The rock mechanical behavior of opalinus clay – 20 years of experience in the mont terri rock laboratory

Abstract

Monitoring the behavior of the rock mass combined with a sound knowledge of rock mechanical properties is of great importance. Already in the very beginning of the Mont Terri rock laboratory in 1996, rock samples were extracted for the assessment of elastic parameters. The transverse isotropic Opalinus Clay exhibits a strong dependence on its orientation with respect to the applied load. Furthermore, rock strength is dependent on water content/suction and microcracking due to desiccation and/or mechanical load. Swelling potential, very low hydraulic conductivity and non-linear temporal behavior are further important properties of Opalinus Clay, which require well-defined sampling procedures, sample conditioning and test setup in the lab. Monitoring deformation and pore water pressure of the temporal behavior of galleries and niches prior, during and after excavation is standard procedure in the Mont Terri rock laboratory. Besides classical survey of deformation as well seismic, and geoelectric methods are applied in order to assess sedimentary heterogeneity, tectonic discontinuities and the extent of the excavation damaged zone (EDZ). Swisstopo has furthermore developed a technique based on resin impregnation, which allows for characterizing geometry, properties and temporal behavior of the EDZ. These datasets allow for establishing and continuously improving conceptual models of the rock mechanical situation and behavior in the Opalinus Clay. Conceptual models and long-term datasets are important for the calibration and validation of HM coupled models. In the framework of the Mont Terri Project, several numerical models have been developed and tested for the specific use in consolidated shales, which are important instruments for future predictions.