Physical simulation of asperity degradation using laboratorial shear tests of artificial fractures

Abstract

Hydromechanical behaviour of sheared rock fractures is complex as it highly depends on the evolution of surface roughness after the degradation of asperities. A new fracture shear cell (FSC) which is able to conduct tests under large normal and shear loads was used in this study to investigate the effects of fracture surface rough-ness on asperity contact degradation and micro-cracking of the intact sample. Fractures with synthetic and real-rock geometries were built in mortar specimens and subjected to shear tests at different normal stresses. Specimens were also tested in two cycles in such a way that the second cycle was repeated at the same level of normal stress after cleaning the produced debris in the first cycle. Damaged regions were marked and it was observed that the significant damage occurs through the large bumps of the fracture surface. The analyses enable investigation of roughness evolution and asperity degradation during the fractures shearing. The results showed that by increasing the normal stress, asperity degradation significantly increases and this will affect the fracture shearing mechanisms. The results also indicated that shear strength is reduced in the second cycle due to a reduced roughness after the first shearing cycle. Assuming the basic friction angle of 31° for the real rock fracture, the average asperity angle estimated using the Patton bilinear equation after the first and second cycles of shear tests were 30.9° and 16.2°, respectively. It was seen that the asperity angle is reduced in the second cycle to twice as much in the first cycle. The analysis confirms the significant effects of roughness on asperity degradation.