Evolution characteristics of acoustic emission and strain energy for deep granite under different damage stages

Abstract

The first underground research laboratory (URL) as the research platform for the deep geological disposal of high-level radioactive waste (HLW) in China is being built in a deep granite rock mass. The damaged characteristics and mechanical properties of the deep granite under loading seriously influence the URL’s construction safety and disposal function for HLW. Therefore, this paper investigates the damaged characteristics and mechanical properties of the Beishan deep granite used in various confining pressure levels through a series of triaxial compression and acoustic emission (AE) tests. Based on the crack volumetric strain model, the crack stress thresholds of the deep granite specimens under various confining pressure levels were acquired. The damage development of the specimens used in five confining pressure conditions was analyzed, and the evolution of AE parameters and strain energy of the deep rock under different damage stages were explored. The studied results shown that the crack stress thresholds of the deep granite rose linearly with the increasing confining pressure, but their stress ratios remained relatively constant under various confining pressure levels. Under the rock loading tests, the designed confining pressure levels influenced the AE hit count and AE accumulative count development under the rock damage process. The high-frequency points of the AE hit count coincide with the AE accumulative count increasing, and the maximum value of the AE hit count of the specimens used in different confining pressure conditions appears in the postpeak stress‒strain stage. Additionally, with the axial strain of the loading rock rising in the prepeak stress‒strain stage, all the strain energy (including total energy U, elastic energy Ue and dissipated energy Ud) continues to rise, and in the postpeak stress‒strain stage, Ue stored inside the rock is quickly released, resulting in the Ue/U decreasing and Ud/U increasing.