Undrained face stability of tunnels in Bangkok subsoils

This paper presents the analysis of undrained face stability of tunnels in Bangkok subsoils. In this study, two-dimensional (2D) and three-dimensional (3D) finite element analyses were employed to analyse the 2D plane strain conditions of transverse (2DT) and longitudinal (2DL) sections as well as the full 3D geometry, respectively. The simplified geometry of 2DT corresponds to the circular face of tunnels subjected to a uniform face pressure, while that of 2DL correspond to the plane strain heading applied by a uniform face pressure at the front plane. For the 2D plane strain transverse and longitudinal sections, the tunnel lining was not considered in the analyses. For 3D cases, tunnel linings were included in the analysis and modelled as the rigid plate element with the fully rough surface along their lengths, while the tunnel face was subjected a face pressure. For all cases, the uniform surcharge was applied all over the ground surface. Strength reduction methods in conjunction with 2D and 3D finite element analyses were employed to determine the stability and the factor of safety of this problem. The stability analyses included ranges of tunnel face pressure measured during the tunnelling process. Effects of three-dimensional stability of tunnel face were also presented and discussed in this paper. Results of analyses were verified and compared with existing methods.