Study On Deformation Characteristics Of Deep And Large Foundation Pit During Excavation And Dewatering In Subway Station

At present,the construction of domestic underground rail transit is in full swing,and a large number of deep and large foundation pit projects have emerged.The surrounding environment of the foundation pit is often complex,with many structures and dense pipelines,which makes the construction process of the foundation pit extremely complicated,and the safety accidents and environmental problems caused by the construction of the foundation pit are overlapping.It can be seen that the safety of the foundation pit engineering structure itself is very important,while ensuring the safety and stability of its surrounding environment can not be ignored.Based on the background of the deep foundation pit project of the Martyrs Cemetery Metro Station of Jinan Rail Transit Line 1,this paper analyzes the deformation of the foundation pit,the settlement of surrounding surface and the impact of building foundation settlement caused by the precipitation excavation of the deep foundation pit through on-site monitoring combined with finite element numerical simulation.(1)Systematically summarize the method and applicable conditions of deep foundation pit precipitations,and analyze the soil deformation mechanism caused by deep foundation pit precipitation,excavation,and related measures to control the deformation of the foundation pit.The study on the deformation characteristics of pit precipitation excavation provides the basis.The study of excavation deformation characteristics provides the basis.(2)According to the monitoring data of the site construction process of the deep foundation pit of this subway station of Jinan Rail Transit Line 1,the paper selects a representative set of monitoring points for specific data analysis.The three aspects of soil settlement around the foundation pit,deformation of the enclosure structure,and foundation settlement of the adjacent building were analyzed.The deformation law caused by precipitation excavation is summarized:the measured value of the surface settlement point on the long side of the foundation pit is larger than the measured value on the shorter side of the foundation pit;In the early stage of precipitation excavation of the foundation pit,the displacement of the envelope structure only occurs at the top of the pile,the location where the maximum horizontal displacement occurs gradually moves to the lower part of the pile as the depth of precipitation excavation increases,and its extreme point finally appears at a position about one third from the bottom of the pile;The maximum point of building foundation settlement is closest to the edge of the foundation pit,and the settlement monitoring point of the building farther away from the foundation pit tends to uplift soon after excavation of the foundation pit.(3)Based on the fluid-solid coupling module of ABAQUS,the article established a three-dimensional finite element model considering the whole process of excavation.The entire simulation process is divided into six steps of precipitation and six steps of excavation,and each time the water level drops below the excavation surface by 0.5m.The numerical simulation results are compared with the measured data and theoretical calculations to verify the accuracy,and the deformation characteristics of the main structure caused by dynamic precipitation and excavation of the foundation pit were discussed in depth.For the deformation of the enclosure structure,the impact of precipitation in the first step is small,and the impact of precipitation in the second step is the most prominent,after the second step of precipitation,the impact of precipitation on the deformation of the enclosure structure is getting weaker and weaker,until the sixth step of precipitation,it played a positive role in the deformation of the enclosure structure and reduced the deformation to a certain extent;regarding the settlement of the surface around the pit,the surface settlement value caused by precipitation is less than that caused by excavation,among them,the fifth step of precipitation has the greatest impact on surface subsidence,and its settlement value has reached 14.2%of the total value of surface subsidence.(4)Through the numerical simulation,the basic laws of deformation of the foundation pit and surrounding environment under the influence factors of precipitation and non-precipitation,different precipitation methods,different precipitation speeds and depths,and different distances of buildings from the foundation pit are analyzed.Compared with the interlayer precipitation and the one-time precipitation method,the layer-by-layer precipitation method has less impact on the stability of the foundation pit itself and the surrounding environment;there is an optimal value for the speed and depth of the foundation pit precipitation,when the precipitation speed and depth reach the optimal value,it has a certain effect on the stability of the foundation pit;compared with the precipitation depth,the precipitation speed has little effect on the deformation of the foundation pit.