Sequential growth of deformation bands in the laboratory

We investigate the formation and evolution of localised faulting in high porosity sandstone by laboratory triaxial compression of intact 100-mm-diameter core samples. Experiments were carried out dry, at constant confining pressure (34 MPa), constant axial strain rate (5 x 10(-6) s(-1)) and increasing axial strain (1.5-11.2%). Tests generated fault zones consisting of sets of distinct pale granulated strands, separated by lenses of apparently undamaged host rock. The sets of strands were sub-parallel to the shear direction 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 strands occurring in a fault zone and the applied axial strain. Mean grain size, however, reached a steady value irrespective of axial strain. This implies that a limited amount of strain is accommodated on each strand with further strain requiring new strands to form. However, no direct evidence for strain hardening was observed in the post-failure macroscopic stress-strain curves. Our laboratory induced deformation zones strongly resemble the key characteristics of natural deformation bands. We show the first laboratory evidence for the sequential development of increasing numbers of discrete deformation bands with increasing strain. (C) 1999 Elsevier Science Ltd. All rights reserved.