Influence of carbonate content and micro fabrics on the failure strength of the sandy facies of the Opalinus Clay from Mont Terri (Underground Rock Laboratory)

The relationship between micro fabric, failure strength, and mineralogical composition is studied on representative samples of the sandy facies of the Opalinus Clay (Mont Terri). An approximately 7 m long drill core of the bore hole BLT-A2 (sandy facies, Mont Terri) was investigated on different scales, mainly with respect to carbonate distribution. analysis showed that the carbonates of the sandy facies mainly occur as i) matrix which in turn acts as cement, ii) in the fine sand fraction, and iii) as biogenic carbonates being present in minor amounts. Mechanical testing indicated a clear relation: the failure strength increased with increasing carbonate content. The carbonates of the sandy facies, therefore, appear to be similar to the carbonates of the Callovo Oxfordian Clay with respect to their possible influence on failure strength.