Similarly, fault zones and shear zones are highly permeable
zones, likely to form easy avenues for ground water passage.
Inclined fault planes and shear zones over the roof and along
the sides introduce additional complications in computation of rock pressure on
the one hand and of rock strengths on the other.
This discussion leads to a general conclusion:wherever tunnel
is intersected by fault planes or shear zones, it is to be considered as
passing through most unsafe situations and hence designed accordingly by
providing maximum support and drainage facilities.
Joints are cracks or fractures developed in rocks due to a
variety of causes. although all types of joints tend to close with depth (due
to load of overburden), their presence and orientation, has to be investigated.
Joints are planes of weakness and must always be suspected
when the rocks are folded and faulted.
Even originally closed joints may become reactive
and open up in the immediate vicinity of tunnel excavation.
Jointed rocks cannot be considered as
self-supporting although these might belong to massive
Ground Water Conditions
Determination of ground water conditions in the region of
tunnel project is not to be under-estimated at any cost.
In fact ground water level vis-à -vis tunnel axis is a major
factor governing computations of overhead loads on tunnels and also in the
choice of method of tunnelling.
conditions effect the tunnel rocks in two ways
Firstly, through its physico-chemical action, it erodes and
corrodes (dissolves) the susceptible constituents from among the rocks and
thereby alters their original properties constantly with the passage of time.
It might have already done much of this type of job when the
tunnel is excavated through such water-rich rocks.
Secondly, it effects the rock strength parameters by its
static and dynamic water heads.
Such an action may become highly pronounced when
an artesian acquifer is actually intercepted by tunnel excavation.