Study On Strata Deformation Induced By Foundation Pit Dewatering Considering The Influence Of Continuous Barrier Of Adjacent Structure

Foundation pit precipitation is a major factor of surrounding environment deformation caused by foundation pit construction in water-rich area.In recent years,a large number of scholars have studied the environmental effect of foundation pit precipitation.Their research mainly focus on the consolidation and settlement of the outer layer of the pit caused by precipitation and its influence on the surrounding underground structure.However,for foundation pit engineering where the water stop curtain completely cuts off the hydraulic connection between the inside and outside of the pit,foundation pit precipitation does not lead to the consolidation and settlement of the soil outside the pit,but the lateral movement of the enclosing structure is induced and the out-of-pit formation movement is caused.Obviously,when there is an existing underground structure outside the pit,the movement of soil is restricted to some extent by the existing underground structure,so that the different deformation response from that without an underground structure outside the pit is presented.However,up to now,there are few reports on this issue at home and abroad.Therefore,this paper focuses on the study of formation deformation caused by foundation pit precipitation considering the influence of adjacent structural barriers.Based on a foundation pit project of Tianjin Metro Line 3,this study carried out 2 sets of scale model tests with geometric similarity ratio of 50 for the foundation pit precipitation process before excavation,and studied the deformation rules and deformation mechanism of the retaining structure and surrounding strata caused by precipitation before excavation when considering the influence of existing structure barriers.At the same time,three-dimensional numerical models of foundation pit precipitation with or without underground structure outside the pit were established based on the measured data,and the numerical models were verified by the measured data.On this basis,the model was further used to conduct parameter analysis to obtain the deformation characteristics of the outer layer of the pit caused by foundation pit precipitation under different conditions of existing underground structure layout.The main parameters considered in parameter analysis include: distance between adjacent structure and foundation pit,length of adjacent structure,and buried depth of adjacent structure.In this paper,the results of model test and numerical calculation are as follows:(1)Regardless of whether there is or no underground structure outside the pit,the precipitation before foundation pit excavation causes cantilever lateral movement of the retaining structure toward the pit,which further leads to the formation deformation behind the retaining structure(this deformation is not consolidation deformation caused by the drop of water level outside the pit,but formation deformation caused by the formation loss behind the retaining structure).When there is an existing underground structure outside the pit,the lateral movement of the retaining structure and ground settlement caused by foundation pit precipitation are both smaller than the results of no adjacent structure under the same precipitation condition.The smaller the distance between adjacent structure and foundation pit(D),the larger the length of adjacent structure(L),and the larger the buried depth of adjacent structure(Z),the smaller the lateral movement and ground settlement of the retaining structure caused by precipitation before excavation.This indicates that the existing structure outside the pit has a barrier effect on the ground deformation between it and the foundation pit,that is,the existing structure can block the development of soil deformation in this part,and the barrier effect is weakened with the increase of D,and strengthened with the increase of the length of adjacent structure L and the buried depth of adjacent structure Z.In addition,the pit of underground structure block outside the pit of foundation pit precipitation caused land subsidence model of “single settling tank”,while outside the pit underground structure for “double settling tank” mode(it means there are settlement grooves not only between the underground structure and the foundation pit,but also behind the underground structure.),which means that the underground structure has traction effect on the formation deformation behind it,and the traction effect decreases with the increase of D,L and Z..(2)Through the statistical analysis of the calculation results of all different working conditions,the relationship between the maximum lateral displacement(?hm)of the retaining structure and the maximum ground subsidence(?vm)between the adjacent structure and the foundation pit was explored.The research found that the change of ?vm with different parameters was basically between ?vm =0.5?hm and ?vm =0.9?hm.At the same time,the relationship between the maximum lateral displacement increment of the retaining structure and the maximum subsidence increment of the ground is fitted,and a power function equation is obtained,which can provide a preliminary reference for the formulation of foundation pit precipitation scheme.(3)The research indicates that the precipitation before excavation makes the soil in the pit consolidated,leading to the horizontal displacement of the soil in the pit pointing to the pit.The major cause of the deformation of the retaining structure are the decrease of the total pressure of the active area within 10 m of the buried depth,the decrease of the total pressure of the passive area within 19 m of the buried depth,and the asymmetry of the negative friction force on both sides of the retaining structure.When there is an adjacent structure outside the pit,the total pressure reduction of the wall and soil in the passive area within the buried depth of 19 m is larger,and the smaller D is,the greater the total pressure reduction is,the greater the L and Z are,and the greater the total pressure reduction is.This also makes the smaller the lateral displacement of the enclosure structure,the more significant the barrier effect of the adjacent structure.