The Stability Analysis Of Shield Boring In Shallow Buried Sand Gravel Stratum In Lanzhou Subway

Nowadays the application of shield machine method in tunnel construction is more and more common,which inevitably leads to movement and deformation of formation in process of construction.At present, the available results for reference of research on deformation rules and controlling methods of shield construction in gravel stratum are lacked. Gravel stratum is a typical mechanical unstable layer, which is characterized as force transmission by particle contact, irritable disturbance, high sensitivity and loose structure without cohesion. Therefore, the study of the stability of shield excavation gravel formation is pretty necessary.Based on the shield construction of Lanzhou subway tunnel, this paper fully considers the characteristic of engineering geology and hydrogeology conditions in Lanzhou area and researches on shield excavation stability in gravel formation in the process of excavation.In the following are the main research content :1.Based on theory of tunnel mechanics and soil mechanics, this paper analyzes the law of deformation and instability of the upper stratum caused by shield construction and proposes the pre-reinforcement anchor technology to reinforce the formation in advance.At present, the concept of reinforcement is less applied in the construction of shield tunnel excavation, which provides the valuable reference to the following tunnel construction of Lanzhou subway and other similar engineering constructions. The concept of reinforcement is full of innovation and high engineering application value.2.Based on Terzaghi effective loose earth pressure calculation theory and combined with ground conditions of Lanzhou subway tunnel, the overburden pressure on the tunnel is calculated. With limit analysis theory and combined with the wedge body model, this paper puts forward calculation formula of supporting force to ensure the stability of excavation face in the process of shield construction and determines the security range of propulsion and excavation supporting forces, which provides the reliable theory basis to reduce the surface subsidence or uplift effectively caused by tunnel construction.3.Based on the Lanzhou subway and combined with theoretical analysis and calculation of the reinforcement mechanism and effect of the anchor bolt pre-reinforcement, it is concluded that setting vertical bolt could improve soil cohesive force and internal friction angle of the formation and reduce the supporting pressure of excavation surface in some degree, which effectively improve the strength of the gravel stratum and the stability of excavation face.4.By means of the calculation software of finite difference FLAC 3D,we simulate and analysis the pre-reinforcement effect of the vertical anchor bolts,and compare the effect of pre-reinforcement by adjust the spacing and the embedded depth of the vertical anchor bolts.