Undrained dynamic response of naturally high-strength sensitive clay to bidirectional cyclic loadings under the plane strain condition

Soils beneath railway embankments can be considered under the plane strain condition. Excessive settlement has been observed on structured clays beneath railway embankments in the coastal areas of China. Compared with the cyclic behaviors of structured clay under the axisymmetric condition, the dynamic responses under the plane strain condition have been less studied. In this study, several one-way undrained cyclic true triaxial and cyclic plane strain tests were conducted on undisturbed structured clay. The test results show that the permanent major principal strain obtained from cyclic plane strain tests is smaller than that obtained from cyclic true triaxial tests under the same conditions, implying that the permanent major principal strain obtained from the cyclic true triaxial test is overestimated. Moreover, the cyclic minor principal stress has a significant influence on the permanent deformation and excess pore pressure in cyclic plane strain tests. Compared to the results of cyclic plane strain tests with the constant minor principal stress, the permanent major principal strain is lower by 9.2% and 20.1% when the ratios of the amplitude of the cyclic minor principal stress to that of the cyclic major principal stress (eta) are 0.3 and 0.5, respectively. As eta increases, the maximum excess pore pressure increases markedly, whereas the change in the minimum excess pore pressure is almost negligible. Furthermore, an empirical formula for predicting the permanent major principal strain under the plane strain condition consid-ering the impacts of bidirectional cyclic loadings was established.