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Technical Manual for Design and Construction of Road Tunnels - Civil Elements

Chapter 10 - Tunnel Lining

10.4 Cast-in-Place Concrete

10.4.1 Description

Cast-in-place concrete linings are used as final linings in two pass lining systems. Initial ground support is installed in the tunnel as the tunnel is excavated and can take any form from steel ribs and lagging to precast concrete segments. A water proofing system or drainage blanket is typically placed between the initial ground support and the cast-in-place concrete lining.

Figure 10-5 shows the typical section for the cast-in-place lining used for the Cumberland Gap Tunnel. The Cumberland Gap Tunnel is a highway tunnel excavated in rock by the drill and blast method. Initial ground support is untreated rock, shotcrete and rock bolts. The initial ground support varied along the length of the tunnel due to varying ground conditions.

Cumberland Gap Tunnel Lining (Unfinished)

Figure 10-5 Cumberland Gap Tunnel Lining (Unfinished)

Figure 10-6 is a photograph of a heavy rail transit tunnel in Washington, DC . This tunnel was excavated in soft ground by a tunnel boring machine. This tunnel utilized a two pass system consisting of rough cast precast concrete segments as the initial ground support and a final lining of cast-in-place concrete. A high density polyethlylene waterproofing membrane was placed between the precast segments and the cast-in-place concrete final lining.

Advantages of a cast-in-place concrete lining are as follows:

  • Suitable for use with any excavation and initial ground support method.
  • Corrects irregularities in the excavation.
  • Can be constructed to any shape.
  • Provides a regular sound foundation for tunnel finishes.
  • Provides a durable low, maintenance structure.

Cast-In-Place Concrete Lining, Washington DC

Figure 10-6 Cast-In-Place Concrete Lining, Washington DC

Disadvantages of a cast-in-place concrete lining are as follows:

  • Concrete placement, especially around reinforcement can be difficult. The nature of the construction of the lining restricts the ability to vibrate the concrete. This can result in incomplete consolidation of the concrete around the reinforcing steel.
  • Reinforcement when used is subject to corrosion and resulting deterioration of the concrete. This is a problem common to all concrete structures, however underground structures can be also be subject to corrosive chemicals in the groundwater that could potentially accelerate the deterioration of reinforcing steel.
  • Cracking that allows water infiltration can reduce the life of the lining.
  • Chemical attack in certain soils can reduce lining life.
  • Construction requires a second operation after excavation to complete the lining.
10.4.2 Design Considerations

In order to maximize flexibility and ductility, a cast-in-place concrete lining should be as thin as possible. There are, however, practical limits on how thin a section can be placed and still obtain proper consolidation and completely fill the forms. 10 inches (25 cm) is considered the practical minimum thickness for a cast-in-place concrete lining.

Reinforcing steel in a thin section can also be problematic. The reinforcement inhibits the flow of the concrete making it more difficult to consolidate. If two layers of reinforcement are used, then staggering the bars may be required to obtain the required concrete cover over the bars. This can make the forms congested and concrete placement more difficult. Self consolidating concrete has been in development in recent years and has been used in unreinforced concrete linings in Europe with some success. Self consolidating concrete may prove useful in reinforced concrete linings, however it recommended that an extensive testing program be made part of the construction requirements to ensure that proper results are, in fact, obtained.

Cast-in-place concrete is used as the final lining. In many cases a waterproofing system is placed over the initial ground support prior to placing the final concrete lining. Placing reinforcing steel over the waterproofing system increases the potential for damaging the waterproofing. In all cases that are practical to do so, cast-in-place concrete linings should be designed and constructed as plain concrete, that is with no reinforcing steel. The presence of the waterproofing systems precludes load sharing between the final lining and the initial ground support. A basic design assumption is that the final lining carries long term earth loads with no contribution from the initial ground support.

Ground water chemistry should be investigated to ensure that chemical attack of the concrete lining will not occur should the lining be exposed to ground water. If this is an issue on a project, mitigation measures should be put in place to mitigate the effects of chemical attack. The waterproofing membrane can provide some protection against this problem. Admixtures, sulfate resistant cement and high density concrete may all be potential solutions. This problem should be addressed on a case by case basis and the appropriate solution be implemented based on best industry practice.

Concrete behavior in a fire event must also be considered. When heated to a high enough temperature, concrete will spall explosively. This produces a hazardous condition for motorists attempting to exit the tunnel and for emergency response personnel responding to the incident. This spalling is caused by the vaporization of water trapped in the concrete pores being unable to escape. Spalling is also caused by fracture of aggregate and loss of strength of the concrete matrix at the surface of the concrete after prolonged exposure to high temperatures. Reinforcing steel that is heated will loose strength. Spalling and loss of reinforcing strength can cause changes in the shape of the lining, redistribution of stresses in the lining and possibly structural failure.

The lining should be protected against fire. Both external and internal protection can be provided. External protection in the form of coatings or boarding is available commercially. These are specialty products that can provide a measure of protection against relatively low temperature fires. Manufacturers should be consulted to ascertain the exact level of protection that they can provide. Including polypropylene fibers in the concrete mix can reduce vaporization of entrapped water. The fibers melt during a fire and provide a pathway for water to escape.

10.4.3 Materials

Mixes for cast-in-place concrete should be specified to have a high enough slump to make placement practical. A slump of 5" (12.7 cm) is recommended. Air entrainment should be used. The moist environment in many tunnels combined with exposure to cold weather makes air entrainment important to durable concrete; 3 to 5 percent air entrainment is recommended.

Compressive strength should be kept to a minimum. High strength concretes require complex mixes with multiple admixtures and special placing and curing procedures. Since concrete lining acts primarily in compression, 28 day compressive strengths in the range of 3,500 to 4,500 psi (24 to 31 MPa) are generally adequate.

Reinforcing steel bars should conform to the requirements of ASTM A615 grade 60 and welded wire fabric when used should conform to ASTM A185.

10.4.4 Construction Considerations

Cast-in-place concrete must attain a minimum strength prior to stripping forms. The concrete must also be cured. Leaving the forms in place can accomplish both these goals, but can inhibit the rate of construction. The concrete should reach some minimum strength prior to stripping the forms. This should be computed by the designer assuming that the tunnel is supported by the initial support and thus the final lining at the time of stripping will be carrying only its own weight. The strength of the concrete in the forms can be verified by breaking field cured cylinders. This will allow the forms to be stripped as soon as possible. Curing can continue after stripping by keeping the concrete moist or by applying a curing compound. Curing compounds should only be used if the concrete is the finished exposed surface. The curing compound will act as a bond breaker if finishes such as ceramic tile are applied to the concrete. Sealants and coating will not adhere to concrete surfaces that have had curing compound applied unless the curing compound is removed via sand blasting or other technique.

The length of pour along the centerline of tunnel should be limited to minimize shrinkage in the concrete. Lining forms are usually designed to be re-used so limiting the length of pour does not impose a hardship on the contractor. Construction joints can be bulkheaded or sloping. Bulkheaded joints provide a uniform appearance, however, depending on how uneven the face of excavation is, construction of the bulkhead may be difficult. Sloped construction joints do not affect the performance of the lining, but can be unattractive and should be rubbed out after the forms are stripped.

Placing concrete in a curved shape overhead will leave a void at the crown. This void is filled after the concrete is cured by pumping grout into the void. Grout pipes are installed in the forms prior to placing the concrete to facilitate this operation. Spacing of the grout pipes along the tunnel should be limited to 10 feet and the pipes should be offset from the crown by 15 degrees on both sides.

When appurtenances are attached to the finished concrete lining, epoxy type anchor bolts should never be used. It is recommended to use undercut mechanical anchors to attach appurtenances to tunnel linings.

10.5 PRECAST SEGMENTAL LINING

10.5.1 Description

Precast segmental linings are used in circular tunnels that are mined using a tunnel boring machine. They can be used in both soft and hard ground. Several curved precast elements or segments are assembled inside the tail of the tunnel boring machine to form a complete circle. The number of segments used to form the ring is a function of the ring diameter and to a certain respect, contractor's preferences. The segments are relatively thin, 8 to 12 inches (20 to 30 cm) and typically 40 to 60 (1 to 1.5 m)inches (cm) wide measured along the length of the tunnel.

Precast segmental linings can be used as initial ground support followed by a cast-in-place concrete lining (the "two-pass" system) or can serve as both the initial ground support and final lining (the "one-pass" system) straight out of the tail of the TBM. Segments used as initial linings are generally lightly reinforced, erected without bolting them together and have no waterproofing. The segments are erected inside the tail of the TBM. The TBM pushes against the segments to advance the tunnel excavation. Once the shield of the TBM is passed the completed ring, the ring is jacked apart (expanded) at the crown or near the springlines. Jacking the segments helps fill the annular space that was occupied by the shield of the TBM. After jacking, contact grouting may be used to finish filling the annular space and to ensure complete contact between the segments and the surrounding ground. A waterproofing membrane is installed over the initial lining and the final concrete lining is cast in place against the waterproofing membrane. Horizontal and vertical curvature in the tunnel alignment is created by using tapered rings. The curvature is approximated by a series of short chords.

Precast segmental linings used as both initial support and final lining are built to high tolerances and quality. They are typically heavily reinforced, fitted with gaskets on all faces for waterproofing and bolted together to compress the gaskets after the ring is completed but prior to advancing the TBM. As the completed ring leaves the tail of the shield of the TBM, contact grouting is performed to fill the annular space that was occupied by the shield. This provides continuous contact between the ring and the surrounding ground and prevents the ring from dropping into the annular space. Bolting is often performed only in the circumferential direction. The shove of the TBM is usually sufficient to compress the gaskets in the longitudinal direction. Friction between the ground and the segments hold the segment in place, maintaining compression on the gasket. When first introduced into the United States in the mid-1970's, segmental linings were fabricated in a honeycomb shape that allowed for bolting in both the longitudinal and circumferential directions. Figure 10-2 shows the lining used for Section A of the Baltimore Metro. After 30 years of service, this lining is still providing a stable dry opening for over a hundred trains per day. Recent lining designs have eliminated the longitudinal bolting and the complex forming and reinforcing patterns that were required to accommodate the longitudinal bolts. Segments now have a flat inside surface as shown in Figure 10-7 and Figure 10-8 . Figure 10-8 shows the segments in the casting bay after being stripped of the forms. Once adequate strength is achieved, the segments are inverted to the position they must be in for erection inside the side the tunnel. Segments are generally stored in a stacked arrangement, with one stack containing the segments required to construct a single ring inside the tunnel. As with segments used for initial lining, horizontal and vertical tunnel alignment is achieved through the use of tapered segments. Figure 10-8 shows the segments stacked in the storage yard awaiting transport into the tunnel.

Advantages of a precast segmental lining are as follows:

  • Provides complete stable ground support that is ready for follow-on work.
  • Materials are easily transported and handled inside the tunnel.
  • No additional work such as forming and curing is required prior to use.
  • Provides a regular sound foundation for tunnel finishes.
  • Provides a durable low maintenance structure.
  • Disadvantages of a precast segmental lining are as follows:
  • Segments must be fabricated to very tight tolerances
  • Reinforcing steel must be fabricated and placed to very tight tolerances.
  • Storage space for segments is required at the job site.
  • Segments can be damaged if mishandled.
  • Spalls, cracked and damaged edges can result from mishandling and over jacking.
  • Gasketed segments must be installed to high tolerances to assure that gaskets perform as designed.
  • Reinforcement when used is subject to corrosion and resulting deterioration of the concrete.
  • Cracking that allows water infiltration can reduce the life of the lining.
  • Chemical attack in certain soils can reduce lining life.

Precast Segments for One-Pass Lining, Forms Stripped

Figure 10-7 Precast Segments for One-Pass Lining, Forms Stripped

Stacked Precast Segments for One-Pass Lining

Figure 10-8 Stacked Precast Segments for One-Pass Lining

10.5.2 Design Considerations

Initial Lining Segments Segments used as an initial support lining are frequently designed as structural plain concrete. Reinforcing steel is placed in the segments to assist in resisting the handling and storage loads imposed on the segments. Reinforcement is often welded wire fabric or small reinforcing steel bars. The segments are usually cast by a precaster or in a yard set up specifically for manufacturing the segments.

Stacked Precast Segments for Two-Pass Lining

Figure 10-9 Stacked Precast Segments for Two-Pass Lining

Steel Cage for Precast Segments for Two-Pass Lining

Figure 10-10 Steel Cage for Precast Segments for Two-Pass Lining

Figure 10-9 shows stacked segments for a two-pass liner system. These segments are used as the initial lining and are not required to be waterproof. Therefore no gaskets are used. No keyway for a gasket in cast into the segment. Note however, the keyway cast into the sides of the segments used to help with placement of the segment and maintaining alignment of the segments in the radial direction. Figure 10-10 shows the reinforcing steel cages for the segments.

Structural analysis is performed by one of the methods described in section 10.3.4 . When using a structural analysis program for analysis, the structural model should include hinges (points where no bending moment can develop) at the locations of the joints in the ring. Using hinges at the joint locations provides the ring with the flexibility required to adjust to the loads, resulting in the predominant loading being axial load or thrust. This is an approximation of the behavior of the lining since joints will transfer some moment. The actual behavior of a segmental lining can be bounded by models that have zero fixity at the joints and full fixity at the joints.

Radial joints in between segments can be flat or concave/convex as shown on Figure 10-11 Convex/concave joints facilitate rotation at the joint, allowing the segment to deform and dissipate moments. Flat joints are more efficient at transferring axial load between segments and may result in less end reinforcement. In either case, the ends of the segments that form the joints should be reinforced to facilitate the transfer of load from one segment to another without cracking and spalling. The amount of reinforcement used should consider the type of joint and the resulting load transfer mechanism. Handling and erecting the segments are also sources of damage at the joints. Reinforcing can mitigate this damage.

Radial Joints, Baltimore, MD

Figure 10-11 Radial Joints, Baltimore, MD

The primary load carried by the precast segments is axial load induced by ground forces acting on the circumference of the ring. However, loads imposed during construction must also be accounted for in the design. Loads from the jacking forces of the TBM are significant and can cause segments to be damaged and require replacement. These forces are unique to each tunnel and are a function of the ground type and the operational characteristics of the TBM. Reinforcement along the jacking edges of the segments is usually required to resist this force. The segments should be checked for bearing, compression and buckling from TBM thrust loads.

Handling, storage, lifting and erecting the segments also impose loads. The segments should be designed and reinforced to resist these loads. The dead weight of the segment with a dynamic factor of 2.0 applied to that dead weight is recommended for design to resist these loads. When designing reinforcement for these loads, the provisions of Chapter 5 of the LRFD specifications should be used. Grouting pressure can also impose loads on the lining. Grouting pressures should be limited to reduce the possibility of damage to the ring by these loads. A value of 10 psi (69 kPa) is recommended as the maximum permissible grouting pressure. The anticipated grouting pressure should be added to the load effects of the ground loads applied to the lining.

Initial lining segments are considered to be temporary support, therefore long term durability is not considered in the design of the linings or materials used.

Final Lining Segments Segments used as a final lining are designed as reinforced concrete. The reinforcement assists in resisting the loads and limits cracking in the segment. Limiting cracking helps make the segments waterproof. The provisions of chapter 5 of the AASHTO LRFD specifications should be used to design the segments. The segments are manufactured by a precaster or in a yard set up specifically for manufacturing the segments. Since the segments are cast and cured in a controlled environment, higher tolerances can be attained than in cast-in-place concrete construction.

Structural analysis is performed by one of the methods described in section 10.3.4 . When using a structural analysis program for analysis, an effective moment of inertia should be used to account for the flexibility induced in the ring at the bolted joints. The effective moment of inertia can be calculated using formula 10-10. When using this effective moment of inertia, no hinges are installed in the beam spring model.

Final lining segments can be fabricated with straight or skewed joints. Figure 10-12 shows a schematic of a lining system with straight joints. The orientation of the joint should be considered in the design of the lining to account for the mechanism of load transfer across the joint between segments. Skewed joints will induce strong axis bending in the ring and this should be accounted for in the design of the ring. Whether using straight or skewed joints, segments are rotated from ring to ring so that the joints do not line up along the longitudinal axis of the tunnel. Figure 10-13 is a picture of a mock-up of a ring of segmental lining.

Schematic Precast Segment Rings

Figure 10-12 Schematic Precast Segment Rings

Mock-up of Precast Segment Rings

Figure 10-13 Mock-up of Precast Segment Rings

Joint design should consider the configuration of the gaskets. The gasket can eliminate much of the bearing area for load transfer between joints (See Figure 10-11 for example). Joints should be adequately reinforced to transfer load across the joints without damage.

The primary load carried by the precast segments is axial load induced by ground, hydrostatic and other forces acting on the circumference of the ring. The presence of the waterproofing systems precludes load sharing between the final lining and the initial ground support. A basic design assumption is that the final lining carries long term earth loads with no contribution from the initial ground support. Loads imposed during construction must also be accounted for in the design. Loads from the jacking forces of the TBM are significant and can cause segments to be damaged and require replacement. These forces are unique to each tunnel and are a function of the ground type and the operational characteristics of the TBM. Reinforcement along the jacking edges of the segments may be required to resist this force. The segments should be checked for bearing, compression and buckling from TBM thrust loads.

Lifting and erecting the segments also impose loads. The segments should be designed and reinforced to resist these loads. The dead weight of the segment with a dynamic factor of 2.0 applied to the dead weight is recommended for design to resist these loads. When designing reinforcement for these loads, the provisions of Chapter 5 of the LRFD specifications should be used.

Grouting pressure can also impose loads on the lining. Grouting pressures should be limited to reduce the possibility of damage to the ring by these loads. A value of 10 psi (69 kPa) is recommended as the maximum permissible grouting pressure. The anticipated grouting pressure should be added to the load effects of the earliest ground loads applied to the lining.

Ground water chemistry should be investigated to ensure that chemical attack of the concrete lining will not occur should it be exposed to ground water. If this is an issue on a project, mitigation measures should be put in place to reduce the effects of chemical attack. The waterproofing membrane can provide some protection against this problem. Admixtures, sulfate resistant cement and high density concrete may all be potential solutions. This problem should be addressed on a case by case basis and the appropriate solution implemented based on best industry practice.

Concrete behavior in a fire event must also be considered. When heated to a high enough temperature, concrete will spall explosively. This produces a hazardous condition for motorists attempting to exit the tunnel and to emergency response personnel responding to the incident. This spalling is caused by the vaporization of water trapped in the concrete pores being unable to escape. Spalling is also caused by fracture of aggregate and loss of strength of the concrete matrix at the surface of the concrete after prolonged exposure to high temperatures. Reinforcing steel that is heated will loose strength. Spalling and loss of reinforcing strength can cause changes in the shape of the lining, redistribution of stresses in the lining and possibly structural failure.

The lining should be protected against fire. Both external and internal protection can be provided. External protection in the form of coatings or boarding is available commercially. These items can provide a measure of protection against relatively low temperature fires. These are specialty products and manufacturers should be consulted to ascertain the exact level of protection that they can provide. Including polypropylene fibers in the concrete mix can reduce vaporization of entrapped water. The fibers melt during a fire and provide a pathway for water to escape.

Appendix G presents a calculation example to illustrate the design process for precast segmental lining.

10.5.3 Materials

Concrete mixes for precast segments for initial linings do not require special designs and can generally conform to the structural concrete mixes provided in most state standard construction specifications. Strengths in the range of 4,000 to 5,000 psi (27 to 35 MPa) are generally adequate. These strengths are easily attainable in precast shops and casting yards. Curing is performed in enclosures and is well controlled. Air entrainment is desirable since segments may be stored outdoors for extended periods of time and final lining segments may be exposed to freezing temperatures inside the tunnel.

Steel fiber reinforced concrete has become a topic of discussion and research for precast tunnel linings. Theoretically, steel fibers can be used in lieu of steel reinforcing bars. The fibers can potentially eliminate the need for fabricating the steel bars to very tight tolerances, provide ductility for the concrete and make the segments tougher and less damage prone during construction. Unfortunately, there is no US design code for the design of steel fiber reinforced concrete. Papers have been written that propose design methods and several European countries have developed design methods. The recommended practice until further research is conducted and design codes are developed is to use steel fibers in segments where the design is conducted as detailed in this manual and the lining is found to be adequate without reinforcing. The steel fibers then can be included in the concrete to improve handling characteristics during construction. A testing program is required by the specifications to have the contractor prove via field testing that the fiber reinforced segments can withstand the handling loads imposed during construction. The fibers then can be used lieu of reinforcement that would be installed to resist the handling loads.

Reinforcing steel bars should conform to the requirements of ASTM A615 grade 60 and welded wire fabric when used should conform to ASTM A185.

Concrete mixes for one pass lining segments have strengths ranging from 5,000 psi to 7,000 psi (34 to 48 MPa). Higher strengths are easily obtainable in precast shops and assist in resisting handling and erection loads.

10.5.4 Construction Considerations

Initial Lining Segments Grout holes are required for contact grouting. Grout holes can also serve as lifting points for the segments. Locate the grout holes symmetrically so that the load to the lifting devices is evenly applied. Grout holes and lifting devices are usually designed by the contractor to loads and criteria specified by the designer. The construction industry is moving toward vacuum erection and handling equipment. This device does not rely on the grout holes to handle the segments. A device of this type can be seen in Figure 10-7. This devices relies on vacuum created between the segment face and the device to produce the reaction required to lift and erect the segments.

Segments should be cast and cured in accordance with the requirements of the standard specifications of the owner. In the absence of standard specifications, the requirements of the Precast Concrete Institute should be used to develop construction specifications for the precast segments. Segments should be stored in a manner that will not damage the segments. Support locations should be shown on the drawings and maximum stacking heights should be specified.

Segments should be detailed to facilitate jacking the rings at the crown or near springline after erection. Space for material to temporarily close off the gap to stop earth from coming into the tunnel is required. A means to jack the segments should be devised and the space remaining from the jacking should be backfilled with concrete and/or contact grouting to complete the ring. The ends of the segments that are used for jacking may require additional reinforcing or steel plates to protect them from the forces associated with jacking.

The number of segments for two pass systems is usually kept at a minimum, with the segments being slightly larger than for a one pass system and the joints in the rings will line up with joints in adjacent rings.

Final Lining Segments (One-Pass System) The same considerations as for initial lining segments apply to final lining segments. Final lining segments, however are not jacked at the crown after erection. Final lining segments must also be detailed to accommodate the gaskets required for waterproofing. Often, final lining segments also receive a waterproofing coating applied to the outside of the segment. This waterproofing coating should be a robust material such as coal tar epoxy since the segments slide along the shield as it advances and damage to the coating will occur.

10.6 Steel Plate Lining

Steel plate lining is a segmental lining system. It is sometimes used for circular tunnels in soft ground mined by TBM or other methods. Several curved steel elements or segments are assembled inside the tunnel or the TBM to form a complete circle. The segments are constructed from steel plates that are pressed into the required shape. The plates have flanges along all four edges. The flanges are used to bolt the segments together in the longitudinal and circumferential directions. Adjacent rings are rotated so that joints do not line up from ring to ring. The segments are fitted with gaskets along all the flanges that are compressed when the bolts are tightened. These gaskets are intended to provide waterproofing for the tunnel. Lining plate is manufactured in standard sizes and in widths of either 12" (25.4 cm) or 24" (50.8 cm). Only the radius changes to meet the requirements of the project. Figure 10-14 shows typical steel lining plate details.

Typical Steel Lining Section

Figure 10-14 Typical Steel Lining Section

Advantages of a steel plate lining are as follows:

  • Provides complete stable ground support that is ready for follow-on work.
  • Materials are easily transported and handled inside the tunnel.
  • No additional work such as forming and curing is required prior to being ready to use.

Disadvantages of a steel lining plate are as follows:

  • Thrust applied from TBM must be limited to the capacity of the plate.
  • Steel is subject to corrosion in the damp environment usually encountered in a tunnel.
  • Fire can cause the lining plate to buckle and/or fail.
  • Cast-in-place concrete will be needed for fire protection.
10.6.1 Design Considerations

Design of steel plate linings should be in accordance with Chapter 12 of the AASHTO LRFD specifications. The required checks for the service condition are included in that chapter. Typically, the design parameters and minimum dimensional requirements are specified on the drawings. The information on the drawings is developed from the AASHTO LRFD requirements. Lining plate manufacturers have standard products that can be selected for use on a project. The contractor will provide a specific product intended for use on the project and supply computations illustrating that the product meets the minimum requirements shown on the drawings.

The steel plate lining must be designed to resist jacking loads imposed by a TBM. A jacking ring or some other method of distributing these loads to the plates must be utilized to avoid damaging the plates during tunneling. Often, stiffeners are required at the center of the plates to resist the jacking loads. These stiffeners along with the flanges at the edges of the plates resist the bulk of this jacking force. The stiffeners and flanges are designed as columns to resist the anticipated jacking loads. Design of steel lining plate linings must include other loads induced by construction activities. Lifting and erection stresses should be checked by the contractor.

Curvature in horizontal and vertical alignments is accommodated with tapered segments just as with concrete segments.

Steel plate linings should be protected against corrosion. The exterior surface can be protected by a coating such as coal tar epoxy. The interior can be protected with coatings such as paint or galvanizing, but the most effective protection is a layer of unreinforced concrete. This concrete layer provides protection against corrosion and against heat damage due to fires. The protective concrete layer is placed after completion of the mining operation to avoid damage that can be caused by the jacking or the shield.

Gasket requirements for steel plate linings are similar to those for concrete segments. However, steel plate linings have far less surface area for gasket installation than do concrete segments.

10.7 Shotcrete Lining

As discussed in Chapter 9, shotcrete represents a structurally and qualitatively equal alternative to cast-in-place concrete linings. Its surface appearance can be tailored to the desired project goals. It may remain a rough, sprayer type shotcrete finish, or may have a quality comparable to cast concrete when trowel finish is specified. Shotcrete as a final lining is typically utilized in combination with the initial shotcrete supports in SEM applications when the following conditions are encountered:

  • The tunnels are relatively short in length and the cross section is relatively large and therefore investment in formwork is not warranted, i.e. tunnels of less than 400-600 feet (150-250 m) in length and larger than about 25-35 feet (8-11 m) in springline diameter.
  • The access is difficult and staging of formwork installation and concrete delivery is problematic.
  • The tunnel geometry is complex and customized formwork would be required. Tunnel intersections, as well as bifurcations qualify in this area. Bifurcations are associated with tunnel widenings and would otherwise be constructed in the form of a stepped lining configuration and increase cost of excavated material.

When shotcrete is utilized as a final lining in dual shotcrete lining applications it will be applied against a waterproofing membrane as presented in Chapter 9. The lining thickness will be generally 10 to 12 inches (200 to 300 mm) or more and its application must be carried out in layers with a time lag between layer applications to allow for shotcrete setting and hardening. To ensure a final lining that behaves close to monolithically from a structural point of view it is important to limit the time lag between layer applications and assure that the shotcrete surface to which the next layer is applied is clean and free of any dust or dirt films that could create a de-bonding feature between the individual layers. It is typical to limit the application between the layers to 24 hours. Shotcrete final linings are applied onto a carrier system that is composed of lattice girders and welded wire fabric mounted to lattice girders toward the waterproofing membrane side. This carrier system also acts fully or partially as structural reinforcement of the finished lining. The remainder of the required structural reinforcing may be accomplished by rebars or mats or by steel or plastic fibers. The final shotcrete layer allows for the addition of micro poly propylene (PP) fibers that enhance fire resistance of the final lining.

Unlike the hydrostatic pressure of cast-in-place concrete during installation the shotcrete application does not develop pressures against the waterproofing membrane and the initial lining and therefore one must ensure that any gaps between waterproofing system and initial shotcrete lining and final shotcrete lining be filled with contact grout. As in final lining applications contact grout is accomplished with cementitious grouts but the grout takes are much higher. To assure a proper grouting around the entire lining circumference it is customary to use longitudinal grout hoses arranged radially around the perimeter. Figure 10-15 displays a typical shotcrete final lining section with waterproofing system, welded wire fabric (WWF), lattice girder, grouting hoses for contact grouting and a final shotcrete layer with PP fiber addition.

Typical Shotcrete Lining Detail

Figure 10-15 Typical Shotcrete Lining Detail

Probably the most important factor that will influence the quality of the shotcrete final lining application is workmanship. While the skill of the shotcrete applying nozzlemen (by hand or robot) is at the core of this workmanship, it is important to address all aspects of the shotcreting process in a method statement. This method statement becomes the basis for the application procedures, and the applicator's and the supervision's Quality Assurance / Quality Control (QA/QC) program. Minimum requirements to be addressed in the method statement are as follows:

  • Execution of Work (Installation of Reinforcement, Sequence of Operations, Spray Sections, Time Lag)
  • Survey Control and Survey Method
  • Mix Design and Specifications
  • QA/QC Procedures and Forms ("Pour Cards")
  • Testing (Type and Frequency)
  • Qualifications of Personnel
  • Grouting Procedures

General trends in tunneling indicate that the application of shotcrete for final linings presents a viable alternative to traditional cast-in-place concrete construction. The product shotcrete fulfills cast-in-place concrete structural requirements. Design and engineering, as well as application procedures, can be planned such as to provide a high quality product. Excellence is needed in the application itself and must go hand-in-hand with quality assurance during application.

Chapter 9 presents detail discussions about shotcrete for initial support. Chapter 16 presents details about applying shotcrete for concrete repairs.

10.8 Selecting a Lining System

Each tunnel is a unique project and has its own combinations of ground conditions, opening size, groundwater condition, alignment and applicable construction technique. Given the wide range of combinations of these variables, guidance on the selection of a lining type can only be made using generalizations. The lining system designed for a project is selected based on the best judgment and experience of the designer. Once the project has been bid and awarded, it is not unusual for the contractor to request a change in the lining type, the mining method or both. The following paragraphs give conditions under which certain lining types make sense and offers caveats to be heeded when selecting a lining type for the project.

Cast-in-Place Concrete Cast-in-place concrete can be used in any tunnel with any tunneling method. It requires some form of initial ground support to maintain the excavated opening while the lining is formed, placed and cured. Cast-in-place concrete is usually used in hard ground tunnels mined using drill and blast excavation and soft ground tunnels mined using sequential excavation. Cast-in-place concrete can be formed into any shape so that the lining shape can be optimized to the required opening requirements.

Cast-in-place concrete is also used in both hard and soft ground tunnels excavated using a tunnel boring machine. Is these tunnels, the cast-in-place concrete lining is the final lining constructed after initial ground support is installed. Using cast-in-place concrete (two pass system) in a TBM tunnel can result in a larger excavated opening than if a single pass precast lining is used.

Cast-in-place concrete linings are cast against a waterproofing membrane. The membrane can be damaged during placement of reinforcing steel and forms. Forms must remain in place until the lining gains enough strength to support itself and curing must take place after forms are stripped.

Precast Segmental Lining Precast segmental linings are used exclusively in soft and hard ground tunnels excavated using a tunnel boring machine. This single pass system provides the ground support required during excavation and also forms the final lining of the tunnel. This system requires gaskets on each edge of the segments to provide a watertight lining. The segments must be manufactured to tight tolerances. The segments require specialized equipment to handle and erect inside the tunnel. Once erected and in place, the lining system is complete.

Steel Plate Lining Steel plate linings can be used in any ground condition with any mining method. The steel plates form the final lining and ground support once in place. This single pass system provides the ground support required during excavation and also forms the final lining of the tunnel. This system requires gaskets on the each edge of the segments to provide a watertight lining. The segments must be manufactured to tight tolerances. The segments require specialized equipment to handle and erect inside the tunnel. The segments are usually thin and not very stiff in the longitudinal direction. This lack of stiffness limits the amount of thrust that can be used to advance the tunnel boring machine. Difficult ground conditions that require high thrusts to advance the TBM may preclude the use of steel lining plate. Corrosion problems associated with steel linings can severely reduce the life of the lining.

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Updated: 06/19/2013
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