Geotechnical characterisation of a weak sedimentary rock mass at CERN, Geneva

The European Organisation for Nuclear Research (CERN) in Geneva has extensive underground facilities, which were built over the past 70 years in a weak layered sedimentary rock called the red molasse. CERN has thus been continuously exploring its underground space and has gathered extensive geotechnical data from both laboratory and field tests. The data shows that the red molasse is composed of marls and sandstones forming 6 different geotechnical units with different geotechnical characteristics. The strength-stiffness relationship of the red molasse is lower than other molasses from other regions, and that the marls are significantly more ductile than the sandstones. Moreover, the intermediate rock units (sandy marls and marly sandstones) have similar strength but a different stiffness, a distinction which is not represented in the standard strength classification system. Although all rock units were subjected to the same diagenesis, one rock unit is found to be very weak with soil-like properties. A mineralogy analysis shows that this unit is composed of high plasticity clay, whilst the other marls units are composed of medium-high and low plasticity clay. The field tests show rapid and progressive transitions between the different rock units, which makes field prediction difficult. This paper presents an overview of the geotechnical data gathered by CERN as well as the geotechnical characterisation of the site The geotechnical characterisation presented in this paper also compares laboratory tests with field tests.