Time Effects in Relation to Crushing in Sand
Time effects have frequently been reported in granular materials, and differ from those observed in clays. Recent studies suggest that particle crushing and static fatigue may be at the root of observed time-dependent behavior in granular material. This study has focused on time effects associated with crushing. Several series of time-dependent triaxial compression tests were performed on dense Virginia Beach sand to explore time-dependent behaviors of granular materials. Furthermore, to exhibit the relation between the static fatigue phenomenon and breakage of individual particles, two series of single particle tests on glass beads were carried out. Although results of single particle tests showed a considerable amount of scatter, the static fatigue phenomenon was clearly observed and beads fractured after some time under stresses smaller than their short-term strength. The stress-strain behavior was not noticeably affected after aging under isotropic compression stresses, while it was slightly influenced under K0 stress states.Time effects were negligible under low confining stresses. However, under high confining pressures, a delay was observed at the beginning of time-dependent phases in the initiation of creep deformations or stress relaxation depending on the prior shearing strain rate; the slower the shearing strain rate was, the longer the delays were. Moreover, time effects were more pronounced at higher deviator stresses. Experimental results after two months suggested that there is no end to creep deformations and stress relaxation. Additional loading at the end of time-dependent phases indicated that there were no structuration effects for dense Virginia Beach sand. Stress drop experiments followed by time dependent phases showed that greater time effects were observed when the initial stress level was closer to the yield surface. Sieve analyses performed after each experiment showed that particle breakage increased with the energy input. Therefore, a significant amount of particle crushing was observed during creep, while negligible crushing was detected during stress relaxation. Specimens sheared for longer periods of time experienced larger amounts of particle crushing and vice-versa. All the aforementioned observations were adequately explained by the static fatigue phenomenon and the proposed mechanistic picture for time effects in granular materials.
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