Understanding and predicting excavation damage in sedimentary rocks: A continuum based approach
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The most widely accepted approach to long-term storage of nuclear waste is to design and construct a deep geological repository, where the geological environment acts as a natural barrier to radio nuclide migration. Sedimentary rocks, particularly argillaceous formations, are being investigated by many countries because of favorable isolating qualities (laterally continuous and low permeability) and the ability of self-sealing of fractures. Underground construction creates a damage zone around the excavation. The depth away from the excavation surface of the damage zone depends on the rock mass properties, the stress field, and the construction method. This research investigates the fracture development process in sedimentary rocks and evaluates continuum modelling methods to predict the damage zone dimensions. At the laboratory scale, a complete classification system for samples of carbonate and siliciclastic rocks has been developed, with geotechnical considerations, which when applied narrows the variability of the mechanical properties. Using this system, crack initiation (CI) shows the most uniform range in each class, particularly for mud rocks. Tensile strength was found to be higher for the Brazilian method than Direct method of testing. Brazilian reduction to Direct values was found to be rock type dependent. Laboratory testing results are also influenced by the orientation of bedding. Bedding and other structures were also found to influence the excavation behaviour as observed at the Niagara Tunnel Project in a mudstone and in excavations in the Quintner limestone of Switzerland. The conceptual stages of damage development and the potential fracture networks in sedimentary rocks are used to summarize the understanding of excavation damage developed in this thesis. Using a continuum based modelling approach, a set of predictive damage depth curves were developed for the different excavation damage zones. This approach was found to be most sensitive to the tensile strength used as an input. Back analysis of the Niagara Tunnel Project and forward prediction of the excavation damage around a shaft in the Queenston Formation are used to illustrate the importance of this research. The prediction methods were also applied to cut-off design analysis. This research has enhanced the understanding of excavation damage development in sedimentary rocks and provided a methodology to predict the dimensions of the excavation damage zones using a continuum based approach.