rock mechanics

rock mechanics Rock mechanics is the theoretical and applied science of the mechanical behaviour of rock and rock masses; it is the branch of mechanics that is concerned with the response of the rock and rock mass to the force fields of their physical environment. It is an extremely complex subject that has only since the 1950s become a science, and it remains a blend of advanced analytical techniques and ‘rules of thumb’.

Throughout history, underground openings have been used as shelter for both the living and the dead. Man has searched avidly for minerals, has tunnelled for storage, for water, to make roads, and to carry out sieges or escape from them. The importance of underground space is increasing as man explores the options for new uses, such as the disposal of radioactive waste or underground storage. Civil engineering projects of all types can entail rock excavation, whether at the surface or at depth.

Several inherent complexities render the scientific aspects of rock mechanics noteworthy. These include rock fracture, size effects, tensile strength, and the effects of groundwater and weathering. Rock masses are generally heterogeneous, containing intact materials of varying engineering pro-perties as well as joints, faults, folds, and other features. This inherent variability makes engineering in rock different from engineering design within most other media.

Civil engineering projects to which rock mechanics is applicable can be divided into seven categories.

Foundations. Rock is usually an excellent foundation material, but near-surface rock can be heavily weathered and fractured as a result of stress-relief. It is important to determine the competence of the rock mass to bear the required load at the acceptable levels of deformation or settlement.

Rock slopes. Rock slopes can fail in various ways: by plane, wedge, direct, or flexural toppling. The potential for failure in any of these modes can easily be identified by rock-mechanics methods.

Shafts and tunnels. The stability of shafts and tunnels depends on rock structure, in situ stresses, stress regime modification, groundwater flow, and construction technique. The fundamental rock-mechanics principles provide the basis for adequate design and mitigation against failure.

Caverns: use of underground space. The risks of underground instability increase as the type of excavation moves from a generally circular tunnel excavation towards a non-circular cross-section and as the size of underground opening increases. Jointing and inhomogeneity within the rock mass become increasingly important as the span of the excavation increases.

Mining. There is a huge range in the geometry of mining operations, but in all cases the mining methods are designed to extract the mineral with the minimum amount of artificial support and in some instances with the minimum amount of natural support.

Geothermal energy. In extracting geothermal energy from hot dry rock, cold water is pumped down into the rock mass to pass through fractures and exit from a borehole or set of boreholes. The optimal configuration for a production system depends on the interactions between rock joints, in situ stress, water flow, temperature, and time.

Radioactive waste disposal. The primary aim of radioactive waste disposal is to isolate waste so that unacceptable levels of radiation do not affect the biosphere. Predicting the safety of a repository requires an understanding of all aspects of the containing rock mass and in addition the ability of the rocks to adsorb radiation.

The two main engineering processes to which rock mechanics applies are excavation and support. The design in both cases relies heavily on an adequate quantification of the properties of the rock mass. Quantitative classification of rock masses provides a rapid means of assessing their quality, comparing qualities, and assessing support requirements. Classification applied on a routine basis can be of great value in mines and other underground projects, and has been shown to be of considerable economic benefit. The two most commonly used applications are the so-called Q system and the Geomechanics Classification System. The Q system is based on rock block size, joint shear strength, and confining stress. The Geomechanics Classification, which derives a rock mass rating (RMR), is based on rock material strength, rock quality designation (RQD), joint spacing, joint roughness and separation, and groundwater. Both systems and a number of variations of these basic approaches are now widely used as the basis for the quantification of rock-mass properties in underground excavation.

Brian J. Mcconnell