Spalling is a failure mechanism which occurs in the walls of underground excavations in rocks with low ratio of tensile to compressive strength (brittle rock). This damage and subsequent failure occurs before induced stress reaches the peak laboratory strength of the rock. While commonly observed in excavation walls, spalling does not always occur in laboratory strength tests due to geometric constraints within the test. However, by monitoring acoustic emission/microseismic activity or meticulous strain measurement during a laboratory compressive test, the onset of crack damage thresholds for the rock can be identified. Crack initiation (CI) and crack propagation (CD) stresses correspond to long-term in situ strength and yielding (short term) strength of the rock respectively. The International Society for Rock Mechanics (ISRM) Commission on Rock Spalling aims to develop an accurate suggested methodology for CI and CD measurement based on acoustic data and measured strain. To evaluate the rationality or subjectivity of the existing algorithms four core sets of the same rock type (Smaland granite from Forsmark, Sweden) was tested in four laboratories in different countries. The estimated CI and CD for Smaland granite by using acoustic data and/or measured strain is discussed in this paper. In addition the repeatability and subjectivity of interpretation for each method will be discussed as basis for a new suggested method.


The failure of more blocky rockmasses can also shift to a brittle phase due to the closure of pre-existing fractures in the rockmass as the result of the rise in confinement as the depth increases (Martin et al 2001). In rockmasses that are categorized as the mid-range of the rock classification systems where their failure is controlled by inter-block shear failure, the rock strength criteria derived by classification systems such as GSI are well adapted (Carter et al 2008).

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