Primitive field stresses are the stresses that prevail today in the rock as a result of the original geologic formative processes including one or more tectonic episodes. It is commonly known in geological engineering that excavations change the primitive field stresses in close proximity. It is also known that the stresses so raised around excavations are a function of the magnitudes of the primitive field stresses and, if in excess of the strength of the rock, give rise to fractures in the rock. The degree of fracturing is directly related to the magnitude of the primitive field stresses. It represents the surface area of newly created fractures and can be assessed by the depth to which fractures are induced into the wall rock and the extent to which they occur around and along the excavation. Stillwater Mining Company developed a system for classifying the degree of observable excavation induced fracturing and for statistically relating such fracturing to the primitive field stresses in terms of the strength of the rock and the stress raising effect of the excavation.
The observed degree of fracturing is empirically related in the literature to the proportion of the maximum stress to strength of the wall rock around an excavation. This proportion can be plotted against a similar proportion calculated from the geometry of the excavation and the rock parameters for a number of excavations. The ratio of the primitive field stresses to one another is one of the parameters on which the calculation is based. The value for the ratio of the primitive field stresses at which the average empirical and calculated wall stress to strength proportions are equal, is an estimate of the actual ratio of the primitive field stresses provided the associated correlation is acceptable. The system has been applied on the mine to three data sets and has resulted in estimates of the primitive field stress ratio that compare well with actual measurement and simulation of the incision of the Stillwater Valley in primitive landscape. The system is refined by ongoing application and extension of actual stress measurements across the Mine.
1. INTRODUCTION
The Stillwater Mine, the largest primary producer of palladium and platinum in the Western Hemisphere, is situated in the Stillwater Igneous Complex in south central Montana, U.S.A. The Stillwater Complex is a 42 km long, layered, approximately 2.7 b.y. old lopolith that hosts a stratiform palladium and platinum rich layer known as the J-M Reef. Stillwater Mining Company has produced palladium and platinum from the J-M reef since 1987.
Production is primarily accomplished by variations on overhand and underhand horizontal cut-and-fill methods with long-hole stoping practices playing an important role.
Studies concerning the degree of mining induced fracturing began in earnest in early 2001. Such fracturing of the rock mass around excavations plays a crucial role in ground control methodologies. Determining the level of mining induced fracturing provides a tool to estimate the field stress state, which, in turn, can provide more reliable input parameters for design purposes.
