Rock mass consists of intact rock blocks, separated by various discontinuities that are formed by weathering and other geological processes. Intact rock is an unjointed piece of rock. Rock fragments and rock cores used in laboratory tests are generally all intact rocks. The intact rock itself is a non-homogeneous, anisotropic and inelastic material. The presence of discontinuities in a large scale makes the situation even more complex. The engineering performance of a rock mass under external loadings is very often governed by the strength and orientation of the discontinuities rather than the pro perties of the intact rock. Other factors that influence rock behaviour are the presence of water and the initial stresses within the rock mass. The discontinuities make the rock mass weaker than the intact rock. In addition, the discontinuities allow access to water, thus compounding the problem. Figure 3.1 shows a relatively steep excavation in a heavily jointed rock.