Micromechanics of failure in porous carbonate and volcanic rocks

Abstract

Laboratory experiments, microstructural observations and micromechanical modeling were conducted to investigate the micromechanics on porous carbonate and volcanic rocks. A suite of limestones, two blocks of Alban Hills tuff and three blocks of Mt. Etna basalt samples were chosen to study. Microstructural observations illustrate that pore collapse first initiates at the larger pores, and microcracking dominates the deformation around the pore surface in porous limestones. To capture these micromechanical processes, we developed a model treating the limestone as a dual porosity medium, with the total porosity partitioned between macroporosity and microporosity. While inelastic compaction is associated with pore collapse in limestones, development of dilatancy and brittle faulting was observed to relate to the initiation and propagation of stress-induced cracks in a compact rock. The Coulomb criterion is used extensively for describing the macroscopic development of shear fracture in a brittle rock. To gain insights into the physics of the Coulomb criterion, we derived analytic approximations for the empirical failure parameters with reference to the sliding wing crack model. Uniaxial and conventional triaxial experiments have been conducted on Alban Hills tuff and Mt. Etna basalt at room temperature. The phenomenological behaviors were observed to be qualitatively similar to that in a porous sedimentary rock. Synthesizing published data, we observe a systematic trend for both uniaxial compressive strength and pore collapse pressure of nonwelded tuff to decrease with increasing porosity. To interpret the compaction behavior in tuff, we extended the cataclastic pore collapse model originally formulated for a porous carbonate rock to a dual porosity medium made up of macropores and micropores or microcracks. Microstructural observations of the intact material of Mt. Etna basalt revealed the presence of thin cracks (probably formed during the rapid cooling of the lava) and quasi-spherical voids formed during degassing. The effects of water, phenocryst and porosity on mechanical behaviors of Etna basalt were systematically investigated. Micromechanical models were employed to elucidate the micromechanics of brittle failure and inelastic compaction.