Study On Influence Of Dewatering In Confined Aquifer On Mechanical Deformation Of Deep Foundation Pit In Shanghai Area

With the development of underground space in Shanghai soft soil area,the influence of shallow and deep confined aquifer must be considered in the construction of deep foundation pits.Different from the traditional excavation foundation pit,the deformation will be affected by dewatering through changing the distribution of seepage field and stress field.Therefore,it is of great significance to study the influence of dewatering on deep foundation pit.Based on the typical geological and projects of Shanghai,this paper simulated the excavation dewatering coupled construction progress of foundation pits.Studying the deformation characteristics and the dewatering-induced additional deformation on a circle foundation pit,analyses focused on its stress and influencing factors.Studies on a square foundation pit with dewatering focused on the deformation spatial distribution characteristics.Basing on these,the net pressure distribution model of the wall and a three dimensional surface settlement distribution model were proposed.Finally,numerical models based on projects were conducted to analyze the deformation characteristics and the numerical results are compared and validated with the field data.The main researches and results are as follows:(1)The numerical model of circle foundation pit in shallow confined aquifer was conducted to analyze the mechanical deformations and dewatering-induced additional deformations.Based on these,the net pressure distribution model of the wall and the surface settlement distribution model were proposed.The results indicate that the excavation dewatering uncoupled results overestimate the wall deformation and surface settlement,and underestimate the horizontal displacement of soil under the wall.The dewatering will change the pressure on the wall.Therefore,the horizontal deformation near the toes will be increased,whereas at the roof of confined aquifer will be decreased.Furthermore,the additional horizontal deformation of the wall is related to the stiffness of the support system.The additional horizontal displacement of soil under the wall is mainly related to unloading degree and partition ratio.The surface settlement beyond 2.0He(where He is the depth of the pit)outside the pit is mainly affected by dewatering.Only the maximum value of surface settlement and the influence range will be affected by dewatering.Excavation alleviate the additional surface settlement within 1.5He outside the pit,which relate to the depth of the confined aquifer,the wall-soil interaction and partition ratio.(2)The numerical model of square foundation pit in shallow confined aquifer was conducted to study the spatial distribution characteristics.Additionally,the settlement distribution model wasextended into a three-dimensional model.The results show that the distribution of seepage field caused by dewatering won’t be affected by excavation.The dewatering will change the size of deformation,instead of the spatial deformation characteristics.The corner effect alleviate the wall deformation and ground settlement in the area about 1.0He at the corner of the wall.In every longitudinal sections beyond these area,the dewatering-induced deformation of the wall is identical.The dewatering reduces the wall deformation near the excavation surface,and increases the wall deformation near the toes.In everywhere,the surface subsidence and its range is increased by dewatering.Within the range of 1.5He outside the pit,the dewatering-induced surface subsidence is significantly alleviated everywhere.(3)Based on the circular foundation pit of Shanghai center and the square foundation pit of the South Qilianshan Road station,the numerical models were performed to analyze the deformation characteristics with the measured data.Furthermore,the surface settlement distribution model proposed in this paper was applied and verified.The results show that the ground settlement of circular foundation pit,which has a wider settlement trough,is mainly affected by dewatering.While the deformation spatial effect is obvious with corner effect in the narrow rectangular foundation pit.Besides,the surface settlement distribution model proposed in this paper can not only fit the subsidence outside the pit in this two types pits,but also fit the vertical deformation of shallow buried pipelines and the distribution of surface settlement after recharge.