Engineering Geology

E n g i n e e r i n g G e o l o g y
created by water entering a tunnel is that of face stability. Secondary problems include removal
of excessively wet muck and the placement of a precision-fitted primary lining or of ribs.
The value of the maximum inflow is required and so are the distribution of inflow along the
tunnel section and the changes of flow with time. The greatest groundwater hazard in underground
work is the presence of unexpected water-bearing zones, and therefore, whenever
possible, the position of hydrogeological boundaries should be located. Obviously, the location
of the water table, and its possible fluctuations, are of major consequence.
Water pressures are more predictable than water flows as they are nearly always a function
of the head of water above the tunnel location. They can be very large, especially in confined
aquifers. Hydraulic pressures should be taken into account when considering the thickness of
rock that will separate an aquifer from a tunnel. Unfortunately, however, the hydrogeological
situation is rarely so easily interpreted as to make accurate quantitative estimates possible.
Sulphate-bearing solutions attack concrete, thus water quality must be investigated. Particular
attention should be given to water flowing from sequences containing gypsum and anhydrite.
Rocks containing iron pyrite also may give rise to water-carrying sulphates, as well as acidic
water.
Most of the serious difficulties encountered during tunnelling operations are directly or indirectly
caused by the percolation of water towards the tunnel. As a consequence, most of the techniques
for improving ground conditions are directed towards its control. This may be achieved by using
drainage, compressed air, grouting or freezing techniques.
Gases in Tunnels
Naturally occurring gas can occupy the pore spaces and voids in rock. This gas may be under
pressure, and there have been occasions when gas under pressure has burst into underground
workings, causing the rock to fail with explosive force (Bell and Jermy, 2002).
Wherever possible the likelihood of gas hazards should be noted during the geological
survey, but this is one of the most difficult tunnel hazards to predict. If the flow of gas appears
to be fairly continuous, then the entrance to the flow may be sealed with concrete. Often, the
supply of gas is exhausted quickly, but cases have been reported where it continued for up
to 3 weeks.
Many gases are dangerous. For example, methane, CH 4 , which may be encountered in
Coal Measures, is lighter than air and can readily migrate from its point of origin. Not only is
478