Stiffness Degradation of Undisturbed Saturated Soft Clay in the Yangtze Estuary Under Complex Stress Conditions

Stiffness degradation will occur due to the generation of accumulated pore pressure in saturated soft clays under cyclic loading. The soil static-dynamic multi-purpose triaxial and torsional shear apparatus in Dalian University of Technology was employed to perform different types of test on the saturated soft marine clay in the Yangtze Estuary. Undisturbed samples of the clay were subjected to undrained cyclic vertical and torsional coupling shear and cyclic torsional shear after three-directional anisotropic consolidation with different initial consolidation parameters. Investigated were the effects of the initial orientation angle of the major principal stress, initial ratio of deviatoric stress, initial coefficient of intermediate principal stress and continuous rotation of principal stress axes on the stiffness degradation. It is found that the degradation index decreases （or degradation degree increases） significantly with increasing initial orientation angle of the major principal stress and initial ratio of deviatoric stress. Compared with the effects of the initial orientation angle of the major principal stress and initial ratio of deviatoric stress, the effect of initial coefficient of intermediate principal stress is less evident and this trend is more clearly reflected by the results of the cyclic torsional shear tests than those of the cyclic coupling shear tests. At the same cycle number, the degradation index obtained from the cyclic torsional shear test is higher than that from the cyclic coupling shear test. The main reason is that the continuous rotation in principal stress directions during cyclic coupling shear damages the original structure of the soil more than the cyclic torsional shear does.Based on a series of experiments, a mathematical model for stiffness degradation is proposed and the relevant parameters are determined.

Effect of Initial Principal Stress Direction on the Dynamic Characteristics of Carbonate Sand

The dynamic characteristics of carbonate sand under wave loads are very important for constructions on the ocean floor. The initial principal stress direction has been known to exert some influence on the dynamic characteristics of sand during cyclic loading. In an effort to investigate this aspect of the problem, several series of cyclic undrained tests were carried out on a saturated and loose sample of carbonate sand using a geotechnical static and dynamic universal triaxial shear apparatus. In this test apparatus, a hollow cylindrical sand specimen is subjected to a simultaneous application of both triaxial and torsional modes of shear stresses, which brings about the continuous rotation of principal stress axes. The test results indicated that the initial principal stress direction has a considerable influence on the dy- namic strength of loose carbonate sand and with the increase of initial orientation of principal stress, dynamic strength will be reduced, the cyclic pore pressure increased, but the residual pore pressure reduced.