The purpose of this paper is to present a viable method of reinforcing rock pillars by utilizing fully grouted reinforcing steel and a cement - fly ash mortal'. Although the fully bonded bolt concept is not new, as exemplified by the present widespread use of resin bolts within United States mines and by the use of cement grouted reinforcing used as back pins in the Kiruna Iron Ore line in northern Sweden,(1)(2) the method of stabilizing mine pillars with fully bonded reinforcing is new. This method, called the "grout-rebar system", was first incorporated at St. Joe Minerals Corporation's Fletcher Mine in December of 1972. The system was used on an emergency basis before extensive rese3rch on the mechanics of the system could be completed. Since December of 1972, however, approximately 206 pillars have been reinforced at Fletcher and the system is currently being used on a regular basis at two other St. Joe mines and at two competitor mines in the area. Basically, this paper is a case study of the efforts, research and results of utilizing the grout-rebar system, on a selective basis, at the Fletcher Mine. In addition to information about the system, this paper will explain in detail the three rock mechanics instruments developed by St. Joe and used to measure and monitor rock movement to validate the groutrebar system.
The Fletcher Mine is rated at 5,000 tons per day (4,539 metric tons per day) of lead ore, including secondary ores of zinc and copper in the form of galena, sphalerite and chalcopyrite, respectively. Mining is done in a stratiform deposit approximately 1,000 feet in depth (304.8 meters) with the host rock being principally dolomite. The ore is mined in large open stopes by the method commonly referred to as room-and-pillar mining. The mine is a trackless mine in that all mining equipment operates on rubber tires. The mining sequence usually consists of mining at the top of the ore horizon, utilizing the burn-cut blasting pattern or slabbing to a free face. Pillar size and spacing is 28 feet in diameter (8.53 meters) x 60 feet center to center (15.3 meters), respectively. Opening size is 32 feet wide (9.8 meters) between pillars x 18 feet average height (5.5 meters). Subsequent mining passes are done by "taking up bottom", utilizing a splitting and lifting pattern of horizontal holes. This process is repeated until the bottom of the ore horizon is reached, leaving rooms up to 56 feet in height (17.1 meters). The extraction ratio of the mine is approximately 78 to 80 percent. Due to many different factors, the pillars at Fletcher experienced deterioration in the form of continual surface spalling to splitting. Convergence of the mine, as measured by numerous rod extensometer points, was excessive in the areas where the pillars were experiencing this deterioration. The individual strength of each pillar was determined by many different parameters, but the failure mode in almost all cases was the same, i.e., shear or splitting axially. A physical examination of the mine was made and all pillars were rated as to their structural competence.