Formation of parallel joint sets and shear band/fracture networks in physical models

Both oedometric and plane-strain tests were performed with parallelepipedic samples made of synthetic granular, cohesive, frictional and dilatant rock analogue material GRAM2. For the first time parallel sets of fractures that have all the characteristics of natural joints were reproduced in the laboratory. The fractures are regularly spaced, normal to sigma(3), and have plumose morphology very similar to that of natural joints. These fractures can form at tensile stress sigma(3) much smaller in magnitude than the tensile strength of material and even at slightly compressive sigma(3). When mean stress sigma exceeds a certain value, the fractures become oblique to sigma(1) (the obliquity increases with sigma), forming networks of conjugate shear bands/fractures. These results of plane-strain experiments are in good agreement with those of better controlled conventional axisymmetric tests on a similar material in Chemenda et al. (2011b) and are closer to real geological situations. Both types of experiments are complementary. Their results lead to the conclusion that at least certain categories of natural fractures (including joints, and conjugate shear fractures/bands) were initiated as deformation localization bands. The band orientation is defined by the constitutive properties/parameters (notably the dilatancy factor) that are sensitive to sigma. (C) 2011 Elsevier B.V. All rights reserved.