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In deep underground excavations in hard rocks where stresses easily exceed the micro-crack initiation stress level inside the rock mass, proper consideration of the behaviour of rockmass during the brittle fracturing process in constitutive modelling is of paramount importance. Current empirical and conventional experimental methods for obtaining the deformational behaviours of hard rocks under loading do not lead to results that can be matched with in situ failure observations. This paper demonstrates that this problem is not necessarily a matter of the general notion of size effect but rather can be related to the different circumstances under which the cohesive and frictional strength components are mobilized in laboratory compression tests and around underground openings. It is also demonstrated that the propagation of the failed or breakout zone (depth and extent) is a function of the strain-dependent brittleness index IBε introduced in this paper, which explicitly considers the relative delay in friction mobilization relative to the rate of cohesion loss as functions of plastic strain. This new brittleness index characterizes the entire stress–strain curve (pre- to post-peak stages) and represents the involved micro-mechanisms during the brittle failure process: that is, initiation, propagation, and coalescence of cracks. This study shows that brittleness of rock is the most dominant factor, in controlling breakout shape, which explains the failure of stress-based criteria adopted by many researchers in predicting the stress-induced breakout depth around openings.