Shear-enhanced compaction in dilating granular materials

• Compactive deformation band populations have low angles to maximum compression.
• Shear band orientation models predict that these bands should have formed in materials with initial positive dilatancy angles.
• Shortening across bands can occur during net volume increase of the material for a wide range of boundary conditions.
• Implications arise for interpretation of bands in the field, in theory, and in modeling applications.

Compaction of loose, granular materials commonly results in the loss of porosity and, hence, a volumetric decrease of the material by the localization of deformation bands. Different types of deformation bands tend to form at a range of specific angles to maximum compression (σ1), as documented in numerous field and laboratory studies. Usually, localization of bands occurs at low angles to σ1 during deformation involving volumetric increase and at higher angles during compaction of the material. Generally in agreement with field and laboratory orientation measurements, several models have been used to obtain the optimum angle deformation band formation with respect to σ1. However, some field and laboratory studies report deformation bands with substantial compaction across them that have orientations requiring material dilation to be in accord with those models. This discrepancy is explored by modeling the orientations of structures under progressive deformation for all combinations of simultaneous pure and simple shear. Our results allow for shear-enhanced compaction at the onset of dilational shear band formation, thus accounting for both the band orientation as well as the observed compaction within the bands. These findings indicate that compaction localized within a deformation band is not simply related to a total volumetric decrease of the material, since the transition between localization of compaction and dilation is found not to coincide with the transition of volume decrease to increase of the material, and hence to the material′s mechanical response at a given stress state.