Instability of loose sand in constant volume direct simple shear tests in relation to particle shape

Post-peak flow instability of six sands notably different in terms of average particle shape characteristics, gradation, and mineralogy was investigated. Tests were carried out under constant volume condition using a NGI-type direct simple shear apparatus. The testing program covers wide ranges of initial void ratios and vertical effective stresses. Experimental findings confirm that the critical state parameters of the test sands depend strongly on the average particle morphologic characteristic factors. For specimens in loose state, it is shown that stress ratio at the onset of flow instability, normalized constant volume peak shear strength, brittleness index, and the normalized pore water pressure of the tested sands are interrelated with the particles' morphologic factors.