Undrained cyclic torsion shear tests on permanent deformation responses of soft saturated clay to traffic loadings

Long-term traffic loadings usually induce huge settlement in the pavement subgrade underlying soft subsoil. Traffic moving loading essentially produces a heart-shaped stress path in the deviatoric stress space. The conventional cyclic apparatus is usually used to simulate the permanent settlement of pavement, neglecting the rotation of the principal stress axis caused by the traffic loading. In order to achieve the heart-shaped stress path, a series of undrained cyclic torsion shear tests are performed on DHCA. In comparison, a series of undrained cyclic triaxial shear tests are also done on DHCA. The saturated soft clay is selected to be tested by applying two types of cyclic loadings, i.e., cyclic heart-shape shear and cyclic triaxial shear. The two types of cyclic shear loadings produce much different permanent undrained responses under the same initial confining and dynamic stress levels. The difference of permanent behaviors becomes more significant with the increase of the dynamic stress level. Compared with the conventional undrained cyclic triaxial shear, the undrained cyclic torsion shear generates larger permanent axial strain and higher accumulated pore pressure. In addition, the effective dynamic stress ratio increases during cyclic torsion shearing and tends to become constant at the occurrence of cyclic incremental collapse. The classical plastic shakedown response observed during a low number of loading cycles gradually becomes an intermediate response of cyclic plastic creep after a high number of loading cycles.