INTRODUCTION
The U.S. Bureau of Mines, in cooperation with the Homestake Mining Co., Lead, SD, and the University of Utah, Salt Lake City, UT, conducted a field demonstration to evaluate the behavior of cable bolt supports at three different sites at the Homestake Mine. This paper presents the results of the demonstration at one of these sites which was located between the 6500 and 6650 levels of the 45-48N stope. The area studied was supported with both conventional and birdcage cable bolts.
45-48N STOPE
The Homestake Mine is located in the Black Hills of South Dakota in the town of Lead. The three major formations in the Lead area, the Poorman, the Ellison, and the Homestake, are all Precambrian in age. The 45-48N stope is located in the Homestake Formation and lies between the 6500 and 6650 levels.
Ground control for the 45-48N stope consisted of double 18.3-m-long conventional and birdcage cable bolt supports and 1.5-m-long rock bolts. Stations for the cable support patterns began at the south end of the stope at station 0 and continued north to station 16 (fig. 1). Conventional cables bolts were installed on 3-m centers between stations 0 and 8 and birdcage cables were placed on 3.4-m centers between stations 9 and 16. A typical cross section of the stope showing the fan-shaped configuration of the cable supports is shown in figure 2.
INSTRUMENTS
Instruments placed around the 45-48N stope consisted of extensometers and strain gauges attached to cable supports. The extensometers had either five or six groutable rebar anchors, depending on the depth of the hole, and were read electronically with 102-mm range linear
potentiometers located at the collar of the hole in the drift on the 6500 level.
The cable support strain gauges were installed to determine the amount of load being placed on selected cables. Each gauge is 0.69 m long and uses a thin (0.25 mm in diameter), nickel- chromium wire that is wound into spiral grooves between the outer strands of the cable.
