Research On Construction Risk And Safety Measures Of Deep Foundation Pit Adjacent To The Subway Tunnel

With the continuous advancement of urbanization in China,the construction of various types of structures in cities is also in full swing.Urban rail transit and large commercial office buildings are constantly surging,and underground space is being fully utilized.At the same time,with the continuous construction of the subway rail transportation,the surrounding environment of the foundation pit project is becoming more and more complicated,and the distance between the deep foundation pit and the subway tunnel is increasing.The situation of they getting closer is also frequent,so the deformation law and construction risk of the construction of the deep foundation pit adjacent to the subway tunnel have become one of the focuses of engineers and scholars.This article relies on the deep foundation pit project of Anhui Huali Technology and Operation Center near the Hefei Metro Line 3 tunnel,on the basis of a comprehensive investigation of the project,established and solved a three-dimensional numerical model of the foundation pit using Midas GTS/NX finite element software,and figured out that the maximum deep horizontal displacement of the foundation pit envelope structure is 15.82 mm and the maximum displacement value of the left subway tunnel lining structure is 1.90 mm,including the deformation laws of surface deformation,pit bottom uplift and internal support axial force.These conclusions are basically consistent with the theoretical law and measured data.According to the engineering characteristics and referring to the simulation results,the risks of deep foundation pit construction are identified,and the risk index system is obtained.In order to evaluate the risks the fuzzy comprehensive evaluation method was used,the weights and risk levels of all kinds of factors in different levels are determined.It is obtained that the total risk level of the deep foundation pit construction is Level 3,in which the risk values of earth excavation,insufficient drainage measures and construction quality of the retaining piles are relatively higher.Guided by the results of risk assessment,this article proposes targeted safety control measures and analyzes part of the monitoring data.Based on the actual project,the impact level division was studied.The results show that the method of dividing the project impact zoning according to the shortest distance directly above or outside the external structure of the newly constructed project and the outside of the tunnel structure is not suitable for the foundation pit project of the adjacent tunnel.This article proposes that when judging the impact of the new foundation pit project on the urban rail transit engineering,the nearest distance between the soil at the bottom or the side wall of the foundation pit and the outside of the tunnel structure should be used as the basis for judgment.Based on this,the impact level is re-divided,and the conclusion is drawn that the actual impact level of the foundation pit on the tunnel in this project is Level 2.The methods and actual engineering case used in this article can provide a certain reference value for the urban municipal engineering construction and the safety impact classification and risk assessment of the existing subway tunnel adjacent to the foundation pit.