Time-Dependent Characteristics of Coral Sand under Triaxial Stress States

Time-dependent effects of soils have been widely recognized as a key issue, which is particularly evident for coral sand due to its high frangibility. To determine the time-dependent behaviors of coral sand under triaxial stress states for enhanced engineering applications, a series of triaxial tests was carried out on two gradations of coral sand. Besides the conventional time-dependent tests, deviator-stress rate, creep with different stress histories, drop creep/relaxation, and postpeak creep/relaxation tests, which have been rarely investigated, have also been conducted. Additionally, the particle size and shape variations in the coral sand after the tests were analyzed to elucidate the time-dependent behavior mechanism. According to the test results, different from the tests with varying axial strain rates, over- and under-shooting phenomena were not clear when the deviator stress rate changed suddenly. Creep behavior was found to be noticeably influenced by the stress history and decreased with increasing preconsolidation pressure. The stress unloading could considerably reduce the subsequent creep or relaxation response, and the response diminished with increasing magnitude of deviator stress drop. The time-dependent behavior of coral sand was mainly determined by the level of particle breakage. The coral sand particles became smaller and more regular due to particle breakage, which would increase the compressibility of specimens and weaken interlocking between the sand particles, leading to more obvious time-dependent behaviors. The influence of particle breakage on the time-dependent behaviors of coral sand could be examined from two perspectives, i.e., the particle breakage during creep or relaxation processes and that during preloading processes. The effects of the unstable broken particles that formed during preloading were larger. In addition, a unique relationship was observed between the relative breakage and input energy for the same coral sand gradation, regardless of the test conditions, which was meaningful for the time-dependent constitutive modeling considering particle breakage.