Fragmentation patterns during rockfall: analysis of the influence of discontinuities and impact conditions through drop tests

Fragmentation of rock blocks upon impacts is frequently observed during rockfall. The increase in frequency of rockfall events due to climate change makes fundamental the knowledge and prediction of possible blocks fragmentation to assess and manage the risk. Although fragmentation occurs regularly and at various scales, the process has not been completely investigated in rockfall. In this specific field, the term fragmentation describes the division of an initial block by either breakage, generating new pieces, or the disaggregation of the blocks delimited by fracture stuck together thanks to a small cohesion or some kind of cementation, or even both phenomena together. Fragmentation of brittle spheres, made of mortar, has been studied experimentally and numerically by some of the Authors through vertical drop tests performed in a fragmentation cell conceived and built at the University of Newcastle. Interesting findings on the breakage of intact rock at a meso-scale level have been revealed and published. The present work deals with an experimental campaign in which discontinuities were created into the mortar spheres. To this aim, a specific specimen assembly technique was adopted. The mortar spheres were casted in separate parts and then glued together with mortar with lower mechanical performances. Different discontinuity patterns were investigated, considering how the position and inclination with respect to the impacted surface affect the fragmentation mechanisms. The generated patterns were studied by detailed image analyses, and they were related to discontinuities’ geometry and impact conditions. Interestingly, the number of hemispherical fractures, when formed, together with the presence of secondary cracks, is strongly related to all the parameters. In specific conditions, disaggregation only occurs. Knowing the state of the rock face and possible released blocks, the results can provide interesting insights on the number and type of possible fragments.