Experimental and PFC2D numerical study of progressive shear behaviour of single rough rock fractures

Abstract

This thesis investigates the progressive shear behaviour and asperity degradation of single rough rock fractures using 2D numerical simulations and laboratory experiments.The particle flow code (PFC) was chosen for simulation purposes in this work. The fracture and intact sample are modelled as an assembly of circular disks in PFC2D. By performing biaxial test simulations in PFC2D, correlations were obtained between micro and macro properties of the intact sample. These results were used later for guidance to establish the micro properties of models corresponding to mortar samples, based on the macro response obtained from the lab experiments. To define a set of micro properties corresponding to fracture particles and investigate their effects on shear behaviour of both planar and rough fractures, a sensitivity analysis of fracture model micro properties including particle size, particle friction coefficient, and contact bond strength were carried out.Several synthetic profiles with triangular and sinusoidal geometries were simulated to study their shear behaviour. A prior knowledge of the shearing response of such simple geometries allowed calibration of the model to be made. The results confirmed the observation of different failure modes, i.e. sliding, asperity cut-off, and degradation, as a result of increasing the normal stress. The simulation results were compared with lab experiments that were carried out on synthetic samples with constant height elevations along their thickness. This was the closest geometry that could be used to calibrate 2D models, and a good agreement was found between the results of the two approaches.The lab shear tests were conducted using a fracture shear cell (FSC). This was a special set up made from modifications on an existing triaxial stress cell. The FSC was capable of applying large shear and normal loads to the sample where the normal load was applied in a constant rate using a pressure cylinder.Both lab experiments and simulations were performed on pseudo-real as well as rock-like fracture specimens. The directionality in shear strength when the sample was sheared in opposite directions along a horizontal plane was also studied. The 1D Riemannian dispersion parameter (DR1) was determined for different profiles’ geometries as a measure of roughness. This parameter showed a good correlation with the profile’s shear strength. The spline fits to the peak shear strength data for triangular profile geometry as well as the number of rock fracture profiles obtained from literature were developed and presented. A detailed discussion on the simulations and lab experiments will be given and the results presented and interpreted.