MICROMECHANICS OF BRITTLE FAULTING AND CATACLASTIC FLOW IN BEREA SANDSTONE

The micromechanics of failure in Berea sandstone were investigated by characterizing quantitatively the evolution of damage under the optica l and scanning electron microscopes. Three series of triaxial compress ion experiments were conducted at the fixed pore pressure of 10 MPa an d confining pressures of 20, 50 and 260 MPa, respectively, correspondi ng to three different failure modes: shear localization with positive dilatancy, shear localization with relatively little dilatancy and dis tributed cataclastic flow. To distinguish the effect of non-hydrostati c stress from that of hydrostatic pressure, a fourth suite of hydrosta tically loaded samples was also studied. Using stereological procedure s, we characterized quantitatively the following damage parameters: mi crocrack density and its anisotropy, pore-size distribution, comminute d volume fraction and mineral damage index. In the brittle regime, she ar localization did not develop until the post-failure stage, after th e peak stress had been attained. The microcrack density data show that very little intragranular cracking occurred before the peak stress wa s attained. We infer that dilatancy and acoustic emission activity in the pre-failure stage are primarily due to intergranular cracking, pro bably related to the shear rupture of lithified and cemented grain con tacts. Near the peak stress, intragranular cracking initiates from gra in contacts and this type of Hertzian fracture first develops in isola ted clusters, and their subsequent coalescence results in shear locali zation in the post-failure stage. The very high density of intragranul ar microcracking and pronounced stress-induced anisotropy in the post- failure samples are the consequence of shear localization and compacti ve processes operative inside the shear band. In contrast, Hertzian fr acture was a primary cause for shear-enhanced compaction and strain ha rdening throughout the cataclastic flow regime. Grain crushing and por e collapse seem to be most intense in weakly cemented regions. Finite element simulations show that the presence of cement at grain contacts alleviates the tensile stress concentration, thus inhibiting the onse t of Hertzian fracture and grain crushing.