24 Geological setting
Figure 2.20 Slickensided fault surface with low shear resistance.be a combination of the two-as in a pressurized water tunnel-where the
tunnel is excavated, the rock stress is redistributed and then a water
pressure applied inside the tunnel (Fig. 2.23). The engineer has to consider
the stability of the structure throughout this process.
Thus, it is very important for the engineer to be aware of the types of
stress state, both natural and applied, that can be present. In particular,
there are two aspects of stress which are perhaps counter-intuitive at first
sight:
(a) in the case of a deep underground tunnel, the floor will be affected in
the same way as the roof by the stresses around the tunnel;
(b) in the majority of stress states measured throughout the world, one
horizontal component of the stress field has greater magnitude than
the vertical component.
The result of (a) may be that in addition to rock bolting the roof, the floor
may have to be bolted down. The result of (b) is often that our primary
High horizontal stress
Figure 2.21 Applying loads to a rock mass which may well already contain a pre-
existing stress state.