Analysis On Design Of The Supporting System Of Foundation Pit And Its Environmental Impact Upon Nearby Subway

Deep excavation and supporting are the hot and difficult problems which are considered by geotechnical engineering.How to control deformation of foundation pit through effective supporting method and ensure the safety of the foundation pit are the constant topics in geotechnical engineering field.The soft clay foundation is widespread in Shanghai area.The strength of foundation soil is very low and the groundwater level is very high,which bring adverse factors to the design and construction of deep foundation pit.The environment of foundation pit engineering adjacent to subway is complex.Although many scholars have made a research on for this in recent years,but there are still many problems in the supporting structure adjacent to the subway foundation pit,displacement of soil,and influence of tunnel.Based on a commercial building project in Shanghai near the subway,this paper analyzes the deformation of the support system and its impact on the environment by using finite element numerical simulation,establishes the design scheme and optimizes the support system structure according to the requirement of environmental protection.In addition,the construction situation of the project in this paper and the field test results and analysis conclusions are compared and analyzed.The results can provide references for similar project design and construction.The conclusions of this study are as follows:(1)In the finite element numerical analysis,the selection of various parameters in the calculation model is very important.Especially in the foundation pit engineering of soil constitutive model selection is particularly important.In this study,the Hardening Soil model Hardening Soil model is used and it can simulate the soft soil and hard soil,and other types of Soil mechanics properties.According to the parameters provided by the geological survey report and the methods provided by the relevant test results,reasonable constitutive model parameters are obtained,and a more reasonable result can be obtained by finite element analysis of foundation pit engineering in Shanghai area.(2)Through finite element analysis,it is can be concluded that the project is near the subway side chooses 800 thick underground continuous wall for supporting foundation pit,metro tunnel deformation duo to excavation near the tunnel(the distance is about 1 times of foundation pit excavation depth)is 8.02 mm,metro tunnel deformation duo to excavation near the tunnel(the distance is about 2 times of foundation pit excavation depth)is 4.36 mm.The additional deformation of the interval tunnel is within the scope of subway protection control requirements.The farther the foundation pit distance is,the smaller the additional deformation of the subway is.(3)In this study,the deformation of the supporting system and interval tunnel was analyzed by adjusting the supporting stiffness and reinforcement width in the pit adjacent to the subway.Respectively analyzing the three kinds of supporting stiffness(800 mm thick and 1000 underground continuous wall,Φ 900 bored piles)and three kinds of pit reinforcement width(5 m,8 m,11 m)combinations,it can be found the underground continuous wall with 1000 mm or 11 m width of foundation reinforcement,maximum horizontal displacement of retaining structure to reduce the amplitude is too small and cost performance is low;While using Φ 900 bored piles or 5 m wide width of foundation reinforcement,supporting structure and its great impact on the environment,but it can not satisfy requirements over the side of the subway.Based on the above analysis,it is decided that 800 thick underground continuous wall is adopted to support the adjacent subway side and the reinforcement width is 8m.Outside the scope of protection of the subway(general for tunnel 50 m)area can decrease the supporting stiffness.Choosing Φ 900 bored piles can ensure the safety of the excavation and reduce the cost of supporting structure.(4)During the construction of foundation pit,the quality of supporting structure and foundation reinforcement construction shall be strictly controlled.The whole process of field measurement should be carried out in the later excavation of foundation pit,and the measured deformation data should be compared and analyzed with the finite element calculation results.The curves of the measured support structure,soil horizontal displacement and ground settlement deformation are basically consistent with the calculated results.For example,the maximum value of the measured slope curve of the support structure CX4 is 20.31 mm,while the simulated results are 18.64 mm.It is proved that the whole process of foundation pit construction is analyzed by finite element method.In addition,it can be seen that the foundation pit of the adjacent subway side is supported by the underground continuous wall with large rigidity and the apron edge is used to reinforce the pit can effectively protect the additional deformation of the tunnel.