Evolution of anisotropic stiffness and damping ratio of soft clay by bidirectional step-amplitude cyclic triaxial tests

Soft clay is the primary soil type encountered in engineering construction in the eastern coastal regions of China. The deformation characteristics of soft clay are closely related to its inherent stiffness. Under the action of long-term geostatic stress and external load, the dynamic behavior and characteristics of soil in vertical and horizontal directions are different, i.e., anisotropy. In this study, the dynamic parameters of saturated soft clay samples were investigated through bidirectional dynamic step-amplitude cyclic triaxial experiments. The anisotropic stiffness evolution of soft clay over a wide strain range was analyzed, and the effects of different consolidation states on the development of dynamic shear modulus and damping ratio were also examined. Under the same confining pressure, the soft clay samples subjected to axial step-amplitude cyclic loading exhibited higher ultimate dynamic stress values in backbone curves compared to those under radial step-amplitude cyclic loading, while the obtained shear modulus showed the opposite trend. The anisotropic stiffness ratio of soft clay samples tended to increase with increasing confining pressure, with an average value of 1.25 in the range of 100–300 kPa. The shear modulus of the samples increased with increasing confining pressure and consolidation stress ratio but decreased with increasing overconsolidation ratio (OCR).