The majority of the tunnels on the New York City subway and transit system need repair. Several inspections of the subway have shown that the major types of tunnel defects are corrosion of the structural steel elements, deterioration of the concrete lining, and excessive leakage of groundwater into the tunnels (Figure 1). Conventional methods of repair such as painting exposed structural steel, injecting concrete cracks, filling construction joints and installing additional drainage, are often neither successful nor cost-effective. These repairs to the lining alone, cannot keep water out of the tunnel as it always finds a new path and the deterioration process continues.
The main disadvantages with the above-mentioned methods are increased corrosion of steel, continued concrete deterioration, increased humidity in the tunnel environment, and the operating costs of pumping. In addition, preventive measures against freezing, near air intakes, are always necessary. The biggest problem with these methods is that the structural steel and the extrados of the concrete lining are exposed to an aggressive environment and are not easily accessible for inspection or repair.
In 1991, and later in 1998, the International Tunnelling Association (ITA) published recommendations and several reports regarding the damaging effects of water on tunnels. These recommendations underline that all tunnels are susceptible to an aggressive environment (rock/soil) on the extrados of the structural lining, which induces damage to the structural elements.
It is stated that any water inflow/leakage into the tunnels and/or deterioration of the structural elements is not the problem of the tunnel lining itself, but the overall system, defined as the surrounding ground/tunnel lining condition. Any repair method applied to just one of the above-mentioned elements of the system will not be effective and will probably fail over some period of time.
The key to successful repair is understanding the surrounding ground/tunnel lining system, as well as the experience of the designers.
Visual inspection methods
The subway tunnels inspected were constructed prior to 1975, as either cut and cover, drill and blast, pre-cast immersed or TBM tunnels. Generally, the immersed tunnels are in good conditions, and are essentially dry except for at the connections between the elements. The cut and cover (trench) tunnels have typical corrosion problems on exposed steel frames, occasional zones of water inflow/infiltration through cracks in concrete lining at the roof, sidewalls and invert where the outer waterproofing system was damaged or has deteriorated.
The TBM tunnels have an almost regular pattern of defects that occur at approximately 3m intervals. These tunnels exhibit severe concrete deterioration, corrosion of reinforcement steel and water infiltration. Typically damaged areas are located at 10 to 2 o’clock locations. The concrete has a lot of cracks and water infiltration to these areas, and in many cases efflorescence, rust and black organic matter can be seen.
The drill and blast tunnels are the most damaged, with linings in very poor condition with numerous cracks, heavy water infiltration and a lot of rust spots.
To develop appropriate and economical methods for rehabilitation it is necessary to identify and classify all of the tunnels’ existing problems. Visual inspections show that existing tunnels commonly exhibit the following defects:
Most of the above-mentioned problems are associated with tunnels constructed in rock.
Geology and groundwater
Selection of a rehabilitation method always starts with an evaluation of the existing environment. The Manhattan subway tunnels cross rock and soil formations belonging to the Manhattan Prong; consisting of schist, gneiss and marble. The network of rock mass fractures controls the groundwater conditions. Proximity of adjacent surfaces, alteration processes that have removed or placed minerals on fracture surfaces, and joint wall material that has been fragmented or crushed by faulting and shearing, can increase the permeability of individual joints. Also, larger scale features, such as mylonite, act as a regional hydraulic barrier whereas major shear zones act as regional storage and conduits for groundwater.
Groundwater chemical test results indicate generally moderate concentrations of chlorides, ranging from 60-100 mg/ltr, rarely reaching the 550mg/ltr level. Tests also indicate low concentrations of sulphates (from common occurrences of 40mg/ltr, to rare cases with 120-180mg/ltr) and pH values from 6.17 to 6.9 (see Table 1).
The concentration of dissolved oxygen, perhaps, is one of the most important factors influencing the rate of corrosion for all structural elements. The typical content of dissolved oxygen in the groundwater ranges from 9-10mg/ltr. Other components of the groundwater, such as CaCO3 and dissolved solids do not play a significant role in corrosion process. The specific conductance of groundwater ranges from 650-2000µmhos/cm.
In addition, dynamic transit of stray direct currents can cause severe corrosion within a very short period of time. One ampere of stray current continually discharging into the electrolyte (soil or groundwater) can remove 9kg of steel in one year. In a year, 45g of metal would be removed if, for example, just 5 milliamperes flowed off a 25mm diameter rock bolt. Test results showed stray currents ranging from a few milliamperes up to 20 amperes in different locations during survey.
Evaluation of Manhattan rocks and analysis of the groundwater samples identify the Manhattan environment as highly corrosive, especially in combination with stray electric current activity.
Possible causes of lining damage
Generally, causes of the tunnel lining damage can be classified as follows:
The design and construction techniques for rehabilitation of the tunnels must be selected based on severity of the above-mentioned causes and on the results of the intensive testing programme.
Testing methods
The type of lining damage has to be identified to develop appropriate methods of rehabilitation. An intensive testing programme has to be performed, in addition to visual inspection, including:
Test selection procedure is based on a combination of factors, such as non-destructiveness, cost, speed and reliability, and may follow the procedure shown in the Table 2. Visual inspection is the least expensive NDT method, however, only well-trained inspectors can provide valuable information. Ultrasonic and Ground Penetrating Radar (GPR) are the most flexible methods for checking concrete characteristics. The accuracy of application of the test methods will influence the selection and quality of the tunnel repair methods.
Existing methods of lining repair
After evaluation, the rehabilitation method can be selected, the following have been used frequently:
Lack of experience in the field of tunnel rehabilitation, together with a neglect of local environmental factors and inappropriate cost analysis, can result in the selection of the repair methods which implements only general tunnel liner surface repair.
As mentioned earlier, these methods just provide cosmetic rehabilitation and can only delay the lining’s deterioration. In this case future tunnel rehabilitation costs will be much higher.
An appropriate way of lining repair is to stop any movement of groundwater in the surrounding rock. The mechanism for water infiltration into tunnels is well known. Predominantly, groundwater comes into the structure via rock joints and faults. The effect of groundwater on the tunnel lining is more than just infiltration. Groundwater can stream towards the tunnel and have a number of mechanical effects such as wash-outs and silting the drainage system, enlarging any voids behind the lining, loosening the surrounding ground and permanently changing stress conditions around the tunnel. It will also continuously dissolve certain chemicals such as carbonate and chloride, which will attack concrete and steel elements.
In addition, groundwater creates several other negative effects, such as frost penetration and consequent thawing, which creates a bursting action in the concrete. Therefore, stopping the flow of water behind the lining is critical.
Understanding the importance of interaction between the tunnel lining and a rock surface is crucial to tunnel repair. The lining is subjected to additional asymmetrical stresses in locations where it is not in full contact with the ground. The ground may become weathered, losing its self-supporting qualities and the volume of voids in contact with the lining may increase. In addition, the lining may break up in this zone, since it is not confined and is subject to high stresses.
The presence of voids may cause much higher ground loads and stresses, especially where there is groundwater. The probability of finding voids behind the lining is very high, but the size, shape, and extent, is generally unknown. Here groundwater accumulates and migrates along the extrados until it finds an entry point through expansion joints, cracks, etc. Complete void filling is essential to ensure repair.
Voids that are caused by concrete shrinkage and/or by deterioration of wood materials, or corrosion of the structural steel elements, exist between the rock and initial support system or the outer surface of final lining and need to be filled for the following reasons:
This can be accomplished using void and contact grouting, which allows a reduction in the permeability of the rock mass and increases rock strength.
Integrated method
The experience accumulated during the last 20 years helped to develop the four-stage “Integrated Method” of tunnel rehabilitation, to produce a fully impermeable tunnel lining.
Stage one of the integrated tunnel repair method includes contact grouting operations. This is best accomplished by drilling holes in a regular grid, approximately 3m x 3m, through the concrete lining directly into voids identified by testing methods. The holes can be drilled through the lining at the invert, walls or crown of the tunnel. The diameter of these holes typically ranges from 35mm- 50mm. Normally the holes should extend into the rock for at least 300mm. An appropriate mix needs to be designed to reduce risk of shrinking cracking. For this reason the use of thin grout it is inadvisable. A grouting pressure of up to 5 bar is recommended.
The second stage is rock penetration grouting. The purpose of this is to fill all joints, cracks, voids or any other discontinuities in the surrounding rock. For this it is recommended that 35mm-50mm diameter drill holes, in a regular pattern of 1.5m x 1.5m, drilled into the rock for up to 3/4 of the tunnel width or a diameter (but not less than 3m), are adopted. The grouting pipes should be fixed into these holes and a cementicious or chemical grout (depending on rock permeability) injected. In this case a fine cement or chemical grout (if economically justified) is recommended. The grouting pressure should to be up to 20 bars. The test grouting procedure should be developed by a specialist.
The last stage of rehabilitation is concrete lining repair and corrosion protection of the exposed structural steel elements. The surface of the concrete has to be free of dirt, dust, grease, oil or other foreign matter. Injection ports for the cracks and joints should be installed at intervals of no less than the thickness of the concrete at that location. An appropriate chemical mix should be used, normally this can be an epoxy or polyurethane mixture. When the cracks or joints are completely filled, the grout should be allowed to cure sufficiently before removal of the surface seal. Cracks must be 90% full to attain a bond strength of approximately 45MPa.
The final step is to coat the rusted structural steel elements with an appropriate anti-corrosive material. The quality of each stage must be confirmed by visual inspection and non-destructive test methods.
Conclusions
Any rehabilitation method designed to allow groundwater infiltration cannot be considered reliable. The tunnels will soon require additional work, which could be much more costly and time consuming.
An extensive testing programme, using modern non-destructive test methods is recommended to increase the quality of the tunnel rehabilitation work.
The “Integrated Method” is a cost-effective and powerful method that can be used for repair or rehabilitation of most of the transportation tunnels, assuring safety and long-term serviceability.
