Initiation and propagation of compaction bands in a porous sandstone with pre-induced shear bands

Compaction bands in porous sandstones have been described as tabular zones of localised deformation that accommodate pure compaction, with no macroscopic evidence of shear. These deformation bands are formed normal or subnormal to the maximum principal stress direction and are accompanied by localised porosity loss. The involved micro-processes are mainly characterised by grain crushing and pore collapse. To gain more insight into the onset and propagation of such deformation structures, so far, a series of experimental studies has been carried out in porous sandstone specimens. In some experiments samples contained notch acting as stress concentrator and, thus, localising the onset of compaction band formation. Here we investigate the impact of a laboratory pre-induced shear band in a Bentheim sandstone specimen on the subsequent compaction band onset and propagation.
Bentheim sandstone has a porosity of 22%. The specimen used in this study had a diameter of 50 mm and a length of 125 mm. Triaxial compression experiments were performed (at GFZ) using a servo-hydraulic loading frame from Material Testing Systems (MTS). Ultrasonic transmission signals and Acoustic Emissions (AE) were recorded throughout the duration of the tests using eighteen P-wave piezoelectric sensors, glued directly on the surface of the specimens and two P-wave sensors incorporated into the top and bottom caps. Moreover, two strain-gages were used to measure vertical displacements. The triaxial compression experiments were performed in two stages: a. isotropic compression with confining pressure increasing up to 20 MPa followed by loading in axial direction using an Acoustic Emission control and then by axial unloading; b. isotropic compression with confining pressure increasing from 20 MPa up to 185 MPa and subsequent loading in axial direction using a displacement control rate of 20 μm/min, followed by a full unloading of the specimen.
AE waveforms and ultrasonic signals were automatically discriminated after the complete experiment. P-wave onset times were picked and AE locations were calculated (4D), considering time-dependent variations in P-wave velocities and employing an anisotropic heterogeneous ultrasonic velocity model, consisting of five horizontal layers. AE events were classified as tensile, shear, and compressive.
4D AE locations indicated that during the first loading stage (a) a shear band developed from the top to the mid-height of the specimen accompanied by a reduction in P-wave velocities. The occurred micro-mechanisms included both shear and compressive events. During the second loading stage (b) a compaction band developed from the tip of the shear band; the latter acting as stress concentrator. P-wave velocities were also decreased in this stage, but in smaller absolute values compared to the previous stage of the shear band formation. During the compaction band onset and propagation the occurred micro-mechanisms were principally characterised by compressive events.