Theoretical And Test Studies On Face Stability Of Shallow Tunnel

The need for mechanized excavation of tunnels in cities has continuously increased in recent years, especially as a result of the global expansion in the number of tunnels being constructed for subways. Tunnels with low covers are often headed using the advanced shield technique. Nevertheless, face collapses during the construction of shallow tunnels still occur. In extreme cases, the collapse propagates up to the ground surface creating a great surface subsidence. Hence, the 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, model tests, numerical modeling and analytical methods were carried out in this thesis. The main contents are as follows:1) Large-scale tests on the face stability of shallow tunnels were conducted using 3D model with the tunnel diameter of 1 m in dry sand. The support pressures on tunnel face and the settlement of ground surface were studied with different cover depths (C/D = 0.5, 1, and 2). The values of ultimate support pressures P_u are given for each test. The earth pressures in the soil above the tunnel were recorded in the test of C/D = 2. The evolution of soil arching during face failure is investigated. The boundaries of the arch zones are also proposed. Finally, the author provides a description of the two-phase failure mechanism.2 ) A finite difference program (FLAC3D) is chosen for numerical analysis of the system described by the tunnel model. The results from the numerical modeling and the measurements from the centrifuge modeling were compared in order to assess their predictions. The change of stress field, displacement field, principal stress direction and plastic zone during the face collapse are investigated for further understanding on the failure mechanism.3 ) Based on the experiment and numerical results, the choice of the wedge model parameters is discussed. According to the experiment and numerical simulation, the rotation of principal stress axes can be found in the arch. Based on the theory of main stress axes rotation, the analytical expression of the lateral earth pressure coefficient of the soil is presented. Compared with the traditional expressions of wedge mode, the proposed one is more in agreement with the experimental results.