Dynamic Behaviour And Elasto-plastic Model Of Frozen Soil Subjected To Reapeated Cyclic Loading

More and more infrastructures are building in cold regions of China in recently years. The frozen soil is often the base or subbase of structures in cold regions and it may be subjected to the vibration induced by traffic, earthquake and wind etc. Hence, the dynamic design method of frozen soil system is becoming a subject of prime importance for relevant engineers or researchers. As a special geo-material, the mechanical behavior including dynamic behavior of frozen soil is different with that of unfrozen one, and it is sensitive to the vibration of environment conditions such as temperature or initial moisture content. It is important to investigate the dynamic behaviors, the evolutions of dynamic behaviors and the constitutive models under dynamic loadings of frozen soils while designing or evaluating dynamic stability of frozen soil structures.In this study, a large number of tri-axial tests under repeated cyclic loading have been conducted. Based on test results, the accumulation behavior and the evolution of dynamic modulus are studied systematically. The evolution models of dynamic modulus are then proposed. The strength behavior, plastic flow behavior and hardening behavior are also investigated based on experimental evidences and then the dynamic bounding surface plastic model is produced. The sub-stepping integration algorithm with automatic error control is employed to solve the bounding surface plastic equations. The performance of this model has been verified and evaluated by test results. Subsequently, the accumulated behavior including accumulated intensity and direction are also studied by the test results of frozen soil under long-term low-level repeated cyclic loading. A constitutive model is formulated by empirical equations derived experimental results and the classical elasto-plastic theory to simulate the elasto-plastic behavior of frozen soil under long-term low-level repeated cyclic loading. The details of this study is organized as follows:(1) A large number of tri-axial tests for frozen soils under high-level and long-term low-level repeated cyclic loading are conducted systematically and the accumulated behavior under different levels of repeated cyclic loadings are studied. Subsequently, the dynamic shear modulus and dynamic bulk modulus are used to evaluate dynamic behavior, and the evolutions of them under high-level and long-term low-level repeated cyclic loading are studied respectively. Based on experimental evidence, the evolution models have been produced. Finally, the formulations of evolution models parameters for frozen soils with different temperatures or initial moisture contents are proposed.(2) The strength behavior, plastic flow behavior and hardening behavior of frozen soil are studied by tri-axial compression test, tri-axial shear loading-unloading test and isotropous loading-unloading test respectively. The function of bounding surface, hardening rule and dilatancy equation is presented respectively derived from the experimental evidence, and hence they are integrated into the bounding surface plastic model. Subsequently, based on the assumption of generalized von-Mises criterion, the bounding surface plastic model is developed into three dimensions. Finally, the mechanism and capture method of model parameters are analyzed.(3) The advantages and disadvantages of integration algorithms for elasto-plastic model including explicit, semi-explicit and implicit are analyzed. The mechanism and implementation process of sub-stepping explicit integration algorithm with automatic error control are studied by the framework of generalized elasto-plastic model. Subsequently, the implementation procedures of sub-stepping explicit integration algorithm with automatic error control for bounding surface plastic model is studied in detail and the computational program has been formulated. In addition, the performance of this algorithm is evaluated and the advantages of that in implementation for strong nonlinear complex elasto-plastic model have been discussed. Finally, the model parameters are calibrated by test result.(4) The influences of initial stress ratio, initial mean stress, repeated stress amplitudes, temperature and initial moisture content on accumulated intensity and accumulated direction of frozen soils are studied systematically based on tri-axial test under long-term low-level repeated cyclic loading. The effect of initial stress ratio on dynamic shear modulus and dynamic bulk modulus has been studied complementally. All the above experimental evidences will be the foundations of elasto-plastic model of frozen soils under long-term low-level repeated cyclic loading.(5) The empirical equations of accumulated shear strain and accumulated direction ratio are formulated derived from test results of tri-axial tests under long-term low-level repeated cyclic loading. Combined with the evolution models of dynamic shear and bulk modulus, a constitutive model which can simulate the deformation behavior of frozen soil under long-term low-level repeated cyclic loading has been presented. Subsequently, a numerical strategy integrating the presented constitutive model is produced for the boundary value problem. Lastly, the simulation performance and the parameters are calibrated by test results.