Manuel Rocha Medal Recipient Wave interaction with underground openings in fractured rock

Abstract

The objective of this work is to lead to improved models of seismic wave propagation around underground openings by studying the interaction of the waves with the fractured rock surrounding these openings. It demonstrates that seismic models can help in stability problems such as rockbursting in deep-level mining, or in the interpretation of micro-fracturing at waste storage sites. A significant emphasis is placed on comparing the models with observations from controlled experiments. These comparisons demonstrate that the wave propagation can be reliably and accurately modelled, and in so doing it motivates their application to the larger rock engineering problems. Seismic wave models are first applied to laboratory experiments on multiple fractures. Simulation through multiple displacement discontinuities yields strikingly similar waveforms to the experiments, while also identifying the need to build stress dependence into the fracture models, such as stress dependent fracture stiffness. The wave-fracture modelling is extended to in situ fractures in rock at the surface of a deep tunnel, using data collected during an acoustic emission experiment at the URL Mine-by tunnel. Waveforms from the velocity scans are compared against those from elastic models and various models of fracture, such as random assemblies of small open fractures (cracks) and larger fractures with fracture stiffness. Results indicate that it is possible to account for the wave-speeds and amplitudes using models with fractures. A generic method is then proposed for calculating the frequency variation of wave-speed and amplitude for any collection of cracks. The models of fracture are then applied to the rockburst problem, to investigate how the excavation affects the amplitude and the distribution of ground motion. The results provide important insights into the causes of the apparent amplification observed by researchers in this field. The thesis also covers the theory of the models used, including novel numerical work on dispersion and new grid schemes. The full detail of the work cannot be covered in this paper which instead seeks to summarize the main achievements.