Particle-size distribution and microstructures within simulated fault gouge

Abstract

This paper presents an investigation of comminution mechanisms and microstructure development within simulated fault gouge. We sheared 4.0 mm thick layers of quartz sand between rough steel surfaces using a triaxial apparatus. The layers were sheared at constant effective normal stress of 100 MPa, under saturated drained conditions, and at 45° to the axis of cylindrical steel samples. Porosity changes were measured throughout shear and microstructural observations were carried out on the deformed layers. Two types of load paths were investigated for shear strains (γ) between 0 and 3.3; either the shear stress was repeatedly cycled from zero to failure or the sample was sheared in a single-load cycle. Multiple-cycle experiments exhibit significantly more compaction than single-cycle experiments deformed to similar strains. Gouge layers from both sets of experiments contain oblique zones of localized shear (Riedel shears bands) after γ = 1.3–1.5. Gouge particles obey a fractal size distribution for the range 12.5–800μm; i.e. particle density vs size follows a power law, N( n) IA = bn - D where N(n) is the number of particles in a size range, A is the area examined, n is the mean of the size range, b is a constant and D is the fractal dimension. D for particles within the bulk material increases with shear strain for γ < about 1.5 after which it remains 2.6 ± 0.15. This value of D agrees with that found for natural fault gouge and with that predicted by a comminution model in which fracture probability depends on the relative size of nearest-neighbor particles. Analyses of particles within shear bands indicate continued size reduction after γ = 1.5. These particles do not obey a fractal size distribution for the range 6.25–100 μm due to a lack of particles larger than 25–50 μm. The rate of comminution within the bulk layer decreases at about γ = 1.5, which coincides with the onset of shear localization. Our data indicate that comminution is driven by relative movement between particles and that gouge layers attain a steady state particle-size distribution at γ = 1.5. The porosity—strain data and microstructural observations show a correlation between the onset of shear localization and the rate of dilatancy with shear strain.