Abstract: In engineering practices, the vertical stress and horizontal stress in the subgrade soil unit constantly change, and the amplitude and direction of shear stress also constantly change, which lead to continuous rotation of the stress path in the soil. To investigate the influence of the confining pressure and the cyclic stress ratio(CSR) on strength, cumulative strain, resilient strain and softening of natural soft clay when the principal stress axis continuously rotates under pure axial compression of traffic load, the GDS hollow cylinder apparatus (HCA) is used to simulate the stress path under a laboratory-simulated traffic load, and tests of continuous rotation of the principal stress axis under different confining pressures and different cyclic stress ratios are carried out. The testing results show that with the accumulation of pore pressure, the samples of natural saturated soft clay gradually softened. The axial and torsional moduli decreased with increasing CSR and confining pressure and then reached a steady value after a certain number of cycles. When the cyclic stress ratio was relatively small, the axial and torsional stress-strain hysteretic curves were linear, and almost coincided with different cycles. With the increase of the number of revolving cycles of the principal stress axis, the axial and shear stress-strain hysteresis curves became more and more nonlinear. The axial and shear stress-strain hysteresis loops of the specimens no longer coincided with the rotating circles, and the hysteresis loops gradually tilted towards the x axis. With the increase of cyclic stress ratio, the axial and shear moduli quickly attenuated at the initial stage of loading and then reached a steady value. Moreover, when the axial and shear moduli of the specimen were stable, the corresponding number of revolving cycles of the principal stress axis increased with increasing confining pressure and cyclic stress ratio.

Key words: natural soft clay, principal stress rotation, soften, hysteresis loops