Modeling of damage, permeability changes and pressure responses during excavation of the TSX tunnel in granitic rock at URL, Canada

Abstract

This paper presents numerical modeling of excavation-induced damage, permeability changes, and fluid-pressure responses during excavation of the TSX tunnel at the underground research 2 laboratory (URL) in Canada. Four different numerical models were applied, using a wide range of approaches to model damage and permeability changes in the excavation disturbed zone (EDZ) around the tunnel. Using in situ calibration of model parameters the modeling could reproduce observed spatial distribution of damage and permeability changes around the tunnel, as a combination of disturbance induced by stress redistribution around the tunnel and by the drill-and-blast operation. The modeling showed that stress-induced permeability increase above the tunnel is a result of micro and macrofracturing under high deviatoric (shear) stress, whereas permeability increases alongside the tunnel as a result of opening of existing microfractures under decreased mean stress. The remaining observed fracturing and permeability changes around the periphery of the tunnel were attributed to damage from the drill-and-blast operation.Moreover, a reasonably good agreement was achieved between simulated and observed excavation-induced pressure responses around the TSX tunnel for 1 year following its excavation. The simulations showed that these pressure responses are caused by poroelastic effects as a result of increasing or decreasing mean stress, with corresponding contraction or expansion of the pore volume. The simulation results for pressure evolution were consistent with previous studies, indicating that the observed pressure responses could be captured in a Biot model using a relatively low Biot-Willis' coefficient, α ≈ 0.2, a porosity of n ≈ 0.007, and a relatively low permeability of k ≈ 2×10 -22 m 2 , which is consistent with the very tight, unfractured granite at the site.

Elastic–Brittle–Plastic Behaviour of Shale Reservoirs and Its Implications on Fracture Permeability Variation: An Analytical Approach

“…Note that all other parameters were the same as those in Rutqvist et al (2009) andSouley et al (2001), where numerical models were employed to replicate these measurements. It should be noted that the estimated fracture compressibility in this paper is within the range of values reported by Rutqvist et al (2009). Although this case is simplified here and many processes are neglected, the good agreement between the model and field data provides a verification of the accuracy of the proposed model.…”

Deep Borehole Field Test Research Activities at LBNL

“…The first alternative has been done in practice by Rutqvist et al (2009) as shown in Figure 4.4.2. In this case a model involving how permeability changes with deviatoric (shear) and mean effective stress for competent (intact) granite.…”

A multiple-code simulation study of the long-term EDZ evolution of geological nuclear waste repositories

“…The parameters in Equation (1) were calibrated to produce reasonable permeability changes as observed near excavated tunnels, in particular at the URL, in Canada, leading to a set of parameters presented in Table 4 (Rutqvist et al 2008c). Figure 5 shows substantial changes in permeability near the top of the tunnel, caused by large deviatoric strain.…”