Strength of Rock

Creep Mechanical Behaviour Outline

Rocks fail in different ways depending on the temperature and pressure. At low temperatures and high strain rates rocks are brittle-elastic. They deform elastically at stresses up to about 70% of their strength then crack propagation becomes dominant and eventually the rock fails as cracks coalesce to form a large fracture or failure surface.

At low confining pressures, shallow depths or near free surfaces, vertical splitting (1) is the usual failure mode. At higher confining pressures (deeper) a single shear plane develops (2). At even higher confining pressures, a network of inclined shears develops (3).

At low strain-rates, elevated temperatures and very high confining pressures the stress strain curve does not have a distinct maximum to indicate failure. Samples show the continuous deformation under load characteristic of ductile-plastic materials. Failed cores have a characteristic "barrel" shape. The transition from brittle-elastic behaviour to ductile-plastic behaviour is favoured by:

The change in behaviour is called the brittle-to-ductile transition. For most rocks it occurs at temperatures and pressures outside the normal range of engineering. However, some shales, fine grained limestones (chalk) and most evaporites (rocksalt, potash, gypsum etc) show ductile behaviour in near-surface, low-temperature environments.

Creep Mechanical Behaviour Outline
(c) Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9