Abstract

This study aims at strengthening the understanding of the mechanical sealing process of the excavation damaged zone (EDZ) in Opalinus Clay, an indurated claystone currently being assessed as the host rock for a deep geological repository in Switzerland. To achieve this goal, hybrid finite-discrete element method (FDEM) simulations are applied to the HG-A experiment, an in-situ test carried out at the Mont Terri underground rock laboratory to investigate the hydro-mechanical response of a backfilled and sealed microtunnel. The mechanical re-compaction of the EDZ is analyzed by accounting for an increase of swelling pressure from the bentonite backfill onto the rock. Simulation results indicate an overall reduction of the total fracture area around the excavation as a function of the applied pressure, with locations of ineffective sealing associated with self-propping of fractures.

1 Introduction

In the field of underground nuclear waste disposal, the excavation damaged zone (EDZ) is defined as a zone with hydro-mechanical and geochemical modifications inducing significant changes in flow and transport properties of the rock mass. A sound understanding of the processes involved in the EDZ formation and temporal evolution is necessary to increase the confidence in performance and safety assessment calculations of deep geological repositories. In the short-term, the EDZ is typically associated with an increase of flow permeability of one or more orders of magnitude. In the long-term, the EDZ will experience complex, time-dependent thermo-hydro-mechanical-chemical processes due to the interaction between the rock mass, buffer materials and heat-producing waste. Experimental data from laboratory and in-situ testing clearly show that sealing mechanisms occur in argillaceous rocks, including Opalinus Clay, leading to a reduction in the effective hydraulic conductivity of the EDZ with time (Bock et al. 2010). In this study, the mechanical re-compaction of the EDZ in response to radial stress acting on the excavation walls caused by swelling of the saturated bentonite buffer was numerically investigated. With its explicit consideration of fracturing processes, a hybrid finite-discrete element (FDEM or FEMDEM) simulation approach was applied to the HG-A experiment. The HG-A in-situ experiment was carried out at the Mont Terri underground rock laboratory (URL) to investigate the hydro-mechanical response of a backfilled and sealed microtunnel (Marschall et al. 2006).

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