Study On The Influence Of Groundwater Table Variation On The Seepage Behavior Of Foundation And Excavation In Coastal Area

Following the national strategy,a large amount of major constructional projects are planned and proceeded in the cities and the new districts in coastal areas,such as high-rise buildings,underground spatial complexes,sea/river-crossing bridges,sea/river-crossing tunnels,harbour,etc.Coastal areas contain abundant groundwater which has some characteristics varying with sea waves.Dynamic variation of groundwater can make the seepage field more complicated in the soil around foundation and excavation,and may affect the deformation and the stability of the excavation detrimentally,leading to failure of retaining structures,piping,etc.Thus,it is of great engineering significant to study the seepage and stability of foundation or excavation affected by variation of groundwater.This study started from investigating the impact of varying groundwater table on the pore water pressure response in the double-layered and stratified foundation soils.Then,a seepage model of excavation was established gradually with the consideration of varying groundwater table.Based on this model,three different approaches,i.e.analytical method,finite element simulation and model test,were used to investigate the impact of groundwater table variation on several aspects,such as the pore pressure response around the excavation,the characteristics of the water/earth pressure on both sides of the retaining structure,and the seepage failure of the excavation.By considering the effects of dynamically varying artesian water head,this study also investigated the excess pore pressure response in the aquitard embedded in the base of the deep excavation and the stability of the excavation against seepage.The major work and novel conclusions are summarised as follows(1)An analytical study was carried out on the propagation of excess pore pressure generated by the varying groundwater table in double-layered soil.Double-layered models,with harmonically or arbitrarily varying groundwater table,were built for analytical derivation,by assigning an impermeable or a permeable bottom.Analytical solutions of excess pore pressures,in the double-layered soils of the different models,were derived by employing Duhamel integration approach,separation of variables,and Fourier series expansion.They were validated by comparing with the solutions from the literature and the finite difference method.A series of parametric study were also conducted,regarding the distribution of excess pore water pressure affected by the dimensionless influential factor,the ratio of permeability coefficient,the ratio of volume compressibility,and the thickness ratio of soil layers.(2)The governing equation and the solution conditions were established for the pore pressure response to arbitrary variation of groundwater table in layered soil,with consideration of factors such as the anisotropic permeability of soil and the depth-dependent compressibility.Several commonly used finite element numerical software were selected to discuss their theoretical principles and numerical solution accuracy in dealing with the problem of pore pressure propagation in soils.The finite element software COMSOL Multiphysics which has the highest numerical precision was emplyed to simulate the pore pressure response to arbitrary variation of groundwater in layered soils.The impacts of the anisotropic permeability,the depth-dependent compressibility and the stratification of soils on the distribution of excess pore water pressure were discussed and analyzed.(3)Based on the simplified one-dimensional model for double-layered soil,the total pore pressures on both sides of the retaining structure were obtained by the method of superposition of the steady pore water pressures(caused by steady seepage)and the excess pore water pressure(caused by varying groundwater table outside the excavation).Furthermore,the vertical effective stress,the effective earth pressure and the total lateral pressure on both sides of the retaining structure were obtained.The exit gradient was derived for the point at the bottom of the excavation and close to the retaining structure.A method was proposed to calculate the seepage stability based on the theory of unsteady seepage.The precision of the analytical solutions was verified by comparing with the numerical results given by COMSOL Multiphysics.Numerical examples were given to analyse the influence of dynamic variation of the groundwater table on the total pore pressure,the effective earth pressure and the total lateral pressure on both sides of the retaining structure.Comparisons were made between the analytically derived exist gradient based on unsteady seepage theory and that based on traditional method.This study also discussed the methods to assess the seepage stability at excavation bottom considering unsteady seepage and its engineering application.(4)In this study,a model test device was developed to simulate the excavation process under complex groundwater conditions,especially the conditions with dynamic variation of phreatic or artesian groundwater table.It provides an efficient way to study and to solve the problems encountered in excavation process under complex groundwater conditions.(5)By using the above-mentioned test device,two sets of model test were carried out to study,respectively,the steady seepage under the condition of constant water table outside the excavation and the unsteady seepage under the dynamic variation of water table outside the excavation.The experimental results were compared with the analytical solutions of simplified one-dimensional steady seepage and the 3-D numerical results from COMSOL.The characteristics of the total pore pressure were discussed,under the conditions of steady seepage induced by constant water table and unsteady seepage induced by dynamically varying water table.(6)By using the one-dimensional leaky foundation model and considering the varying artesian water head,analytical solutions were derived for the excess pore water pressure and the exit gradient in the base aquitard of the large excavation.A new method was proposed to assess the seepage stability considering the unsteady seepage induced by the dynamic variation of artesian water.The validity of the analytical solution of exit gradient was verified.The variation characteristics and influential factors of the exit gradient in excavation caused by the variation of artesian water head were analyzed.Finally,the analytical solution of exit gradient in excavation,which was based on leaky theory,was applied in the engineering.