridge Design Specifications (AASHTO, 2008). The intent of this chapter is to provide guidance in the interpretation of the AASHTO LRFD Specifications in order to have a more uniform application of the code and to provide guidance in the design of items not specifically addressed in AASHTO (2008). The designers must follow the latest LRFD Specifications. A design example illustrating the concepts presented in this chapter can be found in Appendix C. Other considerations dealing with support of excavation, maintenance of traffic and utilities, and control of groundwater and how they affect the structural design are discussed.
5.2 Construction Methodology
In a cut and cover tunnel, the structure is built inside an excavation and covered over with backfill material when construction of the structure is complete. Cut and cover construction is used when the tunnel profile is shallow and the excavation from the surface is possible, economical, and acceptable. Cut and cover construction is used for underpasses, the approach sections to mined tunnels and for tunnels in flat terrain or where it is advantageous to construct the tunnel at a shallow depth. Two types of construction are employed to build cut and cover tunnels; bottom-up and top-down. These construction types are described in more detail below. The planning process used to determine the appropriate profile and alignment for tunnels is discussed in Chapter 1 of this manual.
Figure 5-1 is an illustration of cut and cover tunnel bottom-up and top-down construction. Figure 5-1(a) illustrates Bottom-Up Construction where the final structure is independent of the support of excavation walls. Figure 5-1(b) illustrates Top-Down Construction where the tunnel roof and ceiling are structural parts of the support of excavation walls.
Figure 5-1 Cut and Cover Tunnel Bottom-Up Construction (a); Top-Down Construction (b)
For depths of 30 to 40 feet (about 10 m to 12 m), cut-and-cover is usually more economical and more practical than mined or bored tunneling. The cut-and-cover tunnel is usually designed as a rigid frame box structure. In urban areas, due to the limited available space, the tunnel is usually constructed within a neat excavation line using braced or tied back excavation supporting walls. Wherever construction space permits, in open areas beyond urban development, it may be more economical to employ open cut construction.
Where the tunnel alignment is beneath a city street, the cut-and-cover construction will cause interference with traffic and other urban activities. This disruption can be lessened through the use of decking over the excavation to restore traffic. While most cut-and-cover tunnels have a relatively shallow depth to the invert, depths to 60 feet (18 m) are not uncommon; depths rarely exceed 100 feet (30 m).
Although the support of excavation is an important aspect of cut and cover construction, the design of support of excavation, unless it is part of the permanent structure, is not covered in this chapter.
5.2.2 Conventional Bottom-Up Construction
As shown in Figure 5-2, in the conventional "bottom-up" construction, a trench is excavated from the surface within which the tunnel is constructed and then the trench is backfilled and the surface restored afterward. The trench can be formed using open cut (sides sloped back and unsupported), or with vertical faces using an excavation support system. In bottom-up construction, the tunnel is completed before it is covered up and the surface reinstated.
Figure 5-2 Cut-and-Cover Tunnel Bottom-Up (a) and Top-Down (b) Construction Sequence
Conventional bottom-up sequence of construction in Figure 5-2(a) generally consists of the following steps:
- Step 1a: Installation of temporary excavation support walls, such as soldier pile and lagging, sheet piling, slurry walls, tangent or secant pile walls
- Step 1b: Dewatering within the trench if required
- Step 1c: Excavation and installation of temporary wall support elements such as struts or tie backs
- Step 2: Construction of the tunnel structure by constructing the floor;
- Step 3: Compete construction of the walls and then the roof, apply waterproofing as required;
- Step 4: Backfilling to final grade and restoring the ground surface.
Bottom-up construction offers several advantages:
- It is a conventional construction method well understood by contractors.
- Waterproofing can be applied to the outside surface of the structure.
- The inside of the excavation is easily accessible for the construction equipment and the delivery, storage and placement of materials.
- Drainage systems can be installed outside the structure to channel water or divert it away from the structure.
Disadvantages of bottom-up construction include:
- Somewhat larger footprint required for construction than for top-down construction.
- The ground surface can not be restored to its final condition until construction is complete.
- Requires temporary support or relocation of utilities.
- May require dewatering that could have adverse affects on surrounding infrastructure.
5.2.3 Top-Down Construction
With top-down construction in Figure 5-2 (b), the tunnel walls are constructed first, usually using slurry walls, although secant pile walls are also used. In this method the support of excavation is often the final structural tunnel walls. Secondary finishing walls are provided upon completion of the construction. Next the roof is constructed and tied into the support of excavation walls. The surface is then reinstated before the completion of the construction. The remainder of the excavation is completed under the protection of the top slab. Upon the completion of the excavation, the floor is completed and tied into the walls. The tunnel finishes are installed within the completed structure. For wider tunnels, temporary or permanent piles or wall elements are sometimes installed along the center of the proposed tunnel to reduce the span of the roof and floors of the tunnel.
Top-down sequence of construction generally consists of the following steps:
- Step 1a : Installation of excavation support/tunnel structural walls, such as slurry walls or secant pile walls
- Step 1b: Dewatering within the excavation limits if required
- Step 2a: Excavation to the level of the bottom of the tunnel top slab
- Step 2b: Construction and waterproofing of the tunnel top slab tying it to the support of excavation walls
- Step 3a: Backfilling the roof and restoring the ground surface
- Step 3b: Excavation of tunnel interior, bracing of the support of excavation walls is installed as required during excavation
- Step 3c: Construction of the tunnel floor slab and tying it to the support of excavation walls; and
- Step 4 Completing the interior finishes including the secondary walls.
Top-down construction offers several advantages in comparison to bottom-up construction:
- It allows early restoration of the ground surface above the tunnel
- The temporary support of excavation walls are used as the permanent structural walls
- The structural slabs will act as internal bracing for the support of excavation thus reducing the amount of tie backs required
- It requires somewhat less width for the construction area
- Easier construction of roof since it can be cast on prepared grade rather than using bottom forms
- It may result in lower cost for the tunnel by the elimination of the separate, cast-in-place concrete walls within the excavation and reducing the need for tie backs and internal bracing
- It may result in shorter construction duration by overlapping construction activities
Disadvantages of top-down construction include:
- Inability to install external waterproofing outside the tunnel walls.
- More complicated connections for the roof, floor and base slabs.
- Potential water leakage at the joints between the slabs and the walls
- Risks that the exterior walls (or center columns) will exceed specified installation tolerances and extend within the neat line of the interior space.
- Access to the excavation is limited to the portals or through shafts through the roof.
- Limited spaces for excavation and construction of the bottom slab
It is difficult to generalize the use of a particular construction method since each project is unique and has any number of constraints and variables that should be evaluated when selecting a construction method. The following summary presents conditions that may make a one construction method more attractive than the other. This summary should be used in conjunction with a careful evaluation of all factors associated with a project to make a final determination of the construction method to be used.
Conditions Favorable to Bottom-Up Construction:
- No right-of way restrictions
- No requirement to limit sidewall deflections
- No requirement for permanent restoration of surface
Conditions Favorable to Top-Down Construction
- Limited width of right-of-way
- Sidewall deflections must be limited to protect adjacent features
- Surface must be restored to permanent usable condition as soon as possible
5.3 Support of Excavation
The practical range of depth for cut and cover construction is between 30 and 40 feet (about 10 m to 12 m). Sometimes, it can approach 100 feet. Excavations for building cut and cover tunnels must be designed and constructed to provide a safe working space, provide access for construction activities and protect structures, utilities and other infrastructure adjacent to the excavation. The design of excavation support systems requires consideration of a variety of factors that affect the performance of the support system and that have impacts on the tunnel structure itself. These factors are discussed hereafter.
Excavation support systems fall into three general categories:
- Open cut slope: This is used in areas where sufficient room is available to open cut the area of the tunnel and slope the sides back to meet the adjacent existing ground line (Figure 5‑3). The slopes are designed similar to any other cut slope taking into account the natural repose angle of the in-situ material and the global stability.
- Temporary: This is a structure designed to support vertical or near vertical faces of the excavation in areas where room to open cut does not exist. This structure does not contribute to the final load carrying capacity of the tunnel structure and is either abandoned in placed or dismantled as the excavation is being backfilled. Generally it consists of soldier piles and lagging, sheet pile walls, slurry walls, secant piles or tangent piles.
- Permanent: This is a structure designed to support vertical or near vertical faces of the excavation in areas where room to open cut does not exist. This structure forms part of the permanent final tunnel structure. Generally it consists of slurry walls, secant pile walls, or tangent pile walls.
Figure 5-3 Cut and Cover Construction using Side Slopes Excavation- Ft McHenry Tunnel, Baltimore, MD
This section discusses temporary and permanent support of excavation systems and provides issues and concerns that must be considered during the development of a support of excavation scheme. The design of open-cut slopes and support of excavation are not in the scope of this manual. Information on the design of soil and rock slopes can be found in FHWA-NHI-05-123 "Soil Slope and Embankment Design" (FHWA, 2005d), and NHI-99-007 "Rock Slopes" (FHWA, 1999), respectively. Supports of Excavation are referred to FHWA-NHI-05-046 "Earth Retaining Structure" (FHWA, 2005e). Many of the issues described below associated with ground and groundwater behavior are applicable to side slopes also.
5.3.2 Temporary Support of Excavation
Support of excavation structures can be classified as flexible or rigid. Flexible supports of excavation include sheet piling and soldier pile and lagging walls. A careful site investigation that provides a clear understanding of the subsurface conditions is essential to determining the correct support system. Rigid support of excavation such as slurry walls, secant piles or tangent piles are also used as temporary support of excavation. Descriptions of these systems are provided Section 5.3.3 Permanent Support of Excavation.
A sheet piling wall consists of a series of interlocking sheets that form a corrugated pattern in the plan view of the wall. The sheets are either driven or vibrated into the ground. The sheets extend well below the bottom of the excavation for stability. These sheets are fairly flexible and can support only small heights of earth without bracing. As the excavation progresses, bracings or tie backs are installed at specified intervals. Sheet pile walls can be installed quickly and easily in ideal soil conditions. The presence of rock, boulders, debris, utilities, or obstructions will make the use of sheet piling difficult since these features will either damage the sheet pile or in the case of a utility, be damaged by the sheet pile. Figure 5-4 shows a sheet pile wall with complex multi level internal bracing.
Figure 5-4 Sheet Pile Walls with Multi Level-Bracing
A soldier pile wall consists of structural steel shape columns spaced 4 to 8 feet apart and driven into the ground or placed in predrilled holes. The soldier piles extend well below the level of the bottom of excavation for stability. As the excavation progresses, lagging is placed between the soldier piles to retain the earth behind the wall. The lagging could be timber or concrete planks. The soldier piles are relatively flexible and are capable of supporting only modest heights of earth without bracing. As the excavation progresses, bracing or tie backs are installed at specified intervals. Soldier piles can also be installed in more different ground conditions than can a sheet pile wall. The spacing allows the installation of piles around utilities. The finite dimension of the pile allows drilling of holes through obstructions and into rock, making the soldier pile and lagging wall more versatile than the sheet pile wall. Figure 5-5 shows a braced soldier pile and lagging wall.
Figure 5-5 Braced Soldier Pile and Lagging Wall
Support of excavation bracing can consist of struts across the excavation to the opposite wall, knee braces that brace the wall against the ground, and tie backs consisting of rock anchors or soil anchors that tie the wall back into the earth behind the wall. Struts and braces extend into the working area and create obstacles to the construction of the tunnel. Tie backs do not obstruct the excavation space but sometimes they extend outside of the available right-of-way requiring temporary underground easements. They may also encounter obstacles such as boulders, utilities or building foundations. The suitability of tie backs depends on the soil conditions behind the wall. The site conditions must be studied and understood and taken into account when deciding on the appropriate bracing method. Figure 5-6 shows an excavation braced by tie-backs, leaving the inside of the excavation clear for construction activities.
The design and detailing of the support of excavation must consider the sequence of installation and account for the changing loading conditions that will occur as the system is installed. The design of temporary support of excavation is not in the scope of this manual. The information presented herein is intended to make tunnel designers aware of the impact that the selected support of excavation can have on the design, constructability and serviceability of the tunnel structure. Guidance on the design of support of excavation can be found in FHWA-NHI-05-046 "Earth Retaining Structure" (FHWA, 2005e).
Figure 5-6 Tie-back Excavation Support leaves Clear Access
Use of temporary support of excavation does have the advantage of allowing waterproofing to be applied to the outside face of the tunnel structure. This can be accomplished by setting the face of the support of excavation away from the outside face of the tunnel structure. This space provides room for forming and allows the placement of waterproofing directly onto the finished outside face of the structure. As an alternate, the face of the support of excavation can be placed directly adjacent to the outside face of the structure. Under this scenario, the face of the support of excavation is used as the form for the tunnel structure. Waterproofing is installed against the support of excavation and concrete is poured against the waterproofing. In this case, the temporary support of excavation wall is abandoned in place.
5.3.3 Permanent Support of Excavation
Permanent support of excavation typically employs rigid systems. Rigid systems consist of slurry walls, soldier pile tremie concrete (SPTC) walls, tangent pile walls, or secant pile walls. As with temporary support of excavation systems, a careful site investigation that provides a clear understanding of the subsurface conditions is essential to determining the appropriate system.
A slurry wall is constructed by excavating a trench to the thickness required for the external structural wall of the tunnel. Slurry walls are usually 30 to 48 inches thick. The trench is kept open by the placement of bentonite slurry in the trench as it is excavated. The trench will typically extend for some distance below the bottom of the tunnel structure for stability. Reinforcing steel is lowered into the slurry filled trench and concrete is then placed using the tremie method into the trench displacing the slurry. The resulting wall will eventually be incorporated into the final tunnel structure. Excavation proceeds from the original ground surface down to the bottom of the roof of the tunnel structure. The tunnel roof is constructed and tied into the slurry wall. The tunnel roof provides bracing for the slurry wall. Depending on the depth of the tunnel, the roof could be the first level of bracing or an intermediate level. The excavation would then proceed and additional bracing would be provided as needed. At the base of the excavation, the tunnel bottom slab is then constructed and tied into the walls. Figure 5-7 shows a slurry wall supported excavation in an urban area.
Figure 5-7 Braced Slurry Walls
SPTC walls are constructed in the same sequence as a slurry wall. However, once the trench is excavated, steel beams or girders are lowered into the slurry in addition to reinforcing steel to provide added capacity. The construction of the wall then follows the same sequence as that described above for a slurry wall.
Tangent pile (drilled shaft) walls consist of a series of drilled shafts located such that the adjacent shafts touch each other, hence the name tangent wall. The shafts are usually 24 to 48 inches in diameter and extend below the bottom of the tunnel structure for stability. The typical sequence of construction of tangent piles begins with the excavation of every third drilled shaft. The shafts are held open if required by temporary casing. A steel beam or reinforcing bar cage is placed inside the shaft and the shaft is then filled with concrete. If a casing is used, it is pulled as the tremie concrete placement progresses. Once the concrete backfill cures sufficiently, the next set of every third shaft is constructed in the same sequence as the first set. Finally, after curing of the concrete in the second set, the third and final set of shafts is constructed, completing the walls. Excavation within the walls then proceeds with bracing installed as required to the bottom of the excavation. Roof and floor slabs are constructed and tied into the tangent pile. The roof and floor slabs act as bracing levels. Figure 5-8 is a schematic showing the sequence of construction in plan view. Figure 5-9 shows a completed tangent pile wall.
Figure 5-8 Tangent Pile Wall Construction Schematic
Figure 5-9 Tangent Pile Wall Support
Secant pile walls are similar to tangent pile walls except that the drilled shafts overlap each other rather than touch each other. This occurs because the center to center spacing of secant piles is less than the diameter of the piles. Secant pile walls are stiffer than tangent piles walls and are more effective in keeping ground water out of the excavation. They are constructed in the same sequence as tangent pile walls. However, the installation of adjacent secant piles requires the removal of a portion of the previously constructed pile, specifically a portion of the concrete backfill. Figure 5-10 is a schematic showing a plan view of a completed secant pile wall.
Figure 5-10 Completed Secant Pile Wall Plan View
In general, rigid support systems have more load carrying capacity than flexible systems. This additional load carrying capacity means that they require less bracing. Minimizing the amount of bracing results in fewer obstruction inside the excavation if struts or braces are used, making construction activities easier to execute. Rigid wall systems incorporated into the final structure can also reduce the overall cost of the structure because they combine the support of excavation with the final structure. Waterproofing permanent support walls and detailing the connections between the walls and other structure members are difficult. This difficulty can potentially lead to leakage of groundwater into the tunnel. The design and detailing of the support of excavation must consider the sequence of installation and account for the changing loading conditions that will occur as the excavation proceeds and the system is installed.
5.3.4 Ground Movement and Impact on Adjoining Structures
An important issue for cut-and-cover tunnel analysis and design is the evaluation and mitigation of construction impacts on adjacent structures, facilities, and utilities. By the nature of the methods used, cut-and-cover constructions are much more disruptive than bored tunnels. It is important for engineers to be familiar with analytical aspects of evaluating soil movement as a result of the excavation, and the impacts it can have on existing buildings and utilities at the construction site. Soil movement can be due to deflection of the support of excavation walls and ground consolidation:
- Deflection of support of excavation walls: Walls will deflect into the excavation as it proceeds prior to installation of each level of struts or tiebacks supporting the wall. The deflection is greater for flexible support systems than for rigid systems. The deflections are not recoverable and they are cumulative.
- Consolidation due to dewatering: In excavations where the water table is high, it is often necessary to dewater inside the excavation to avoid instability. Dewatering inside the cut may lead to a drop in the hydrostatic pressure outside the cut. Depending on the soil strata, this can lead to consolidation and settlement of the ground.
Existing buildings and facilities must be evaluated for the soil movement estimated to occur due to the support wall movement during excavation. This evaluation depends on the type of existing structure, its distance and orientation from the excavation, the soil conditions, the type of foundations of the structure, and other parameters. The analysis is site specific, and it can be very complex. Empirical methods and screening tools are available to more generally characterize the potential impacts. The existing buildings and facilities within the zone of influence must be surveyed (Chapter 3) and monitored as discussed in Chapter 15 Geotechnical and Structural Instrumentation.
Measures to deal with this issue include:
- Design of stiffer and more watertight excavation support walls.
- Provide more closely spaced and stiffer excavation support braces and/or tiebacks.
- Use of pre-excavation soil improvement.
- Underpinning of existing structures.
- Provide monitoring and instrumentation program during excavation.
- Establish requirement for mitigation plans if movements approach allowable limits.
5.3.5 Base Stability
Poor soil beneath the excavation bottom may require that the excavation support structure be extended down to a more competent stratum to ensure the base stability of the structure. This may depend upon whether the earth pressures applied to the wall together with its weight can be transferred to the surrounding soil through a combination of adhesion (side friction) and end bearing.
Soft clays below the excavation are particularly susceptible to yielding causing the bottom of the excavation to heave with a potential settlement at the ground surface, or worse to blow up. High groundwater table outside of the excavation can result in base instability as well. Measures to analyze the subsurface condition, and provide sufficient base stability must be addressed by the geotechnical engineer and/or tunnel designer. Readers are referred to FHWA-NHI-05-046 "Earth Retaining Structure" (FHWA, 2005e) for more details.
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