Theoretical And Experimental Study On Nonlinear Finite Strain Consolidation

One-dimensional(1-D) consolidation theory was proposed by Terzaghi in1920s. Then consolidation theory is one of basic subjects in the research field of soil mechanics and it has been studied for nearly one century. In order to make consolidation theory closer to actual conditions, many researchers gradually abandon the assumptions proposed by Terzaghi. They develop the finite strain consolidation theory, the consolidation theory of non-Darcian flow, the consolidation theory of unsaturated soils and greatly enrich the theoretical system of consolidation. In our country, soft clay is widely distributed along rivers and in coastal areas. Engineers find that the consolidation displacement of soft clay ground is very large. Therefore, the hypothesis of small strain in conventional theory is not applicable and the hypothesis of finite strain is suitable. The finite strain consolidation theory has been studied for decades. However, nonlinear compressibility, nonlinear permeability, non-Darcian flow, secondary consolidation behavior and unsaturated condition are not yet to be fully considered in this theory. Consequently, in this thesis theoretical and experimental study is performed to do further research on the finite strain consolidation behavior of soft clay. Main works of this thesis are as follows:(1) Governing equations of3-D finite strain consolidation are derived and simplified. On the basis of the balance equation, the constitutive equation, the geometric equation, the principle of effective stress, the Darcy’s law and the continuity equation, the governing equations are derived in Euler coordinates. Afterwards, the above equations are simplified under3-D condition,2-D condition and1-D condition, respectively. This work establishes the relationship between multi-dimensional theories and1-D theory.(2) Considering nonlinear compressibility and nonlinear permeability, the finite strain consolidation theory is studied. Based on the experimental study for Ningbo soft clay, the nonlinear equations of permeability coefficient and effective stress are obtained as functions of void ratio. Taking nonlinear compressibility and nonlinear permeability into account, the new governing equation can be derived. Considering the weight of soft clay accurately, an example is analyzed by using the new governing equation and the Gibson governing equation in order to compare these two methods. Considering the weight of soft clay simply, an example is analyzed by using the new governing equation, the Gibson governing equation and the Terzaghi governing equation in order to compare these three methods.(3) In nonlinear finite strain consolidation, the permeability index, hydraulic gradient and influence of the weight of soils are further studied. According to the existing experimental data of Ningbo, Shanghai and Hangzhou soft clay, the empirical relationships between permeability index and initial void ratio are summarized for the three kinds of soft clay. Afterwards, the variation of hydraulic gradient in nonlinear finite strain consolidation is studied. The influences of external load and depth of soil on the variation of hydraulic gradient is focused on. The variation law of hydraulic gradient is summarized and compared with existing experimental results. Furthermore, the changing curves of hydraulic gradient can be roughly divided into five phases. This five-phase model can be employed to explain the migration of pore water during consolidation. Finally, the influences of different considerations for the weight of soils on finite strain consolidation behavior are studied. Considering different magnitudes of applied load and different thicknesses of soft clay layer, examples are analyzed. Then the calculated results of different methods are compared. In the end, the applicability of different methods is discussed.(4) Based on the exponential flow law, the nonlinear finite strain consolidation theory is studied. The experimental study for Hangzhou soft clay is carried out to obtain the expressions of nonlinear compressibility and nonlinear permeability. Taking the exponential flow law, nonlinear compressibility and nonlinear permeability into account, the finite strain consolidation theory improves and the governing equation is derived. Based on the experimental results, the influences of the exponent on the calculation results of excess pore water pressure, settlement and consolidation degree are researched.(5) Taking the secondary consolidation behavior into account, the nonlinear finite strain consolidation theory is researched. The experimental study for Hangzhou soft clay is carried out to obtain the secondary consolidation coefficient and some other test data. On the basis of some assumptions about the secondary consolidation behavior, the continuity equation of pore water in the existing finite strain consolidation theory is developed. Considering the nonlinear compressibility and nonlinear permeability, the governing equation is derived. According to the test data, an example is analyzed with the new method and other two methods. The calculation results of excess pore water pressure with different methods are compared. Moreover, The calculation results of settlement with different methods are compared with the observed data of Hangzhou soft clay sample.(6) Preliminary study on finite strain consolidation theory for unsaturated soils is carried out. The deformation of unsaturated soil under external load is divided into two stages. At the first stage, neither air nor water is expelled from the pores and the deformation is only induced by compression of air. At the second stage, the deformation is caused by pore air and pore water expelling. Considering the geometric nonlinearity, the balance equation, the continuity equations for pore fluid and pore water, the effective stress principle proposed by Bishop and the soil-water characteristic curve equation proposed by Fredlund&Xing, the governing equations for1-D finite strain consolidation of unsaturated soils can be derived during the second stage.(7) Macro and micro experimental study is carried out to study Hangzhou soft clay in dual scale. X-ray Energy Dispersive Spectroscopy (X-ray EDS), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests are performed on some representative natural samples to show the pore size distribution and the micro morphological characteristics. Finally, oedometer tests for standard consolidation and rheological consolidation are performed. MIP and SEM tests are carried out on consolidated samples to research the micro mechanical characteristics.(8) According to the engineering background of a vacuum preloading project in Lianyungang of Jiangsu Province,3-D numerical simulation of soft clay ground with plastic drainage plates is performed. The3-D model of soft clay ground with long and short plastic drainage plates is established. Under different engineering conditions, the above model is analyzed to study the influence of long and short plastic drainage plates on the consolidation behavior of soft clay ground. Afterwards, the3-D model of soft clay ground without plastic drainage plates is established. In order to approach the calculated result of settlement by using the model considering long and short plastic drainage plates, the permeability coefficients of the soil layers in the treated area are changed time after time, then the equivalent permeability coefficient can be achieved. Finally, according to the actual size of the vacuum preloading ground, a large3-D model is built. Applying the equivalent permeability coefficient, the large model is analyzed. Ultimately, the calculation results of settlement are compared with the measured results.