Denver, CO: U.S. Department of the Interior, Bureau of Mines, TN 383, 1991 Jun; :1-2
Objective: Improve underground mine roof boIt support systems by alleviating problems associated with certain ground support systems currently being used. Approach: Convert a passive resin-grouted roof bolt system into a uniform, tensionable roof bolting system. Background: Resin-grouted roof bolts are one of the most common forms of ground support in U.S. underground coal mines. These support systems can be installed with a full-column of grout and be loaded with subsequent roof movements after installation. This is called a "passive" support system. An alternative to the passive support system is the mechanically-anchored, tensioned roof bolt system. This system is installed with resin grout. This system is designed to load or tension the support during installation by using a mechanical anchor, coupling, or threaded nut after the resin has properly cured. It is called an "active" support system and is used extensively for improved strata control in a laminated geologic deposition or in difficult ground conditions. To ensure that active bolts have been properly installed, the bolt loads must be checked by using a torque wrench or other suitable equipment. Unfortunately, this practice presents potential safety hazards and has limited accuracy. Laboratory and field experiments also reveal that the torque-to-tension ratio developed on active support systems can vary by 50% from bolt-to-bolt. Variations in bolt load installations can induce stress concentrations and lead to roof degradation or failure. To alleviate problems associated with tensionable "active" bolt installations, the Bureau of Mines has developed a support system that is independent of mechanical tensioning devices subject to frictional losses and system bleed-off. This support system also offers added advantages of providing pre-determined uniform installation load, and reducing support cost by at least 30%. How It Works: The Bureau-developed support system utilizes resin grouted rebar bolts installed in such a manner that the system becomes "active" immediately upon insertion. The system requires minor bolter modifications, special bolt installation stingers, and a calibrated load cell. The roof bolt machine is modified by installing a valve that permits 10,000 lb of upward force to be developed at the installation head, on demand, through the operation console. The stingers were designed to withstand enormous repetitious axial and bending loads generated between the installation head and the roof bolt. The calibrated load cell is utilized to calibrate the upward thrust to any desired level prior to support installation. To install the support system, the following method is prescribed: Drill the hole for the roof bolt as the manufacturer recommends for the particular diameter and length of support, ensuring that the hole is 1-inch longer than the bolt tendon. Install a cartridge tube of 15-second resin into the hole and insert the bolt in compliance to the manufacturer's specifications. After the resin has been adequately mixed, the roof bolt operator throws the lever on the bolter that pushes the bolt upward with the full predetermined "thrust" and maintains the pressure until the resin has cured. When the upward force is removed, the support system is immediately placed into the predetermined tension. Additionally, the bolt operator is no longer subjected to the hazards associated with required tension determinations. Test Results: Laboratory testing and massive underground tests at sites in room-and-pillar and longwall gateroads have demonstrated that this method is capable of installing bolts under uniform tensions equivalent to levels as high as 300 ft-lb of torque. The test site support systems were subjected to adverse underground conditions such as high levels of pillar sloughage, floor heave, and stress concentrations from an adjacent full-extraction pillar section. The support systems maintained the immediate roof, and no indications of strata separation or excessive deterioration were measured.
Denver, CO: U.S. Department of the Interior, Bureau of Mines, TN 383