The undrained cyclic behaviour of a carbonate sand

A series of undrained cyclic triaxial tests was carried out on a carbonate sand that had previously been the subject of extensive research for monotonic loading. It was found that the generation of pore pressures under cyclic loading depended on the same factors as they did for monotonic loading, so that the state of the soil in the plane of specific volume against logarithm of mean effective pressure was of particular significance. For states that were not initially on the normal compression line, the pore pressure changes were also found to be highly dependent on how the initial state was achieved: whether it was by compaction and first loading, or by overconsolidation. Failure was found to occur for a much lower number of cycles from states on the one-dimensional normal compression line than for states on the isotropic normal compression line, and cyclic loading to failure was found to be associated with much less particle breakage than monotonic loading. When the stiffnesses measured during cycling had been normalised for those factors that influence the monotonic stiffness of the soil tested, it was still found that for two-way cyclic loading there was a component of the reduction in stiffness that might be attributed to changes of fabric due to the alternation in the direction of the major principal stress.