ABSTRACT
Terms used when describing time-dependent behaviour of rocks are creep, stress relaxation, and fatigue. To avoid confusion in the use of these terms, they are defined via Figure 9.1 in which a rock specimen is illustrated being loaded in compression through an adjacent rock element represented by the spring. Creep is defined as increasing strain while the stress is held constant. Stress relaxation is defined as decreasing stress while the strain is held constant. In practice, and especially for the rock around an underground excavation, a rock element will be loaded via the stiffness of an adjacent element and so the time-dependent behaviour will be somewhere between the ideal conditions of creep and stress relaxation, as illustrated in Figure 9.1. Fatigue refers to an oscillation in the applied stress producing permanent strain. Of course, these definitions are similar when the force-displacement curve is being considered.
