Cataclastic fault rocks in underground excavations : a geological characterisation

The present study makes part of a research project on the geological and geomechanical characterisation of weak cataclastic fault rocks encountered in underground excavation sites in the Alps. The project is composed of a geological and a geomechanical part, split up in two PhD theses and realised in parallel at the Laboratory of Geology (GEOLEP) and the Laboratory of Rock Mechanics (LMR) at the Swiss Federal Institute of Technology in Lausanne. By their very contrasting and unfavourable mechanical behaviour in underground excavations, weak cataclastic fault zones influence directly the excavation method, the safety of the working site, the choice of tunnel support, the long-term behaviour and the costs of the infrastructure. The construction of galleries across cataclastic fault zones bears furthermore the potential risk to modify the local and regional hydrogeological regime. Weak cataclastic fault rocks show a large scatter of geological and geomechanical properties. Due to their low cohesion, they alter rapidly when exposed to weathering. They can hence be studied and sampled in natural outcrops and exploratory boreholes only with great difficulty. Analyses and laboratory tests are laborious and require special sample preparation techniques and equipments. For these reasons still little is known about their geological, hydrogeological and geomechanical behaviour. The purpose of this study is to develop a detailed and objective differentiation and characterisation method for weak cataclastic fault rocks. Classifications and terminologies of (weak) cataclastic fault rocks are in fact still controversial. After a review of fault rock definitions and rock deformation mechanisms, the term kakirite has been retained in agreement with Heitzmann (1985) and Wyder (1997) as a general designation for weak, cohesionless cataclastic fault rocks. This term has been approved recently by the Swiss Society of Engineers and Architects (SIA) to designate fault zones made up of "loose, structureless material" (SIA Norm 199, 1998). Cataclastic fault zones have been studied and sampled in galleries currently under construction in different geological and tectonic contexts in the Alps. Research has been concentrated mainly on fault zones occurring in quartzo-phyllitic host rocks. For comparison, fault rocks from greenstones and serpentinite have been analysed too. To improve the understanding of the geomechanical and hydrogeological behaviour of weak cataclastic fault rocks, a new quantitative characterisation method has been developed. In a simplified approach, the geomechanical behaviour of (fault) rocks is considered to be controlled by two factors, namely the: mineralogical composition (mwVh), and rock fabric (TC, MC) The proposed mineralogical and structural characterisation of cataclastic fault rocks is referring to the meso- to microscopic scale, respectively to a scale comparable with common geomechanical laboratory tests. Mineralogy is expressed quantitati