ABSTRACT: Mechanical discontinuities are defined as any surface through which some property of the rock mass is not continuous. Usually, the properties of the bulk rock separated by a discontinuity are used to predict the behavior of a formation. However, an analysis of the discontinuities as such is uncommon. The main property of a discontinuity is its shear strength. The shear strength has frictional parameters, such as basic friction angle and roughness, and may have a cohesion parameter. In this work, we evaluated the mechanical properties of artificial mechanical discontinuities. The artificial discontinuities were numerically designed. The mechanical performance of these discontinuities was initially assessed using published failure criteria that depend on the properties of the rock and the discontinuity morphology obtained by visual and 3D image analysis. Finally, triaxial direct shear tests under high confining pressure were performed to validate the criteria and assess their validity under different stress field conditions, properties of rock, and discontinuity morphology. 1. INTRODUCTION The deployment of underground fluid injection and storage projects, such as CO2 underground storage, can only succeed if the fluid permanence on the targeted formation is warranted. This could only be achieved if the formation’s characteristics are understood. The main risks associated with fluid migration are related to discontinuities in the reservoir or caprock (Damen et al., 2006), therefore, these must be carefully identified, characterized, and included in the rock mechanics models. There are many definitions of discontinuities in literature, as Hencher and Richards (2014) summarize in their work. A definition adopted in the cited work is that discontinuities consist of relatively weak planes in rock volumes. Since in this work we mainly focus on the rock mechanics, we will refer to them as mechanical discontinuities, considering any surface through which some mechanical property of the rock volume is not continuous.