Research On Construction Technology Of Urban Tunnel Entrance Section With Small Clear Distance In Backfill Area

The rock and soil in the portal section are subject to natural weathering for a long time,most of which are weak and low in strength,especially the slope with backfill on the surface is more seriously eroded by weathering.In the case of adverse surrounding rock conditions,in order to prevent various engineering diseases in construction,it is very important to strengthen the portal section of the slope,the construction mode of the tunnel and the selection of supporting measures.Based on the background of Lei Jiapo overpass tunnel project,this paper makes a lot of monitoring and measurement on the construction site,and obtains the data needed for the research and analysis,which provides the data base for the development trend and comparative analysis of the deformation of the tunnel structure in the later period.In addition,the research and analysis are conducted for the problems of reinforcement of existing retaining structure,tunnel excavation method and staggered distance of the tunnel The corresponding numerical models are established to analyze the difference between the theoretical and the actual monitoring values and the reasons.The main research contents are as follows(1)The model of the existing retaining structure before and after reinforcement is established by using the finite element software.The vertical and horizontal displacement of the retaining wall before and after the reinforcement of the existing retaining structure and the axial force,shear force and bending moment of the existing anti sliding pile are analyzed.The technical feasibility of the scheme is determined by comparing the values before and after reinforcement,and then the tunnel construction technology is studied.(2)The model of small clear distance tunnel excavation under three different construction technologies is established.The displacement and deformation of surrounding rock,the main stress of surrounding rock,plastic area of surrounding rock,stress of supporting structure and settlement of road pavement are compared comprehensively,so as to ensure that the results obtained by each construction method meet the requirements of design regulations.On the premise of meeting the requirements,the comprehensive economy and technology,the comparison of three construction methods,finally determined a reasonable construction method,which provides the technical basis for the subsequent construction.(3)The optimal method of excavation is used to excavate the tunnel,and the numerical model of different distance is established.The settlement of arch roof,bottom uplift,plastic zone of surrounding rock and support force obtained by the numerical model of staggered distance are compared,and the reasonable distance of staggered holes is analyzed.(4)According to the monitoring scheme,the tunnel portal section of Lei Jiapo overpass is comprehensively and detailedly monitored and measured.The environment and geological conditions of the project and the reinforcement support are observed in advance.The horizontal convergence around the tunnel,the settlement of arch roof and the settlement of the surface are monitored.The data are sorted out to analyze whether the settlement of the surface,the settlement and horizontal convergence of the arch roof are in the safety standard The results show that the law of the change of surrounding rock and supporting structure is in line with the actual engineering change condition.