Instability of granular soils under strain path testing

Recent experimental studies revealed that when the drainage condition of loose sand is switched from drained to undrained it can manifest runaway instability prior to attaining the failure stress state. To study this type of unstable behavior (referred to as prefailure instability) under more-general conditions, the strain-path testing technique was used to constrain the strain development of a triaxial sample along a constant strain-increment-ratio path. Medium-loose to dense samples were used in this study. Provided certain conditions are met, prefailure instability can occur even for a dense sample. The factors affecting the occurrence of instability are established from test results. Two instability conditions are established for predicting of instability under a general stress-controlled condition. These two conditions require input parameters that are relatively simple to obtain yet can accurately predict the occurrence of instability. For the special case of undrained condition, these two instability conditions degenerate to those proposed by previous researchers.