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 an d 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 wel l as their hydrogeological behaviour under natural conditions. Several cata clastic zones crossed by the galleries have been sampled and analysed. In t he first part of the paper, the complex cataclastic rock terminology is rev iewed. A simplified, schematic classification is proposed for engineering g eology purposes. Special techniques for sampling and treatment for tests an d analyses are described. The cataclastic zones encountered in a metamorphi c context showed rather irregular geometrical forms which explains the diff iculties in making precise geological prognosis. Most of these zones were o f fine-grained composition with an impermeable central part acting as a gro undwater 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 o ur study. (C) 1999 Elsevier Science B.V. All rights reserved.