ABSTRACT
Since 1958, the Sifto Salt Division of Domtar, Inc., has operated the Goderich (Ontario) salt mine in Michigan basin bedded halite, with shafts located on Lake Huron's eastern bank. Initial mining utilized a conventional room and pillar pattern. However, roof deterioration developed. Incidence of progressive upward failure of saltback and overlying shaley-dolomite layers intensified with entry aging. Studies were therefore initiated on new mining patterns utilizing the Serata Stress Control Method. Pursuant thereto, three test areas, indicated in the mine plan of Fig. 1, were designed and excavated. Full conversion to Stress Control mining was effected in January, 1983. This paper focuses on the instrumentation work in the third test area. Test Area 3 was designed to validate the excavation geometry of the new stress-controlled main entry system. The testing of that design was accomplished, in part, by comprehensive in situ stress, property, and deformation measurements on the performance of the model main entry section of Test Area 3. The paper describes: (1) the theoretical background of the SPDR/Stress Control Method; (2) specific problems and objectives addressed by the 1983 program of in sttu measurements; (3) measurement results; and (4) conclusions drawn therefrom.
BACKGROUND
The excavation dimensions, or "excavation geometry", of the main entry system designed for the Goderich mine were derived as a site-specific adaptation of the general principles of underground mine design which inform the Serata Stress Control Method. The Stress Control Method, in turn, resulted from application of a generalized method of geological engineering to the particular case of underground mine design. This general method is known as the "integrated" or "SPDM" method.
FIG. 1. Plan View of Entire Mine, Showing Excavation History, 3 Stress Control Method Test Areas, and the Stress Controlled 4-Room Main Entry Circumscribing.(available in full paper)
The SPDM Method in General
The scientific basis for the engineering design of earth structures, with respect to geomechanical structural stability, can be symbolized in the following heuristic notation: (mathematical equation)(available in full paper) This signifies that the stress state of the ground (S), as active cause, and the "strength" (material properties) of the ground (P), as passive or resistive cause, interact to produce ground deformation (D) -- including extreme ground deformation or structural failure -- as their joint effect. Of course, in general, ground deformation or "strain,, acts back upon its causes, both stresses and properties, changing them in turn, in spiraling succession, propagating the behavior or performance of the geological structure as a time-dependent process.
Fig. 2 depicts the interconnection of stresses, properties, and deformations, in determining the behavior of geological structures. The linear, solid arrows denote the Joint action of S and P inproducing D. The non-linear, broken arrows denote the back reaction of D upon S and P. This includes damage or deterioration of material strength due to shear strain or dilatancy, dissipation of stress energy through ground movement, etc. The double-headed arrow between S and P indicates the tight coupling of this pair. Stresses and properties are not completely independent.
