| The enclosure wall and environment will be deformed when the foundation pit is pumped.Recharge is often used to control water level drop outside the pit to reduce environmental deformation.At present,there are a lot of studies on the above aspects,but there are still some shortcomings as follows.Firstly,in a leaky aquifer,when there is hydraulic connection between the inside and outside of foundation pit,the variation of water level in each soil layer outside pit may be different due to the barrier of aquitard.But the complex distribution and development laws of enclosure wall deflection and ground settlement caused by above have not been well revealed yet;Secondly,The current pumping / recharge model tests cannot simulate the free deformation of the retaining wall,and cannot get the laws of groundwater seepage and soil deformation caused by foundation pit pumping / recharge when considering the retaining deformation.Thirdly,the existing underground structures near the new foundation pit will affect the groundwater seepage and block the soil movement to some extent(Barrier soil and Barrier water).However,the current studies have not revealed the coupling mechanisms of barrier water and barrier soil of existing underground structures during pumping/recharge.In a leaky aquifer area with dense underground structure,it is of great significance to solve the above research deficiencies for foundation pit design and construction.Therefore,the studies carry out the following researches:Firstly,finite element models are established by ABAQUS based on a practical dewatering test.The distribution and development laws of enclosure wall deflection and ground settlement caused by pumping in a leaky aquifer is analyzed.Secondly,the similarity model test system for the seepage and deformation of the retaining wall during pumping is designed.The pumping-recharge models test are carried out according to the similarity ratio and the laws of groundwater seepage and soil deformation caused by foundation pit pumping / recharge is shown.Thirdly,the coupling mechanisms of barrier water and barrier soil of existing underground structures during pumping/recharge is revealed,and the recharge design is optimized.The main results and conclusions of the study are as follows:(1)From the distribution laws of soil settlement during pumping,the maximum settlement location caused by foundation pit dewatering is related to the hydraulic connectivity in strata,pumping time,pumping location and the motion state of enclosure wall.First,as to the condition of no enclosure wall deflection(e.g.,the enclosure wall is effectively supported by strut system),when the hydraulic connectivity between the pumped aquifer and the strata above it is weak,pumping-induced maximum settlement would be always near the top of the pumped aquifer;when the above-mentioned hydraulic connectivity is strong,the maximum settlement location would initially appear near the top of the pumped aquifer,but shift upward with the extension of pumping time,and finally approach the ground surface;Second,as to the condition of considering the enclosure wall deflection(e.g.,during the stage of the pre-excavation dewatering),the evolution of the maximum settlement caused by pumping of confined aquifer are the same as the above conditions without considering the enclosure wall movement;however,during the pumping of phreatic aquifer(or unwatering dewatering),the maximum settlement position caused by pumping is always close to the ground surface,regardless of the hydraulic connectivity in strata.(2)From the development laws of enclosure wall and soil deformation during pumping,when a foundation pit is hydraulically connected with surrounding environment,the enclosure wall deflection increases first,then decreases,and gradually tends to be stable during pumping from phreatic aquifer.However,when there is no hydraulic connection between the inside and outside foundation of pit,the displacement of the enclosure wall increases and then gradually stabilizes.No matter whether the foundation pit is hydraulically connected with surrounding environment,there is tensile deformation of soil near the surface ground during pumping from phreatic aquifer,which make the maximum soil settlements caused by pumping are almost at deep positions in the early pumping stage.(3)The top of enclosure lateral deflection and surface ground settlement outside the pit increases continuously by pumping during the model test.After simultaneous recharge outside the pit,the surface settlement outside the pit remains basically unchanged,but enclosure lateral deflection is further increased.So it is feasible,economical and repeatable to carry out the pumping-recharge models test considering the enclosure wall deformation in the laboratory environment.This test can provide a reference for the subsequent pumping-recharge model test under laboratory conditions.(4)When the foundation pit is pumped,The barrier soil effect would reduce surface settlement outside the pit,while barrier water effect would increase surface settlement outside the pit.The effect of the barrier effect on the surface subsidence depends on relative strength of barrier water and barrier soil.The closer the distance between the existing pile foundation and the foundation pit and the deeper the existing pile foundation,the stronger the barrier water and barrier soil will be.In addition,the barrier effect of existing pile foundations on groundwater seepage during recharge will reduce surface settlement,which is stronger than that during pumping.Further,the recharge designs are optimized based on the barrier effect: when there is pile foundations outside the pit,the spacing of recharge wells can be appropriately increased and the total amount of recharge is basically equal to the total amount of pumping. |
