Brittle spalling is the failure mechanism that is commonly observed in the walls of underground excavations in rocks with high ratio of compressive to tensile strength (brittle rock). Extensile crack damage that is controlling spalling in brittle rock is rarely capable of leading the failure mechanism of the rock in laboratory strength tests due to geometric constraints within the test. The onset of crack damage thresholds in brittle rock can be identified by acoustic emission monitoring or rigorous strain measurements during a laboratory compressive test. Crack initiation (CI) and crack propagation (CD) stresses correspond to long-term and yielding (short-term) in situ strength of the rock. The repeatability and subjectivity of interpretation of the existing algorithms for calculation of crack damage thresholds is evaluated through inter-laboratory comparisons for Smaland granite and since initially these algorithms were all developed for granitic rocks, their accuracy and robustness is determined for Lindsay Cobourg limestone as a sedimentary rock. This paper presents the latest guidelines that are to be included in the ISRM “Suggested Method” for defining damage thresholds in brittle ro


Brittle spalling is a failure mechanism that occurs in the walls of underground excavations in rocks with low ratio of tensile to compressive strengths (brittle rock). The same concept is true for moderately jointed rockmasses at great depth where the pre-existing fractures tend to close as the result of increase in confinement [1]. The overall progress of failure in this type of rocks is dominated by the brittle failure of the intact rock rather than inter-block shearing. The rock strength criteria derived by rock classification systems such as GSI [2] are well adapted to rockmasses that are categorized as the mid-range of classification systems and in which the failure is controlled by inter-block shear failure [3].

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