Study On Blow-out Face Pressure Of Shield Tunnels In Sand

For the sake of saving space and relieving traffic pressure, the subway becomes an important means of transportation, especially as a result of the China's urban population continuing to expand. The shield tunneling method are widely used in the construction of tunnels, because of its security, high mechanical degree, well operational environment, less earth volume, quick progress, strong adaptability, less influence and high security. In the construction of shield driven tunnel, the too large support pressure may lead to the upheaval of ground surface, even blow-out failure of tunnel face. Hence, the passive failure mechanism and the estimation of the required support pressures for shields have been a topic of research until the present day.To solve the problem of face stability of shallow tunnels shield in sand, firstly, a finite element program (ABAQUS) is chosen for numerical analysis. The analysis results of a large number of cases reveal that passive failures are often occurred on shallow tunnels. The passive failure mechanism is also studied in this chapter.Secondly, a centrifuge test on passive failure of tunnel face, which is carried out by Zhejiang University and The Hong Kong University of Science & Technology, is presented in detail. The relation between tunnel face pressure and displacement is obtained to determine passive failure pressure, and the deformation mechanism is observed by incremental displacement field to reveal passive failure mechanism. Then, the finite element method is used to simulate this test, whose results are coincident with the test.According to the numerical and centrifuge test results, a modified wedge model, which is based on Mohr-Coulomb model, is proposed to calculate the passive failure pressure. Many cases are calculated to analysis the response of passive failure pressure as the change of soil parameters.In conclusion, the numerical method and the 3D wedge model can well predict the passive failure pressure of tunnels shield in sand, which can provide reference for determining upper limit support in the construction of shield driven tunnels.