Possible mechanism of spallation in rock samples under uniaxial compression

In uniaxial compression, rocks are often observed to have spallation failure at the lateral surface of the sample developing parallel to the direction of loading. While easily explained by a 2D concept of axial crack growth in uniaxial compression, the real 3D mechanism cannot be explained this way, as 3D crack growth in uniaxial compression is restricted to the size of the initial defect due to the phenomenon of wing wrapping. The wing wrapping can only be overcome in biaxial compression with the magnitude of the intermediate principal stress exceeding 5-8.5% of the axial load (depending on the type of crack-producing defect). In uniaxial compression, the role of the intermediate principal stress is played by the circumferential compressive stress induced due to the end constraint owning to the presence of end friction. The results of Finite Element modelling show that zones of biaxial compression are formed at the lateral surface near the sample ends. The sizes of the zones and the maximum magnitude of the circumferential stress depend upon the friction coefficient and the Poisson's ratio of the rock. The modelling also revealed the presence of zones of tensile circumference stress which can potentially induce splitting failure.