Unexcavated areas of rock left within %he limits of a mine or system of underground openings are referred to as pillars and, as such, they are the most important element in stabilizing the underground structure. In underground excavations in competent rock the pillars may remain intact and support the structure indefinitely; in operations involving induced caving, the pillars are generally reduced to a size such that they will crush and fail at some predetermined rate; and in some incompetent rock, especially in salt and potash mines, the pillars usually undergo continuous plastic and/or viscoelastic deformation (creep). The creep rate depends on the average pillar stress (which in turn depends on the extraction ratio) and is usually limited to a value such that the pillar deformation and attendant effects will not interfere with mining operations. A number of researchers have investigated the strength of model pillars where the limiting strength was determined from the load at which the model pillars ruptured or fractured with a sudden release of applied load, that is, as a brittle material. The typical uniaxial compressive test specimen is in effect a model of a prototype pillar having the same scaled dimensions. A number of empirical relationships have been developed that relate specimen strength to specimen size and shape (height-to-width or diameter ratio). It has been noted in these investigations that, in addition, the specimen strength also depends on other factors, especially the rate of loading and the end constraint, that is, the resistance offered by the bearing plate to the lateral deformation of the specimen. In considering the mechanical properties of materials, it is generally assumed that failure not only depends upon brittle fracture but, for more ductile materials, on some limiting strain or strain rate. Although the magnitude of the strain or strain rate that limits the utility of the mine pillar is not a unique value but rather a value that will depend on the system of mining and other factors, it is a priori evident that any rational basis for designing a mining operation in a rock that exhibits plastic or viscoelastic effects would require a knowledge of the de formational characteristics of supporting pillars. The objective of this investigation is directed toward this end. Specifically, the investigation considers the deformational behavior of model pillars made from salt, trona, and potash ore (sylvite plus halite) tested under a constant applied load. As the load in the pillar prior to reaching this constant value is less than that after the pillar has assumed its maximum load, any design based on constant load conditions would be on the conservative side.
Preliminary to the development of a model pillar for creep tests, a limited investigation was made to determine the effects of specimen shape and end constraint on the strength and deformational behavior of salt and trona specimens subjected to moderate rates of loading (approximately 100 psi/ minute).
