ABSTRACT
The existing surface of exposed rock that may be found where soil cover is absent can be considered as nature’s disturbed zone. Steep mountainsides in rocky terrain, steep gorges or valley sides where dams may be founded or glaciated terrain are typical examples. As we have seen earlier the special coincidence of low stress, weathering effects and possibly more frequent jointing (with joint apertures above the limits of ‘acoustic closure’) make such zones extremely ‘visible’ in seismic refraction or sonic logging of shallow boreholes. Velocities may be up to several km/s lower in the near-surface zone than at greater depth, in some cases even when joint frequencies remain unchanged. In an attempt to get away from the weathered zone, investigators of dam sites and deep foundations may often construct adits or shafts for conducting deformability and other geomechanical tests. Unfortunately, however much care is taken, even to the extent of non-blasting methods, a disturbed zone results. The removal of stressed rock and its usual replacement by air at atmospheric pressure (a convenient ‘definition’ of tunnelling), results in a radial stress (r) that approaches zero at the excavation walls. The tangential stress () may assume many values (including negative) depending upon the existing stress anisotropy, joint orientation, rock strength and the disturbance caused by the excavation method.
