Geomechanica is launching a guest blog series, written by experts and numerical modellers in the field of rock engineering. In the first post of this series, we’re showcasing the work of Andrés Alcolea on “A pragmatic methodology to abstract the EDZ around tunnels of a geological radioactive waste repository – Application to the HG-A experiment in Mont Terri.”

Andrés Alcolea is a Civil Engineer and Hydrogeologist with more than 20 years of experience in numerical modelling of groundwater, contaminated aquifers and geothermal reservoirs. He is the Head of the “Groundwater and Geothermal” Division with TK Consult AG, a consulting company in Zürich (Switzerland).

The Excavation Damaged Zone (EDZ) around the backfilled underground structures of a geological repository represents a possible release path for radionuclides, and corrosion and degradation gases which needs to be addressed adequately in Safety Assessments (SA). The hydro-mechanical phenomena associated with the creation and temporal evolution of the EDZ are of high complexity, precluding the detailed representation of the EDZ in conventional SA modelling tools. Thus, simplified EDZ models able to mimic the safety-relevant functional features of the EDZ, are required.

In this context, TK Consult AG has developed a versatile and heuristic modelling approach with the goal of representing the creation and evolution of the EDZ in an abstracted and simplified manner. The key features addressed are the stochastic character of the excavation-induced fracture network and the self-sealing processes associated with the re-saturation after backfilling of the underground structures.

The methodology consists of three main steps (Figure 1). Firstly, using Geomechanica’s hybrid finite-discrete element method (FDEM) code, the geometry and geomechanical conditions of the discrete fracture networks forming the EDZ are simulated (Figure 1a). Secondly, the geometry and hydro-geomechanical properties simulated by the FDEM are mapped onto a finite element mesh (Figure 1b), which allows the fluid motion equations in the excavation near-field to be solved. A prominent feature of the methodology is that hydraulic parameters of both fracture and matrix evolve with time as a response to resaturation of the tunnel surroundings. Finally, an abstraction of the complex model is made upon the late time behavior (after full resaturation) of the system (Figure 1c). The main outcomes are:

• Spatio-temporal distributions of hydraulic parameters and corresponding specific fluxes towards the tunnel, with special emphasis on the late time behavior (i.e., the one relevant for SA).
• The abstraction of the EDZ at late times: a piece-wise homogeneous model with hydraulic behavior identical to that of the complex initial FDEM model is defined.

Figure 1: Concept of the EDZ abstraction process for Safety Assessment (SA) applicable for a circular tunnel: (a) Representative fracture patterns are simulated for relevant repository configurations with a discrete element model (FDEM by Geomechanica Inc.); (b) The discrete fracture patterns are converted to heterogeneous porosity and conductivity distributions; (c) In a final abstraction process, the heterogeneous porosity / conductivity distributions are converted to a shell defined by a radius and homogeneous porosity / conductivity.

The abstraction methodology relies on a good initial discrete representation of the fracture network. The methodology has been tested on a range of generic repository settings in the context of a sensitivity study, aimed at investigating the impact of repository depth and in-situ stress conditions on the hydraulic significance of the EDZ after repository closure. Finally, the model has been benchmarked with a data set from an in-situ self-sealing experiment at the Mont Terri Underground Rock Laboratory (URL), demonstrating the ability of the modelling approach to mimic the hydraulic response of the EDZ around a backfilled tunnel section during the re-saturation phase. Only two model parameters are calibrated: the rate of fracture closure and the skin factor around the tunnel. An example of the fit attained with such a simple model is displayed in Figure 2.

Fig. 2: (a) Pressure fits at observation borehole HG-A2 for simulations HG-A and HG-A-F; (b) number of closed fractures vs. time. In the inset, the discrete fracture networks.

The fitting could be improved by using a 3D extension of the modelling approach (on-going work) or highly parametrized sophisticated models. However, the fact that such a simplistic (and under parametrized) 2D model is capable of reproducing the main experimental trends can be considered a hopeful step towards the abstraction of the Excavation Damaged Zone.

Further reading:

• Alcolea, A., Kuhlmann, U., Lanyon, G.W. & Marschall, P. (2014): Hydraulic conductance of the EDZ around underground structures of a geological repository for radioactive waste – A sensitivity study for the candidate host rocks in the proposed siting regions in Northern Switzerland. Nagra Arbeitsbericht NAB 13-94, Nagra, Wettingen, Switzerland.