Effect of heterogeneity on failure of natural rock samples

A carbonate sample extracted from the depth of about 10 kft was subjected to uniaxial loading while the confining stress remained constant. Post-experiment inspection of the sample showed an inclined crack at an angle less than 20° to the horizontal. This subhorizontal crack orientation was contrary to the expected 45° inclination, the plane of the maximum shear stress. Coincidentally, as shown by CT-scan prior to loading, there was a boundary between two layers of different density inside the sample located almost exactly where the crack appeared. This density difference has arguably translated into the contrast in the elastic properties at the boundary. The hypothesis is that because of this elastic heterogeneity, an incipient crack developed at the boundary due to the unavoidable tensile stressing of the sample as it was brought to the benchtop from its original state of high confining stress at depth. Controlled uniaxial compression made the sample slip along this crack, which then developed into a prominent feature. This assumption was corroborated by a numerical experiment showing a strong von Mises stress concentration at the elastic contrast boundary during hydrostatic tensile loading. Another sample, from the same formation, but without strong density heterogeneity, exhibited a classic 45° crack after uniaxial loading. These results provide a novel and important insight into the mechanics, breakage, and strength of natural rock.