ABSTRACT
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 12- and 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 2.8- by 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.
