Experimental and numerical investigation of brittle sandstone specimens containing different shapes of holes under uniaxial compression

Defects, such as joints, fissures and holes, are common in natural rocks. These pre-existing defects have significant effects on the mechanical properties of rock masses. Researchers have performed several studies on jointed or fissured rocks; however, rock specimens with one hole of different shapes have not been studied systematically. In this research, the mechanical properties and cracking behaviors of specimens with a hole under uniaxial compressive loading are explored through laboratory experiments. The corresponding numerical simulations using two-dimensional particle flow code (PFC2D) are conducted and the force field distribution before and during cracking, which is significant for exposing the cracking mechanism but hard to be measured in laboratory tests, were captured. The results demonstrate that the shape of hole has a considerable impact on the uniaxial compression strength and failure mode but minimal effects on the elastic modulus. The force field distribution before crack initiation varies with the shape of the preexisting hole, and differences primarily exist in the configuration of the pure tension triangle and the low force region. During compressive loading, the crack initiation and propagation are influenced by the force field distribution, which will be affected by the newly formed cracks in return. The macro-cracks, generated in the specimens with a hole under compression, can be assigned to two categories: tension-induced cracks and compression-induced cracks. The former type of crack is typically thin and comparatively orderly, and it generally propagates along the maximum loading direction. The latter type of crack is relatively disorderly and thick, and it typically forms from the coalescence of several short but thick cracks. The compression-induced crack is a mixture of tension and shear cracks whereas the tension-induced crack is a pure tension crack.