All engineering construction activities on ground and underground are executed in soil or rock. These geological materials are formed through very complex natural processes. One may be the outcome of the other. Understanding their engineering responses is essential to evolve economical and rational designs and adopt appropriate construction methodologies. A geotechnical engineer is supposed to understand the responses of both these materials equally well. This article very briefly deals with characterizing rock mass as an engineering material. A number of correlations established recently are presented for solving rock engineering problems more realistically under equivalent continuum approach with joint factor, a weakness coefficient in rock mass.
The engineering responses of soils and rocks differ significantly. A soil is a particulate material existing over a wide range of particle size as clay, silt, sand, gravel, cobble and boulder. A soil is considered to be a less competent material in comparison to a rock. A soil mass is often treated as a continuum, homogeneous and isotropic mass. On the contrary, a rock, which may exist in an intact form in a limited volume, is often found to exist as a discontinuous mass due to the action of tectonic forces resulting jointing and anisotropy, which control the engineering behavior. Rocks have joints / joint sets. These joints could be tight or open; they may have gouge material formed as a result of shear along the fractures as in the case of faults or soil may be deposited in the open joints by flowing water. These discontinuities play a decisive role in the rock mass behavior and the displacements take place primarily along these weak planes. The permeability is governed by the discontinuities and the gouge material. Seepage and pore water pressures are governed by anisotropy in the mass. Test results from a small specimen of rock cannot be directly applied to solve engineering problems unlike what is done in the case of soils, except rockfills. The combined influence of joints, their configuration and the strength along them will have to be considered in the analysis and design. The rock mass is, therefore, a discontinuum, anisotropic and inhomogeneous naturally occurring pre-stressed medium Hence, strictly speaking continuum approaches are not appropriate to solve rock engineering problems. But depending upon the ratio of the extent of mass considered to the spacing of joints, (i.e. joint frequency), it is often treated as an equivalent continuum to assess the overall mass response.
The anisotropy in rocks is due to the shape of crystals and their orientation, planes of weakness due to schistosity, joints, fractures, shear plans and fault zones; it significantly influences the engineering response. Anisotropy in rocks may be either inherent, induced or both. may be either inherent, induced or both. The orientation (β0) of these planes of weakness with respect to the direction of loading greatly influences compressive strength, shear strength, tensile strength, modulus and stress-strain responses.