Research On The Bearing Mechanism Of Transverse Pipe Curtain And Surface Settlement Prediction Method Of Ultra-shallow Buried Underground Excavation Station Tunnel Pile Method Constructio

With the acceleration of urbanization in China,the contradiction between urban development and urban land use is becoming more and more obvious,and the space resources on the ground are difficult to meet the needs of urban scale development.In order to solve this contradiction,vigorously carry out the development of urban underground space such as underground rail transit construction has gradually become an inevitable trend in the development of large cities.However,subway construction inevitably crosses the bustling sections of the city,important government centers and historical and cultural monuments.As a kind of advanced support method,the pipe roof method can effectively reduce the surface settlement.The transverse pipe roof has large bearing capacity and convenient construction,and is gradually applied to the subway construction of urban busy sections.Based on the transfer station center project of the Olympic Sports Center of Jinan Rail Transit Line 4,this paper studies the bearing mechanism of the transverse pipe roof in the construction of super-shallow underground excavation station by means of field test,numerical simulation and theoretical derivation.Based on this,the prediction method of surface settlement under the action of transverse pipe roof is proposed.The results show that :(1)Through field test and numerical simulation,the surface settlement and the stress change of the pipe roof during the excavation of the station are studied.The results show that the surface settlement and the deformation of the pipe roof are large at the excavation of the lower soil,and the surface settlement and the deformation of the pipe roof structure are relatively small at the unexcavated part.In the lateral direction of the station,the surface settlement curve shows a wave-like U-shaped to a smooth U-shaped change,and the surface settlement curve shows a serrated settlement law in the longitudinal direction of the station.The lower part of the pipe roof is subjected to tensile stress at the excavation of the pilot tunnel,and the upper part is subjected to compressive stress.During the construction of the underground excavation station,the excavation of the underground layer has a great influence on the surface settlement and the stress of the pipe roof.(2)The bearing capacity of the pipe curtain structure is mainly reflected in its own structure and the soil arch between the pipes.Firstly,the structural stiffness of the pipe roof is large,which can share most of the loose earth pressure in the upper layer.On the one hand,it can resist the upper load through its own deflection deformation.On the other hand,through the coordinated deformation of the pipe roof structure,it can transfer the load to both ends,and its bearing mechanism is similar to that of the beam structure.Secondly,due to the large difference between the stiffness of the pipe curtain structure and the soil,the upper soil produces uneven settlement after being loaded.There is a soil arching effect between the steel pipes of the pipe curtain structure,and the soil arch between the pipes can bear a part of the overlying soil load to a certain extent.(3)Based on the evolution law of soil arch,a theoretical analysis model of multi-layer parabolic arch is established.Based on this model and the stress deflection theory,the relationship between the arch foot,vault and average soil pressure of any layer of soil arch is studied.Finally,the influence range of soil arch in the stratum and the loose soil pressure of the lowest soil arch are obtained.The vertical load of the pipe-roof structure due to excavation is the average loose earth pressure of the lowest soil arch.Through the elastic foundation beam theory,considering the influence of excavation disturbance,the deflection calculation formula of the pipe-roof structure after single pilot tunnel excavation,full pilot tunnel excavation and middle soil breaking is further deduced by Matlab programming software.(4)Under the action of transverse pipe roof,the surface settlement of super-shallow underground excavation station mainly comes from three aspects : pipe roof jacking loss,pipe roof structure deflection deformation and soil arch collapse between pipes.Based on the Peck formula,a surface settlement prediction method considering pipe roof jacking,pipe roof structure deflection deformation and soil arch effect is proposed.Based on this method,from three aspects of controlling the attitude of the pipe curtain,improving the stiffness of the pipe curtain structure and enhancing the soil arching effect,the surface settlement control measures for the construction of the super-shallow buried underground excavation station under the action of the transverse pipe curtain are proposed.