Excavation-induced damage studies at the Underground Research Laboratory

The Underground Research Laboratory (URL) is situated in the Lac du Bonnet granite batholkh. in Canada. The URL was developed to study issues related to the deep geologic disposal of used fuel front 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 by the excavation method and by redistribution of the in situ stress field around the excavated openings. 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; if, rock, damage will develop around underground openings even when a low-energy excavation method is used and that designing the excavation geometry and orientation plays a major role in constructing stable openings. Two zones affected rock have been shown to exist front studies at the URL; (1) a zone of irreversibly damaged rock 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 the damaged rock (ultrasonic velocity transmissivity) using a variety of instruments have shown that a less intensively damaged outer zone surrounds a more darnaged inner zone. The inner damage zone may have zones of failed rock depending on stress concentrations relative it) 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 zonewhere the stress redistribution is too small to permanently change rock propel-ties (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. (C) 2004 Elsevier Ltd. All rights reserved.