The influence of system compliance on collapse of triaxial sand samples

The observed occurrences of dynamic instability of triaxial samples of loose sand presented in the literature are considered in relation to the existence of energetically possible departure paths involving homogeneous deformations. The existence of energetically possible departure paths is investigated under small strain conditions through the sign of the second-order variation in the total energy of the whole mechanical system, comprising both the sample and the loading devices, thus taking the influence of system compliance into account. The Severn-Trent sand model is used to evaluate the second-order deformation work of the sample and no ad hoc assumption is found necessary to improve the model's response. The evolution of elastic anisotropy is measured and taken into account, revealing negligible effects. The theoretical findings prove consistent with experiments and highlight the important part played by the compliance of the loading apparatus, which can deeply affect the onset of dynamic instability, as further confirmed by new experimental evidence.