A proper selection of the distance to the face to install support is of paramount interest in tunneling. This selection can be based on the Longitudinal Deformation Profile (LDP), in the frame of the Convergence Confinement Method (CCM). However, obtaining the actual LDP of a tunnel is not a simple matter. Elastic approaches have been widely used in the past. Recently, new techniques accounting for perfect plasticity have been proposed. The aim of this study is to assess the influence of considering strain-softening behavior on the longitudinal deformation profiles for tunnels by means of simple numerical techniques. The obtained results show that the fact of accounting for strain-softening behavior does not significantly affect LDPs.
The CCM consists of three basic graphical components: the longitudinal deformation profile (LDP), which relates tunnel deformation to distance to the tunnel face; the support characteristic curve (SCC), representing the stress–strain relationship in the support system; and the ground reaction curve (GRC). The LDP is a graphic representation of the radial displacement of point in the periphery of the tunnel (or tunnel wall deformation) in regard to the distance between this point and the tunnel face. An accurate description of this LDP is needed to estimate the optimal distance to the face for installing the support. Estimates of pressure-displacement curves can be compared to estimate the factor of safety against overload or over-deformation. The support needs to be ‘installed’ at the appropriate location or distance from the face. In order to calibrate the model so that the internal pressures or the displacements are correlated to a real distance from the face, a longitudinal displacement profile or LDP is required (Vlachoupoulos & Diederichs, 2009; V&D, in what follows). The corresponding graph is shown in Figure 1.