Conical Failure Surface of Medium-Dense Hostun Sand in Undrained Multidirectional Simple Shear Tests

Realistic seismic shearing is usually not limited to a single direction. A number of studies on the effect of multidirectional shearing have proven that soil behaves differently under multidirectional loading compared with unidirectional shearing. One of the many differences refers to the inability of excess pore pressure to reach initial overburden stress under certain loading conditions, even if large deformation has occurred. To investigate the mechanisms behind the phenomenon, undrained cyclic simple shear tests were conducted on clean Hostun sand with various loading paths. Based on the test results, a conical failure surface is hypothesized and validated in (σv′, τX, τY) space. The failure cone can explain why the strike-direction stress paths in bidirectional tests cannot reach failure lines and also applies to unidirectional tests. An inverse correlation between limiting excess pore pressure ratios and total shear stress ratios can be derived from the failure cone equation.