Microstructural control on compaction localisation in granular materials

Field observations and laboratory experiments have demonstrated strain localisation can develop in porous rocks in response to an applied stress field. Shear fractures and compaction bands are strain localisation features that can form at relatively low confinement during brittle deformation and at higher confinement during shear-enhanced compaction, respectively. Previous experimental studies suggested that the formation and geometry of compaction bands also depends on the microstructural attributes of the rock.

We investigated the influence of microstructure on compaction localisation in porous rocks using sintered glass bead samples, which allowed for a tight control on grain size and shape and sample porosity. During the fabrication process, populations of solid glass microspheres of predetermined size and size distribution are heated above their glass transition temperature. Above this temperature, the glass beads act as viscous liquid droplets. Time-dependent coalescence of droplets that share contact then causes the bead-pack to evolve into a connected system, producing a porous granular material of known microstructural geometries and final porosity.

We previously conducted hydrostatic compaction and triaxial compression tests on synthetic samples of porosity ranging from 10 to 38% with a monodisperse grainsize (diameter ranging from 0.15 to 1.3 mm). Experimental results showed remarkable reproducibility for the same experimental conditions and concurrence with the phenomenology of mechanical behaviour of natural sandstones. After these validation tests, we conducted systematic experiments on monodisperse synthetic samples of 25 and 35% of porosity prepared using glass beads of mean diameter 0.25, 0.525 and 1.15 mm. Triaxial deformation tests were conducted on water-saturated samples, in drained conditions (with a fixed pore pressure of 10 MPa), at room temperature, at a constant strain-rate and at effective pressures corresponding to the regime of formation of compaction bands. Our mechanical data provide indirect evidence for compaction localisation. We have focused our attention on the influence of porosity and grain size on the formation and microstructural attributes (such as thickness, length and tortuosity) of the compaction bands.