The engineering mechanics problem posed in all structural design is the prediction of the performance of the structure under the loads imposed on it during its prescribed functional operation. The subject of engineering rock mechanics, as applied in mining engineering practice, is concerned with the application of the principles of engineering mechanics to the design of the rock structures generated by mining activity. The discipline is closely related to the main streams of classical mechanics and continuum mechanics, but several specific factors identify it as a distinct and coherent field of engineering.
A widely accepted definition of rock mechanics is that first offered by the US National Committee on Rock Mechanics in 1964, and subsequently modified in 1974: Rock mechanics is the theoretical and applied science of the mechanical behaviour of rock and rock masses; it is that branch of mechanics concerned with the response of rock and rock masses to the force fields of their physical environment.
Rock mechanics itself forms part of the broader subject of geomechanics which is concerned with the mechanical responses of all geological materials, including soils. The learned society for geomechanics in Australia, the Australian Geomechanics Society, defines geomechanics as “the application of engineering and geological principles to the behaviour of the ground and ground water and the use of these principles in civil, mining, offshore and environmental engineering in the widest sense”.
This definition of geomechanics is almost synonymous with the term geotechnical engineering, which has been defined as “the application of the sciences of soil mechanics and rock mechanics, engineering geology and other related disciplines to civil engineering construction, the extractive industries and the preservation and enhancement of the environment” (Anon, 1999). The term geotechnical engineering and the adjective geotechnical will be used in this sense in this text.
Rock differs from most other engineering materials in that it contains fractures of one type or another which render its structure discontinuous. Thus a clear distinction must be made between the rock element or rock material on the one hand and the rock mass on the other. Rock material is the term used to describe the intact rock between discontinuities; it might be represented by a hand specimen or piece of drill core examined in the laboratory. The rock mass is the total in situ medium containing bedding planes, faults, joints, folds and other structural features. Rock masses are discontinuous and often have heterogeneous and anisotropic engineering properties. The nature and distribution of structural features within the rock mass is known as the rock structure. Obviously, rock structure can have a dominant effect on the response of a rock mass to mining operations. It can influence the choice of a mining method and the design of mining layouts because it can control stable excavation spans, support requirements, subsidence, cavability and fragmentation characteristics. At shallow depths and in de-stressed areas, structurally controlled failures may be the prime concern in excavation design. At depth and in areas of high stress concentration, the influence of structure may be less marked, and limiting the induced boundary stresses or energy release rates may be more important considerations.
