Creep in model pillars of saskatchewan potash

Abstract

Results are reported of creep tests on model pillars of Saskatchewan potash ore. The first series of tests, each having duration of 1000 hr, shows the effect of changes in temperature and vertical stress on the creep behaviour of pillars in a homogenous material. In the second series, each having a duration of 1500 hr, are incorporated the effects of the presence of clay seams in the roof and floor. The test conditions in both series of tests represent those existing at subsurface depths of between 3400 and 4500 ft in Saskatchewan, for extraction ratios of 0·32–0·51 at 3400 ft and 0·10–0·36 at 4500 ft. For the first series of tests, with a pillar diameter-to-height ratio of 4, the vertical natural strain ϵ on the pillars could be related to the time T by a simple power law of the form, ϵ = aT b , for T greater than 200 hr. Thus the rate of vertical creep of the model pillars was found to decrease with an increase of time. For the second series of tests it was found that a pillar diameter-to-height ratio of at least 8 was required to prevent brittle failure at vertical stresses equivalent to depths of 3400 ft. With this diameter-to-height ratio of 8 the creep behaviour for test equivalent to this depth was very similar to that of pillars in a homogenous material at the same depth, but having a diameter-to-height ratio of 4. However, the effect of the increase in temperature associated with an increase in depth to 4500 ft was very much more pronounced when discontinuities were present.