ABSTRACT:

Opalinus Clay is considered to be a potential rock mass for the disposal of high level nuclear waste in Switzerland. This relatively weak over-consolidated clay shale has several benefiting characteristics, like low permeability and the potential for self-sealing that make it a first priority candidate as a host formation in Switzerland. However, the low strength of the anisotropic rock and the associated ground behavior is apparent and meaningful for both, the constructability and the long term integrity of the disposal site. Observations made during a mine-by project in the Mont Terri Underground Rock Laboratory showed brittle rock mass behavior. In order to understand the observed failure mechanisms a detailed laboratory investigation has been initiated comprising servo-controlled direct shear and unconfined compressive strength tests.

1 INTRODUCTION

The argillaceous rock Opalinus Clay is considered as a potential host rock for nuclear waste disposal in Switzerland. This relatively weak over-consolidated clay shale has several benefiting characteristics, like low permeability and the potential for self-sealing that make it a first priority candidate as a host formation in Switzerland. However, the low strength of the anisotropic rock and the associated ground behavior is apparent and meaningful for both, the constructability and the long term integrity of the disposal facilities. In order to understand the mechanical behavior of Opalinus Clay the Mont Terri underground research laboratory (URL) has been constructed to perform full scale in-situ geomechanical testing for potential repository scenarios. In order to verify observed failure mechanisms during the mine-by experiment, an innovative laboratory investigation has been initiated comprising servo controlled direct shear and unconfined compressive strength tests (UCS).

2 METHODS
2.1 Sampling and sample preparation

Laboratory samples from the Opalinus Clay shaley facies were obtained with a 89mm diameter 28m long borehole utilizing triple tube core barrels with the air rotary drilling technique.

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