Pre-failure strain softening and pre-failure instability of sand: A comparative study

Strain softening in the form of decreasing shear resistance and instability, the inability to sustain given loads, can occur before the failure state is attained. This so-called prefailure strain softening or pre-failure instability has been observed to occur for very loose sand in undrained tests and for dense sand in strain path controlled tests. Experimental data are presented in this paper to compare the pre-failure strain softening and pre-failure instability behaviour. It is established that the factors that control the occurrence of pre-failure strain softening and pre-failure instability are essentially the same, although the two types of behaviour are different. The similarities and differences between strain softening and instability are discussed for both loose and dense sand.