Study On The Influence Of Shallow Buried Large Span Tunnel Construction On Ground Settlement

With the rapid development of social economy and the accelerating pace of urbanization,the construction of urban rapid rail in our country has entered a golden age of development.The construction of the tunnel excavation will affect the tunnel strata,cause it to be disturbed and cause damage to the rock and soil,thus forming different depth and range of surface subsidence,When the settlement or deformation exceeds a certain limit,it will affect the safety of tunnels,surface buildings and underground facilities,especially for the city center area of shallow buried tunnel excavation,often wear dense buildings,underground pipe network(such as gas pipelines,oil pipelines,water pipes etc.),road traffic(such as roads,railways)and so on,the construction environment is complex,the construction conditions are limited,the objects that need to be protected are complex and diverse,so the demand for surface settlement is more stringent.So far,the design,calculation theory and construction technology of medium and sub span tunnels have been relatively mature both at home and abroad,with the continuous emergence of shallow buried large span tunnel engineering,the problems encountered in construction are increasing,the problem of surface subsidence is the most prominent,and the current design and construction standards have been difficult to meet the requirements of the design and construction of shallow buried large span tunnel.At present,there are not many researches about the design and construction of shallow buried large span tunnel at home and abroad,there are few experiences that can be used for reference,and they are still in the stage of exploration and summary.In order to reduce the damage caused by the surface subsidence caused by the tunnel construction to the ground buildings and underground facilities,it is necessary to analyze the influencing factors of surface subsidence,therefore,this paper takes a tunnel mileage from D4K20+275 to D4K20+560 as the engineering background,FLAC3 D three-dimensional finite difference software is used to study the influence factors of surface settlement during shallow excavation of large span tunnel,the research contents are as follows:Firstly,summarizes the shallow large span tunnel construction projects at home and abroad and the results of previous studies,it lays a foundation for the innovative research in this paper,and the harm,cause,influence factor and construction method of surface settlement of shallow buried largespan tunnel are expounded;Second,the shallow buried large span tunnel excavation method commonly used full section method,step method,bench method,and the method of double side wall(CD and CRD)were reviewed in detail,to provide theoretical basis for studying the influence factors of surface settlement in shallow large span tunnel construction;Three,using FLAC3 D numerical simulation analysis of different sections(60 square meters,80 square meters,100 square meters,120 square meters,140 square meters,160 square meters),different depth(10m,20 m,30m,50 m,70m,100m),different construction methods(full section method,three step method,three steps and six steps)and blasting vibration influence on surface subsidence,combined with field blasting vibration monitoring and road base settlement measurement experiment study the influencing factors of surface subsidence and its settlement law,at the same time,it is verified whether the three step six step blasting method is feasible for the project.Through the analysis of influencing factors of surface settlement in shallow buried large span tunnel construction,draw the following conclusions:Firstly,study on the influence factors of surface settlement of shallow large span tunnel excavation,tunnel construction will lead to different levels of settlement at the top of the tunnel,blasting vibration,tunnel depth,section size,excavation method and blasting vibration are the important factors of surface subsidence,the construction of the tunnel will lead to different levels of settlement at the top of the tunnel,rock and soil at the bottom of the tunnel in different degrees of uplift,the buried depth of tunnel has the most serious influence on the settlement at the top of tunnel and the uplift of rock and soil at the bottom of tunnel,the greater the depth,the greater the value.Second,different construction methods have different effects on surface settlement of shallow buried large span tunnel,in this paper,the surface subsidence caused by the three methods of construction is as follows: full section method,three step method and three step six step method.Three,the tunnel is subjected to blasting load,the densification and settlement of surrounding rock stratum have been accelerated,the surface subsidence increased obviously and the range of plastic zone enlarged obviously.Four,the surface settlement of the lower section of the highway is basically consistent with the simulated land subsidence law,the cumulative maximum settlement is within the allowable range,and the measured data of blasting vibration are within the prescribed range,in the complexconstruction environment,the three steps,six steps and support measures are feasible,effective,reasonable and safe.