Chapter 18: Rock behaviour

Rocks are naturally occurring, polycrystalline materials that play a wide range of roles in civil engineering. Their behaviour can be understood in terms of their intrinsic textures (grains, cement, voids and discontinuities) and through the application of standardised testing methods. Rocks and soils display a wide range of rheological properties, even under near-surface conditions. The void space in rocks is generally filled with water, at some pore pressure. For many engineering applications rocks may be considered as poroelastic, with a proportion of the applied load being supported by the pore pressure. Thus, their deformation is related to the effective stress (= total stress – pore pressure) as is generally found for soils. Rocks have a wide range of stiffness, strength and permeability, all of which will determine their suitability for different engineering applications. Most importantly, rocks are heterogeneous, displaying various forms of layering or grain fabrics, and are almost ubiquitously fractured. Thus the rock mass properties vary over a range of scales (heterogeneity) and orientations (anisotropy).