Study On Failure Criterion For Intact Soft Clay Under Complex Stress Path

Through a summary of research results belonging to domestic and foreign scholars on stress state change for foundation soil unit in the process of excavation unloading of pit and under traffic loads, seismic loads and wave loads, I found that rotation of principal stress axis was salient features in stress state change for foundation soil unit. Clearly, characteristics of shear failure and failure criterion in stress path for rotation of principal stress axis were one of the important issues related to practical engineering design. Based on this, according to the needs of construction projects in southeastern coastal areas of widely distributed soft clay, Hollow cylinder apparatus, Instrument for triaxial test and Scanning electron microscopy were used to carry out series of macro-micro tests for Hangzhou intact soft clay including the rotation of principal stress axis and the initial stress state changes. The main work and achievements were as follows:1. Undrained tests including one-way. forward& reverse, continuous cycle for rotation of principal stress axis and meanwhile changes for amplitude of shear stress and direction of principal stress axes had been done, and it was found that(1) the level of stress and strain was very small during rotation of principal stress axis. and major principal stress direction angle of late shear stage with fixed principal stress direction that was given by early rotation of principal stress axis controlled stress-strain characteristics for the complete shear stage.(2)one-way rotation of principal stress axes carried out little impact for pore pressure characteristics of the complete shear stage, but forward& reverse, continuous cycle for rotation of principal stress axis and meanwhile changes for amplitude of shear stress and direction of principal stress axes carried out greater impact for that.(3)rotation of principal stress axis carried out little impact for shear strength characteristics of intact soft clay in the complete shear stage, and the anisotropy of shear strength was controlled by major principal stress direction angle in late shear stage with fixed principal stress direction.2. Series of macro-micro tests for rotation of principal stress axis considering changes that included the initial principal stress direction angle, coefficient of intermediate principal stress, initial shear stress and over-consolidation ratio had been done, and it was found that(1)the initial principal stress direction angle determined the characteristics of shear deformation, and the initial principal stress direction angle equal to 45 degrees was the cut-off point.(2)when coefficient of intermediate principal stress was increasing, the increment of pore pressure generated was increased after the first decrease, but normalized shear strength of clay had not reduce or increase regularly, because micro-change factors which influenced anisotropy of normalized shear strength of intact soft clay were different in different coefficients of intermediate principal stress.(3)the changes of initial shear stress and over-consolidation ratio had a greater impact to pore pressure characteristics, so the establishment of failure criterion from the perspective of pore water pressure should consider the effects of changes of initial shear stress and over-consolidation ratio.3. Based on the analysis of impacts of rotation of principal stress axis and the initial stress state changes to shear characteristics of intact soft clay, failure criterion for intact soft clay under complex stress path was researched, the conclusions was that(1)Lade-Duncan failure guidelines after the authors had done a further revision might analysis characteristics of shear failure in stress control system of this test.(2)revised Lade-Duncan failure guidelines was used to calculate clay parameters in different shear directions, and the limit pore water pressure, the peak of effective principal stress ratio, the generalized shear strain and the peak of principal stress difference as failure criterion was carried out a comparative analysis, then the shear test for intact soft clay under complex stress path should use the peak of principal stress difference as failure criterion.