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Influence of confining pressure on the mechanical and structural evolution of laboratory deformation bands

We present experimental observations of the influence of confining pressure on the mechanical behavior and structural style of damage in porous quartz rich sandstones. Large (100‐mm diameter) samples of sandstone are deformed in a triaxial deformation apparatus, resulting in deformation expressed as pale interweaving bands of granulated material with finite shear offset and associated microcracking. The deformation fabrics evolve systematically from localized to more pervasive geometries with increasing confining pressure, associated with a systematic reduction in dynamic stress drop. The initial failure envelope is consistent with Mohr‐Coulomb frictional behavior for all tests. These mechanical and structural observations confirm a gradual transition between brittle and semi‐brittle behavior below the threshold for bulk cataclastic flow in a porous granular medium. Our experiments demonstrate that deformation band formation is strongly pressure‐sensitive. The resulting structures are likely to have a strong anisotropic influence on permeability.