Stress Measurement
The direct measurement of stress is an important source of information on the state of stress in the lithosphere. At shallow depths, the state of stress is strongly affected by the presence of faults and joints, and stress measurements near the surface yield little useful information on tectonic stresses in the lithosphere. At sufficiently large depths, the lithostatic pressure closes these zones of weakness, allowing stresses to be transmitted across them. Stress measurements made at depth are thus directly interpretable in terms of large-scale tectonic stresses. Stress measurements at depth are carried out in mines and in deep boreholes. The two principal methods of making in situ stress measurements are overcoring and hydrofracturing. The first step in overcoring is to drill a hole in rock that is free of faults and joints. Strain (deformation) gauges can be installed in three perpendicular directions on the base of the hole and on the side, as illustrated in Figure 2–17a (strain or deformation in response to stress is discussed quantitatively in the following section). Alternatively two holes are drilled at right angles, and strain gauges are installed on the bases of the two holes.
