Field evidence of the existence of the high horizontal stresses at shallow depths is seen most clearly in areas of massive igneous rocks which contain very few tectonically induced fractures. These rocks usually contain “sheet joints’’ near-parallel to the ground surface, as shown in Figure 1. The sheet joints show rough, irregular, plumose surfaces indicating that the rock failed in tension, by buckling or spalling, the tension being induced by the high horizontal compressive stress. Some sheet joints show slickensides
near their extremities indicating local shear failure which should be expected here (Figure 1).
The spacing of sheet joints is often 0.3 m to 1 m near the ground surface and becomes progressively wider with depth.
From all of these characteristics, sheet joints are considered to be stress-relief features. The linear decrease in horizontal stresses as the ground surface is approached is due to the progressive relief of horizontal stresses partly by buckling and spalling, as the overlying rock load is removed by erosion. Holzhausen (1989) provides a comprehensive account of the characteristics and origin of sheet joints.
In fractured rock masses, i.e. those which are already weakened by defects of tectonic origin, the effects of horizontal stress relief are not so obvious but are always present, as opening up of the existing defects, as shown in Figure 2b. The destressing effects usually extend to greater depths in jointed rock than in massive rock. However, in most geological situations it can be assumed that beyond the near-surface effects, the tectonically induced joints will become progressively tighter (less open) with greater depth (Snow, 1970).