ABSTRACT
In determining the rock strength for each of these applications, it is most important to account for
the presence of discontinuities, such as joints, faults or bedding planes. For most conditions this requires that the rock mass strength properties, rather than the intact rock properties be used in design. The rock mass is the in situ, fractured rock which will almost always have significantly lower strength than the intact rock because the discontinuities divide the rock mass into blocks. The strength of the rock mass will depend on such factors as the shear strength of the surfaces of the blocks, their spacing and continuous length, and their alignment relative to the load direction. For example, the wedge of rock at the downstream toe of the dam foundation shown in Fig. 3.1(b) could fail in shear along a surface lying partially through intact rock and partially along existing discontinuities. Furthermore, if the loads are great enough to extend discontinuities and break intact rock, or if the rock mass can dilate resulting in loss of interlock between the blocks, then the rock mass strength may be significantly diminished from that of the in situ rock. Foundations located in fractured rock which are designed using the strength values of intact samples tested in the laboratory are likely to be significantly under-designed.
