In the early stages of the development of the science of mining rock mechanics all the theories and concepts were based on the assumption that the rock material was elastic. Fortuitously, this assumption was a close approximation for most rocks encountered in mining, especially the so-called "hard-rocks". The success of techniques based on an understanding of phenomena described by the arch theory, beam theory, etc. attests to the usefulness and general applicability of elasticity theory. Recent work has shown that all rocks, even the hardest granites, are not completely elastic 1, although the assumption of elastic behavior remains valid enough for most practical purposes. Some rocks, however, notably rock salt and potash, are so anelastic that most of the previously accepted principles of rock behavior must be completely disregarded. This paper describes part of a rock mechanics investigation carried out in the potash mines of the Alsace district, France 2. One of the basic objectives of this investigation was the understanding and description of the behavior of long pillars of potash with respect to time and the manner in which they perform their task of supporting the overlying strata.
One of the most useful measures of anelasticity is "creep", that is the time-dependent strain of the material under constant stress. Creep in all theological materials, including rocks, appears to be similar and can be depicted by a generalized creep curve (Fig. 1). Depending on the values of stress and temperature and the material being tested, three phases of creep can be distinguished. The primary phase includes the instantaneous elastic strain and a creep strain in which the strain rate decreases with time. In the secondary phase, the strain rate has become constant and may, at sufficiently low temperatures and stresses approach zero. The tertiary phase is typified by an increasing strain rate and rapidly increasing strain leading inevitably to failure.
The research was performed at the Mine Fernand, Mines Domaniales de Potasse d'Alsace at Mulhouse, France, in an experimental area comprising three panels and their interjacent pillars in a district of partial extraction by "strip workings" (Fig. 2). The panels were extracted by a combined longwall-room and pillar technique whereby small pillars were developed to provide support for the working area. As the panel advanced, these small pillars were successively robbed and finally removed completely allowing the roof over the width of the panel to cave. The rock mechanics instrumentation was installed in maintained roadways adjacent to and just outside of the working panels. The principal instruments used in the investigation were a borehole extensometer, which measures axial borehole deformations and the "stressmeter" which measures diametral deformations. These instruments were developed by Prof. E.L.J. Potts at the University of Newcastle, England and have been described in detail in the literature 3, 4. Figure 2 shows the location of all of the instrumentation for the entire investigation and illustrates some of the ideas underlying the overall approach to the investigation.