Theoretical And Experimental Study On One-dimensional Nonlinear Consolidation Of Soft Soil Based On Continuous Drainage Boundary

Consolidation theory has always been one of the key problems in soil mechanics,for it is closely related to soil deformation,strength,stability and permeability.Consolidation theory includes three aspects:consolidation equation,initial condition and boundary conditions.Since Terzaghi put forward the consolidation theory,the research on soil consolidation mainly focuses on the optimization of consolidation equation and initial conditions,but the research on boundary conditions is relatively few.However,as the input condition for solving the consolidation equation,the influence of boundary conditions on soil consolidation cannot be ignored.The traditional Terzaghi's drainage boundary has been widely used to solve consolidation problems.Its form is simple and easy to use,but it can only be expressed as completely permeable and completely impermeable.In practical engineering,because pore water discharge is impeded,the boundary excess pore water pressure is time-dependent.Although semi-permeable drainage boundary reflects the changing process of permeable boundary,its boundary condition is a function of pore pressure,which makes the solving process and results of consolidation equation more complicated.The form of continuous drainage boundary is simple,which can reflect the changing process of boundary pore pressure with time,and the state of boundary drainage capacity from impermeable to fully permeable.At present,the research on linear consolidation of soil based on continuous drainage boundary has been relatively perfect,but the research on consolidation of soil considering nonlinearity of material(i.e.nonlinear compressibility and permeability)and nonlinearity of flow(i.e.non-Darcian flow)under continuous drainage boundary is still relatively rare.In order to enrich the study of one-dimensional nonlinear consolidation of soft soil under continuous drainage boundary,this thesis adopts continuous drainage boundary,and obtains the solution of soil consolidation by using analytical method,finite difference method and semi-analytical method,and analyzes its consolidation law in depth.In view of the fact that there is no consolidation test related to continuous drainage boundary at present,a large-scale stress-strain oedometer is used to carry out consolidation test related to boundary pore pressure,and the interface parameters of continuous drainage boundary are retrieved by combining the theoretical solution obtained in this thesis with the test results.The main conclusions are as follows:Considering the continuous drainage boundary condition and the nonlinearity of soil material,the one-dimensional nonlinear consolidation problem of soil with a fixed consolidation coefficient c_v is studied.The analytical solutions of one-dimensional nonlinear consolidation of single-layered soil and multi-layered soil with continuous drainage boundary are obtained by variable substitution,separation of variables and Laplace transform,respectively.The correctness of the analytical solutions are verified by comparing with the nonlinear consolidation analytical solution based on the traditional drainage boundary.Based on the proposed solution,the consolidation behavior of soil is detailedly analyzed for different interface parameters or nonlinear parameters.The results show that,with the increase of interface parameters,the drainage capacity of continuous drainage boundary is stronger,and the consolidation solution based on continuous drainage boundary is closer to that based on traditional drainage boundary.The permeability of different soil interfaces can be approximately simulated by adjusting the interface parameters.Further,in view of the non-constant consolidation coefficient c_v,the one-dimensional nonlinear consolidation problem from single-layered soil to layered soils under continuous drainage boundary condition is studied by using finite difference method and semi-analytical method.The correctness of the proposed solution is verified by comparing with the traditional drainage boundary solution and the results obtained by different methods.The influences of nonlinear parameters,boundary parameters and soil stratification on one-dimensional nonlinear consolidation are analyzed in detail.It is found that,considering the nonlinear characteristics of soil,the continuous drainage boundary as a impeded drainage boundary weakens the influence of permeability coefficient on the consolidation rate of soil.The consolidation rate of soil considering the real distribution of soil self-weight is faster than that assuming the average distribution of self-weight.Therefore,ignoring the true distribution of gravity in the engineering calculation will result in a slow settlement rate of foundation.Adopting the non-Darcian flow model proposed by Hansbo,the finite difference method and semi-analytical method are used to study the one-dimensional consolidation problem of single-layered soil to layered soils considering the influence of non-Darcian flow under continuous drainage boundary.Compared with the traditional drainage boundary solution,the Darcy's flow solution and the results obtained by different methods,the correctness of the proposed solution is verified.Subsequently,the one-dimensional consolidation law of soft clay considering the influence of non-Darcian flow with continuous drainage boundary is studied.The results show that,with the increase of flow index of non-Darcian flow,the flow velocity in soil becomes smaller,the excess pore water pressure is harder to dissipate,and the consolidation rate of soil is slower.Meanwhile,the consolidation degree based on non-Darcian flow is more different from that based on Darcy's flow.Ignoring the influence of non-Darcian flow will overestimate the consolidation rate of soil.The consolidation test was carried out by using a large-scale stress-strain oedometer.The change of boundary pore pressure in the consolidation process was measured by setting a pore water pressure gauge at the specimen boundary.The results show that the boundary pore pressure gradually dissipates over time in the consolidation process,but the peak pore pressure is far less than the external load value,which verifies the rationality of the continuous drainage boundary,and also indicates that the interface parameters cannot be inverted only by the boundary pore pressure.In addition,it is found that the compression characteristics of the sample conform to the nonlinear compression model.Finally,the interface parameters of continuous drainage boundary are obtained by inversion by combining one-dimensional nonlinear consolidation theory of continuous drainage boundary with experimental consolidation degree curve,which provides an idea for the inversion of interface parameters of continuous drainage boundary.