The influence of rock fabric on excavation damage in the Lac du Bonnet granite

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Everitt, R. A.
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The Canadian concept for disposing of nuclear fuel waste involves placing and sealing it in excavations 500 to 1000 m deep in plutonic rocks of the Canadian Shield. Investigations pertaining to the Canadian concept for used nuclear fuel waste disposal in plutonic rocks were conducted at AECL's Underground Research Laboratory (URL). The URL is excavated within the Archean granite of the Lac Du Bonnet batholith (LDBB), approximately 120 km northeast of Winnipeg, Manitoba, at the western edge of the Canadian Shield. At the URL, AECL has undertaken a wide range of experiments to test the feasibility of burying nuclear fuel waste in granitic rocks. The investigations have included detailed mapping of geological features, stress measurement, and a recording of excavation response. The purpose of this thesis is to show how the geological conditions affect excavation stability, in particular how the fabric of granite influences excavation damage development including overbreak around tunnel or shaft perimeters. Most of the design of the excavations has been based on the assumption that the host granite is homogeneous (i.e. texturally and mechanically homogeneous). This thesis demonstrates, through a program of field observations and laboratory testing, that the Lac du Bonnet, and probably most granites, are highly heterogeneous and anisotropic at both the macro and the micro scale. Field investigations included detailed mapping. This had four objectives: (1) to define the general site characteristics including: (a) the geology of the site, (b) the fault and fractures domains, and (c) the in-situ stress domains. This was supported by field and laboratory investigations including fracture, foliation and microcrack analyses. This work defined the nature and types of rock mass heterogeneities in the rock mass. (2) to locate and describe evidence where fabric anisotropy has influenced rock mass response. This was not restricted to only present-day excavation response, but also included observations from ancient faults and fractures, whose distribution and orientation provided the first hint of the anisotropic behaviour of the LDBB granite. (3) to characterize areas where excavation damage, including overbreak, occurre , and (4) to select and characterize areas where samples were collected for rock properties testing. (Abstract shortened by UMI.)