In 1982, Atomic Energy of Canada Limited (AECL) carried out the first in situ stress measurements at the proposed site for the Underground Research Laboratory (URL). These hydraulic fracturing measurements were taken between the 12and 540-m depth in the general area of the proposed access shaft to the URL. The URL shaft was constructed in two stages. Stage 1 (upper shaft) was excavated from the surface to a depth of 255 m between 1983 March and 1985 April. This stage was excavated as a nominal 2.8by 4.9-m-rectangular shaft by the traditional drill and blast benching method. Stage 2 (lower shaft) was excavated as a 4.6-m-diameter circular shaft using a full-face drill and blast technique. Since 1985, an extensive in situ stress research program has been ongoing to characterize the in situ stress state around the URL access shaft and the main development level at 240-m depth, and to address some of the commonly asked questions about in situ stress results: 1) Are in situ stresses dependent on the scale of the measurement technique? 2) What is the influence of geological features on the results? 3) Is residual stress a major component of the stress results? 4) Will different measurement techniques provide the same results? and 5) How can the stress data be presented in a useful form to the design engineer? To date, some of the above concerns have been investigated using overcoring (USBM, CSIR, and SSPB), hydraulic fracturing, under-excavation, microseismic monitoring and observations (shaft-wall failure and core discing), and our results are published or in press. This paper is a summary of our findings to date.
Atomic Energy of Canada is constructing an Underground Research Laboratory (URL) near Lac du Bonnet, Manitoba as part of its research into the concept of safe disposal of nuclear fuel waste in stable plutonic rock. The URL lease area is shown in Figure 1.
Knowledge of the in situ stress conditions at this site is important in assessing the impact of various stress conditions on the overall disposal concept. One typical concern is the effect of stresses around an excavation on the nature, extent, and connected permeability of the excavation damage zone surrounding the opening, and ultimately the resulting effect on the processes involved in solute transport. To gain an understanding of the stress conditions at the URL, we have employed several different techniques to measure the in situ stress state, with varying degrees of success. This paper describes the approaches to stress measurement we have taken to date at the URL, the relative merits of each technique, and the results of these measurements. Details of the instrumentation used in the stress determinations are not presented here; several reports dealing with the specific instrumentation used at the URL are available or in press.
