Creep

In a conventional strength test, the axial stress on a rock core is gradually increased until the rock fails. Rocks will also deform under constant stress by a process called creep. Creep is time dependent deformation under constant stress.

The conventional test (red line) shows an increase in strain proportional to the applied stress up to the peak strength when the rock fails. In a creep test, (black lines) the stress is raised rapidly and held constant. The rock strains until it fails. Notice that the nearer the creep stress is to the peak strength, the smaller the amount of creep strain to failure.

The strain-time curve for a creep test has a very characteristic form. Initially, as the load is applied the elastic strain occurs (virtually instantaneously). As time passes under constant stress, the rate of strain reduces. This period of decelerating strain-rate is called primary creep. The primary creep phase is followed by an extended period of slow (almost steady-state) deformation called secondary creep . At the end of this stage, the strain-rate begins to accelerate and the material rapidly fails. The final stage of accelerating deformation is called tertiary creep.

Creep in rock masses is associated with crack propagation. During the primary creep phase the rock "aclimatises" to the applied stress and crack propagation slows to a stable, almost constant rate. During the "steady" secondary creep stage, the material is damaged more and more until finally, in the tertiary stage, uncontrolled accelerating crack propagation leads to failure.

Creep is important at low temperatures and pressures only in a few rock types: shales, soft chalks and evaporite rocks. It is a major factor in the design and construction of potash mines in Saskatchewan.