Cataclastic rocks in underground structures—terminology and impact on the feasibility of projects (initial results)

Strongly tectonized zones of cataclastic rocks can exert great influence on excavation works, on long term behaviour of underground infrastructures and on local and regional hydrogeological conditions. The construction of two large galleries in the Swiss Alps offers the possibility of analysing the geological composition and structural characteristics of these zones as well as their hydrogeological behaviour under natural conditions. Several cataclastic zones crossed by the galleries have been sampled and analysed. In the first part of the paper, the complex cataclastic rock terminology is reviewed. A simplified, schematic classification is proposed for engineering geology purposes. Special techniques for sampling and treatment for tests and analyses are described. The cataclastic zones encountered in a metamorphic context showed rather irregular geometrical forms which explains the difficulties in making precise geological prognosis. Most of these zones were of fine-grained composition with an impermeable central part acting as a groundwater flow barrier. An example of a large cataclastic zone crossed by a gallery is presented at the end of the paper with some initial results of our study.