Effects of bedding and foliation on mechanical anisotropy, damage evolution and failure mode

Abstract

AbstractIn this study we review recent advances in our understanding of anisotropy in rocks, focusing on dilatant and compactant failure in sandstones and in a foliated metamorphic rock. In sandstones, the anisotropy can be associated with bedding, as in the Rothbach sandstone, or it can also be due to shape anisotropy of the grains and/or the pores, as in the Bentheim sandstone. Two scenarios are proposed for the development of anisotropy in these two end members. In a metamorphic rock with strong foliation like the Four-mile gneiss, it has been commonly observed that the brittle strength is minimum at a foliation angle of about 30°–45°. A damage mechanics model is proposed that underscores the dominant role of biotite foliation in the development of microcracking. In contrast it is often observed in sandstones with strong bedding that the strength is minimum in the direction parallel to bedding. New results for the Rothbach sandstone showed that compared to parallel-to-bedding samples: (i) in the brittle faulting regime the perpendicular-to-bedding samples have both a higher strength and dilatancy stress; and (ii) in the cataclastic flow regime the compactive yield envelope for the perpendicular-to-bedding samples expands significantly towards higher stress values.