THE INFLUENCE OF GRAIN-SIZE AND POROSITY ON CRACK INITIATION STRESS AND CRITICAL FLAW LENGTH IN DOLOMITES

The influence of rock texture on crack initiation stress (sigma(i)) an d critical flow length (L-i) is studied by a series of triaxial tests performed on monomineralic dolomites. The critical flaw length, as pre dicted by analytical models, is shown to be larger than the measured m ean grain size (d(m)) by two-three orders of magnitude. This discrepan cy is explained by rock texture variations, which influence the fractu re propagation mode and consequently fracture initiation stress. The q uantification of rock texture is accomplished using porosity. Fracture initiation stress is shown to be inversely related to both porosity a nd mean grain size. When porosity is low, the sensitivity of sigma(i) to mean grain size is high. This effect is reduced with higher porosit y values. A model for initial flaw length is developed by a synthesis of Griffith initiation criteria with our empirical model for fracture initiation stress. Initial flaw length is found to be directly proport ional to the elastic modulus, mean grain size and porosity of the rock . When porosity and mean grain size decrease simultaneously, the initi al flaw length vapidly decreases and approaches the mean grain size va lue. Therefore, the classical assumption that grain size scales initia l flaw size is shown to be valid only in the very restricted case of l ow porosity-low grain size rocks. In such textures, where void space i s minimal, available crystal faces function as truly initial flaws, an d variations in mean grain size influence crack initiation stress sign ificantly. In more porous textures, however, the initial flaw length i s shown to be up to two orders of magnitude higher than the mean grain size in the rock, depending upon the porosity and mean grain size val ues. In such textures crack initiation sti ess is much less sensitive to variations in mean grain size, indicating that the role of individu al grains is less significant. (C) 1997 Elsevier Science Ltd.