In the past ten years, several large-scale underground experiments have been conducted at AECL's Underground Research Laboratory (URL) to address geomechanics issues related to the disposal of nuclear fuel waste in Canada. Two key rock mechanics issues that have been studied are the development of near-field excavation damage and the stability of underground excavations in granite under various boundary conditions. This paper presents an overview of four of these in situ experiments and highlights the main findings in terms of rock mechanics considerations for nuclear waste disposal.
In Canada, research into the disposal of nuclear fuel waste has focused on the concept of deep geological burial in an underground disposal vault. In designing excavations associated with such a vault, a key objective is to maintain stable rock mass conditions around each opening throughout the vault's life cycle, while at the same time minimizing excavation damage. During this time period, the rock mass will be subjected to mechanical loads generated by underground excavation, support loads introduced by buffer and backfill materials, and thermal loads generated by the emplaced nuclear fuel waste. Other loads, such as those associated with glaciation, are also possible over the long term. As a first step to designing stable openings, the rock mass response to excavation and to subsequent loading, and factors that may affect this response, must be understood.
To address these and other related issues, Atomic Energy of Canada Limited (AECL) constructed the Underground Research Laboratory (URL) approximately 120 km NE of Winnipeg, Manitoba, Canada within the Lac du Bonnet granite batholith near the western edge of the Canadian Shield. As shown in Figure 1, within the first few hundred metres of the surface at this site, the granite contains subvertical joint sets and several major low-dipping thrust faults (called Fracture Zones), and has undergone secondary alteration. Below Fracture Zone 2 and its splays, the granite is interrupted by granodiorite dykes, but is sparsely fractured and relatively unaltered.