Cataclastic fault rocks in underground works: project AlpTransit-Lötschberg and other sites in the Alps

Weak cataclastic fault rocks (kakirites) in underground works have been studied in order to improve the knowledge about their geological, hydrogeological and geomechanical properties. A new quantitative method is proposed to differentiate fault rocks in regards of their microscopic rock fabric. It is based on a combined clast-matrix model which has proven to be appropriate to classify a wide range of structural features. Two opposite fabric types are distinguished. First: a granular fabric, bearing clasts in an homogeneously fine-grained matrix; secondly a schistose fabric, characterised by dense discontinuity systems but bearing no clasts. Important mineralogical variations of quartz and sheet silicate proportions between kakirites, host rocks, sieved clast- and matrix-fractions have been observed. The mineralogical fractionation is the result of the mineral dependent cataclasis, favoured further by the schistose fabric of the host rocks. High contents of phyllitic minerals in the matrix can explain the often very low permeability of cataclastic fault zones.