Abstract

In indurated clay formations, the Excavation Damage Zone (EDZ) around underground structures is of a brittle nature. Detailed knowledge is required concerning the structural inventory of the EDZ fracture network and the relevant hydro-mechanical phenomena and processes during excavation and the subsequent operations phase, which forms the geomechanical basis for the development of conceptual and numerical models of the EDZ. A synopsis of empirical and experimental evidence from the Mont Terri Underground Research Laboratory (URL) is provided, summarizing the key phenomena and processes associated with the creation and evolution of an EDZ in the Opalinus Clay. In particular, the significance of rock mass anisotropy is highlighted both as a likely origin for far-reaching pore fluid pressure changes and as a controlling factor in EDZ development during the excavation process.

1 Introduction

An Excavation Damage Zone (EDZ) represents a rock volume around a tunnel with significant hydro-mechanical modifications resulting from excavation works. These modifications may involve fracturing of the rock and an increase of flow and transport properties (Tsang et al. 2005). The creation and evolution of the EDZ is controlled by multiple factors, comprising rock properties and prevailing geological conditions (e.g. stress field, pore pressure) as well as engineering aspects, such as the applied excavation technique, tunnel geometry, tunnel support and ground interaction. In indurated clay formations, further phenomena and processes may affect the evolution of the EDZ during the operations phase. These include changes of mean effective stress due to drainage processes and tunnel ventilation, operations-related ground motion, and swelling of the rock mass during construction work due to wetting. For a radioactive waste repository, the requirements derived to warrant the long-term safety of backfilled underground structures impose limits regarding the acceptable size and intensity of the EDZ generated during excavation. Underground structures for radioactive waste therefore differ from conventional infrastructure.

Opalinus Clay is the preferred host rock for spent fuel and high-level radioactive waste and one possible host rock for long-lived and intermediate to low-level waste in Switzerland. It is a moderately to heavily overconsolidated claystone of Jurassic age with relatively uniform thickness of approximately 100±30 m across the Molasse Basin and the Jura Fold and Thrust Belt in Northern Switzerland.

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