Abstract: Crack initiation stress is one of the key stress thresholds in the process of rock failure under compression conditions. Accurate determination of crack initiation stress is very important for characterizing mechanical properties of rocks and for evaluating the stability of underground excavations. At present, the ISRM has established a commission on rock spalling, and one of the important objectives of the commission is to develop suggested guidelines for determining the crack initiation stress of rocks. By analyzing and summarizing the previous approaches to identifying crack initiation stress of rocks, a new method based on accumulative acoustic emission (AE) hit number is proposed to determine crack initiation stress. Compared with other strain methods, a significant advantage of this developed method is to remove the user's subjective judgment. Using rock mechanics testing machine and AE monitoring system, uniaxial compression tests on granodiorite samples collected from the Tianhu rock block in Xinjiang Autonomous Region, have been conducted to obtain stress-strain relationships and corresponding AE data. According to the comparison of crack initiation stress values obtained from different methods, the effectiveness and reasonability of the proposed accumulative AE hit method are verified. Due to the fact that ISRM has not provided clear guidelines for establishing crack initiation stress of rocks, it is suggested that the strain in combination with AE measurements be used to identify this critical stress threshold of rocks subjected to compressive conditions.

Key words: rock mechanics, compression condition, crack initiation stress, acoustic emission measurement, accumulative acoustic emission hit number, geological disposal for high-level radioactive waste