Laboratory investigation of shear and compaction bands: compaction and dilation identification

Shear bands and compaction bands have been obtained in Vosges Sandstone cylinders tested in the laboratory under confining pressures of 30 to160MPa. The specimens have been heavily instrumented under load and subjected to pre- And post-deformation ultrasonic and Xray tomography. The primary purpose of this study is to add a detailed knowledge of the small-scale development of permanent dilational and compactional strains to our understanding of the mechanisms operative in shear/compaction band development and associated mechanical behaviours. This will inform predictions of changed fault zone permeability and lead to better prediction of fault seal or fault leakage under subsurface conditions. Samples were strain gauged and acoustic emissions were also recorded for some samples. Most were subjected to pre- And post-deformation ultrasonic and Xray tomography. Provisional assessment of the shear bands shows that material has densified but the velocities have also decreased. Open fractures are also seen on the processed Xray images. The samples show bulk compaction. The compaction band studies are less advanced, with acoustic emissions showing development of separated discrete bands that may have a shear component. This is an ongoing study and more robust conclusions, including thin section work, are expected by the Meeting.