A Rock Mechanics Case History of Elliot Lake

The Elliot Lake area is characterized by Proterozoic sediments containing uranium-bearing conglomerates separated by quartzite beds 10 to 100 ft (3.0 to 30.5 m) thick. The geological structure consists of a broad syncline with an east–west axis plunging about 5 °W, cut by northwest trending faults, and with steeply dipping east–west extension joints. All the mines use a stope-and-pillar method of extraction with narrow rib pillars about 250 ft (76 m) long on dip and sill pillars on strike.After the Elliot Lake Laboratory was established, detailed studies were undertaken to evaluate the methods that were available for the determination of the mechanical properties of the rock mass and its state of stress before mining. Practical studies were then made on the pillars, roofs, and abutments.Testing techniques were improved for the rock substance and the rock mass; however, much remains to be done to be able to characterize adequately the mechanical properties of the rock mass. A novel random sampling approach produced a suite of specimens many of which included fractures, with a mean uniaxial strength that was surprisingly little lower than the mean of only the solid specimens. The dispersion of values in such a suite was, of course, quite large. Of the other tests used, Brazilian tests were found to be useful for quality control of stress determinations using a strain recovery technique.The use of borehole pressure cells, seismic velocity, and borehole penetrometers as techniques for the determination of the mechanical properties of the rock mass remains questionable.The tectonic history of the region was resolved; it provides an explanation for the existence of horizontal stresses greater than vertical stresses and for the major principal stress to be oriented parallel to the axis of the syncline. It was also shown that the major principal stress axis is essentially parallel to the strike of extension joint surfaces, even when the strike deviates from the predominant 090° azimuth direction.After considerable experience with mining in these geologic conditions, which probably are more uniform than in most metal mines, the determination of stable spans of stopes and breadths of pillars can be done very well by judgment. However, for examining new layouts relatively simple theoretical analyses, particularly for the determination of stable pillar sizes, were found to provide a rational and useful basis for extrapolation.The stresses determined in pillars and abutment zones and the deformations of the roofs corresponded fairly well to values predicted by analytical and model techniques. The increased stress in the abutment zones extended into the solid for a relatively limited distance, which, in this relatively hard rock, seems to be related substantially to the span of the adjacent stope. All field measurements were subject to dispersion. The electrolytic analogue, which takes into account the three-dimensional aspects of the geometry of the tabular orebodies, showed that irregular mining boundaries have a distinct contribution to the variance of the pillar stresses. The finite element method was found to be flexible and useful in studying specific questions, particularly related to novel mining plans.