Study On The Mechanism Of Land Subsidence Caused By Groundwater Level Variations

Land subsidence has become one of the most serious environmental geohazards and influenced an extremely wide range for a long time.It is hardly found at the initial stage,but may easily induce a serious disaster chain in the latter stage,becoming an obstacle for the long-term stable operation of infrastructures and the fast development of city economy.In the coastal soft soil areas of China,the population is increasing significantly every year.This problem adds burden to urban water supply and also brings a challenge to the subsidence control,since more and more groundwater resources are exploited for domestic and industrial purposes.Therefore,this dissertation aims to explore the mechanism of land subsidence induced by groundwater level variations and the effects of different pumping-recharging schemes on subsidence by theoretical analysis and numerical simulation.Main research contents and achievements are as follows.(1)The axisymmetric consolidation model of single soil layer is established based on Biot's coupled consolidation theory by considering the anisotropic permeability.Laplace-Hankel integral transforms are used to obtain the general solutions in the transformed domain for this model.The stress,displacement and pore pressure boundary conditions are used to solve the undetermined coefficients in the general solutions and the analytical expressions for these components are then obtained.The effect of soil properties and pumping boundary conditions such as constant water drawdown and constant water flow velocities on the consolidation behavior of single soil layer are studied in detail.(2)Axisymmetric consolidation of multi-layered soils induced by groundwater extraction from a dewatering well is studied with consideration of the anisotropic permeability and the well length.Laplace-Hankel transforms are utilized to solve the governing equations.The analytical layer-element method is used to build relationships between displacements,stresses,excess pore pressure and seepage velocity in the transformed domain.The real solutions can be obtained by the inversion of Laplace-Hankel transforms.A series of parametric studies,especially the length of a dewatering well and the combined effect of pumping and recharging,are conducted to analyze the consolidation behaviors of layered soils.The results show that recharging shallow aquifer and taking intermittent pumping and recharging can help restrict the settlement.(3)An anisotropic bounding surface model is used to describe the plastic anisotropy of natural soft clay.Combined with the isotropic hardening rule,the rotational hardening rule is incorporated into the bounding surface formulation with an associated flow rule.The developed model is programmed in ABAQUS software by writing UMAT subroutine,which is integrated in return mapping algorithm.A series of triaxial tests are performed on undisturbed samples of soft clay soil to determine materials parameters and also to validate this model.The results show that the anisotropic bounding surface plastic model is able to describe the accumulative deformation characteristics of soft clay soil under the effect of cyclic loads.(4)Three dimensional fully coupled FE model of land subsidence is established by combining anisotropic bounding surface plastic model and non-linear permeability model.The effects of soil properties and different pumping-recharging schemes on the development of land subsidence are studied in detail.The results show that the deformation of soft clay layer is much larger than that of sand layer and the plastic deformation makes up the most of the former.Recharging after pumping or taking cyclic pumping and recharging can help slow down the subsidence.