Abstract

While seismic waves are commonly treated as horizontally polarised, the earthquake is not limited to only one direction on the horizontal plane. This presentation discusses the results of undrained cyclic simple shear tests with multi-directional loading paths. A conical failure envelope is hypothesised and validated in the 3D τ-σ’ space. This failure cone hypothesis explains the unsolved questions, including why stress paths seem not able to reach the failure line in bi-directional linear tests and why, in multi-directional shearing scenarios, liquefaction can happen without the excess pore pressure ratio reaching the unity. The concept of conical surface, however, cannot be transferred conveniently to the phase-transformation states, where a bi-modal phenomenon is seen. Based on the testing data, discussions are made regarding liquefaction criteria, liquefaction resistance, the development of excess pore pressure and shear strain, as well as the degradation of shear modulus in multi-directional loading conditions.