Undrained anisotropy and cyclic resistance of saturated silt subjected to various patterns of principal stress rotation

Although the undrained anisotropic behaviour of sand has been extensively studied, little is known about undrained anisotropy and liquefaction susceptibility of silt soil. This paper presents a systematic experimental study on the undrained response of a silt soil under different patterns of principal stress rotation, including the 90 degrees jump of principal stress and the continuous rotation of principal stress. Particular attention has been focused on the influence of the cyclic stress path and the influence of cyclic stress ratio (CSR) on the pore pressure ratio r(u) and the deviatoric strain amplitude gamma(qa). A remarkable finding of this study is that, for all loading patterns investigated, the pore pressure ratio r(u) is uniquely related to the deviatoric strain amplitude gamma(qa), rather than to the CSR. Based on the experimental data, an explicit expression is proposed to relate gamma(qa) and r(u), which can be used to predict the onset of failure as well as the build-up of pore water pressure at a specific strain level. The study also found that the relationship between the CSR and the number of cycles required to failure (N-f) is dependent on cyclic loading patterns and stress paths. However, by defining a unit cyclic stress ratio (USR) as a new index for cyclic resistance, a virtually unique relationship between USR and N-f can be established for all cyclic loading patterns and stress paths considered.