Numerical Application And Centrifugal Modeling Study Of A Non-coaxial Constitutive Model

In some practical geotechnical problems, such as excavation of foundation pit, bearing capacity and deformation characteristic of shallow foundations and the effect of earthquake or wave load to the seabed, the changing of the initial state of the principal stress caused by the rotation of the principal stress orientation should not be ignored. In the process of the rotation of the principal stress orientation, the variations of the rotations of the orientations of the principal stress and the principal plastic strain rate are not coincide, which is called non-coaxial phenomenon. The constitutive model of the non-coaxial phenomenon is given a lot of attentions in the current geotechnical research area. It is meaningful to research the non-coaxial phenomenon and its influence basing on a soil constitutive model, and to verify the numerical calculation results of the non-coaxial model by the centrifuge modeling test results. These research works could compliment the traditional elastic-plastic constitutive model and supply design basis for the important engineering structures and foundations, which have important theory value and practical meaning.The present research gives a detail analysis of the non-coaxial phenomenon and its influence on the relationship between stress and strain, the numerical simulations of sand simple shear test are performed to research the non-coaxial phenomenon and its influence factors, the bearing capacity and deformation characteristics of shallow foundations are research in numerical calculations and centrifuge modeling tests, the rationality of the numerical results of non-coaxial model are verified by the results of the centrifuge modeling tests. The main research contents and achievements are as follows:1. The yield vertex non-coaxial theory is adopted into the traditional elastic-plastic model and a non-coaxial constitutive model is suggested accordingly. The numerical simulations of the simple shear tests of sand are performed to give a detail analysis of the non-coaxial phenomenon and its influence on the relationship between the stress and strain. The research results show that:the variations of the orientations of the principal stress and the principal plastic strain rate are not coincide at the early stage of the shearing process in the non-coaxial model numerical results, and they approach to each other gradually along with the development of the shearing. The non-coaxial phenomenon gives a softening effect to the curves of stress-strain, which means that the increasing trend of the shear stress rate in the non-coaxial model’s results is behind that in the coaxial model’s results. Compared with the numerical results obtained in the case of perfect elastic-plastic, the non-coaxial phenomenon gives more influence on the stress-strain relationship in the case of strain hardening. When the shear strength is obtained by the peak stress for dense sand, the difference between the results of coaxial and non-coaxial models should be considered; when the shear strength is obtained by the residual strength at the critical state, there is not difference between the results of coaxial and non-coaxial models. The non-coaxial phenomenon is influence by these factors such as flow rule, static lateral pressure coefficient and vertical pressure.2. In the case of perfect elastic-plastic, strain hardening and softening, numerical analysis of the load-deformation characteristic of shallow foundation problems are performed with coaxial and non-coaxial models, the non-coaxial phenomenon and its influence on the load-settlement relationship are researched accordingly. The research results show that:the non-coaxial phenomenon gives a softening effect to the load-settlement relationship, which means that the increasing trend of the vertical load in the non-coaxial model’s results is behind that in the coaxial model’s results. When the lateral location of the soil element under the footing increases, the rotation of the principal stress orientation increase obviously, and also, the non-coaxial phenomenon becomes more obvious. The non-coaxial phenomenon is more obvious at the higher position under the corner of footings. The non-coaxial phenomenon is more obvious in the case of strain hardening and softening than that of the perfect plastic, and also, it is more meaningful to research the non-coaxial phenomenon basing on the strain hardening and softening model. The changes of footing shape, depth of embedment and eccentricity should be taken into consideration when the non-coaxial phenomenon and its influence are researched.3. The bearing capacity and deformation characteristics of shallow foundations are researched with centrifuge modeling tests, these researches are mainly focus on the characteristic of the load-settlement relationship, the relationship between the bearing capacity factor Ny and the footing width, and the footing size effect. The research results show that:in case of medium dense sand, there is not inflection point or slope mutation section in the curves of load-settlement; when the settlement increases, the initial slope of the load-settlement curve increases according to the increasing of the footing width. When the settlement according to the characteristic value of the bearing capacity fak increases to0.015-0.05, the experimental results approach to the theoretical results. The footing size effect is obvious when the settlement is obvious, but the experimental results of Ny is close to the theoretical results; when the settlement is small, the footing size effect is not obvious and is lower than the theoretical results. In case of dense sand, the load-settlement curves of the centrifuge test results have obvious inflection points. The bearing capacity increases with increasing of footing width. When the footing width is small, the bearing capacity increases with increasing of footing depth obviously. The settlement δ according to the bearing capacity gives little influence to Nq, but gives more influence to Nγ. When δ is considered, the value of Nγ is lower than that without consideration of δ. The changing of footing shape gives an obvious influence to Nγ, but it gives a little influence to Nq. The footing size effect is more obvious in the experimental results of the circular footing than that of the square footing.4. The numerical simulations of the centrifuge tests are performed, and the numerical calculation results are compared with test results. The rationality of the numerical results of the non-coaxial model is verified through load-settlement relationship and bearing capacity characteristic. The research results show that:when the settlement is small, the slope of the load-settlement curves of the numerical results is higher than that of the centrifuge test results, and the difference between the results of coaxial and non-coaxial models are not obvious. The difference between the results of the coaxial and non-coaxial models increase with increasing of settlement, and the centrifuge test results distribute between the results of non-coaxial model. The values of the bearing capacity factors are overestimated by the coaxial model, and the values of these factors obtained from the centrifuge test results are distributed between the results of the non-coaxial model.