LAUSR

Drained triaxial axisymmetric isotropic and subsequently shear compression tests are performed on fully water-saturated short cylindrical samples of nearly monodisperse glass beads, initially assembled in a loose state by moist tamping and undercompaction technique. Both fully drained isotropic and shear tests are affected by dynamic instabilities triggered under uncontrolled isotropic or deviatoric stress. These dynamic instabilities encompass isotropic collapses, compression stick-slips and especially the unexpected isotropic liquefaction. The most reasonable but imprecise scenario points to a possible dynamical collapse of the granular structure triggered by a spontaneous failure of the force chains, generating an instantaneous surge of excess pore pressure in a fully saturated system. This paper aims in exploring the key role of pore pressure outburst on some global macroscopic parameters of these dynamic events. It focuses on the unforeseen, short-lived and stabilised excess dynamic pore pressure and proposes an in-depth assessment of the validity and the reliability of pore pressure measurements. Irrespective of loading condition (i.e. isotropic or drained compression shear) the ephemeral stabilised excess dynamic pore pressure controls the evolution of the axial and volumetric strains in an essentially dynamic consolidation process. Some appearance conditions for these unexpected dynamic instabilities are discussed.