WV Seal (7255 bytes)West Virginia Office of Miners' Health Safety and Training

Roof Control Information

Note: The information obtained from these documents is accurate to the best of our knowledge and is not a substitute for current mining laws or regulations.  The state is not liable for any damages resulting from any information that unintentionally may be inaccurate or untimely.


The  names  of  roof  bolt  systems can be confusing.   Point   Anchor, Torque Tension, Double Lock, Tension Rebar, Combination, etc., are all names of roof bolt systems named by the manufacturer much the same as Ford or GM would name cars. However, there are only four basic types of roof support systems commonly used to support the mine roof. (1) Conventional Roof Bolts have an expansion shell or mechanical anchor that allows the bolt system to be placed in tension. (2) Full Grout Resin Rods made of deformed rebar has a resin or chemical anchor the length of the bar that locks the rod and roof together and resists movement. (3) Mechanical Anchor, Resin Assisted type bolts use both an expansion shell and one foot of resin, to enhance the anchor zone. These are normally used in soft roof to accomplish higher torque with stronger bolt systems. (4) Tensionable Deformed Rebar with approximately 4” of threads and a nut are normally used with at least two feet of resin that is mixed and held for a few seconds. A second spin is utilized to tighten the nut to the desired torque. This bolt can also have resin installed the full length of the bolt when a quick set resin is used on the top and a slow set resin is used on the bottom portion of the bolt.  The slow glue allows tension to be applied, while achieving full bolt grout - a great idea!

(Photo shows correct installation of steel strap "Bolt up with flanges down.")

Resin is used in conjunction with roof bolts that have an expansion shell to enhance the anchor zone of the roof bolt. Most expansion shells make contact with approximately 3 to 4 inches of the wall of the drilled hole. Normally, the equivalent of 1-foot of resin is used to increase the contacted surface at the anchor point of the roof bolt. This increases the capacity of the anchor and helps to maintain proper torque in soft shale conditions.  A dry hole (a bolt without resin) should be installed at the beginning of the shift to determine if the bolts are being installed at the proper torque.  If any resin-assisted bolt is torqued after installation, no more than approximately 150 ft.lbs. should be applied to the bolt head. If more torque is applied, the resin will break from around the bolt damaging the anchor.

 

Abutment Loading

Normally a mine will not have to deal with abutment loads if second mining is not performed by the mine operator. But more and more mines are being opened underneath, over and between older mines that did perform second mining. When second mining occurs, barrier blocks large enough to control the weight induced by the second mining are left behind. Some of the coal pillars that were intended to be mined could not because of the conditions in the area of these pillars. When a mine passes under, over, or between these areas, the load carried by these barriers or unmined pillars is transferred through the interburden to the working mine. These loads are not limited to directly under or over these areas. Normally the roof breaks and falls to an arch; which is normally a 21 to 25 degree angle. This is why weight is experienced a short distance before and after the mine developments are actually over or under the barrier or unmined pillars. In most mines, the worst squeeze is experienced when going onto or coming off these unmined areas of the older mines. The higher the depth of cover and the smaller the interburden between the old and new mines, the more problems with weight can be expected. In some of the combinations, the area becomes unminable. In many cases, narrowing the entry widths and increasing the pillar sizes will provide a stable environment through these areas. Longer roof bolts may not help with the abutment load, but may be necessary because of breaking roof. Since the mine roof and floor are converging, a floor-to-roof type support will be necessary to help control the weight.  It might be wise to begin additional roof support measures approximately one crosscut before and after entering these areas. Study over and underlay maps to locate and make preparations for these areas.

“SuperBolts”

Mechanical anchor, resin-assisted type roof bolts, commonly known as “Double Lock “ or “Super Bolts” have allowed coal to be mined safely that otherwise may not have mined at all. These roof bolts come in 7/8 and 1 inch diameter systems made of high strength steel, Grade 75 or higher, and in different length sections, (2 and 4 feet), that can be coupled together to achieve a variety of depths into the mine roof, even in thin coal seams. These bolts have an expansion shell and are normally installed with two equivalent feet of resin. The bolts have a friction reducing teflon washer that allows higher installation torque and still maintains the integrity of the bolt. Roof bolts 12 and 16 feet long won’t suspend 10 feet of roof. Sometimes, however, strong enough strata can be found at these depths which - when combined with one of these super bolt systems - will adequately support the immediate mine roof. (Several mines have found success when these bolts were installed in a star pattern as supplemental roof support at questionable intersections.)

ARMPS Program

There is now available an easy to use computer program from NIOSH that estimates the load carrying capacity of coal pillars. Analysis of Retreat Mining Pillar Stability (ARMPS) is user friendly even for the newest computer users. The program requests information that is easy to obtain and expresses a safety factor represented by a number like 1.3 or 1.5. Many options for mine layouts and the step-by-step data input will provide accurate results within seconds. One thing the user must always keep in mind is that ARMPS is a tool that will assist in mine planning, but like most tools, improper use can be a costly experience. Using a large pillar for a mine that doesn’t plan second mining is a waste of reserves. On the other hand, using a pillar too small to adequately support the mine roof also ends with wasted reserves, and possibly, loss of equipment and lives. However, ARMPS does not account for the effects of multiple-seam mining, therefore site specific evaluations are an important part of the formula to success. When plans do not include second mining, a safety factor of 1.5 for a developing load has provided a stable environment in most mines. When plans do include second mining, a safety factor of 2.0 for a developing load will provide a large, or strong enough pillar to accept the loads placed on the pillars (excluding multiple seam mining).  Also, accuracy of ARMS is reduced when depth of cover exceeds 1,000 feet.

Highwalls and Auger Holes.  Opening deep mines on strip benches with auger holes is nothing new. The mining code requires 50’ of natural strata between drift openings. In the 70’s,miners crawled to the back of the auger holes and timbered their way to the surface. In the 80’s, we stopped entering the auger holes and began filling the entrance of the holes with timbers 10’ in length. This prevented the miners from having to enter these holes, but did not always provide the support necessary to have a stable highwall in all cases. Grouting the entire length of the auger holes  now appears to be the best method. The strength of the grout can be changed at the time of mixing to suit the needs of the highwall. There is no reliable way to determine the strength of the webbing between the auger holes. The quality of the mine roof normally weakens as the coal seam outcrops with mud seams and cracks. Although the auger operator didn’t intend to cut the auger holes together, it sometimes happened. Some of these conditions can be observed from the surface of the face-up, but most often the holes cut together deep under the mountain and cannot be seen from the surface. Grout is thin enough to flow into all the voids creating a stronger foundation for the highwall to rest upon. Today, no one is exposed to the danger of entering the auger holes



Operator Protection System (OPS) An alternative means to safely support the mine roof when the automated temporary roof support system (ATRS) is unable to reach the roof has been developed. This re-usable roof support system was tested in Consolidation Coal Company’s Buchanan No. 1 mine, near Big Stone Gap, Virginia. The Virginia Division of Mines, MSHA, Heintzmann Corporation, and several mine operators developed the system based on 50-ton Heintzmann roof jacks. The system is assembled in an area of supported roof and is used for bolting overcasts, belt channels and cavities created by roof falls.  Channel beams are used as a base similar to sled runners so the system can be pushed into the unsupported area with a scoop or roof bolter. Four 50-ton jacks support a network of steel beams and sliding canopies to protect the drill operators. Elk Run Coal Company and Heintzmann Corporation provided a demonstration of the OPS in the parking area of the training center. Roof specialists from MSHA’s Mount Hope and Madison offices and MHS&T’s Oak Hill and Danville offices were in attendance.

 


Roof Control Seminar

All MHS&T roof control inspectors attended a roof control seminar at the Mine Health and Safety Academy, Beckley WV on May 30 and 31. Representatives from NIOSH, MSHA, and MSHA’s Technical Support group provided information about remote control mining, ATRS extensions, pull tests, multiple-seam mining, and a roof bolt jam nut problem that could only be observed after a roof fall occurred. The jam nuts on some of the roof bolts are so strong that it caused the expansion shell to twist upon installation and render the roof bolt ineffective. If you observe this problem, (indicated by twisted leafs on expansion shells after roof falls occur) it should be discussed with the roof control inspector in your region. Representatives from Jenmar, Fosroc, Fletcher, Excel Mining, and American Commercial introduced new technology that their companies are developing for the coal industry. They displayed roof and spider caps that are used in conjunction with the roof bolt plate to reduce the amount and size of scaling roof materials. New flexible mine sealants are now available to keep the air from the mine roof that is susceptible to scale. A new computerized roof drill system now determines the type of roof material being drilled, records the location and number of separations experienced while drilling and downloads the information to a computer for later use.