Effect of the Principal Stress Direction on Cyclic Cumulative Deformation and Pore Pressure of Soft Clay

Permanent settlement of the subsoil induced by traffic load has been paid more and more attention recently. Predicting the traffic-load-induced permanent settlement correctly especially on saturated soft clay ground has been an important issue for the highway construction. For this propose, simplified methods based on empirical models for predicting the accumulative deformations of soft clays are usually preferred. Numerous experimental studies have been conducted by cyclic triaxial tests. However, little is known concerning the effects of principal stress direction, where the major principal stress direction is rotated away from the vertical by an angle due to undrained embankment loading. In this paper, a series of undrained cyclic tests with the hollow cylinder apparatus have been performed on Shanghai soft clay samples that involved different directions of the principal stress. In order to create explicit (empirical) models for predicting cyclic cumulative strain and pore water pressure, undrained static tests were also conducted under different directions of the principal stress. Test results show that the cumulative plastic strain and pore water pressure depend not only on the effective confining pressure, applied cyclic stress ratio and number of cycles, but also on the direction of principal stress. The improved explicit (empirical) models previously proposed to calculate the cumulative axial plastic strain and pore water pressure of saturated clay from the cyclic triaxial tests are further verified by the cyclic hollow cylinder tests under fixed directions of the principal stress. Such improved explicit (empirical) models will provide an effective approach to calculating the long-term settlement of highway embankment built on the soft ground caused by traffic load.