Experimental investigation of the conditions at which the collapse line coincides with the critical state line

Abstract

The conditions at which the collapse line coincides with the critical state line were investigated by means of a new ring shear device. Natural and artificially constituted sands were tested in undrained conditions to evaluate their responses to variations in relative density. The collapse and critical state lines merged in specimens at the critical density in which the shear resistance and pore pressure briefly reached a condition of zero rate of change and remained momentarily constant and equal. At a given normal stress, the friction angle at the phase transformation was equal to the friction angle at the steady state in specimens at the critical density. At densities below the critical density, the specimens exhibited purely contractive behaviour; above the critical density, the specimens’ behaviour was defined by three distinct stages of deformation of phase transformation, peak and steady states. A comparison of the friction angle at the phase transformation, steady state and threshold state in all of the soils revealed only slight differences.