The transition from brittle faulting to cataclastic flow in porous sandstones: Mechanical deformation

Abstract

Abstract. Triaxial compression experiments were conducted to investigate the inelastic and failure behavior of six sandstones with porosities ranging from 15% to 35%. A broad range of effective pressures was used so that the transition in failure mode from brittle faulting to cataclastic flow could be observed. In the brittle faulting regime, shear-induced dilation initiates in the prepeak stage at a stress level C' which increases with effective mean stress. Under elevated effective pressures, a sample fails by cataclastic flow. Strain hardening and shear-enhanced compaction initiates at a stress level C* which decreases with increasing effective mean stress.The critical stresses C' and C* were marked by surges in acoustic emission. In the stress space, C* maps out an approximately elliptical yield envelope, in accordance with the critical state and cap models. Using plasticity theory, the flow rule associated with this yield envelope was used to predict porosity changes which are comparable to experimental data. In the brittle faulting regime the associated flow rule predicts dilatancy to increase with decreasing effective pressure in qualitative agreement with the experimental observations. The data were also compared with prediction of a nonassociative model on the onset of shear localization. Experimental data suggest that a quantitative measure of brittleness is provided by the grain crushing pressure (which decreases with increasing porosity and grain size). Geologic data on tectonic faulting in siliciclasfic formations (of different porosity and grain size) are consistent with the laboratory observations.

Shear Band Formation in Numerical Simulations Applying a Continuum Damage Rheology Model

“…16-20) could not be satisfied for m \ 0.2. Homogeneous compression and weak localization (or lack of it) in compression tests with high confining stress (i.e., [200 MPa and/or m & 0.2 and therefore n & n 0 ) are in agreement with experimental results (e.g., WONG et al, 1997;WU, 2000;MENENDEZ, 1996), with recent studies of compaction bands and compacting shear bands (CHALLA and ISSEN, 2004;ISSEN 2008) and with theoretical and numerical analysis of a coupled damage-porosity model (HAMIEL et al, 2004b(HAMIEL et al, , 2005. The conditions in which strain is not localized along shear-bands could represent a transition from brittle failure to distributed cataclastic flow in which compaction, pore collapse, strain-hardening and intensive grain crushing is dominant (WU 2000;MENENDEZ 1996).…”

Contractional deformation of porous sandstone: Insights from the Aztec Sandstone, SE Nevada, USA

“…Permanent deformation (plastic yielding) occurs when the stress path intersects the yield cap or the critical state line. The elliptical shape of the cap is defined experimentally (e.g., Zhang et al 1990;Wong et al, 1997;Grueschow and Rudnicki, 2005), and this experimental work has shown that its intersection P* with the horizontal p-axis (Fig. 10a) depends on grain radius R and porosity φ through the relation P*= (φR) -1.5 .…”

Micromechanical modeling of cracking and failure in brittle rocks

“…Studies have also been conducted on the micromechanics of brittle failure in porous rocks, specifically sandstone [Menendez et al, 1996;Wong et al, 1997]. These studies show that in sandstones, shear localization usually does not develop until after the peak stress has been reached.…”