Excavation-induced damage studies at the Underground Research Laboratory

Abstract

The Underground Research Laboratory (URL) is situated in the Lac du Bonnet granite batholith, in southeastern Manitoba, Canada. The URL was developed to study issues related to the deep geologic disposal of used fuel from nuclear reactors and has developed into an International Atomic Energy Agency recognized geotechnical center of excellence. Extensive rock mechanics research has been conducted, including work to understand the character and extent of excavation damage. Experience at the URL has shown that damage exists around underground openings and that the damage develops from the energy imparted to the rock by the excavation method and by redistribution of the in situ stress field around the excavated openings. Subsequent near-by excavations, removal of loose material from the existing tunnel, thermal loads and pore pressure changes will all influence the development and extent of rock damage, as does the rock type and its fabric. Studies at the URL have shown that in highly stressed rock, damage will develop around underground openings even when a low-energy excavation method is used and that properly designing the excavation geometry and orientation plays a major role in constructing stable openings. Two zones of excavation-affected rock have been shown to exist from studies at the URL; (1) a zone of irreversibly damaged rock surrounding the excavation, which may include failed zones, inner and outer damage zones and (2) a zone of excavation disturbance where the in situ stresses are influence by the excavation no damage is measurable. Measurements of the properties of the damaged rock (ultrasonic velocity, transmissivity) using a variety of instruments have shown that a less intensively damaged outer zone surrounds a more highly damaged inner zone. The inner damage zone may have zones of failed rock depending on stress concentrations relative to rock strength and damage imparted by excavation method. The inner damage zone is attributed to the effects of the excavation method and stress redistribution and the outer damage zone is attributed to the effects of stress redistribution alone. In lower stressed rock the extent of the outer zone is much less than for an opening in highly stressed rock with the same excavation shape and orientation relative to in situ stresses. The excavation disturbed zone describes the volume of rock surrounding the damage zone where the stress redistribution is too small to permanently change rock properties (unless some other stress increase, such as thermal loading, is added), and it is also characterized non-permanent but potentially long-term changes in hydraulic head.