Highway & Rail Transit Tunnel Maintenance & Rehabilitation Manual
Chapter 4: Rehabilitation Of Structural Elements
This chapter describes various methods for repairing specific deficiencies in structural elements within a tunnel. Water infiltration is the most common cause of deterioration. However, deficiencies could be the result of substandard design or construction, or the result of unforeseen or changing geologic conditions in the ground that supports the tunnel. Another common reason for repairs is the fact that many tunnels have outlived their designed life expectancy and therefore the construction materials themselves are degrading. Due to the fact that there are different causes for the degradation, the method of repair could vary.
This chapter provides in-depth discussions and recommendations for repairing tunnels that are being deteriorated by water infiltration (Section A). In addition, a detailed explanation of the different types of concrete deficiencies and methods for their repair is provided in Section B. Section C addresses the issue of repair for specific types of liner construction.
A. Water Infiltration
Since many tunnels are constructed deep in the ground and often below the groundwater table, controlling water infiltration is of great concern to tunnel owners. Consequently, water infiltration is the underlying cause of most deterioration of the tunnel structure and components. Water infiltration can occur in all types of tunnel construction. Even tunnels that are designed to be waterproof, such as immersed tube tunnels that are placed in a trench at the bottom of a body of water, can develop leaks due to inadequate connection/joint design, substandard construction, and deterioration of the waterproof lining due to chemical or biological agents in the water or from tears caused by tunnel settlement. Most tunnels are designed with the foreknowledge that water will exist in the ground, but it is prevented from entering the tunnel by providing drainage mechanisms around the exterior of the lining or embedded within the joints. As ground water flow patterns change over time and drains become clogged with sediment, the water is bound to find its way into the tunnel through joints or structural cracks.
Another scenario that may occur in a few urban settings is that the elevation of the ground water table may rise due to the accumulating effects of basements of surrounding buildings being made relatively waterproof and the city's water supply needs being met by reservoirs many km (miles) away instead of through groundwater extraction. This could cause a tunnel that was designed to be above the water table to experience hydrostatic forces that it is unable to resist and subsequently water infiltration becomes a problem.
It should be noted that in the 1960s some tunnel owners began to develop maximum allowable rates of water infiltration to be used as a guide to determine original design and subsequent repairs if the amount of infiltration increases. One such owner was the Bay Area Rapid Transit (BART) system in California; they set a limit of 0.8 liters/minute per 75 linear meters (0.2 gpm per 250 linear feet) of tunnel. This translates to 3 liters/minute per 300 linear meters (0.8 gpm per 1000 linear feet) of tunnel. Other tunnel owners have adopted this criteria while still others may use a limit of 3.8 liters/ minute per 300 linear meters (1 gpm per 1000 linear feet) of tunnel. These limits are for reference purposes only, with the main emphasis for determining repair needs placed on the location of the leak and the condition of the tunnel components that are affected.
2. Consequences of Water Infiltration
As can be expected, nothing positive occurs when water infiltrates into a tunnel. The negative consequences can vary from minor surface corrosion of tunnel appurtenances to major deterioration of the structure and thus decreased load carrying capacity of the tunnel. Most tunnels have problems that fall somewhere in between. Below is a list of possible forms of tunnel degradation or safety risks that can result from water infiltration.
- Cement and sometimes aggregates of concrete liners are eroded causing the structure to be weakened.
- Reinforcement steel with poor or inadequate cover corrodes and causes delamination and spalling of the concrete cover.
- Bolts that connect segmental linings can corrode and fail.
- Masonry units and mortar can be very susceptible to water deterioration and can swell or become brittle depending on chemicals in water.
- Steel segmental liners or steel plates can experience section loss if exposed to both moisture and air.
- Fine soil particles can be carried through cracks with the water, creating voids behind the liner, which can cause settlement of surrounding structures and/or cause eccentric loading on tunnel that can lead to unforeseen stresses. These fine particles can also clog drains in or behind the lining.
- Fasteners of interior finishes or other appurtenances (fans, lights, etc.) can corrode and pose danger to a motorists or trains traveling through the tunnel.
- Water may freeze on roadway and safety walks or form icicles from the tunnel crown, all of which endanger tunnel users (Figure 4.1).
- Frozen drains can cause ground water to find or create a new location to enter the tunnel, which may be undesirable.
- Road salts carried by vehicles into highway tunnels, along with the presence of infiltrated water, can increase deterioration of the structure, especially the invert.
- Rate of corrosion for tunnel components of rail transit tunnels can be increased by the presence of stray current from electrified traction power systems.
Figure 4.01 - Ice formation at location of water infiltration in plenum area above ceiling slab.
3. Remediation Methods
In general, there are three options that a tunnel owner must consider for remediation of a water infiltration problem. The three alternatives are: short term repairs, long term repairs or, as a last resort, reconstruct all or portions of the tunnel lining that is causing the problem using methods of waterproofing that incorporate newer technologies. It should be noted that the alternative classifications are given for descriptive purposes and that overlaps between them do exist.
Since the first and second alternatives are the most common and usually most cost effective, a more detailed development of the current methods and some associated details will be given. The third option will not be discussed in as much detail as the first two but will include a brief discussion of some current technologies being used on new tunnel construction. To determine the most cost efficient method of repair for a particular situation, a specific cost analysis should be performed that considers the costs over the life of the tunnel. For guidance in this effort, a brief explanation of life-cycle cost methodology is given in Appendix A.
a) Short Term Repairs
For certain situations, it might be necessary to redirect infiltrated water to the tunnel drainage system on a temporary basis until further investigation can be performed and a more long term solution implemented. It should be noted that certain tunnels, whether due to deficiencies in design or construction or a change in the ground water table, will not be able to stop the water infiltration completely without a total restoration or reconstruction of the tunnel lining or at least significant portions where water infiltration is a problem. Therefore, some tunnels may have to rely on a long term system that conveys the water rather than prevents the water from entering the tunnel. Long term systems will be discussed more in-depth in the next section of this chapter, but the following paragraphs cover a few methods for temporarily diverting the infiltrated water.
(1) Drainage Troughs
If leaks are occurring in joints at the tunnel crown in a direction perpendicular to the tunnel length, then neoprene rubber sheets can be attached to the tunnel lining with aluminum channels. The sheets can be directed to channel the water to the side of the tunnel where it can flow into the tunnel drainage system (Figure 4.2). A similar method utilizing metal drainage troughs is sometimes used to redirect isolated areas of infiltration to the drainage system.
Figure 4.02a - Temporary drainage systems comprised of neoprene rubber troughs and 25 mm (1 in) aluminum channels.
Figure 4.02b - Temporary drainage systems comprised of neoprene rubber troughs and 25 mm (1 in) aluminum channels.
(2) Plastic Pipe Network
Another rather rudimentary method is to use plastic piping with one end inserted into the concrete at the main concentration of the leak. The piping can be hooked together in a network that conveys the water to the primary drainage system (Figure 4.3).
Figure 4.03 - Temporary drainage system comprised of 50 mm (2 in) plastic pipe.
b) Long Term Repairs
Since water infiltration is an ongoing problem for tunnel owners, there have been a wide variety of methods and materials used to prevent the water from entering the tunnel and causing undesirable degradation. Multiple techniques have not performed favorably over the long term, but that does not necessarily mean that the method utilized was the problem. Many different factors are involved in determining which method should be used that are site specific in that the cause and volume of the water infiltration will help determine how to properly prevent it. One method might work very well for one tunnel but not another. Therefore, it is suggested that a detailed study be performed on major leaks to determine the source and amount of water leakage, and the cause and exact location of the leak. This, along with knowing the type and condition of the materials that make up the tunnel lining structure, will help determine how to address the problem. Also, the method of preparing the surface and the procedure for installing the waterproofing system should be investigated to help determine which system should be used. The following paragraphs describe a few methods that have been used to address water infiltration problems for the long term.
(1) Insulated Panels
Insulated panels have been successfully used to line exposed rock tunnels to allow the water to flow behind the insulation down to the primary drainage system, while being insulated to prevent water from freezing. An example of this installation is a tunnel in the Pennsylvania Mountains that used two-inch-thick, 2.4 m by 9.6 m (8 ft by 32 ft) panels of Ethafoam insulation that was secured to the rock using 12 mm (½ in) diameter galvanized steel pins set into the rock on a .9 m (3 ft) square grid (Figure 4.4). It should be noted that use of this type of system would reduce the interior clearances within the tunnel.
Figure 4.04 - Insulated panels used as a waterproofing lining to keep infiltrating water from freezing. (Photo courtesy of Tunnels & Tunnelling International)
(2) Waterproofing Membrane
As an addition to the method given above, a continuous, flexible membrane can be used as the waterproofing layer that allows the water to flow towards the main tunnel drainage system. The specific process that has been effectively used involves placing a geotextile material against the existing tunnel interior, then a PVC waterproofing membrane, followed by a layer of material that will protect the membrane, such as shotcrete or other fire-retardant and protective materials. The term geotextile stands for a wide variety of materials which are normally synthetic and whose main purpose is to provide a drainage gallery outside the waterproofing membrane through which the infiltrating water can freely pass. The geotextile layer also provides a physical protection of the waterproofing membrane. Refer to Figure 4.5 for a detail of this system.
This system requires a relatively smooth surface to attach the membrane to, without projections that could potentially puncture the membrane. It is suggested that mock-up trials be performed to ensure that the components of the system achieve adequate bond to each other, especially the application of a protective layer on the inside of the membrane. If shotcrete is used a minimum membrane thickness might be required as well as limiting the aggregate size in the shotcrete. If a fire retardant protective material is applied in sheets then the connection of this material through the membrane must be properly sealed to prevent water infiltration through this joint.
This system can also be supplemented by inserting pressure relief holes into the surrounding soil/rock that provide a path of least resistance for the infiltrating water, so that adverse hydraulic pressures are not allowed to build up behind the liner. Additionally, a temperature controlled heat strip can be attached to exposed drainage pipes that prevents freezing of water in pipes and subsequent back up of water.
It should be noted that material types other than those stated have been used successfully, which include both preformed sheet materials and liquid applied materials for the waterproofing membrane layer. Therefore, research of current material technology should be performed prior to selecting the individual components of the waterproofing membrane system. The system chosen may need to be site specific given the possible presence of hydrocarbons or other chemicals that could adversely affect the membrane material. Some of the other materials available include polyolefin, which includes polyethylene and polypropylene, and sprayable polymer membranes. The manufacturer of the materials should be consulted and they should be able to supply material specifications and case histories of where the material might have been used successfully. Committee 515 of the American Concrete Institute (ACI) has also developed a guide for the use of waterproofing membrane systems. This is recommended as an additional source of information; however, it does not specifically describe tunnel applications.
The success of this system is primarily dependent on the ability to install a continuous membrane and whether a proper connection of this membrane to the tunnel drainage system is achieved. The membrane chosen must also be able to withstand any future movement of the structure without reflective cracking and must be resistant to chemical or biological attack from the infiltrating ground water.
(3) Crack/Joint Injection
The most common method for preventing water infiltration in concrete linings is to inject the crack/joint with a particle or chemical grout. Particle grouts are very fine cementitious grouts that produce nonflexible fillers that prevent water from penetrating the crack/joint. Since these grouts are nonflexible, they are not recommended for any location that might experience structural movements in the future. Chemical grouts on the other hand can be highly flexible and also have low viscosities that enable them to be injected into very thin cracks. Chemical grouts are expensive, sometimes toxic or flammable and require a high degree of skill for proper application; therefore, an understanding of the chemical properties and their suitability for the desired application is essential.
Even with the drawbacks of some chemical grouts, their performance in stopping water infiltration is significantly superior to particle grouts; therefore, they are used more frequently. It is important to note that if chemical grouts are allowed to dry out they may not be as effective. This could happen if the source of the water infiltration is diverted or the ground water elevation drops below the crack location. In the event of a dry crack, repair methods discussed in Section B of this chapter should be considered.
Of the chemical grouts developed to date, the polyurethane, reactive grouts have performed the best for tunnel applications. This type of grout expands into a foam at the presence of water and subsequently seals off the crack, not allowing water to pass through. This foam is also moderately resistant to tensile forces; therefore it can expand when and if a crack/joint continues to open further. Figure 4.6 shows and explains the procedure for properly injecting a vertical or overhead crack/joint with a chemical grout. It has been found that when applying pressure to inject the grout that low pressure for an extended period is better than high pressure for a short period. The latter can result in further damage to the concrete.
In addition to polyurethane chemical grouts, acrylate esters are also being used to inject cracks. The esters have an advantage over the polyurethanes in that they form a gel upon reaction with the water and serve as a barrier to water penetrating a crack. The esters will also not dry out as can occur with polyurethane grouts as described earlier. For this reason, a site specific investigation will need to be conducted to determine which material is most cost-effective over the long term.
It should be noted that cracks in masonry liners can also be injected, but often times other methods of repair are more effective for masonry over the long term. These methods will be discussed in Section C of this chapter.
(4) Soil/Rock Grouting (Back-Wall Grouting)
As an alternative to injecting a crack/joint (which is generally successful for stopping the leak through the injected crack/joint, but can force the water along the path of least resistance towards another crack/joint), similar materials can be injected through the liner into the soil/rock beyond. The goal of this method is to provide a protective barrier on the outside of the tunnel lining either in specific crack/joint locations or over an entire segment of the tunnel. The material that is injected can form this protective barrier or the injected material can introduce cohesion into the soil, which makes the soil itself impermeable.
The procedure for this method consists of drilling holes perpendicular to and through the liner on a predetermined pattern (based on ground conditions and amount of water present), and installing mechanical injection packers. Then, a grout is injected into the soil/rock and maintained at a constant pressure for a prescribed amount of time to allow the grout to penetrate small cracks in the soil/rock. There are different grouts that are available and a site-specific investigation is necessary to determine which one is best suited for the particular conditions. Some of the available grouts are:
- Microfine cement grouts
- Polyurethane chemical grouts
- Acrylate ester resin chemical grouts
- Acrylamide-based chemical grouts (highly toxic).
Typically the chemical grouts are more expensive; therefore, the cement grouts can be used for areas where voids exist behind the liner and large volumes of grout are required.
In the case of a steel or cast iron liner, the existing grout plug holes should be used as the location for the new grout placement, since the liner would not have been designed to handle additional holes being drilled through it.
One example of this system would be the Bay Parkway Bridge in New York City which is 45 m (150 ft) wide and has soil cover over a rail line running underneath that essentially forms a tunnel. The New York City Department of Transportation chose to utilize the acrylate ester resin chemical grout as the injection material and they used a .6 m to .75 m (2 ft to 2-½ ft) center-to-center spacing for their injection pattern. To date, this repair has performed well and other similar applications are being considered. Another example of this system being used successfully would be in the subway tunnels of the Toronto Transit Commission (TTC). For their situation they chose to use an acrylamide based chemical grout and they had specialty grouting work cars fabricated to condense and mobilize the operation for brief nighttime work periods when the tunnel could be closed. Their experience has shown that this can be a reliable method of stopping water infiltration.
It should be noted that this system could be used in conjunction with other systems. An example would be to back-wall grout a particular area and therefore force water to flow to a predetermined point where a drainage system could be installed. More details for installing drains within the liner are given in the next method.
(5) Crack/Joint Repair
If water infiltration through cracks/joints in concrete linings cannot be stopped by injecting the crack/joint as described previously because of excessive movement which surpasses the tensile strength of the grout material used, then another approach is to convert a crack into a joint that allows differential movement of the concrete, and add waterproofing components to the existing joints. Figure 4.7 portrays a method of routing out the crack or joint to a specific depth and then properly sealing off the water infiltration with successive layers of different impervious materials. The finished product will look and behave like a joint in that it will allow for some differential movement and will be watertight. As with the other repair techniques, a registered professional engineer should review and approve the application of this method to the specific site location. This is especially true for this method due to the possible weakening of the structural capacity of the lining depending on where and what direction the crack is located.
Figures 4.8 and 4.9 deal specifically with cracks and joints respectively and begin by routing or cleaning in the case of a joint. The difference with this method is the addition of a semi-perforated pipe that is inserted into the crack/joint, which enables the infiltrating water to be collected from the exterior side of the pipe and exported into the tunnel drainage system at the bottom of the crack. The pipe can be covered with a neoprene rubber sheet (liquid neoprene is also applicable) on the exterior of the concrete or mastic and impervious mortar can be used to make the repair look just like a normal joint.
(6) Segmental Joint Repair
Segmental liners can be made of either precast concrete, steel, or in the case of older tunnels-cast iron. Water infiltration generally occurs at the joint location where the original lead, mastic, or rubber seal has failed. This can be corrected by repacking the joint with new sealing material and installing new gaskets at boltholes. Cracks and joints can also be injected with particle or chemical grouts as discussed previously. In the case of precast concrete segments, the cracks are injected similar to method (3). In addition, for single-pass liner systems with any of the three segmental liner types, the processes described in method (4) can be implemented on the exterior of the liner with the precautions noted.
c) Reconstruction and New Construction
If the tunnel degradation has advanced to a point where repairing numerous localized areas of the liner becomes cost prohibitive, it may be necessary to reconstruct larger areas using different techniques. This could include shotcrete or pumping plasticized concrete within a form liner. There are several relatively new technologies that are being used for new tunnel construction that can also be incorporated into reconstruction procedures, with some modifications. These methods generally attempt to prohibit the water from infiltrating the final liner and thus entering into the tunnel space. This is accomplished by collecting the water and draining it away either within the liner or on the exterior of the tunnel. The latter method is less common because the drains can become clogged with fine soil particles. In addition, using an exterior drainage system in a tunnel below the ground water elevation is normally not effective over the long term because of the ability for water to penetrate very small cracks that develop between drains.
There are various detailed techniques that will only be explained briefly, although many of these are complex in nature. Furthermore, should an extensive repair be needed, it is recommended that a specialized consultant be obtained to develop possible solutions that are specific to the tunnel in question. The following paragraphs describe available systems for extensive lining reconstruction or that are also applicable for new tunnel construction.
(1) Shotcrete Applications
The use of shotcrete in tunnel construction has greatly increased since the advent of the Sequential Excavation Method (SEM) and the improvement of the shotcrete materials and application processes used. A few of the general material classifications for shotcrete are-cementitious, latex/acrylic-modified, or two-component epoxy. Shotcrete can also be used in tunnel rehabilitation in various forms. One method is to simply coat the entire interior of the tunnel walls and ceiling with a mix design that makes the cured shotcrete relatively impervious to water. This method has some drawbacks that include decreasing the tunnel clearances and trapping the moisture inside the original liner. Trapped moisture can lead to deterioration due to chemical reactions between the water and the liner material, especially in masonry.
Another more in-depth procedure is to remove all or portions of the existing liner, replace it with a structural layer of shotcrete, then place a geotextile layer and waterproofing membrane (either sheet membrane or sprayable polymer membrane), and finally provide a protective, non-structural finish liner of shotcrete on the inside that initially adheres to the waterproofing membrane during curing. As mentioned previously, the membrane thickness and shotcrete aggregate size may have restrictions placed on them in order to ensure that the membrane is not damaged during the shotcreting procedure. It is possible to place another geotextile layer or other protective material on the inside of the membrane, but attachment of this layer is difficult since the attachment mechanism has to puncture the membrane The thickness of this liner is dependent on the tunnel size and shape and the amount of water infiltration that is expected. It is recommended that a detailed site investigation be performed to determine if this final lining will need to resist any hydrostatic loadings. This method allows water that penetrates the initial liner to be directed down the tunnel along the waterproofing membrane to the primary tunnel drainage system. The existing liner can be removed with traditional demolition techniques or, depending on the depth of removal desired, a modern laser-controlled cutterhead mounted on a boom as shown in Figure 4.10 can be used to remove precise depths of masonry, concrete or rock.
Figure 4.10 - Laser controlled cutter for removing portions of existing tunnel liner. (Photo courtesy of Tunnels & Tunnelling North America)
(2) Joint Control
Deteriorated joints can be repaired as described previously in Chapter 4, Section A, Part 3b(5). It is not often that there is an opportunity to completely reconstruct a joint in an existing tunnel. However, when there is a complete tunnel reconstruction or new tunnel construction, the joints can be fitted with a new system that allows the joint to be initially injected with chemical or particle grouts and to be reinjected at any future time that the joint might begin to leak due to settlement of the structure. Also, products exist that can be inserted at anticipated crack locations that actually facilitate crack development at that location. Once the crack occurs, the product can be injected with a chemical or particle grout to stop water infiltration.
(3) Concrete Design
One of the most effective methods of preventing water infiltration in reconstruction or new construction is to properly design the concrete or shotcrete mix to approach impermeability and to not be as susceptible to cracking. This is primarily done by ensuring adequate reinforcement and limiting the water/cement ratio to 0.45. Other considerations include the use of water reducing and shrinkage reducing admixtures. Another admixture that is increasing in usage is a waterproofing additive. This admixture reacts with the fresh concrete to produce crystalline formations throughout the cured concrete that resist the penetration of water.
When major repairs or reconstruction is required, a detailed site-specific investigation should be undertaken to determine what methods and materials can be applied based on current research and experience.
B. Concrete Repairs
As concrete deteriorates, it is important that proper repairs be made to avoid further degradation of the structure. The repairs must be durable, easy to install, capable of being performed quickly during non-operating hours, and cost-effective. The repairs included in this manual are commonly used and have been performed in various tunnel locations.
The defect must first be evaluated to determine the cause and the severity of the deterioration, in order to select the best repair method. Factors affecting the repair are the severity to which the concrete has deteriorated, whether water infiltration is the cause, location of the repair to be completed, and the structural impact of the defect. Repairs should not be made until the cause of the defect has been determined and the situation remedied, or the same problem may repeat itself in the newly repaired concrete.
It should be noted that many concrete linings in highway tunnels have an additional tunnel finish that covers the concrete and therefore may hide the extent of the deterioration. This finish commonly is porcelain tile or prefabricated metal or concrete panels. Therefore, a repair analysis will need to account for the replacement or repair of the finish as well. Concrete deterioration in tunnels may be caused by any of the various factors listed below.
- Water Infiltration - Refer to Chapter 4, Section A for a discussion of the negative effects of water infiltration and suggested methods of repair.
- Corrosion From Embedded Metal - Several factors contribute to accelerate the corrosion of embedded steel, such as oxygen, water, stray electrical currents, chemicals, chlorides, and low pH (acidity). Once the corrosion has begun, signs of this problem are delamination (a separation of the concrete from the embedded steel), surface spalling, or cracking. Cracks may have existed previously that permit deleterious elements access to the reinforcement steel.
- Disintegration of Material - Certain chemicals like acids, alkaline solutions, and salt solutions are common enemies of concrete. Acid attacks concrete by reacting with the calcium hydroxide of the hydrated Portland cement. This reaction produces a water-soluble calcium compound, which is then leached away.
Porous concrete will absorb water into small capillaries and pores. Once there, the water freezes, then expands and exerts tension forces on the concrete. Near the surface small flakes of concrete will break away causing further exposure and eventual spalling and removal of aggregate with the process continuing inward.
- Thermal Effects - Thermal loads cause the concrete to expand and contract putting undue stress on the concrete. This expansion and contraction can lead to cracking. However, due to the relatively uniform environment within a tunnel, this form of degradation of the concrete is limited to areas near portals and possibly within air plenums where temperature fluctuations are more likely.
- Loading Conditions - Load placement will have varying effects on concrete. For continuous concrete spans in roadway slabs over air ducts, cracks may develop over the underlying steel support on the slab topside. In the center of the span, cracks will develop on the underside of the slab. Shear cracks may also develop near the support.
- Poor Workmanship - Workmanship is critical to overall concrete performance. If the reinforcement steel is placed improperly, if there is insufficient vibration to consolidate the concrete, if the concrete is permitted to segregate when placing, or if the concrete is not finished or cured properly, then the strength and long-term durability of the concrete will be affected.
Once the defect has been evaluated and the cause determined, one of the following potential repairs should be implemented:
The most common defect found in concrete is a crack. For cracks where water infiltration or moisture is present, see Chapter 4 Section A for methods of repair. For cracks that are void of water, and movements are not expected, the crack can be filled with an epoxy resin. For cracks on a horizontal surface, the crack may be gravity filled with epoxy by constructing a temporary dam (see Figure 4.11). However, the underside of the concrete surface may need to be sealed, if it is accessible, to prevent the resin from running completely through the crack. For vertical and overhead cracks, a paste gel is placed on the surface of the crack, around the injection ports to contain the resin that fills the crack. See Figures 4.11 and 4.12 for examples of this repair.
A spall is an irregular shaped depression in the concrete in which the fracture is parallel, or slightly inclined, to the surface. It is caused by the separation and removal of a portion of the surface concrete, typically due to corroded reinforcement steel, where the tensile stresses in the concrete exceed the tensile capacity. However, some spalls may occur that do not have any exposed steel. Depths of spalls vary and for repair purposes can be classified as either shallow or deep. A shallow spall typically penetrates less than 50 mm (2 in) into the concrete, whereas deep spalls penetrate 50 mm (2 in) or more into the concrete and usually expose the reinforcement steel within. Reinforcement steel can also be exposed in a shallow spall if it was originally placed too close to the surface of the concrete, resulting in a pop off of the concrete cover.
Special attention needs to be given to determining the cause of any corrosion on the reinforcement steel. If corrosion is due to water infiltration from the exterior of the tunnel, then the methods and materials given in this section may not be adequate to resist the effects of future infiltration. For this situation, it is necessary to address the water infiltration using methods given in Chapter 4, Section A. But, if a complete restoration of the original concrete surface is desired, the following methods can be used.
If the inspector recommends that the spalls should be repaired to preserve the integrity of the concrete, the following procedures may be utilized:
a) Shallow Spall With No Reinforcement Steel Exposed (See Figure 4.13)
This repair is typically performed for aesthetic reasons and not necessarily for structural integrity of the lining. Suggested steps include:
- Remove all loose or delaminated concrete on the spall surface.
- Clean the concrete surface of deleterious materials.
- Sawcut around the spalled area on a 20 degree` angle.
- Place polymer repair mortar in the spall to original concrete depth.
b) Shallow Spall With Reinforcement Steel Exposed (See Figure 4.14)
If the exposed reinforcement steel is only slightly corroded with no significant section loss, then this repair method can be used. If, however, the corrosion appears to be deeper than the current spall depth, or if the spall extends behind the reinforcement steel, it is recommended that the extent of the corrosion be determined and the spall be repaired by the method given in Part c). Suggested repair steps include:
- Remove all loose or delaminated concrete from around the exposed reinforcement steel.
- Clean the reinforcement steel of any corrosion.
- Coat the reinforcement steel and the concrete surface with an anti-corrosion coating.
- If replacing the spalled concrete is recommended, then, prior to application of anti-corrosion coating, perform sawcut as described in Part a) and place polymer repair mortar as final step. Make sure that anti-corrosion coating and polymer repair mortar are chemically compatible.
c) Deep Spall With Reinforcement Steel Exposed (See Figures 4.15 and 4.16)
Generally, any exposed reinforcement steel in a deep spall will be corroded. The extent of this corrosion should be determined and the concrete should be removed around the effected reinforcement steel to a width of a least one half the existing reinforcement steel spacing and to a depth of at least 25 mm (1 in) behind the back of the reinforcement steel. It is recommended that the sawcut around the perimeter of the spalled area be at least 25 mm (1 in) deep to accommodate a repair material with aggregate. If the material being used does not include aggregate, that depth can be reduced to 6 mm (¼ in), given that a proper bonding agent is used. As for bonding agents, experience has shown that separate, manual application is often not performed correctly and insufficient coverage is obtained. Therefore, a bonding agent admixture can be substituted for a certain percentage of the water in the mix. Specific repair recommendations are as follows:
- Remove all loose or delaminated concrete from the spalled surface and face of the reinforcement steel.
- Clean the concrete and steel surfaces of deleterious materials.
- Sawcut around the spalled area.
- Provide new reinforcement steel where necessary and overlap with existing steel according to current American Concrete Institute (ACI) standards.
- Coat the reinforcement steel with an anti-corrosion coating.
- Place polymer repair mortar in the spalled area unless the area is very large such that the use of shotcrete or plasticized concrete pumped with a form is more cost-effective. Where shotcrete is used, additional welded wire fabric is recommended to help support the shotcrete.
C. Liner Repairs
A general note that applies to all the liner repairs suggested in the following sections, is that a registered professional engineer should evaluate and approve suggested repairs and methods used. Particular attention should be directed to determining if structural components need to be temporarily shored so that the component to be repaired is unloaded.
1. Cast-in-Place (CIP) Concrete
CIP concrete liners are common in both highway and transit tunnels because of strength, cost, adaptability to site conditions, durability and resistance to corrosion (if designed and constructed properly), and ability to obtain a smooth surface for the final tunnel finish (tile, metal panels, etc.) application. Although there are many benefits for using CIP concrete liners, they may also have extensive repair needs to remedy cracking, spalling, and reinforcement steel deterioration. These effects could be due to water infiltration, inadequate design/construction, age, or unforeseen changes in ground conditions surrounding the tunnel.
The methods for repair of CIP concrete liners are the same as those given for general concrete in Chapter 4, Sections A and B, but will briefly be reiterated below for reference.
- If water infiltration is occurring, then methods of water redirection, crack injection, soil grouting, or membrane application should be performed prior to actual concrete repair.
- If dry cracks need structural repair, epoxy resins can be injected, but a determination must be made if there are active movements at the crack. If actively moving cracks are epoxy grouted, then subsequent cracks adjacent to original crack may occur depending on the elastic capacity of the epoxy material. Other materials with cellular structures can be used for active cracks.
- Spall repair is dependent on the size and depth of the spall and can be repaired with a polymer mortar for smaller spalls or with a plasticized concrete or shotcrete for larger spalls. Care must be given to cleaning or replacing exposed steel that has experienced corrosion and section loss.
Oftentimes in highway tunnels, the CIP concrete liner is covered with a reflective material such as tile or metal panels; therefore, the repair technique must take into account the attachment requirements of the final tunnel finish.
Another retrofit method that is being used more often for strengthening concrete tunnel linings is carbon fiber, polyaramide glass fiber sheet products. This method is performed by first completing crack and spall repairs, and then the concrete surface is prepared as per the manufacturer's instructions. An epoxy coating is applied to the concrete surface and the fiber sheets are installed in two layers, the sheets with fibers in the transverse (circumferential) direction are installed first followed by sheets with fibers in the longitudinal direction. The sheets are impregnated with epoxy.
This type of retrofit can be used to increase the load capacity of the tunnel arch when there is increase in the weight of overburden, this will also help to prevent further cracking and exfoliation of the concrete lining. Caution: This method should not be used in areas where fires or excessive heat may occur, due to the possible flammability and toxicity of the materials used. In the event of a fire, these materials will fail and therefore the concrete lining will lose any structural improvements provided by the carbon fiber sheets. Also, this method is not recommended in areas of the tunnel that might experience water infiltration.
2. Precast Concrete
Precast concrete liners are often used as a primary liner that is placed by the TBM or manually within the shield of a driven tunnel. They are used because of their easy adaptability to site conditions, and speed of erection. In highway tunnels, the precast concrete liners are often covered with an interior cast-in-place concrete liner for supporting the tunnel finish as described previously. Conversely, transit tunnels, which do not have the same visibility constraints, sometimes use a single precast concrete liner with no interior finish; therefore repairs are made to the precast directly. Generally precast segmental liners are bolted together to compress gaskets in the joints to prevent water infiltration and to provide overall structural stability to the liner.
Repair of precast segmental concrete liners is often related to degradation of the joints, especially in tunnels subject to water infiltration. The joint material can fail and corrosion of the bolts can lead to spalling of the concrete, which can expose the reinforcement steel, subsequently subjecting it to corrosion effects as well. Obviously, the ideal is to repair the joint before the corrosion becomes too extensive; therefore a routine inspection is crucial. As mentioned previously in the water infiltration section, the method of repairing a joint consists of repacking the joint with new gasket material and replacing any bolts that have lost their structural capacity. Also, the joints can be injected with grout to help seal them off to water.
Other defects such as cracks and spalls that can occur within a precast panel can be repaired using the same methods given in either Chapter 4, Section A or B depending on whether water infiltration is present.
Within a tunnel, structural steel is used for two main purposes: as segmental steel liners and as structural columns or beams. Structural columns and beams are mostly found in transit tunnels although steel beams are also used in structural slabs for support of roadway or overlying buildings and tunnels. As with structural steel in other uses such as bridges and buildings, the primary method of failure is by corrosion caused by moisture, which in transit tunnels can be enhanced by the presence of stray current from the rail electrification system. It is also possible for steel to develop cracks due to improper design/erection, fatigue, or from defects in the material.
To repair these defects, it is necessary to determine the cause and actual extent of the damage. Typically this will be done during the inspection process and will be recorded for reference in determining the type of repair. One general note is that for older structures the weldability of the steel must be determined due to the wider range of chemical composition allowed in their fabrication. Table 4.1 illustrates the changes in steel weldability over time.
Table 4.01 - Weldability of Steel Dates Weldability Prior to 1923 Steel should be tested 1923-1936 Generally weldable After 1936 Weldable
If the existing steel utilizes welded connections, then it is safe to assume that the steel is weldable. But, if there is any doubt, then the steel should be tested according to American Welding Society (AWS) standards.
Below are examples of repair procedures that can be used for steel defects:
- If beams or columns made from W-shapes, T-shapes, or channels have significant section loss (greater than 20 percent), then consider welding or bolting plates to flanges or webs to increase the capacity in the area of the section loss.
- For steel segmental liners that have section loss or considerable corrosion of the panels, then plates can be welded on the interior surface to replace the area of section loss.
- If liner joints and bolts are corroded, then new joint material must be installed along with new bolts. If stray current is suspected, then install an insulating sleeve over the bolt to prevent current from passing between dissimilar metals.
- Painting the steel is the best method for preventing corrosion. Research should be conducted to determine the best paint type for the given situation. Traditionally, epoxy paints have performed well for steel. Prior to painting, existing steel should be blast cleaned of all present corrosion - down to white metal.
- If clearance is adequate, headed studs may be welded to the liner and then a layer of reinforced concrete or shotcrete can be constructed inside the steel liner. If welding is not practical, steel bolts that connect the liner segments may be replaced by threaded rods that anchor the re-bars.
4. Cast Iron
Cast iron is similar to steel in the extent of its use for tunnel construction, such as the primary tunnel liner segments and columns (usually tubular) in open areas of transit tunnels. Cast iron differs from steel in that it is not as susceptible to corrosion. Generally cast iron is far less ductile than steel and therefore brittle failure and cracking can be more common.
Repair of cast iron defects is much more difficult than for steel and therefore a detailed, site-specific investigation is required to determine the proper method for repair. However, there are some general comments that can be made about repair methods that can be used.
It is possible to bolt new cast iron members over existing cracks, or areas of corrosion. When doing so, a watertight connection must be accomplished. If the repair is at a joint between liner segments, then the joint itself could be made watertight by inserting gasket material or by injecting the joint with a chemical grout. If the repair is the addition of a plate over a crack or area of section loss in the panel, then a waterproofing material will need to be applied between the new piece and the existing lining.
In general cast iron that is used in tunnels should not be considered weldable. Depending on the type of cast iron (grey, nodular, white, malleable, etc.) and the accessibility of the item to be repaired, some welding techniques can be attempted. Significant expertise is required and preheating is necessary, which is difficult since the cast iron components in a tunnel are not usually removable. Therefore, other methods of repair will usually be recommended.
c) Concrete Liner
Similarly to steel liners, if clearance is adequate, a layer of reinforced concrete or shotcrete can be added inside the cast iron liner. However as mentioned above welding is not usually an option, so replacing bolts or rivets that connect the liner segments with threaded rods to anchor the re-bar is suggested.
d) Metal Stitching
Technology does exist to stitch the cast iron in a manor illustrated in Figures 4.17 - 4.19. It is recommended that if the cast iron cannot be repaired using other methods, that this method be investigated. Currently, this method is being used with much success on high-pressure castings such as water pumps, valves, compressors and pipes, which demonstrate that the method provides a high strength, watertight repair. This process can restore the original strength to the casting without the problems associated with on-site welding such as stress, distortion, hardening and additional cracking because heat is not used in the repair process.
Figure 4.17 - Metal Stitching Detail - (Figure courtesy of Lock-N-Stitch Inc.)
Figure 4.18 - Metal Stitching Procedure - (Figure courtesy of Lock-N-Stitch Inc.)
Figure 4.19 - Metal Stitching Completed - (Photo courtesy of Lock-N-Stitch Inc.)
Shotcrete is a material that is gaining increasing usage for tunnel construction as materials and methods of application continually improve. Another terminology that is sometimes used is "gunite," which refers to fine-aggregate shotcrete. There are various uses for shotcrete in tunnel construction and each use may require a different mix design and application method. Primarily it is used as a primary support liner for the excavation prior to the construction of the final liner. This procedure can be supplemented with rock bolts, lattice girders, or wire mesh for additional strength. More recently with the addition of steel or synthetic fibers and fine-aggregates, shotcrete has been able to be used as a final liner, which can achieve significant strength in thin, smooth layers. Shotcrete can be used to cover and protect a waterproofing liner or as a repair liner for tunnel rehabilitation.
Generally, cured shotcrete will behave similarly to standard cast-in-place concrete and will be susceptible to cracking, spalling and delamination, even though the mix designs were intended to reduce those effects. If repairs need to be made to shotcrete liners, they can be performed in the same manner as the methods given in Chapter 4, Section A and B, depending on whether water is present at the defect.
The term "masonry" refers to materials such as stone or brick that are connected together in the field with mortar, which in the case of brick tunnel liners could be five or more courses thick. In older tunnels-generally those built in the 19th century-masonry was the construction material that was most readily available and economically possible for construction of the liners. Oftentimes, even after concrete and steel began to be used as construction materials for cut-and-cover tunnels, masonry was still used as a protective liner for the mastic waterproofing that was used on the outside of the finished lining. Masonry liners that are still in existence today range from very good condition to very poor condition, depending on the severity of any ground water presence. If they were constructed within geologic conditions that kept them relatively free from the presence of ground water, the masonry itself could last without much attention for a very long time. This is proven by the fact that most of the world's historic tunnel structures were constructed with masonry and still exist today.
Another reason that masonry tunnel liners remain in good condition is that the original method for waterproofing against, or draining of, the ground water was and remains very effective. The original waterproofing system typically consisted of a timber primary support lining and void space between the timber and masonry that was filled with tunnel debris, which formed a drainage channel for ground water. Over time, the timber lining rots and the water erodes the material that filled the void, causing the masonry itself to be exposed to the water.
When masonry is exposed to water, it and the mortar can swell and become brittle depending on the firing temperature of the brick and the chemical make up of the mortar. This, in conjunction with possible ground collapse in the space behind the brick lining, can induce stresses into the lining that cannot be resisted and therefore, structural cracking occurs, which further exacerbates the water infiltration problem.
If it is determined that repairs are needed, then the actual cause of the deficiency needs to be determined in order to select the proper repair method. If the problem is caused by extensive water infiltration, then methods given in Chapter 4, Section A should be considered, otherwise the following are suggested:
- Inject cementitious grout into known large voids behind liner to stabilize ground material. If waterproofing is needed, use methods described in Chapter 4, Section A, Part 3(b)(4).
- Replace cracked or brittle masonry units in localized areas.
- Repoint mortar by removing existing mortar to depth of twice the joint thickness or 18 mm (¾ in) minimum and replacing with new mortar of equal strength and color but increased water impermeability.
- Provide horizontal reinforcement steel embedded in the joint across the crack prior to repointing, for added strength.
- Inject cracks with chemical or particle grouts (take care to use grouts that are suited for the moisture content present in the crack).
- Apply a shotcrete lining if vertical and horizontal clearances can be reduced. However, underlying causes of cracks and water infiltration must be addressed first.
One repair that is not recommended is to apply an impermeable coating-such as a paint or epoxy-to the interior surface of the masonry. This practice is discouraged because any moisture or water that enters the masonry will be trapped and cause swelling; inevitably the face of the masonry will delaminate and fall off.
7. Exposed Rock
Many older tunnels that were constructed through dry, sound rock conditions, were left unlined except for zones near the portals or where the rock was incompetent to carry the loads. These tunnels may function without need for repair long into the future, but it is more likely that ground movements will either cause pieces of rock to fracture and fall to the invert, thus endangering the tunnel occupants, or they will open up cracks in which water will eventually infiltrate into the tunnel space. For the latter situation, some type of waterproofing liner, membrane, or pipe network will most likely need to be installed at the location of the leak to divert the water towards the tunnel drainage system.
If water infiltration is not a concern, then there are methods that can be used to structurally support the exposed rock, so that it does not pose a threat to the tunnel occupants. Listed below and shown in Figure 4.17 are some examples of those methods:
- Metal plates attached with short anchor bolts can be used to support surface defects. The plates can vary in width and length as needed to cover fractured rock.
- Rock bolts can be used to secure thicker sections of fractured rock to a competent layer behind. Examples of rock bolts are standard rock bolts, cable bolts, or friction bolts (dowels). They can sometimes be prestressed, but normally the stress is induced during future ground movements. Also, they are normally grouted into the drilled hole using chemical or particle grouts, but can be anchored mechanically for short-term applications.
- To protect from small spalls or pieces of fractured rock, wire mesh (chain link) can be attached to the surface using rock bolts.
- To completely protect against falling debris and to increase the structural capacity of the liner, shotcrete or a thin cast-in-place liner can be installed. However, this process does reduce the interior clearances.
Figure 4.20 - Rock Bolt Types