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Groundwater control in rock can take many forms depending on the nature and extent of the problem. In fact, for many cases experience has proven that a combination of control methods may be the best solution. For a given tunnel it may also be found that different solutions apply at different locations along the alignment.
1. Dewatering at the Tunnel Face
Dewatering at the tunnel face is the most common method of groundwater control. This consists simply of allowing the water to drain into the tunnel through the face, collecting the water, and taking it to the rear by channels or by pumping. It then joins the site water disposal system. Note that if there are hydraulic or other leaks or spills at the TBM or other equipment in the tunnel such contaminants are in this water.
2. Drainage Ahead of Face from Probe Holes
Probe holes ahead of the tunnel may be placed to verify the characteristics of the rock and hence to provide information for machine operation and control. These holes will also predrain the rock and provide warning of (and drain) any methane, hydrogen sulfide or any other gas, petroleum, or contaminant that may be present. In areas where there are such known deposits of gas or other contaminants it is common (and recommended) practice to keep one or more probe holes out in front of the machine. When such materials are encountered, the probes alert the workers to the need to increase the frequency of gas readings, to increase the volume of ventilation or to take other steps as required to avoid the problem of unexpected or excessive gas in the tunnel.
3. Drainage from Pilot Bore/Tunnel
Pilot tunnels can provide a number of benefits to a larger tunnel drive, including:
- Groundwater drainage
- Gas or other contaminant drainage
- Exploratory information on the geology
- Grouting or bolting galleries for pre-support of a larger opening
- Rock behavior/loading information for design of the larger opening
The question of location and size of pilot tunnel always leads to a spirited discussion such that no two are ever the same. They are typically six to eight feet in general size and may be located at one or more of several locations. As one example, on the H-3 project in Hawaii there was concern that huge volumes of water might be encountered. This is a 36 ft + highway through the mountain to the opposite side of the island. Borings were limited, but did not indicate huge volumes of water. However it was common knowledge that similar sites contained water-filled cavities large enough for canoe navigation and there was concern that a similarly large volume would be found in the H-3 tunnel. The pilot tunnel, proved that water was not a major concern and at the same time provided a second, unexpected benefit: by being able to see and analyze the rock for the whole tunnel bore, the winning contractor determined that he could perform major parts of the excavation by ripping with state-of-the-art large rippers in lieu of using drill and blast techniques. Because of this evaluation he was able to shave off millions of dollars in his bid and accelerate the construction schedule by several weeks. As an added benefit, the pilot tunnel was enlarged slightly and now is the permanent access (by way of special drifts) for maintenance forces to access the entire length of tunnel with small pickups without using the active traffic lanes.
4. Grouting
Groundwater inflow into rock tunnels almost exclusively comes in at joints, bedding planes, shears, fault zones and other fractures. Because these can be identified grouting is the most commonly used method of groundwater control. A number of different grout materials are used depending on the size of the opening and the amount of the inflow.
The design approach is first to detect zones of potentially high groundwater inflow by drilling probe holes out in front of the tunnel face. Second, the zones are characterized and, hopefully, the major water carrying joints tentatively defined. Then, third, a series of grout holes are drilled out to intercept those joints 10 feet to a tunnel diameter beyond the tunnel face or wall. Fourth, using tube-a-machetes, cement and/or water reactive grouts are injected to seal off the water to a level such that succeeding holes are drilled as the fifth step and injected with finer, more penetrating grouts such as micro-fine or ultra-fine cements and/or sodium silicate can be injected to complete the sealing off process. Based on evaluation of the grouting success additional holes and grouting may be required to finally reduce the inflow to an acceptable level. Typically it will be found that steps four and five must be repeated, trial and error, until the required reduction in flow is achieved.
5. Freezing
On rare occasions, it may become necessary to try freezing for groundwater control in a tunnel in rock. This might occur, for example, at a shaft where it was necessary to control the groundwater locally for a breakout of a TBM into the surrounding rock. If upon beginning excavation of the TBM launch chamber it were found that the water inflow was too great the alternative control methods would be to grout as discussed above or perhaps to freeze.
The authors are not aware of any examples in the U.S. where freezing has been used in a rock tunnel, probably for a very simple reason that high inflow encountered into a rock tunnel would be concentrated at the joints present in the rock. The concentration would usually result in a relatively high velocity of flow. Such velocity would typically exceed six feet per day, the maximum groundwater velocity for which it is feasible to perform effective freezing. Thus, for the most part freezing would not be used in rock tunneling except very locally, as discussed above, and even then it might be necessary to use liquid nitrogen to perform the freezing.
6. Closed Face Machine
A closed face machine could be used for rock tunneling in high groundwater flow conditions over short lengths. In reality such a machine would be more like an earth pressure balance (EPB) machine with sufficient rock cutters installed to excavate the rock. For any extended length (greater than a few hundred feet) this would typically be uneconomical. The machine would have to grind up the rock cuttings and mix, the resulting "fines" with large quantities of conditioners and the existing water to result in a plastic material. This is necessary for the EPB to control the face in front of the bulkhead and to bring the material from its pressurized state at the face down to ambient by means of the EPB screw conveyor (See Chapter 7).
For these reasons, one would not normally plan to build a closed face rock machine but to equip an EPB with rock cutters for driving short stretches in rock within a longer soft ground tunnel. An exception to this general statement would be a rock tunnel in weak or soft rock such as chalk, marl, shale, or sandstone of quite low strength such that it essentially behaved as high strength soft ground.
7. Other Measures of Groundwater Control
The groundwater control methods discussed above probably account for more than 95% of the cases where such control is required in a tunnel in rock. For the odd tunnel (or shaft) where something else is required the designer may have to rely on experience and or ingenuity to come up with the solution. A few suggestions are given here, but really inventive solutions may have to be developed on a case-by-case basis.
Compressed Air once was a mainstay for control of groundwater or flowing or squeezing ground conditions but it is used very infrequently in modern construction. Where the tunnel (or shaft) can be stabilized by relatively low pressures (say 10 psi or less) it may still be used. However, it requires compressor plants, locks, special medical emergency preparation and decompression times.
Panning may be attractive in some cases where the water inflow is not too excessive and is concentrated at specific points and/or seams. In this case pans are placed over the leaks and shotcreted into place. Water is carried in chases or tubes to the invert and dumped into the tunnel drainage system
Drainage Fabric is now frequently used in rock tunnels. These geotechnical fabrics can be put in over the whole tunnel circumference or, more often, in strips on a set pattern or where the leaks are occurring. Fastened to the surface of the rock with the waterproof membrane portion facing into the tunnel, this fabric is then sandwiched in place by the cast-in-place concrete lining. The fibrous portion of the fabric provides a drainage pathway around and down the tunnel walls and into a collection system at the tunnel invert.
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