Effects Of Initial Stress And Stress Path On The Static And Dynamic Shear Behavior Of Saturated Loose Sand

In the actual site, initial consolidation stress state is complicated due to the influence of geological sedimentation or by human activities, reflected by different initial deviatoric stress ratio caused by the deviation of the normal stress and the deflection Angle of the initial principal stress caused by shear stress deviation. Take seismic load as an example, a lot of researches reflects that it will be far from reality to regard earthquake, especially near-field earthquake that causes serious harm to vertical-incident earthquake. Dynamic stress path far more complicated than the situation of vertical incidence will be formed when the seismic wave oblique incidences into the soil unit cell, which is reflected by inclined ellipses with different angle and shape in deviatoric stress plane. Simulation of foundation soil initial stress state and complex clipping path, the deformation and stability of the foundation soil in actual working condition added as well, have been key topics in the study of geotechnical engineering, which are related to the people life and property safety and national infrastructure construction and development, Have important practical significance.In order to study the influence on the static and dynamic characteristics of saturated loose sand, the strain control static tests under different deviatoric stress ratios, orientations of initial principal stress, guillotine actions and drainage methods have been carried out with the help of GDS-HCA, aiming at the Fujian standard loose sand; Cyclic torsional shear tests under different orientations of initial principal stress have been carried out; Dynamic tests under cyclic triaxial path, cyclic circle path and cyclic inclined elliptical path with the condition of isotropic consolidation. The following conclusion-can be obtained:(1) Monotonic shear tests under different orientations of initial principal stress have been carried out. In order to describe the softening degree of soil, the concept of softening coefficient has been put forward. The softening degree of soil increases with the orientations of initial principal stress a0. When a0=90°, softening coefficient can reach to0.75. To the non-directional monotonic shear, it’s orientations of initial major principal stress during shearing to influence the monotonic shear properties of saturated sand. When the orientation of initial major principal stress gets higher, sand is easier to get shearing shrinkage. Otherwise, it’s easier to get dilatancy.(2) The concept of critical breakdown point on the basis of monotonic shear tests under different orientations of initial principal stress has been put forward, aiming at the cyclic torsional shear tests with the same consolidation stress state, which can be regarded as failure criteria of cyclic torsional shear tests under different a0. Compared with commonly used failure criteria of pore pressure and strain, this standard can describe the time of Loss of strength, sharp development of strain much more accurately.(3) In the cyclic torsional shear tests, strain development is closely related to the orientation of initial major principal stress. To test specimens that a=0°and90°without orientation of initial principal stress, its failure strain is mainly axial strain. Besides, to test specimens that a=30°,45°and60°which have orientation of initial principal stress, its failure strain is mainly shear strain. During cyclic torsional shear tests, the principal strain direction presents the appearance of getting closer to orientation of initial major principal stress, which reflects the influence that preloading brings to the flow of strain accumulation advantage.(4) Dynamic stress path is closely related to the pore pressure’s development of saturated loose sand and dynamic strength of soils. The development of elastic pore pressure can be classified as "sharp dropping type" and "sharp rising type". The normalizing pore pressure depends on the ellipticity of cyclic stress path and stress amplitude CSR. Under different dynamic stress paths, the cyclic torsional shearing path and cyclic triaxial path have the highest dynamic strength, while the strength under standard inclined elliptical loading path is lower and that under cyclic circular path is the lowest. If we regard seismic wave as vertical incident S wave, the soil liquefaction strength will be overestimated. Under the same dynamic stress ratio, dynamic strength of soil will be nearly20%less than the situation without the condition of oblique incidence if we take the most unfavorable situation as a factor.