Characterization and DEM modeling of shear zones at a large dam foundation

Cataclastic rocks and fault gouge present highly variable properties, from soil-like materials to competent rock mass. In most cases the entire rock mass, or internal major shear zones, can be represented as a 'bimrock' (block-in-matrix rock), i.e. rock fragments of different shape and size, embedded in a finer matrix. In this paper a tectonized metamorphic rock mass in the site of a planned gravity dam is investigated. The dam foundation is crossed by numerous major discontinuities, often grouped to form wider shear zones and characterized by high persistency, waviness and abundant cataclastic filling. Systematic measurements of local orientation and filling thickness along the discontinuity traces made a detailed morphological analysis possible. The most frequent type of filling is a lightly cemented matrix-supported gouge. Undisturbed sampling of filling materials was successful only in some special cases. Laboratory, as well as in situ mechanical tests, were therefore difficult and not exhaustive. Micromechanical DEM modelling was instrumental in obtaining a more comprehensive framework of deformation and failure mechanisms of cataclastic materials. The mechanical properties of the particulate model were first analyzed by numerical simulation of biaxial and direct shear tests. The relationship between the overall constitutive parameters and micromechanical properties, such as inter-particle friction, contact stiffness, cement bond strength, volumetric block proportion and block shape, was determined. The complex interplay between the geometric features of a major discontinuity (joint-wall roughness and waviness) and the properties of the infilling bimrock were then investigated and finally the overall strength of the infilled discontinuity was analyzed.