Microcrack Classification of Rock Salt Under Quasi-static Compression Using Acoustic Emission

Rock salt under a generalised state of stress is either under the compaction or dilatancy domain. The compaction domain is more favourable for engineering use than the dilatancy domain. This study explores the possible application of the AE technique to identify the favourable or unfavourable state of rock salt. The present study analyses AE data of high-halite percentage Khewra rock salt (> 95% halite mineral) under a quasi-static unconfined compressive strength test. The study employs a unique approach to identifying AE events using a detection function. It has the advantage over a conventional threshold-based method of isolating individual events from the short temporal burst of larger numbers of AE events. The source mechanisms of isolated AE events are identified using the AF–RA method. The results from the analysis conclude that 79% of AE events are due to tensile microcracks, and 21% of AE events are due to shear microcracks. Tensile microcracks release 85% of AE energy. The fracture source of AE events in the case of rock salts is unique, and it differs from the reported literature of other geomaterials where shear microcracks constitute a significant amount of AE events. In the present study, the results of the AF–RA analysis are divided into three stages. The stress path of the three stages is compared with the established dilatancy boundary of rock salts. It is observed for the Khewra rock salt that the stress path of Stage I closer to the compaction domain (favourable, stable) reveals that 42% of AE events originated from shear microcracks, and 58% of AE events originated from tensile microcracks. In the case of Stages II and III, the stress path is in the dilatancy domain (unfavourable, unstable); more than 80% of AE events sources originated from tensile microcracks. The results indicate that in-situ AE monitoring and its AE source mechanism identification can be used to identify rock salts’ favourable and unfavourable conditions.