Study On Numerical Modeling And Consolidation Problem Of Clay

Recently, surface settlement owing to seepage of groundwater has been widely concerned. The settlement of saturated clay called consolidation is an important research subject in geotechnical engineering. Actually the law of consolidation of saturated clay is complicated because it is a coupling process between the deformation field of soil skeleton and the pressure field of pore pressure. The coupling effect has a close relation with the constitutive models of both the two physical fields. Especially, the rationality of constitutive relations of soil is a key to this problem.With the development of the theory of constitutive model of soil, the elastic constitutive model was displaced by some non-linear constitutive models such as the Duncan-Chang model, Cam-clay model, etc, which leads to that the Biot′s consolidation theory has developed from elastic into nonelastic. At present, most of constitutive models of soil obtained based on the test results under some specified stress paths could not represent the effects of different loading histories, stress paths on stress-strain relation. Consequently, the present methods of soil consolidation is not satisfactory, for instance, some nonlinear constitutive models obtained under a specified stress path are applied to practices with difference paths, which must result in some greater errors.To solve this problem, elastoplastic models under different paths are established by using the numerical modeling method and applied to elastoplastic consolidation. The details of the theoretical and numerical approaches are given as follows:Firstly, elastoplastic models are established based on the triaxial tests for clay under different stress paths. The initial consolidation states are not similar in different speciments of sedimentary clay, some in isotropic consolidation state whereas most in K0 consolidation state, which denote the different loading histories or stress paths and have diffenet influences.on stress-strain relation. In this dissertation, the triaxial tests for clay in Wuhan under four stress paths under different initial consoldation states and drainage conditions are carried out. Moreover, comparing the test curves with sand under similar paths, some basic machanical characteristics of clay based on which the model framework is choosed and elastoplastic models under multiple paths are constructed by inversion of test data space. Then the three-dimensional surfaces of the stress-strain relations in the whole stress field( p , q ) are drawn through visualization and the corresponding yielding locis also are displayed. It can be clearly seen from the three-dimensional surfaces that the influences of different loading histories, stress paths on the constitutive models of soil are obvious.Secondly, a unified matrix expression of constitutive relation suitable for axial symmetry and plane strain is derived based on the elastoplastic models in this dissertation. Numerical simulations of the triaxial tests under the two different consolidation states by using this model are compared with test curves and numerical results obtained by using some nonlinear models such as Duncan-Chang model and Cam-Clay model. The comparisons show that the models constructed in this dissertation exhibit better stability, fault-terrace and coincidence with the test curves than the others, in which local errors in test data would not seriously affect the final results.Thirdly, the elastoplastic models constructed in this dissertation are inserted into the Biot consolidation equations. Then the incremental frame of governing partial differential equations is established to be suitable for this problem. Two approachs are put forward. The first one is decoupling solution; the other is coupling solution.In the decoupling solution, the supposition of nonlinear relation ofΔp ~Δεv is adopted for the case that dilatancy in the stress-strain relations of some soil mediums is not obvious. According to practical stress path, the decoupling condition of these equations is discussed. Based on the method, an incremental diffusion equation and uncoupling governing equations are presented. Moreover, the method is applied to numerical analyses of ground settlement. The numerical simulation for the evolvements of settlements and pore pressure under different stress paths are compared with the results of this problem by using Cam-Clay model. The results show that this decoupling method can predict the evolvements of pore water pressure and settlement, and reflect the effect of stress path and dilatancy.In the couping solution, based on the characteristics of the models in this dissertation, each of loading step is finished in unit time. Thus the volumetric strains on the current increment step as analysis parameters are inserted into the equation of continuity and a physical equation for pore water pressure is presented. The fundamental solutions of two kinds of classic problems (axial symmetry and plane strain) are derived. By use of the fundamental solutions of pore water pressure combined with finite element method of soil mechanics, a semi-analytical and semi-numerical method for nonlinear consolidation equations is presented. Moreover, a systematic numerical method of numerical modeling and simulation for soil consolidation is established. The method has been applied to numerical analyses of consolidation. Through comparisons of some examples under different stress paths it is indicated that this method is reasonable, which can reflect the effect of stress path on horizontal displacement,settlement and pore water pressure.Based on the constitutive models of clay and sand and the above coupling solution, consolidation of layered subgrade is investigated. Some laws of consolidation of two kinds of double layers subgrade are found: (1) the influence of stress paths on the consolidation of two layers subgrade is similar to monolayer subgrade; (2) the mechanical characteristics of the superstratum are dominant.Summarly, it is very important in the constitutive modeling to reflect the influence of stress path, loading history and dilatancy of soil. Especially, the effect of laoding histories and stress path should be taken into account in the simulation of the consolidation question. The elastoplastic models constructed in this dissertation play an important role in realizing the end.