Study On The Infulence Of Foundation Pit Excavation On The Deformation Of Adjacent Metro Tunnel

In recent years,the construction of underground rail transit in China has developed rapidly.Underground metro tunnels crisscross the city and extend in all directions.The areas along the metro have become the key areas of urban development.As a result,there are more and more foundation pit projects adjacent to the existing metro tunnels;the relative position between the tunnel and the foundation pit is more and more complex;the protection measures for tunnels adjacent to deep and large foundation pit are more and more abundant.In addition,because the theory of foundation pit engineering lags behind the engineering practice,it takes quite a long time for the existing engineering construction technology and experience to form a systematic theoretical system.Therefore,it is necessary to further study and analyze some new characteristics of foundation pit engineering,and strive to obtain some research results with certain guidance and reference value for similar projects.Based on the foundation pit project of Ningbo Greenland center,this paper uses numerical simulation method to study the influence of foundation pit excavation on the deformation of adjacent tunnels.The main contents are as follows:(1)Based on the investigation and design data of Ningbo Greenland center foundation pit project and the special protection measures of adjacent tunnels,a three-dimensional numerical model is established to study the surface settlement,the deformation of retaining structure and the influence on the deformation of adjacent tunnels during the construction of large-scale and multi zone foundation pit.The numerical simulation results are compared with the field measured data to verify the rationality of the model and the feasibility of further research.In addition,according to the tunnel protection measures relying on the project,different tunnel protection countermeasures are formulated,and the effectiveness of each countermeasure is analyzed.(2)This paper studies the influence of the change of foundation pit size and the relative position of the tunnel longitudinal axis and the foundation pit sideline on the tunnel deformation under the condition of non parallel relationship.It is concluded that the change of the angle between the tunnel axis and the side line of the foundation pit and the change of the horizontal distance between the tunnel and the foundation pit will affect the deformation of the tunnel.The former not only affects the deformation value of the tunnel,but also changes the location of the maximum deformation of the tunnel,while the latter only affects the deformation value of the tunnel.;when the angle between the tunnel axis and the foundation pit sideline is different,the sensitivity of tunnel deformation to the change of foundation pit length is different.The smaller the angle is,the greater the influence of the change of foundation pit length on the tunnel deformation is.However,the sensitivity of tunnel deformation to the change of foundation pit width is similar under different angles.(3)This paper studies the effect of three reinforcement plane layout forms(skirt reinforcement,strip reinforcement and unilateral reinforcement near the tunnel side)in the passive area of foundation pit on controlling the deformation of adjacent tunnels and studies the influence of reinforcement area of different reinforcement forms and the reinforcement height of soil above and below the pit bottom on the deformation of adjacent tunnels.The results show that the effect of unilateral reinforcement on controlling tunnel deformation is the most significant,and the deformation control effect of single side reinforcement area A_C=0.2A is better than that of skirt and strip reinforcement area A_C=0.6A;the optimal reinforcement height of the soil below the pit bottom is about 6 m,and the optimal reinforcement height of the soil above the pit bottom is about 4 m.(4)This paper studies the influence of different zoning construction sequence,different zoning schemes,the number of zoning in the partition and partition wall thickness and insertion ratio on the adjacent tunnels,puts forward the guidance and suggestions for the zoning construction of foundation pit for the purpose of controlling the deformation of adjacent tunnels,and obtains that the optimal length of small zoning in the partition is 1.6 times of the excavation depth.