Analysis On Two-dimensional Consolidation Process Of Unsaturated Soil Under Semi-permeable Boundary Conditions

Most of the soft clay,loess and sandy soil commonly used in engineering are in the form of unsaturated soil.The addition of vertical shafts in the consolidation of conventional unsaturated soil can make the pore water and pore air in the soil layer permeate along the vertical and horizontal direction at the same time.Due to the fact that sand cushion and shaft filling medium have certain hindrance effect on pore water and pore air in practical engineering,the permeability characteristics of consolidated boundary to pore water and pore air are often between complete permeability and impermeability,showing semi-permeability characteristics.At present,studies on unsaturated soil consolidation mainly focus on the study of consolidation characteristics under one-dimensional or fully permeable boundary conditions,while there are relatively few studies on the consolidation characteristics under two-dimensional or three-dimensional and semi-permeable boundary conditions.In view of this,this paper based on the existing consolidation theories of unsaturated soil and considering vertical semi-permeable and bidirectional semi-permeable boundary conditions,conducts theoretical and numerical studies on the two-dimensional consolidation characteristics of unsaturated soil.The main works are as follows:Firstly,the two-dimensional consolidation equation of unsaturated soil is solved under the condition that the vertical boundary is semi-permeable.Using the two-dimensional consolidation governing equation and Laplace transform technique for unsaturated soil proposed by Dakshanamurthy and Fredlund,the semi-analytical solutions of excess pore-pressure and soil layer settlement are derived in the Laplace domain.On this basis,the inverse Laplace transform is used to calculate the results in the Laplace domain,so as to solve the excess pore-pressure and soil settlement.Secondly,the finite difference solution of two-dimensional consolidation equation of unsaturated soil under vertical semi-permeable boundary is established.The correctness of the solution in this paper is verified by comparison with the solutions in other literatures and the finite difference method.The two-dimensional consolidation characteristics of unsaturated soil under vertical semi-permeable boundary conditions are studied by using the semi-analytical solution established in this paper,the influence of parameters such as vertical boundary permeability coefficient,horizontal and vertical permeability coefficient ratio,soil layer thickness and vertical shaft spacing on the dissipation law of excess pore air pressure and excess pore water pressure as well as the characteristics of soil settlement in the two-dimensional consolidation process of unsaturated soil was analyzed.Thirdly,considering that soil in nature often exists in the form of stratified soil,it is necessary to study stratified soil for unsaturated soil consolidation.Based on the two-dimensional consolidation governing equation of unsaturated soil proposed by Dakshanamurthy and Fredlund,a two-dimensional consolidation model of two-layer unsaturated soil with vertical semi-permeable boundary was established using COMSOL Multiphysics.On the basis of verifying the correctness of the finite element model,the two-dimensional consolidation characteristics of the double-layer unsaturated soil under the vertical semi-permeable boundary condition are analyzed by using the finite element model.Fourthly,considering the semi-permeable characteristics of upper and lower bedding and shafts,COMSOL Multiphysics is used to establish a two-dimensional consolidation model of bidirectional semi-permeable boundary unsaturated soil.The two-dimensional consolidation characteristics of unsaturated soil under bidirectional semi-permeable boundary conditions were studied,and the effects of boundary permeability coefficient,horizontal and vertical permeability coefficient ratio and different positions of soil layers on the excess pore air pressure and excess pore water pressure were analyzed.Finally,based on COMSOL Multiphysics simulation software,the actual engineering cases are calculated and analyzed,the validity of the two-dimensional consolidation model of unsaturated soil is verified.