Study On The Influence Of Foundation Pit Deformation Triggered By Water-Rich Strata Precipitation

In recent years,with the increasing shortage of land resources,the number of high-rise buildings in China is increasing,the underground development is increasing,the number of foundation pits is large,and the depth is bound to become the trend in the future.Along with the increase of foundation pit depth,the precipitation depth also increases,the precipitation time continues to grow,and the pumping volume gradually increases,which leads to the deformation of the foundation pit more and more,and has a huge challenge to the stability of the foundation pit itself.At the present stage,when researching the deformation caused by foundation pit dewatering,most scholars only study the deformation of foundation pit caused by dewatering after foundation pit excavation.At the same time,monitoring the deformation of retaining structure during foundation pit construction also starts from the foundation pit excavation,ignoring the impact of dewatering before foundation pit excavation on the deformation of foundation pit.However,some scholars found that precipitation before excavation can make the deformation of foundation pit reach cm level,which shows that precipitation before excavation has a greater impact on the deformation of foundation pit,so it is necessary to conduct an in-depth study on the impact of deformation caused by precipitation before excavation of foundation pit.In this paper,based on the torch station pit project of Nanchang Metro Line 4,a study on the influence of precipitation-induced deformation in the pit of water-rich strata is carried out through a combination of model tests,numerical simulations and field measurements to explore the deformation law of the precipitation enclosure structure before excavation and the surface settlement outside the pit as well as the deformation law of the pit under different influencing factors,with the aim of providing some guidance for similar projects.The main contents and results are as follows.(1)With the actual project as the background,the indoor model test with reduced scale is established,and the three aspects such as the depth of precipitation,the spacing of precipitation wells and the pumping speed are analyzed through the model test on the deformation of the enclosure structure,the settlement of the surface outside the pit,the change of water level outside the pit,the pore water pressure and the change of soil pressure,and the influence of precipitation on the deformation of the pit and the change of water level outside the pit before excavation of the pit is investigated.The test results show that.1)Under the same conditions,with the increase of pumping speed V and precipitation depth He,the amount of water pumped,water level outside the pit,lateral shift of the enclosure structure and surface settlement outside the pit gradually increase,and decrease with the increase of precipitation well spacing d.The water level outside the pit shows a "concave and then convex" distribution,and the lateral shift of the enclosure structure and the settlement of the surface outside the pit show a "cantilever" distribution.2)The pore water pressure decreases both inside and outside the pit,but the decrease of pore water pressure inside the pit is obviously larger than that outside the pit,and the two are different in the change pattern.The soil pressure curve in the pit under different working conditions is "Z"-shaped distribution,the upper soil pressure shows an increasing trend,the lower shows a decreasing trend,and the overall trend is mainly decreasing,which induces the lateral movement of the enclosure structure into the pit;the lateral soil pressure outside the pit decreases in the whole burial depth direction,and the closest to the top of the wall,the soil pressure decreases more,which is similar to the enclosure structure.This is exactly in line with the cantilever mode of "big at the top and small at the bottom" of the enclosure structure.(2)Midas GTS NX finite element software was used to establish a three-dimensional real dimensional model of the actual project,and the correctness of the model and the reasonableness of the parameter selection were verified by collecting the water level outside the pit for the field pumping test and the surface settlement monitoring data outside the pit at the SK35+043 section for comparison and analysis with the model results.(3)Based on the above-mentioned model and the selected reasonable parameters,the law of deformation caused by different precipitation conditions was studied by numerical simulation from five aspects,including precipitation depth,spacing of precipitation wells,pit size,insertion depth of enclosure structure and split precipitation.The results show that.1)Under different precipitation factors,the overall deformation curve of the enclosure structure is "cantilever type",and the maximum lateral shift of the enclosure structure appears at the top of the enclosure structure;the surface settlement curve outside the pit is "hook type",and the maximum surface settlement outside the pit appears at about 10 m from the enclosure structure.The maximum surface settlement outside the pit appeared at about 10 m from the enclosure structure.2)The lateral shift of the enclosure structure and the surface settlement both increase gradually with the increase of the precipitation depth,but the increase of the lateral shift of the enclosure structure decreases gradually and the surface settlement increases linearly;the maximum lateral shift of the enclosure structure and the maximum surface settlement outside the pit decrease non-linearly and linearly with the increase of the spacing of the precipitation wells,indicating that the spacing of the precipitation wells has a significant influence on the deformation of the pit,so the spacing of the precipitation wells is chosen reasonably;increasing the length of the pit The lateral shift of the enclosure structure and the maximum surface settlement outside the pit both increase gradually and then tend to a stable value,indicating that there is a critical value for the pit length when the pit width is certain;the maximum lateral shift of the enclosure structure increases gradually with the greater the insertion depth of the enclosure structure,while the surface settlement outside the pit decreases with the increase of the insertion depth of the enclosure structure,indicating that the insertion depth of the enclosure structure has a better effect on the control of the surface settlement outside the pit.Compared with one-time precipitation,the maximum lateral shift of the enclosure structure is reduced by 31.2%,and the maximum surface settlement outside the pit is reduced by 24.3%,indicating that split precipitation has a significant effect on the control of foundation pit deformation.