Evolution of shear bands and cracks in multi-stage triaxial tests with water-saturated sandstone: A study of micro-tectonics with a fractal perspective

Abstract

A series of multi-stage triaxial compression tests revealed quite chaotic reactions of water-saturated Permotriassic sandstones on changing pore water pressures. Not before focussing the various test stages with respect to occurring critical phenomena, which strike the fabric structures due to their fractal nature during any triaxial testing, eventually an approach became tractable: Pore systems with cracks and shear bands can be widened in a quasi-static way, but only up to a stability limit. Thereafter the fabric collapses with a seismogenic contraction of the pore system causing a rise of the pore water pressure. Changes of structural patterns occur as formation either of fissures up to collapse with acoustic emission, or of a rather fractal pattern of crossing shear bands generated without audible noise but with fine grit due to comminution. Dilation and contraction of the pore system are visible from overall volume changes. Both modes of pattern formation can be understood as critical phenomena of the mineral solid. Scale-independent features of these changes of fabric patterns can be transferred to the meso- and macro-scales in the field and in the lithosphere, respectively. However, questions of scale-dependent features related to cohesion should be further investigated.