Abstract

Numerous research studies have shown that clay rock formations exhibit favourable characteristics for the deep geological disposal of radioactive waste. However, one main concern is that the favourable long-term isolation properties of the intact rock mass could be negatively affected by the formation of an excavation damaged zone (EDZ) around the underground openings. The goal of this paper is to present new rock mechanics insights into the phenomena involved in the EDZ formation and evolution processes in Opalinus Clay, a clay shale currently being assessed as a potential host rock for an underground repository in Northern Switzerland. More specifically, discontinuum-based computer simulations, using a hybrid finite-discrete element method (FDEM) approach, were applied to two case studies considering circular excavations in an anisotropic Opalinus Clay formation. In the first case study, numerical modelling was applied to the HG-A experiment, a long-term in-situ test carried out at the Mont Terri underground rock laboratory to investigate the hydro-mechanical response of a backfilled and sealed microtunnel. A mechanistic analysis of the fracturing processes involved in the EDZ formation was carried out. In the second case study, the long-term effect of the bentonite backfill swelling on the EDZ around an emplacement was numerically analyzed. Overall, the simulation results highlight short-term distinctive rock failure kinematics due to the bedded rock structure, as well as a long-term reduction of the excavation-induced fracture porosity in response to mechanical loading.

Introduction

Clay shales are currently being assessed as host rocks for the underground disposal of radioactive waste. The key characteristics that make these argillaceous rocks suitable for nuclear waste storage include: very low hydraulic conductivity, low diffusion coefficients, high retention capacity for radionuclides, and self-sealing of fractures (Blümling, Bernier, Lebon, & Martin, 2007). However, the formation and temporal evolution of an excavation damaged zone (EDZ) around the underground structures, including emplacement tunnels, shafts, and caverns, need to be considered when assessing both short- and long-term safety of an underground repository. Recent research efforts have been focusing on the development and application of a discontinuum numerical model, based on the hybrid finite-discrete element method (FDEM), to investigate the formation and development of the EDZ in Opalinus Clay, a shale formation selected to potentially host a geological repository in Northern Switzerland. In this context, the goal of this study is to gain new rock mechanics insights through the explicit simulation of fracturing processes in two case studies in Opalinus Clay.

In the first case study, the EDZ formation process around the HG-A microtunnel at the Mont Terri underground rock laboratory (URL) was considered (Marschall, Distinguin, Shao, Bossart, Enachescu, & Trick, 2006). The HG-A experiment is a long-term, in-situ test carried out to investigate the hydromechanical response of a microtunnel representing, at a 1:3 scale, a backfilled seal section of an emplacement tunnel for High Level Waste (HLW) according to the Swiss concept for a geological HLW repository. As shown in Figure 1, the EDZ around the microtunnel is characterized by the formation of two distinct breakout zones at two symmetric locations where the bedding planes are approximately tangent to the excavation boundary.

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