Localization of dilatancy in Ohshima granite under constant uniaxial stress

Abstract

The localization of dilatancy during creep of Ohshima granite under uniaxial compression was observed. Hypocenters of 3933 acoustic emission (AE) events were accurately located. It was found that the mechanical behavior of Ohshima granite was controlled by the localization of microcracks. During the stage of loading up to the creep stress, which is 83% of the average short‐term fracture strength, the hypocenters of AE events were randomly distributed throughout the specimen. As soon as the primary creep began, abrupt migration and clustering of AE hypocenters into several near‐surface zones were observed. AE events formed volumetric concentrations. This migration and clustering strongly suggested the rapid localized development of dilatancy at the very beginning of the primary creep stage. The distribution of AE hypocenters observed in this stage was unchanged until final faulting. By the end of the primary creep stage, AE events began to concentrate into one of these clusters, while the activities of other clusters gradually decayed. This change spread broadly and continuously in time during the creep. In the most active cluster, clustering of AE events by itself gave rise to more AE events. The shape of this cluster was spheroidal with the long axis parallel to the loading axis. No evidence directly related to planar focusing of dilatancy was found. Surface strains were mapped. The axial strain distributions in the loading interval showed that the state of stress within the sample was homogeneous. A large change in both axial and circumferential strain fields occurred during the early stage of the primary creep. After this drastic change, the pattern of strain distribution remained unchanged in the subsequent stage of the creep. The accelerated increase in one of the circumferential strain gauges during the tertiary creep stage showed strongly localized deformation preceding faulting. The development of localized dilatancy identified by hypocenter locations was confirmed by the surface strain mapping. The position where the anomalous acceleration in circumferential strain was observed was close to the active, remaining cluster. The migration and clustering of AE hypocenters to the circumferential surface at the beginning of primary creep in the uniaxial unjacketed specimen were explained in terms of stress corrosion. Since the surface is the most favorable for accessing atmospheric moisture, stress corrosion in the vicinity of the surface was facilitated.