Sequential growth of deformation bands in the laboratory

We investigate the formation and evolution of localised faulting in high po rosity sandstone by laboratory triaxial compression of intact 100-mm-diamet er core samples. Experiments were carried out dry, at constant confining pr essure (34 MPa), constant axial strain rate (5 x 10(-6) s(-1)) and increasi ng axial strain (1.5-11.2%). Tests generated fault zones consisting of sets of distinct pale granulated strands, separated by lenses of apparently und amaged host rock. The sets of strands were sub-parallel to the shear direct ion but showed complex anastamosing geometry in perpendicular section. The individual strands had reduced grain size, porosity and sorting compared to undeformed rock. A strong correlation was found between the number of stra nds occurring in a fault zone and the applied axial strain. Mean grain size , however, reached a steady value irrespective of axial strain. This implie s that a limited amount of strain is accommodated on each strand with furth er strain requiring new strands to form. However, no direct evidence for st rain hardening was observed in the post-failure macroscopic stress-strain c urves. Our laboratory induced deformation zones strongly resemble the key c haracteristics of natural deformation bands. We show the first laboratory e vidence for the sequential development of increasing numbers of discrete de formation bands with increasing strain. (C) 1999 Elsevier Science Ltd. All rights reserved.