Influence of pre-existing discontinuities on ground behavior in the Opalinus Clay – a new mine-by project at the Mont Terri rock laboratory

The Opalinus Clay can be characterized as an overconsolidated clay formation consisting primarily of thinly laminated, silty and sandy shales, which can be grouped into three main facies: the shaly facies, the sandy facies and the carbonate-rich sandy facies (Thury & Bossart 1999). Detailed geological mapping in the Mont Terri rock laboratory revealed three different tectonic fault systems: a bedding-parallel SSE dipping fault system, a low angle SSE to SW dipping fault system, and a N to NNE trending fault system (Bossart & Thury 2008). The most significant tectonic structure identified in the rock laboratory is a 0.8 to 2 m thick fault zone associated with the bedding-parallel SSE dipping fault system. Most of these fault planes show slickensides indicating a thrusting movement in NNW direction (Bossart & Thury 2008).