Influence Of Working Conditions Change On Foundation Pit And Surrounding Environment In Top-down Construction

With the development of city construction,the scale of foundation pit engineering is becoming larger and larger,so the excavation of foundation pit brings new challenges to the design and construction of conventional retaining structure.As a new construction method,upside-down method emerges as the times require.It takes the floor,beam and column of the main structure as the supporting system of foundation pit excavation,which is conducive to saving expenses and is gradually widely used in deep foundation pit engineering.Reverse construction method is a complex construction method,if involved or improper construction will cause significant economic losses and casualties,so it is necessary to study and analyze the reverse construction method of foundation pit engineering.Based on the foundation pit project of Wanjili Block in Tongde Plaza of Kunming,this paper establishes a three-dimensional finite element model of soil,structure and surrounding environment by using finite element software.The construction process of the foundation pit is simulated and analyzed.The calculated results are basically consistent with the actual monitoring data,which shows that the analysis model is reliable.The results of numerical simulation are analyzed in detail.The main contents and conclusions are as follows:Based on the foundation pit project of Wanjili Block in Tongde Plaza of Kunming City,this paper establishes a three-dimensional finite element numerical model of the interaction between soil and retaining structure by using finite element software,and simulates and analyzes the construction process of the foundation pit project with the reverse construction method.The calculated results are basically consistent with the actual monitoring data,which indicates that the model is suitable for this project.The parameters of the model are reasonable,and the basic model is reliable.On this basis,the change of working conditions during the construction process of reverse engineering foundation pit is analyzed.The main contents and conclusions are as follows:(1)In the excavation of foundation pit,the lateral displacement curve of diaphragm wall changes from cantilever to convex.The ratio of maximum lateral displacement to excavation depth is between 0.08% and 0.11% under the corresponding working conditions,and the location moves from the top of the wall to the vicinity of the excavation face.(2)The ratio of surface subsidence curve outside the pit to the excavation depth of the corresponding working conditions decreases from 0.29% to 0.05%.The position of the maximum subsidence moves from 0H behind the pit to 0.57 H.The main influence area of surface subsidence is 2H.When there are shallow foundation buildings around the foundation pit,it is more dangerous than when there are pile foundation buildings.When there are shallow foundation buildings,the settlement value increases by 50.4%,and when there is pile foundation,the settlement value increases by 37.9%.(3)On the basis of the above-mentioned engineering examples,the influences of the factors such as diaphragm wall rigidity,floor support rigidity and excavation mode on the foundation pit characteristics and surrounding environment are analyzed respectively.The results show that the deformation of foundation pit and the settlement of soil behind the pit can be well controlled by increasing the Gan degree of retaining structure and bracing in a certain range,and the bending moment of wall will be greatly changed if the stiffness is increased.When the excavation depth is greater than 1.5m,it is not conducive to the deformation control of foundation pit.(4)Through the analysis,it is concluded that the reverse construction method can effectively reduce the impact on the surrounding environment by excavating the foundation pit in complex environment and selecting reasonable working procedures.In view of the relevant factors affecting the reverse construction method,the change of working conditions in the construction process is controlled,and the relevant solutions and suggestions are put forward,which also provide a certain reference for similar projects.