Study On Support Pressure At Excavation Face Of Shield Tunneling In Composite Ground With Soft And Underlying Hard Strata

The stabilization of the shield tunnel face and control standards of support pressure are very important problems in tunnel projects. Because of the obvious stiffness difference between the upper and lower strata, it is more difficult to control the support pressure of excavation face for the tunnels passing through the composite ground with soft and underlying hard strata in contrast with those passing through the homogeneous ground. Improper control of support pressure may lead to significant ground settlement or even failure accidents, which will severely affect the security of the existing structures. Therefore, it is very significant to study the support pressure of excavation face of shield tunnels in the composite ground with soft and underlying hard strata.The main objective of this thesis is to study the stabilization pressure of face and the influence law of support pressure on ground deformation in the composite ground with soft and underlying hard. The main contents are summarized as follows:(1) The finite difference program, FLAC3D, is adopted to study the failure mechanism of the excavation face for two different cases, i.e. the upper stratum is mucky soil and sand, respectively. And the corresponding limit support pressures of the excavation face are obtained. The variation laws of the soil mass deformation with the support pressure are analyzed and summarized. Based on the conventional wedge model, theoretical formulas of limit support pressure of the excavation face in the composite ground with soft and underlying hard are derived. The theoretical results are compared with numerical simulation results, which shows that the results have good agreement.(2) A three dimensional simulation model is established for studying the entire process of soil excavation of shield tunnels. A series of parametric studies with different tunnel depths and soil characteristics are carried out to investigate the influence of support pressure on ground deformation. The results indicate that the ground settlement increases nonlinearly with the increase of tunnel depth and the decrease of support pressure while it decreases with the increase of strength of the soft upper stratum. (3) Based on the site conditions of Guangzhou-Foshan Mass Transit Railway and the control standards of ground deformation in urban areas, reasonable support pressure of the excavation face is determined. The achievements in this thesis can provide valuable references for the design and construction of tunnel projects.