The Mechanical Properties of Naturally Deposited Soft Soil under True Three-Dimensional Stress States

Foundation instability and collapse accidents often occur during construction in soft soil regions. The mechanical properties of naturally deposited soft soil in Shanghai were discussed in light of experimental observations from a series of major principal strain-controlled true triaxial undrained shearing tests performed on cubical undisturbed soft soil specimens. The results show that the three-dimensional stress state has a strong influence on the stress and pore pressure properties of soft soil. The initial gradients of deviator stress–strain curves and strength of soft soil obviously increase with the increasing intermediate principal stress and confining pressure. The measured mechanical responses of soft soil are subject to the influence of stress path effects, changes in the initial mean effective stress, and relative magnitudes of intermediate principal stress b. The effective stress paths are manifested by concave or convex shape caused by excess pore pressure, which increases with the increasing b value. The failure envelopes in the p′- q space are a series of straight lines with various slopes at various b values that intersect at the same point in the p′-axis. Considering the effects of the intermediate principal stress and structural characteristics, the failure function with a shape function g(b) for Shanghai soft soil is presented. The results calculated by the equation g(b) presented are in good agreement with experimental data.