ABSTRACT
Advances in computer systems have allowed complex conditions to be accurately modeled by means of numerical methods, thus providing new approaches for characterizing the face stability of TBM tunnels constructed in soft soils. Front stability is usually maintained by bentonite slurry or pressure balance shields. Recently, several studies are developed to deal with the face stability of the TBM tunnels. Many approaches can be applied to tunnel subject to seepage forces during tunnel construction which are located under the ground water level.The upper bound solution by modifying the failure mechanism was provided by Leca and Dormieux (1990).The wedge stability model that taking full account of the three-dimensional geometry at the tunnel face, was proposed by Anagnostou and Kovari (1996-1997), using limit equilibrium principles. This model is not valid when different soils are present within the tunnel face, and those soils have different angles of internal friction and cohesion. Also, there is no straightforward way to approximate or average the soil properties to obtain the correct support pressure. Two slice methods are proposed according to the shape of sliding surface, In-Mo Lee and Seok-Woo Nam (2004).The first sliding surface is employed to be a logarithmic spiral while the second sliding surface is assumed to be a straight line. The critical support pressure at the tunnel face can be determined by iteratively altering the inclination angle between the sliding surface and the horizontal. Figure 1 shows the critical
surfaces for the methods of the analysis of tunnel face in longitudinal section.
