Mechanics of spallation in circular holes under biaxial compression

Spallation is a type of rock failure observed on the walls of underground excavations, which manifests itself by the ejection of surface parallel rock slabs. The mechanism of spallation involves a recurrent process of extensive crack growth from pre-existing defects in rocks under compression, creating surface parallel fractures that subsequently buckle, producing the ejecta. In this study, we tested cubic Donnybrook sandstone and mortar specimens of various compositions, each containing a central circular hole. These tests were conducted under biaxial compression, without applying stress along the hole axis—a stress typically present in underground spaces. Under this loading condition, it was believed that the immediate hole sidewalls (where radial stress is zero) were subjected only to tangential stress, i.e. uniaxial compression. The spallation was observed on the hole sidewalls in all tested specimens. It is known that the intermediate (second) principal stress is essential for the formation of extensive crack growth (surface parallel fractures). Through finite element modelling, we demonstrate that due to the Poisson’s ratio effect and the non-uniform distribution of tangential stress around the hole boundary, an additional normal compressive stress was induced along the hole axis. This stress, combined with the tangential stress, produced biaxial compression zones at the immediate hole sidewalls, which allows the extensive growth of pre-existing cracks and is sufficient to form a mechanism of spallation.