In this paper, a theoretical stability analysis of a tunnel face, reinforced by fiber-glass inclusions, is proposed. Previous works have already treated the stability of a unreinforced tunnel face in the case of isotropic and homogeneous medium. Based on yield design theory, these studies lead to lower or upper bound estimates of the safety margin. Within the framework of this efficient stability approach, the present work takes into account the effect of reinforcement at the face by the homogenization method. Both the lower and the upper bound approaches are investigated in the cases of a purely cohesive or a cohesive-frictional soil. Examples of design charts are provided to allow a bracketed estimate of the admissible limit loads, depending on the level of reinforcement at the face.
The construction of tunnels in soft ground often requires face reinforcement. Due to the high longitudinal strength of the bolts, it also increases the stability; their relative brittleness to shearing ensures easy removal during excavation. Practicability of such method has well been demonstrated in the past (Pelizza and Peila, 1993; Wong et al., 2000). Nevertheless, the overall behavior of the reinforced tunnel face is not well understood, and the optimization of bolt-quantity requires adequate design tools. Concerning the modeling of heterogeneous media at the face, numerical methods have been proposed, modeling each of the linear inclusions individually or using either a hypothetical fictive frontal pressure or an isotropic increase of strength (Peila, 1994; Dias, 1999). Over these numerical approaches, the homogenization method of periodical media (De Buhan and Salençon, 1993) allows to replace the heterogeneous mixture (soil + bolts) by an equivalent homogeneous medium, leading to analytical solutions under simplified geometry, and has been applied to analyze the displacement behavior of a reinforced tunnel face under spherical symmetry (Wong et al., 2000).
