Influences of rotation of principal stress axis on accumulative deformation of soft clay under subway cyclic loading

In order to investigate the influences of continuous rotation of principal stress axis on the accumulative deformation of soft clay lying below subway tunnel under subway dynamic loading, the finite element method (FEM) combined with field measurement data is used to analyze the complex stress path in subsoil induced by subway loading, and then the hollow cylinder apparatus (HCA) is used to simulate the stress path in laboratory and to carry out comparison tests. Experimental results show that: the continuous rotation of principal stress axis increases the accumulative deformation of soft clay; and the effect is more significant at higher effective principal stress ratio (i.e. the ratio of the effective maximum principal stress to the effective minimum principal stress) or loading frequency. Under the realistic subway loading and the usual tunnel depth, the continuous rotation of principal stress axis magnifies the accumulative deformation of soft clay between 1.09 and 1.23 times (the higher the loading frequency is, the greater the magnification is). The actual accumulative deformation of soft clay under the realistic subway loading (i.e. with continuous rotation of principal stress axis) can be estimated by multiplying the accumulative deformation obtained from traditional cyclic triaxial tests (i.e. without principal stress axis rotation) by the ratio of axial strains between tests with and without principal stress axis rotation. The preliminary study shows there exists power function between the ratio and the maximum effective principal stress ratio of soil.