Study On Dynamic Undrained Behavior And Multiple Mechanism Boundary Surface Plasticity Model Of Saturated Sand

Two modes have been identified in the earthquake induced damage to soil structures and foundations associated with excess pore water pressure rise in saturated sand. One involves complete flow failure due to reduction in soil strength of saturated sand. This mode of damage is essentially analyzed with a simple static approach by comparing post-earthquake residual strength of sand with initial stress due to gravity. The other involves limited, but often large, amount of deformation due to significant reduction in soil stiffness but without involving mechanism of failure in soil. The mechanism inducing this mode of damage is called cyclic mobility. The cyclic mobility occurring in the loose sand is often called "liquefaction" in practice but in the present study the mechanism which does not involve the failure in soil will be called cyclic mobility. The cyclic mobility, often called liquefaction, plays a significant role in earthquake induced deformation in soil structures and foundations as often revealed in the case histories during past earthquakes.In assessing amount of earthquake induced deformation in soil structures and foundations, a simple but physically feasible model is needed for representing essential features of cyclic mobility. Many approaches have been proposed for modeling of cyclic mobility but, up to the present, none of the proposals seem to be so successful as to be readily applicable in practice of soil dynamics and earthquake engineering. The main problems widely seen in the previous studies are (1) inadequate modeling of initially anisotropically consolidated soil and (2) numerical vulnerability associated with the stress path in the vicinity of the failure line.An attempt is made in this study to solve these problems. Based on the plasticity theory, combined with test results in laboratory, the deformation disciplinarians of sand in the earthquake induced liquefaction are discussed, a new model is proposed, and validation & analysis are performed with the new model in this study. The innovation of Multiple Mechanism Boundary Surface Plasticity Model is that the effect of rotation of principal stress axial directions and large shear displacement of sand in the earthquake induced liquefaction are taken into account at the same time in the model.Saturated sand are testes in the laboratory under the undrained cyclic loading condition with static-dynamic dual purpose triaxial compression test apparatus, which is designed under the cooperation of Hohai University and Marui Co. Ltd Japan. The results indicate that the stress path approaches from a initial stress point to the failure lines as cyclic loading goes on, and eventually converges on a certain limiting stress path in the vicinity of the failure lines. The stress-strain curve, however, does not converge on a closed hystersis loop; the shear strain rapidly increases in looser sand or gradually and continually increases in denser sand.The shear strain increases beyond several percent. Effect of initial mean effective stress on the behavior of soil is examined by tests under undrained cyclic loading at several initial confining stresses with the same stress ratios. The results indicate that the stress path is not affected by changing the initial effective mean stress if the shear stress ratio remains the same. Liquefaction resistance defined by excess pore water pressure rise is approximately independent of the initial confining pressure. While the strain is affected by the initial mean effective.The proposed model is of a generalized plasticity type defined in the strain space. The concept of multiple mechanism is used as a vehicle for decomposing the complex mechanism into a set of simple mechanisms defined in one dimensional space. Each virtual simple shear mechanism is simulated by boundary surface plasticity model in the virtual stress and strain space. These virtual simple shear mechanisms play a key role in idealizing the soil behavior under the initially anisotropic consolidation condition.In order to overcome th...