Method Of Barrier Effect Of Waterproof Curtain On Groundwater Seepage During Dewatering In Aquifers

This study was undertaken to evaluate the groundwater variation caused by pumping in an aquifer when a waterproof curtain is used in a foundation pit.In order to investigate the mechanism of the barrier effect of the waterproof curtain in the aquifer,theoretical analysis,numerical simulation and regression analysis were conducted.A new method to calculate the difference between the water levels on the two sides of the waterproof curtain,a new simple analysis model for describing the spatial distribution of the water level in a confined aquifer,and a new method for calculating hydrogeological parameters are proposed.Ground settlement caused by leakage through the waterproof curtain above or below the excavation was investigated using a three-dimensional finite element model(FEM).The concluding remarks of this study are summarised as follows.(1)A new method to calculate the difference between the water levels on the two sides of the waterproof curtain is proposed,in which the anisotropy of the aquifer is considered.A three-dimensional finite difference model(FDM)was established,and the distribution of drawdown along the longitudinal direction of the confined aquifer was analysed.Results show that: i)When the observation point is above the waterproof curtain base,drawdown varies quickly with an increase in depth;when the observation point is below the waterproof curtain,drawdown varies slowly with an increase in depth.ii)The nearer the observation point to the top of the aquifer,the greater the drawdown in the excavation and the smaller the drawdown outside of the excavation.The calculation formula for water level difference was established by analysis of the numerical results.(2)This paper presents a simple analysis model of pumping in a confined aquifer when a waterproof curtain is used.In accordance with Darcy's law and the water equilibrium principle,an analytical solution determining water levels due to the barrier effect below the curtain was obtained.A simple analysis model was then obtained by correcting the analytical solution.The proposed model is able to calculate the spatial distribution of water levels in a confined aquifer.(3)A new approach to calculating hydrogeological parameters(transmissivity,hydraulic conductivity,and storage coefficient)is proposed.When pumping was performed in the excavation with a waterproof curtain,the characteristics of drawdown with time were investigated using a numerical method,and the distribution of drawdown along the longitudinal direction of the confined aquifer was analysed.Results show that variation of drawdown with time can be divided into four periods,and in each of these time periods there is a relationship with screen length of pumping well,thickness of the confined aquifer,insertion depth of the waterproof curtain in the confined aquifer and screen length of the pumping well.Transmissivity can be calculated using drawdown information from the drawdown curve in the third period,and the storage coefficient can be calculated using a section of straight line in the second period.(4)A three-dimensional finite element model(3D-FEM)was established to analyse the influence of leakage through the waterproof curtain on ground settlement.First,a self-defined one-dimensional leakage element,which has previously been applied in a study of tunnel leakage,was introduced to simulate punctual and linear leakage behaviour,and a solid element was used to simulate planar and homogeneous leakage.Groundwater level and ground settlement caused by leakage above or below the excavation surface was analysed using a 3D fluid-solid coupled model.The influence of permeability of the waterproof curtain at the leakage location,leakage length,leakage area,longitudinal distribution of leakages,characteristics of soil and depth of waterproof curtain are discussed.The main conclusions are as follows: i)Leakage in the waterproof curtain will cause a variation in flow direction,drawdown of groundwater level outside of the excavation and significant additional deformation in the longitudinal direction;ii)leaked volume,groundwater level,and ground settlement are related to permeability at the location of the leak,leakage length,leakage area,permeability of soil,anisotropy and deformation modulus of soil;iii)when the leakage is above the excavation surface,the nearer the leakage location to the aquifer base,the greater the settlement caused by leakage;when the leakage is below the excavation surface,the nearer the leakage location to the water-crossing section,the greater the settlement caused by leakage.(5)Analysis of waterproof curtain leakage in two case studies was carried out.The effect on groundwater seepage and on the surrounding environment caused by dewatering,based on measured data from a metro station in Hangzhou and a transportation hub project in Tianjin,is analysed.A 3D groundwater seepage model with 1D consolidation and 3D coupled analysis,considering the leakage of cut-off walls,is established.Results show that hydrogeological parameters and deformation parameters of soils,and hydraulic conductivity of cut-off walls,can be obtained by back calculation based on observation data.If calculated parameters of soils are reasonably accurate,hydraulic conductivity of cut-off walls can reflect the leakage location and the extent of the cut-off walls.