Engineering Geology

Chapter 9
and the dissipation of residual stress, both eventually leading to failure. Soft and
medium clays display squeezing behaviour. Other materials in which squeezing
conditions may obtain include shales and highly weathered granites, gneisses and
schists.
6. Swelling ground. Swelling ground also expands into the excavation but the movement
is associated with a considerable volume increase in the ground immediately surrounding
the tunnel. Swelling occurs as a result of water migrating into the material of the
tunnel perimeter from the surrounding strata. These conditions develop in overconsolidated
clays with a plasticity index in excess of about 30% and in certain shales and
mudstones, especially those containing montmorillonite. Swelling pressures are of
unpredictable magnitude and may be extremely large. For example, the swelling pressure
in shallow tunnels may exceed the overburden pressure and it may be as high as
2.0 MPa in overconsolidated clays. The development period may take a few weeks
or several months. Immediately after excavation, the pressure is insignificant but the
rate of swelling increases after that. In the final stages, the increase slows down.
Boulders within a soft ground matrix may prove difficult to remove, whereas if boulders are
embedded in a hard cohesive matrix, they may impede progress and may render a mechanical
excavator of almost any type impotent. Large boulders may be difficult to handle unless they are
broken apart by jackhammer or blasting.
Water in Tunnels
The amount of water held in a soil or rock mass depends on its reservoir storage properties
(see Chapter 4) that, in turn, influence the amount of water that can drain into a tunnel.
Isolated heavy flows of water may occur in association with faults, solution pipes and cavities,
abandoned mine workings or even pockets of gravel. Tunnels driven under lakes, rivers and
other surface bodies of water may tap a considerable volume of flow. Flow also may take place
from a perched water table to a tunnel beneath.
Generally, the amount of water flowing into a tunnel decreases as construction progresses.
This is due to the gradual exhaustion of water at source and to the decrease in hydraulic gradient,
and hence in flow velocity. On the other hand, there may be an increase in flow as construction
progresses if construction operations cause fissuring. For instance, blasting may
open new water conduits around a tunnel, shift the direction of flow and, in some cases, even
cause partial flooding.
Correct estimation of the water inflow into a projected tunnel is of vital importance, as inflow
influences the construction programme (Cripps et al., 1989). One of the principal problems
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