Relation between crack initiation-damage stress thresholds and failure strength of intact rock

The analysis of a wide literature dataset of mechanical parameters related to intact rocks from more than 480 unconfined compression tests, coupled with new laboratory tests on 132 specimens, is proposed herein with the aim of analyzing the mechanical behavior of a great variety of rock types, mainly focusing on their crack initiation (sigma(ci)) and crack damage (sigma(cd)) stress levels. These thresholds can be employed as warning indicators for rock mass damage and breakouts and represent important input parameters for numerical models. International literature lacks in a detailed analysis on the mutual dependence existing between the main mechanical properties of intact rocks and their crack stress thresholds. In this paper, the study of the correlation between crack initiation-crack damage stress levels and the failure strength of sedimentary, metamorphic and igneous rocks is carried out through single and multiple regression approaches aimed at finding reliable prediction models, which can be useful when time-consuming laboratory experimental procedures need to be avoided. The correlation between predicted and measured values demonstrates that defined models represent a good tool for the empirical estimation of sigma(ci) and sigma(cd), and can be useful for preliminary engineering design dealing with stress-induced brittle fracturing, especially when the definition of warning indicators for rock mass damage and breakouts is needed. In fact, it is known that underground instability mainly depends on the redistribution of stresses around the excavation, which can produce induced stress concentrations, resulting in sudden release of stored energy and causing stress-induced brittle failure phenomena.