Abstract
Based on observations from constant strain rate experiments and from creep and stress relaxation experiments initiated at different stress levels it is found that sand exhibits patterns of time effects different from those observed in clays. It appears that time effects in sand may be associated with crushing of particles, and a mechanistic picture of time effects in granular materials is constructed in which time effects depend on interparticle friction, grain crushing and grain rearrangement. This mechanistic picture is based on measured behavior in drained triaxial compression tests on three different sands in which strain rate effects are observed as small to negligible. While creep and relaxation are caused by the same underlying phenomenon, it appears that results of creep tests cannot be obtained from results of relaxations tests, and vice versa. The phenomenon of static fatigue of individual particles seems to be at the root of time effects in sand. A review of previous studies of static fatigue is presented. Triaxial tests on a beach sand incorporating creep and stress relaxation are followed by grain size analysis to prove that grain crushing relate to the observed time effects. Additional triaxial tests are presented in which the effect of water is demonstrated in support of the static fatigue mechanism. Load-controlled tests on individual sand particles in the form of spherical glass beads (quartz) were performed by maintaining constant loads lower than the short term crushing loads. As do rock and concrete specimens in triaxial compression, the glass beads show effects of time to crushing.
