Strengthening and stress relaxation of Opalinus Clay

Abstract

In the near-field of underground openings the rock will undergo damage. Closure and re-establishment of the initial rock integrity may be assisted by long-term creep of the un-disturbed clay stone in the far-field, thus promoting self sealing processes in the dilated clay-portions by convergence. Therefore, the understanding of creep is essential for the assessment of the long-term post-closure behaviour in a waste repository. Stress relaxation testing on pre-strained Opalinus Clay specimens from the Mont Terri Rock Laboratory was performed to check if a time dependent, though non-dilatant deformation is present. The stress relaxation behaviour of the four tested specimens was investigated during triaxial loading using different sample orientations, i.e. referring to the bedding, and under drained or und-drained conditions, respectively. To suppress artefacts as far as possible, e.g. due to sampling and rock preparation, several loading cycles were performed. After these repeated turns of loading, the stress relaxation was found to be independent from loading geometry or draining anymore. In general, the normalized stress relaxation curves plot in a narrow band. The experimental results are discussed on the basis of common equations for the stress–strain-behaviour which may be appropriate to model the recorded time dependent stress relaxation.