The paper analyzes the mechanical properties of a metamorphic rock mass (metabasite) at a prospective dam site. The formation is crossed by largely persistent and wavy major discontinuities filled with a cataclastic material. The structural setting was reconstructed through scanline surveys. Surveys included detailed measurements of local orientation of the discontinuity walls and infilling thickness. Since the mechanical behaviour of the rock mass is largely influenced by the infilling material, this was carefully sampled on site. Where sampling conditions were particularly favourable, undisturbed specimens were directly retrieved from the gouge material. Laboratory tests on the infilling material included: grain-size distributions and direct shear tests on both undisturbed and reconstituted specimens. A limited number of large-scale in situ shear tests on filled discontinuities were carried out. Field geophysical investigations and rock quality estimates through the survey of exposed rock faces are also reported and discussed. Significance of shear strength parameters at different scales has also been evaluated by comparing shear strength results from small scale laboratory tests and in situ tests.
Intense tectonic deformations can result in a pervasive shearing of the rock mass. Typically, a shear zone is characterized by a complex network of important discontinuities, which can interact with large engineering works, such as dam foundations. In this context, the overall mechanical behaviour of the rock mass exhibits a scale effect, whereby many relevant properties depend on the ratio between the characteristic size of the engineering structure and spacing of discontinuities.
Several case histories where the design and construction of a dam has been influenced by single extremely persistent discontinuities can be quoted (e.g. Duffaut 2013). Moreover there are cases where rock mass structure is formed by hard rock blocks interspersed in a cataclastic matrix (e.g. Scott Dam, Goodman & Ahlgren 2000), the so-called ‘bimrocks’ (block-in-matrix rocks) (Medley 1994). In the latter case the overall mechanical behaviour is primarily dependent on the contrast in strength between blocks and matrix and by the volumetric block proportion (VPB).
Intermediate between these two extremes are rock masses formed by rock blocks included in a cohesive matrix (e.g. Kusan-3 dam, Glawe & Upreti 2004), whose properties are dependent on the type and degree of cementation, but are also influenced by the geometry of the networks of faults filled by gouge material. Sometimes the blocks are arranged to form a relatively regular pattern. In general, the mechanical properties of the rock mass depend on some characteristics of the faults: spacing and orientation, geometry (roughness, waviness and persistence), type of gouge material and possible bonding at the interface between rock walls and infilling.
