1 INTRODUCTION
A shaft is truly the "lifeline" of an underground mine. Damage to the shaft lining and guides as a result of ground movement can result in serious loss of production and extensive and continual repair. In some mining districts, annual shaft repair and maintenance costs resulting from high rock stresses and excessive displacement are in the millions of dollars per year. For many years, the Bureau of Mines has conducted research to characterize the rock mass and to develop structural guidelines to improve the design of accessways in deep mines. Much of this work has centered on determining the in situ conditions that affect the structural stability of shafts in the Coeur d'Alene Mining District of northern Idaho. Recent work by Beus and Board (1984) has concentrated on measuring the rock and support behavior during sinking of the Silver Shaft at the Lucky Friday Mine. These results are currently being used as a comparative data base for further basic research as well as to establish structural design criteria. The factors affecting the design and structural stability of deep mine shafts are numerous. A design approach that uses field data as input and directly compares field measurements with numerical models can validate the procedure and provide realistic design criteria. Naturally occurring "fixed" conditions such as magnitude, direction, and ratio of in situ stresses, geologic environment, and physical properties and constitutive relationships of the rock mass are obviously basic to numerical modeling. Design variables include those factors that have an impact on shaft stability and that may be changed by the designer either before or during construction of the shaft. These are: (1) size, shape, and orientation (if noncircular), (2) type and dimension of support, and (3) excavation and lining sequence. Field data collected from the Silver Shaft verifies the influence of these factors. With present-day technology, it is unrealistic to attempt to incorporate all rock mass behavioral factors and design variables into a single, all-encompassing numerical model. Obviously, some factors have more of an impact on stability than others. The intent of the present effort is to: (1) develop a model that incorporates the effects of opening and support geometries of an actual structure, (2) validate the model by comparing it with actual field measurements, and (3) refine the "displacement reduction factor" in the specific case of the Silver Shaft for use in detailed two-dimensional analyses.
2 FIELD MEASUREMENTS
The Silver Shaft is located about 350 m west of the existing No. 2 Shaft, as shown in Figure 1. Preliminary support of the wall rock was pro- vided by 2-m Split-Set rock bolts and mesh carried to within 1 m of the working face. The liner, nominally at 30 cm thickness, was carried 6 to 10 m behind the face and advanced in 5-m increments. The 6 to 7 m diameter opening was advanced by benching the north and south halves of the shaft at an average rate of 10 ft/d.
