Centrifuge Modeling of Face Excavation in Tunnels with a Deformable Lining

Abstract

This study presents the results from a series of centrifuge modeling experiments performed on tunnels in sand. The goal of these experiments was to evaluate the role of tunnel face movements on both tunnel lining deformations and collapse of the overburden soil. Specifically, a half-space tunnel was modeled, in which a strain-controlled stepper motor was used to withdraw a tunnel face at a constant displacement rate during centrifugation under a target acceleration level. In these experiments, the effects of overburden height, tunnel lining stiffness, and staged construction using the New Austrian Tunneling Method (NATM) were investigated. Relatively narrow shear bands were noticed in all of the collapse tests. Moments and shear forces measured within the tunnel lining indicate that construction staging has the greatest impact on tunnel lining behaviour. The results of the experiments are suitable for investigating whether the design approaches and partial safety factors suggested in EuroCode 7 are appropriate for ultimate state analysis of tunnel faces. Further, they are useful to identify whether modifications of the safety factors for soil and shotcrete lining are needed to achieve low and consistent failure probabilities. Introduction During construction of tunnels in soils or rocks using the New Austrian Tunneling Method (NATM), the tunnel face is the most susceptible to collapse. Collapse failures can lead to surface deflections and tunnel cave-ins, which can create a variety of problems for tunnels constructed in urban areas. Despite the high potential for loss of life and property, the safety factors currently used in tunnel design are mainly based on experience, with little basis in field measurements or validated modeling results. Previous work on centrifuge modeling of tunnels focused on face stability and face support measures (Meguid et al. 2008). With very few exceptions (Konig et al. 1991), centrifuge models used rigid tunnel lining and could not follow the soil deformation during spin-up of the centrifuge. Whereas the stress level in the lining is of minor importance in connection with excavation by means of a tunnel boring machine, the stability of the lining near the face is crucial in connection with cyclic excavation using shotcrete as primary support. To address the effects of the tunnel lining on face stability and to investigate strains and stresses in the lining, physical modeling experiments have been conducted in the geotechnical centrifuge at the University of Colorado in Boulder. Specifically, a half-space model was constructed in which a tunnel face is used to vary the pressure on the tunnel face during centrifugation under a target acceleration level. An acrylic lining was used in the model because the material properties are comparable to those of young shotcrete. The face displacements and strain distribution in the tunnel lining both during spin-up of the centrifuge to the desired g-level as well as during reduction of the face pressure were measured. Care has been taken that the boundary conditions of the lining allow displacements and deformations together with the soil while enforcing symmetry and boundary conditions close to reality. The experimental study also assessed the impacts of overburden, the length of an unlined section close to the face (where the shotcrete has not hardened yet), and the effects of staged excavation. In this paper some details of the model setup are described, and selected results are presented. Additionally, shortcomings of the current model are discussed and some improvements are suggested. The results presented in this paper will be used to validate three dimensional numerical models of soil-structure interaction in tunnels.