20 years of excavation response studies at AECL's Underground Research Laboratory

Incremental development of a rock mechanics, rock fracturing, and excavation stability knowledge base has been ongoing at Atomic Energy of Canada Limited's (AECL's) Underground Research Laboratory (URL) since shaft excavation commenced in 1982. Excavation response experiments conducted as part of shaft construction recorded displacements, shaft convergence, stress changes, and microseismic events in the rock as excavation progressed. An excavation response test at the 240 Level of the URL involved similar monitoring, as well as pore pressure measurements, of a horizontal tunnel excavated through a subvertical water-bearing fracture. These precursor studies led to a series of experiments in the more highly stressed rock at the 420 Level of the URL to investigate the formation of rock damage around tunnels, and to assess the factors that influence the stability of excavations. The first of these experiments was the Mine-by Experiment, an excavation response study involving a mechanically excavated cylindrical tunnel in a pre-instrumented rock volume. The Heated Failure Tests were subsequently conducted in the same rock volume to assess the influence of thermal loading on damage development. These investigations were complemented by studies of borehole breakouts in adjacent excavations. The Excavation Stability Study, which involved excavating ten tunnel segments of different geometry to assess the effects of excavation design on stability and damage development, followed these experiments. The Tunnel Sealing Experiment, focused on developing sealing technologies, also provided insight into the excavation response of the rock mass. The excavation response experiments at the URL culminated in the Thermal-Mechanical Stability Study (TMSS), a comprehensive study to link characterization, numerical modeling, monitoring, and design of underground excavations. This paper provides an overview of the various experiments and studies leading up to the TMSS, highlighting the advances in our fundamental understanding of rock mechanics related to underground excavations, and in our means of designing stable underground openings with minimal excavation damage.