Study On The Influence Mechanism And Control Measures Of Excavation Construction On Adjacent Existing Tunnels

The excavation of foundation pit and shield tunneling are two common construction items in our country currently.The excavation load changes the initial stress state of the surrounding soil,and brings negative effects to the existing subway tunnels in the neighboring area.Therefore,to accurately predict and control the additional internal forces and deformation of adjacent existing tunnels during underground excavation is a major issue to be solved.The structural characteristics of the existing tunnel,the layered characteristics of the foundation,the envelope structure,the stratum reinforcement,the reinforcement of the uplift pile,etc.,all have a great influence on the longitudinal deformation of the existing tunnel.However,the research in this aspect is not perfect.Taking S City metro tunnel adjacent to the foundation pit as an example,numerical simulation method is used to simulate and analyze the influence of excavation support construction of adjacent tunnel foundation pit.The main contents and conclusions of this paper are as follows:(1)Foundation pit deformation mechanism,the foundation pit supporting structure deformation,pit bottom uplift,ground subsidence and other deformation are analyzed,and the relevant theory and calculation of deformation are expounded.In this paper,from the three aspects of foundation pit construction,foundation pit,foundation pit excavation,the possible causes of tunnel deformation caused by foundation pit construction are analyzed,and the foundation pit excavation tunnel deformation is studied theoretically and method-based.(2)Use Li Zhengsheng foundation pit software to design and calculate the foundation pit support scheme,and check the safety and deformation of the foundation pit,to test the safety of the scheme;Using FLAC3 D software,the numerical modeling of the section adjacent to the foundation pit of the subway tunnel is carried out,and the influence of the deformation of the foundation pit in the construction project and the deformation of the adjacent tunnel is simulated and analyzed.Adhering to the principles of "safety,economy,and convenience",in order to accelerate construction,shorten the excavation period of the foundation pit,and reduce the impact on adjacent existing tunnels during the excavation process;Before drawing the enclosure structure,contact the design institute in a timely manner to optimize and adjust the corresponding design parameters.(1)Adjust the original design pile diameter from 1.2m to 1m;(2)Adjust the spacing between the retaining piles from the original design of 1.5m to1.7m;(3)Reduce the 4 rows of anchor cables in the original foundation pit to 3 rows;(4)Adjust the length of the original anchor cable from 25 m to 20 m and 21 m.(3)Compare the data such as the horizontal displacement of the side wall of the foundation pit,the surface settlement and the uplift of the bottom of the pit with the foundation pit software FLAC3 D and the measured data,and compare the results to confirm the correctness of the established model.(4)The excavation of the foundation pit causes the adjacent area and the tunnel to move laterally towards the foundation pit,the maximum horizontal displacement is 3.05 mm,and the tunnel moves towards the ground,the maximum vertical displacement is 3.26 mm,and the deformation in the tunnel reaches the prescribed safety.In order to minimize the impact of excavation on adjacent tunnels,the following specific measures have been taken during excavation construction to prevent excessive deformation of the foundation pit.(1)The retaining structure of the foundation pit adopts high-grade underwater concrete,and the middle of the retaining piles adopts rotary jet grouting piles for water stopping.During pile foundation construction,pile jumping construction is adopted;(2)During the excavation process,timely observation and monitoring feedback of the strata should be conducted,and compared with the geological survey report;(3)Take measures such as setting up a collection pit inside the pit for timely drainage and setting up intercepting ditches outside the pit to prevent rainwater from penetrating into the pit;(4)During the excavation process,the principle of "vertical layering,vertical segmentation,support first and excavation later" shall be strictly followed,and excavation shall be carried out step by step,with strict control over the excavation height of each layer;(5)Stacking is strictly prohibited within a range of 2 meters from the edge of the foundation pit to prevent the collapse of the foundation pit;(6)Assign dedicated personnel to conduct daily inspections of the foundation pit,monitor projects that affect the safety of the foundation pit,and promptly report any abnormal situations.By taking the above measures,the impact of foundation pit deformation on adjacent tunnels has been effectively reduced,providing useful reference for structural design and analysis of similar projects.