It is important to accurately identify the stress at which micro-fractures initiate in rocks under compression when studying phenomena like excavation damage zone (EDZ) formation around deep underground structures, rock burst dynamics, stability of rock slopes, etc. This stress threshold, popularly termed as the point of Crack Initiation (CI), can be determined from strain measurements or acoustic emissions monitored during laboratory compression tests. In terms of strain-based approaches, the reversal of crack volumetric strain and the point of non-linearity in the axial stress-lateral strain curve are commonly used to identify CI. There is, however, a lack of understanding as to whether these approaches are applicable/effective for CI determination under higher confinements. To shed some light on this topic, this study utilizes the two aforementioned strain-based approaches to determine CI over a wide range of confinements in context of a granitic rock, a limestone and a coal. We discuss difficulties associated with using the two approaches for cases when the lateral strain curve is non-linear from the start of the tests. Ultimately, it was found that both approaches yield similar CI estimates at low confinements, but the discrepancy between the results obtained using these approaches increases as a function of confinement. In particular, the stress at the point of axial stress-lateral strain non-linearity can be 1.5 times higher than the stress at the point of crack volumetric strain reversal.

1. INTRODUCTION

When brittle rocks are loaded in compression under a high ratio of major to minor principal stress (σ13; e.g. unconfined or low confinement conditions), they undergo progressive damage starting with the initiation of extensile micro-cracking, followed by microcrack interaction and coalescence that ultimate leads to the formation of a failure plane at peak strength. These distinct phases of damage are illustrated on a typical stress-strain curve in Figure 1. The stress levels at which extensile micro-cracks initiate and interact/coalesce are popularly known as Crack Initiation (CI) threshold and Crack Damage (CD) threshold, respectively. Unlike the peak strength (Hudson et al. 1972; Diederichs and Martin, 2010), CI and CD are true characteristic material parameters, meaning that they do not depend on the loading conditions employed in the test (Martin and Chandler, 1994).

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