The ISRM Commission on Spalling Prediction is tasked with standardizing a practical approach to the prediction of spall initiation, propagation extent and overbreak. This process begins with accurate and repeatable determination of critical limits obtained from lab testing. The first priority is to provide guidance for obtaining critical data from existing testing such as the UCS test. This data includes CI (Crack initiation threshold) and CD (Crack damage threshold). This paper outlines the new recommended terminology and procedures for measuring these two key thresholds. Spall potential as indicated by the ratio of compressive to tensile strength is discussed as are a number of ongoing initiatives of the Commission.


The process of stress induced rock spalling leads to damage and overbreak in tunnels at depth in hard rocks (low porosity). Spalling is the result of visible extension fractures under compressive loading near the boundary of an excavation. Spalling is one mechanism that can lead to bursting (high-energy release during rock failure) although spalling can be violent or non-violent and in some cases can be time-dependent. In unsupported conditions and under an anisotropic in situ stress field, the process of spalling can form notch-like geometries. Extensive research over the last few decades, as summarized by Martin (1997), Kaiser et al. (2000) and by Diederichs (2003,2007), has demonstrated that brittle non-porous rocks exhibit an maximum long term tunnel wall strength of no more than half of the laboratory test value for unconfined compressive strength (UCS). As shown in Figure 1, below the lower bound field strength envelope (CI) no damage occurs within the rock although stress changes occur (Excavation influence Zone, EIZ). Above this envelope, micro-cracks initiate at the grain scale. Upper bound strength (CD envelope) is controlled by shear fractures formed by microcrack coalescence.

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