Influence of carbonate microfabrics on the failure strength of claystones

Claystones are currently being investigated as a potential host rock for radioactive waste disposal. The relevant parameters are hydraulic conductivity, swelling and shrinkage, water uptake, rheological, and mechanical behavior. Much attention is paid to the Opalinus Clay (OPA) in Switzerland and the Callovo-Oxfordian Clay (COX) in France. One of the scientific challenges is to develop numerical models to predict the mechanical behavior of claystones under repository-like conditions. Recent studies have demonstrated the influence of carbonates on selected mechanical parameters (e.g. failure strength). In this study we show that OPA and COX behave differently with respect to their failure strength. The failure strength of OPA decreases with increasing carbonate content, whereas it increases with increasing carbonate content in case of the COX. The different behavior is caused by microstructural variations. Results of carbonate microfabric investigations verify that OPA mainly contains coarse-grained shell fragments, while COX shows a more homogeneous distribution of fine-grained, mainly non-biogenic carbonates. In conclusion, not only the carbonate content but in addition their grain size distribution, their shape and the spatial distribution control the mechanical behavior of both clays.