Micro-texture and petrophysical properties of dilation and compaction shear bands in sand

Abstract

We have studied micro-textural and petrophysical properties of dilation and compaction shear bands (zones) created during deformation of two quartz sands (Ottawa and Hostun sands), differing in angularity, under triaxial compression experiments. Image processing of in-situ CT-scan images taken during the experiments allowed both qualitative study of micro-texture, and quantitative study of porosity and dilatancy. Our results indicate that, in both sands at low confining pressure, i.e. 100 kPa, a zone of enhanced porosity (dilation shear band) is initiated with dilatancy factor ∼0.5–0.6, and towards the end of the tests the porosity continuously increases, especially at the center of the zone (from ∼33% to ∼45% for Ottawa sand and from ∼40% to ∼52% for Hostun sand). However at high confining pressure (7000 kPa), the tested sands reveal different dilatancy and porosity evolution. In rounded Ottawa sand, first a wide dilated zone (dilatancy factor ∼0.26) forms, and by progressive shear loading, a compacted zone is developed inside this dilated zone (dilatancy factor ∼−0.11). For the angular Hostun sand the dilatancy is negative both at the initiation and the final stage, which is reflected by a wide zone of compaction.