Localized failure modes in a compactant porous rock

Since dilatancy is generally observed as a precursor to brittle faulting and the development of shear localization, attention has focused on how localized failure develops in a dilatant rock. However, recent geologic observations and reassessment of bifurcation theory have indicated that strain localization may be pervasive in a compactant porous rock. The localized bands can be in shear or in compaction, and oriented at relatively high angles (up to 90 degrees) to the maximum compression direction. Here we report microstructural characterization of the spatial distribution of damage in failed samples which confirms that compaction bands and high-angle conjugate shears can develop in sandstones with porosities ranging from 13% to 28%. These failure modes are generally associated with stress states in the transitional regime from brittle faulting to cataclastic ductile flow. The laboratory results suggest that these complex localized features can be pervasive in sandstone formations, not just limited to aeolian sandstone in which they were first documented. They may significantly impact the stress field, strain partitioning and fluid transport in sedimentary formations and accretionary prisms. While bifuraction theory provides an useful framework for analyzing the inception of localization, our data rule out a constitutive model that does not account for the activation of multiple damage mechanisms in the transitional regime.