PRELIMINARY DESIGN FOR NATM TUNNEL SUPPORT IN SOIL

Design of tunnels constructed with the new Austrian tunneling method (NATM) is considered. With this method the tunnel is generally excavated using hand-mining equipment and immediately supported with a continuous shell of shotcrete. The shotcrete acts as a preliminary support during the construction. A liner made of concrete is usually installed later to ensure the long-term stability of the tunnel wall. The construction process of NATM tunnels is analyzed using the finite element method. The softening technique, currently used for the analysis of NATM tunnels, is improved and applied to a parametric study of ovoid transportation tunnels constructed in undrained clays. Based on this study, a simplified method is provided for the preliminary design of the NATM tunnel support. The method allows the estimation of moments and thrusts acting on the liner for a range of commonly encountered conditions. Design values are shown to be primarily dependent on soil deformation properties, initial stress conditions, and amounts of stress release in the ground prior to shotcreting.