Simplified convergence confinement analysis of tunnels in squeezing ground is often based, at the preliminary design stage, on an equivalent homogenized rockmass condition (expressed as an average GSI, for example) in the face and surrounding ground. This approach has significant limitations when the face is intersected by multiple rock units and discrete structures with contrasting properties or when the geology of the face is anisotropic. In this paper, typical face profiles from the Saint Martin-la-Porte edit (access tunnel for the Torino-Lyon LGV base tunnel project) are used to explore the differences in modeled response using the equivalent average material approach versus tunnel response analysis incorporating full representation of near-tunnel geology and geomechanical variability.
The St. Martin-la-Porte tunnel, located in southeastern France near the Italian border, is an edit to the TGV tunnel currently under construction that will connect Lyon and Turin.
Throughout excavation of this edit, a conventional static arch & shotcrete liner design experienced extensive failure due to a tunnel section with heterogeneous geology and extreme squeezing under anisotropic stress. Consequently, complex multi-stage liner with multiple sliding joints was designed and installed to control displacements. Typical closure analysis at an early design stage for such a tunnel would involve a homogenization of the tunnel geology and subsequent analytical or numerical analysis using an equivalent homogenous simulated rockmass. This edit study provides the opportunity to study the sensitivities and limitations of this conventional approach to closure analysis in highly heterogeneous ground. This paper presents a discussion of numerical modeling of three examples of geologically contrasting faces encountered during initial excavations at St. Martin-la-Porte. The geometry of the tunnel is simplified and an isotropic stress state is assumed in order to isolate the contribution of heterogeneity to tunnel deformation and liner response.