Deformation Monitoring And Numerical Simulation Analysis Of Deep Foundation Pit Dewatering Excavation In Water-rich Sandy Cobble Ground

With the rapid development of urban rail transit construction,the deep foundation pit of metro station is increasing day by day.Changsha area is located in water-rich sandy gravel stratum,the groundwater level is high and the soil layer is complex,the dewatering excavation of deep foundation pit has a great impact on the deformation of the enclosure structure and the surrounding environment,more importantly,the surrounding buildings and personnel have some potential safety hazards.Therefore,the construction of foundation pit in water-rich sand and pebble stratum,in-depth study on deformation and surface settlement of enclosure structure is helpful to put forward control measures to prevent engineering accidents.This paper relies on the deep foundation pit dewatering excavation project of Wan Bao Da Dao Metro Station in Changsha.Analysed the deformation of retaining structure,settlement of surrounding surface and variation of groundwater level during dewatering and excavation of deep foundation pit,at the same time,sensitivity analysis of the design of foundation pit construction parameters is carried out.The main research contents are as follows:(1)Deformation mechanism analysis of deep foundation pit in water-rich sandy pebble stratum.Focus on two main deformations of enclosure piles and two typical forms of ground surface settlement distributions.Elaborating the influence factors of foundation pit deformation and effective control measures.(2)The horizontal displacement of the retaining structure,the settlement of the surrounding surface and the variation of groundwater level caused by dewatering and excavation of deep foundation pit are studied.Firstly,according to the monitoring scheme,the process of dewatering and excavation of the station foundation pit is monitored and measured on the spot.The main monitoring items are the horizontal displacement of the enclosure structure,the settlement of the surrounding surface and the groundwater level.Secondly,according to the monitoring data compiled chart.Analysis of horizontal displacement of enclosure structure,surrounding surface subsidence and variation of groundwater level.Finally,by comparing the numerical simulation results of Wan Bao Da Dao Metro Station with the actual monitoring results.It is found that the simulation is basically consistent,which verifies the rationality of the simulation.(3)Based on the numerical simulation is the same as the actual monitoring.Studied the sensitivity of influence factors of deep foundation pit deformation under different working conditions.Discussed the sensitivity of design parameters such as permeability coefficient of waterproof curtain,stiffness of retaining pile,depth of retaining pile embedded in rock mass and support position to deformation of deep foundation pit.The results show that the permeability coefficient of confined aquifer has little effect on the deformation of foundation pit due to the complete isolation of confined aquifer by waterproof curtain.When the elastic modulus of retaining pile reaches 4.0×10~7kPa,the increase of elastic modulus has limited control effect on the deformation of foundation pit.For the dewatering excavation of deep foundation pit in the stratum environment of this project,it is advisable to keep the depth of waterproof curtain embedded in rock mass from 3m to 7m.Upward displacement of the second support has an extremely adverse effect on the deformation of the enclosure structure.