A Field and Textural Study of Failure Modes in Granular Rock and Their Distribution In Space and Time

Stanford University, Stanford CA

Investigators

Abstract

Failure modes of sand and sandstone play an important role in construction of engineering structures, optimization of water resources and related contaminant remediation, production of oil and gas, well bore stability analyses, and earthquake hazard mitigations. Two distinct failure modes are recognized in terms of their geometry and micromechanics: sharp planar discontinuities and tabular zones of localized deformation. The tabular zones of localized deformation include shear, compaction, and dilation bands and are the focus of this study. Tabular failure structures in unconsolidated sand and sandstone from a variety of depositional, burial, and regional tectonic settings are mapped and sampled. These samples are studied using thin-section petrography, scanning-electron microscopy, and X-ray computerized tomography to characterize the failure modes and the associated grain fracture and porosity variation. The relationships between grain size, porosity, cementation, loading conditions, burial depth and deformation history on the failure processes are then established. This study is designed to constrain the effect of the environmental and mechanical conditions on the natural failure modes of unconsolidated sand and sandstone by comparing them with those of experimentally derived constitutive laws and those predicted by mathematical and numerical models. The ultimate objective of this study is to provide a predictive tool for the failure behavior of granular rock in various environments, which is so relevant to many societal issues.