Abstract
A series of undrained and drained constant p triaxial compression and extension tests were conducted on Nevada sand at a relative density of about 70%. Most drained tests exhibited a peak followed by a drop of deviatoric stress. The drop in stress corresponded to a drop in dilatancy rate and the formation of a shear band. In most undrained tests, negative pore pressures developed and deviatoric stress increased during the constant volume phase of the test. Eventually, cavitation of the pore fluid occurred and the deviatoric stress stabilized. The maximum stress ratios (and maximum mobilized friction angles) achieved in drained, constant p′ tests were significantly larger than in the constant volume portions of the undrained tests. Rowe (1969) and Bolton (1986) proposed that the peak friction angle has a component due to the critical state friction angle and a component due to the dilation rate. In undrained shear, there is no net dilation, yet the peak mobilized friction angle exceeds the critical state friction angle. The friction angle mobilized in undrained shear appears to be a function of the plastic dilation rate. The plastic dilation rate was estimated in the undrained tests by setting the sum of elastic and plastic dilation rates to zero. The difference between drained and undrained friction angles is consistent with the difference between plastic dilation rates in drained and undrained tests.
