Characterizing in situ stress domains at the AECL Underground Research Laboratory

Abstract

The Underground Research Laboratory access shaft was excavated from the surface to about the 185 m depth in jointed pink granite. Below this depth to the 443 m depth the shaft was excavated in massive grey granite. The grey granite is essentially unjointed, except for a major low-dipping thrust fault and associated minor splays. Overcoring, hydraulic fracturing, convergence measurements, microseismic monitoring, and observations of shaft-wall failure and core discing indicate that unusually high in situ stresses can be associated with large volumes of massive, unjointed granite at fairly shallow depth. The database of in situ stress measurements collected at the Underground Research Laboratory indicates that major geological features, such as thrust faults, can act as boundaries for in situ stress domains and that both the magnitude and direction of the in situ stress state can change when these geological features are traversed. Key words: in situ stress, anisotropy, stress domains, thrust faults, overcoring, hydraulic fracturing, convergence measurements, excavation damage zones.