Research On Improvement Of Non-coaxial Theory And Constitutive Model Of Sand

Constructing constitutive model that can reasonably describe the non-coaxial property of sand has always been a hot topics and most difficult problems in the research field of sand constitutive.The systematic and in-depth research on it has important theoretical significance and engineering application value.So far,although some corresponding sand models have been developed by using different theories and methods,there are still some key problems to be solved.Based on the previous research results,this dissertation makes an in-depth study from four aspects: the proposal of method,the construction of model,the numerical implementation of model and engineering application,and explores a new way to reasonably describe the non-coaxial characteristics of sand and quantitatively evaluate its engineering value.The main achievements of this dissertation are as follows:1.It is proved that the plastic strain rate tensor determined by the traditional potential theory must have the same principal directions as the current stress tensor by means of coordinate transformation,which clearly explains the reason why the coaxial hypothesis is implied in the traditional elastic-plastic models.In order to make up for the defect of the coaxial hypothesis,a non-coaxial model of sand is established based on the traditional non-coaxial theory and state dependent dilatancy theory.The model is used to simulate the simple shear test.The results show that the model can properly describe the noncoaxial behavior of sand during the simple shear test.In addition,the introduction of noncoaxial plasticity will also soften the shear stress-shear strain curve.Furthermore,the implicit stress integral formula suitable for the model is derived.The numerical test results of the simple shear test show that the implicit integration algorithm has good applicability to the non-coaxial model.The model is used to analyze the bearing characteristics of a sandy soil foundation.The results show that considering non-coaxial plasticity will soften the load-displacement curve.If the influence of non-coaxial plasticity is ignored,it may lead to unsafe engineering design.2.A novel orthogonal decomposition method for symmetric second-order tensor is established.Using this method,the composition of non-coaxial stress rate defined in traditional non-coaxial theories is clarified.In view of the deficiency of linear hypothesis in traditional non coaxial theory,the nonlinear loading mechanism is used to describe the plastic deformation induced by each non-coaxial stress rate component,and the total noncoaxial plastic deformation is regarded as the sum of non-coaxial plastic components,so a generalized non-coaxial theory is established.The applicability of the generalized noncoaxial theory in sand constitutive modeling is verified by stress and strain detection tests.The results show that the model based on the generalized non-coaxial theory meets the stability requirements of numerical calculation,and the generalized non-coaxial theory can provide a good theoretical basis for the construction of non-coaxial constitutive model of sand.3.Based on the generalized non-coaxial theory and the boundary surface plasticity theory,a cyclic plasticity model of sand is constructed.The advantage of the cyclic plastic model is that it describes the mechanical response of sand under different loading modes through independent loading mechanisms,which significantly enhances the flexibility and applicability of the model.The model is used to simulate the rotation loading test on the deviatoric plane,fixed direction shear test and pure principal stress axis rotation loading test.The simulation results show that the model can reasonably describe the mechanical response of sand under various complex loading modes.The more important significance of the model is to explore a new way to reasonably describe the non-coaxial characteristics of sand.4.A parallel finite element program for solving dynamic problems of saturated soil is developed by using FORTRAN 95 language and Open MP parallel scheme.The cyclic plastic model is embedded into the developed finite element program by using explicit integration algorithm.Then,the seismic response of a saturated sand foundation model is analyzed by using the model.The results show that considering non-coaxial plasticity will accelerate the accumulation of pore pressure,the reduction of effective stress and the attenuation of shear modulus,increase the permanent lateral deformation of the foundation at the end of vibration,and expand the range of liquefied soil layer in the foundation.If the influence of non-coaxial plasticity is ignored,the risks faced by foundations and surface buildings during earthquake may be underestimated.