 This video production, roof evaluation and decisions, is presented as part of Images commitment to the REAP program. Good roof evaluations and making better decisions in response to these evaluations can do much in helping prevent roof fall accidents. If you've ever wondered why falls of roof and ribs are the number one cause of fatal underground coal mining accidents, consider the fact that whenever a miner cuts into the earth's strata, he uncovers a new opening or area no one has ever been before. This makes him and his job quite unique. He has become an explorer, a discoverer of new territory or space. But unfortunately he has also exposed himself to the possibility that the strata of rock and coal surrounding this new opening might fall on him. To better understand what causes a mine roof to fall, it is necessary to know something about the strata or overburden of rock and earth that overlies the coal seam. When a coal seam is mined, open areas such as haulageways and rooms are created. The roof directly above the coal seam is called the immediate roof. This roof is made of layers of rock that can vary in thickness from a few inches to several feet. The strata above the immediate roof is called the main roof. This roof can range in thickness from several feet to many thousands. When coal is mined, the immediate roof may begin to sag from the pressure of its own weight as well as from the weight of the overburden. Activity taking place in the mine roof can cause tension cracks to appear. When this occurs, the immediate roof may then break, causing pieces of the roof rock to fall into the workplace. Since this roof usually consists of thin layers of rock, it is weak and will not take much stress or pressure from above, and will therefore require constant support during the mining operation. Saging and cracking of the immediate roof not only causes pieces of roof to fall, but also causes the strata to separate, forming a space which can fill with methane, gas or water. This creates the potential dangers of an explosive gaseous atmosphere or excessive moisture that can weaken the mine support. The main roof, which is part of the overburden, is supported by the natural pillars of coal which are left standing during the mining operation. As mining continues, the roof load is distributed on the pillars surrounding the opening. The more coal that is removed, the more pressure the main roof puts on the pillars. If this pressure becomes too great, large pieces of coal may begin to break off the outside edges of the pillars. This is called a rib fall or a rib roll. If all the coal pillars in a section were removed, the roof of the entire section would fall, eventually if not immediately. Roof falls covering large areas occur less frequently today, but falls covering small areas or involving only a few pieces of rock still occur, and miners are still being injured or killed by these falls. To prevent such accidents from occurring, every person who works in an underground coal mine, regardless of what their job is, needs to know more about mine roof. They need to know how to evaluate any signs or visual indications of a potentially hazardous roof. Evaluating the mine roof involves making a careful visual inspection before entering a workplace, while listening for any sounds that might indicate the roof is working. Sounds of activity and change taking place inside the roof such as rock strata shifting and cracking. Some of the things you should look for when evaluating a working place are stress cracks in the roof, in the ribs, and in the floor. Look for any unusual rock formations such as kettle bottoms, those smooth rounded pieces of rock sometimes cylindrical in shape, which can drop out of the mine roof without warning. Also, look carefully for clay veins or any other abnormal roof conditions such as slips or faults. Other indications that a problem may be developing are changes in conditions, a formerly dry place that has now become wet. There may be signs of spalling, small chips of rock on an otherwise clean floor. In supported areas, you should visually inspect for crossbars, timbers, and posts that are bent or cracked from heavy pressure. Look for the squeezing of cribs, header or cap pieces squeezed down and over posts. And notice if there are any timbers that have decayed or become unsafe. If the area is roof bolted, check to see if the roof bolt bearing plates show signs of stress or if any of the bolts have worked out. Knowledge of rock structures, mine roofs, and the reason roofs fall will help you identify potentially hazardous roof conditions and will help you make accurate roof evaluations. The next step is to make the proper decisions on what you do as a result of your evaluations. In many mines, kettle bottoms are a common occurrence. These smooth rounded pieces of cylindrical shaped rock are thought to be the remains of a tree stump which has been replaced by sediments so that the original form has been well preserved. Often, a kettle bottom will be difficult to detect in the mine roof because of a thin layer of shale laying just underneath it. At other times, kettle bottoms are readily visible and easy to identify. They can vary in size and some have been observed up to three feet in diameter. The method used for supporting a kettle bottom depends upon its size. Most small kettle bottoms can be held in place by two roof bolts with header blocks installed on either side of the kettle bottom. If the kettle bottom is large, two roof bolts should be installed on each side with a strap extended between them under the bottom of the kettle bottom. An important point to remember, do not try to support a kettle bottom by installing a roof bolt through it because you have no idea how far the kettle bottom extends into the mine roof. In dealing with mine roof, two other conditions that present a problem similar to horse backs or kettle bottoms are slips and cutters. A slip is a joint on the coal seam or mine roof that may occur when the roof strata cracks or fractures because of loading and movement of the different layers of rock. In evaluating a slip, it is important to consider the location of the slip plane, the angle in which the slip plane extends into the mine roof and the angle of any other slip planes that might intersect them. The location of a slip or cutter with respect to cross cuts and intersections of entries shall also be considered in the decision of selecting the necessary support for the area. If roof bolts are used, they must be of the proper length and installed at the correct angle. Other supplemental supports such as posts, crossbars, or cribs can be added as needed to help support the area affected by the slip or cutter. If the immediate roof were level, continuous, and consisted of nothing but one type of rock, mining would be much simpler and safer. But this is not the case. Differences in the makeup of the rock strata, cracks or faults, and unusual rock structures found in the immediate roof are some of the conditions that create hazards when coal is mined. In addition, the roof strata may not be the same throughout a mine and changes from one type of rock to another often create the potential for a roof fall. Here in this example, an area is shown where the roof changes from sandstone to shale. Because these layers are made up of different materials, they do not adhere or stick together readily, and as a result often separate and fall unless the condition is properly evaluated and appropriate roof supports installed. Similar roof conditions exist adjacent to faults, rolls, and sand channels. In such areas, the sedimentary layers are not continuous, and supporting the roof is difficult. Determining the proper support for these areas must be done on a site-to-site basis, and the experience of the support personnel will greatly help in the situation. Nevertheless, whenever mining is conducted near these hazardous conditions, additional precautions and roof supports may be necessary. Often during mining operations, there is a soft rock, shale or slate above the coal, which falls with the coal or soon after the coal is removed. Such rock usually ranges from 2 to 24 inches in thickness and can create a special hazard to miners, especially where temporary supports are manually set prior to the installation of roof bolts. The proper safety procedure in this situation is to take down the loose draw rock or support it if it has sufficient strength. This is especially applicable where the roof bolting machines are equipped with ATRS. In any event, the presence of draw rock necessitates special consideration and roof support procedures. These include 1. How will the temporary supports be installed? 2. Will it be necessary to install permanent roof supports on reduced spacing? And 3. Will it be necessary to use oversized bearing plates, straps, headers, or other support devices to obtain additional bearing surface in conjunction with the roof bolts? After the roof evaluation is made, whatever decision is made must be for the main purpose of providing a safe working place for the miner. His health and safety is of primary importance. Another common occurrence in mining that causes roof control problems is rider coal seams, a thin coal seam above a workable seam separated by a bed of rock. Many things have to be considered when determining how to support the roof in an area where a rider seam is present. Without being able to actually see the condition underground, there is no one who can tell you what to do. The nature and thickness of the strata between the coal bed and the rider seam have to be carefully evaluated. This can only be done by drilling test holes into the rider seam in every work place. Any change in the location of the rider should be passed on to the foreman and to the next crew coming to the section. Generally speaking, where a test hole indicates the rider seam is as much as 10 feet or more above the coal bed and the rock between the rider is fairly competent, it is likely that existing roof support methods will work. When a rider seam gets closer to the coal bed, the importance of roof evaluations increases dramatically, since the detrimental effect on roof stability will also increase. This usually results in the need for additional roof support. Water is another factor to be considered when discussing roof control problems. When excessive moisture exists in the roof or coal, it compounds all the problems mentioned before. Water can act as a lubricant and allows for easier movement along slips and other discontinuities. Laboratory studies have shown that the strength of shale or sandstone can be much less when it is wet, as compared to the same rock when it is dry. Generally, the water will flow greater where the roof is highly jointed and fractured. In a highly fractured roof, stability would be critical even without the presence of water, but the water compounds the problem. Therefore, special precautions are necessary when wet conditions are encountered. At times, full grout resin bolts can be beneficial in these areas, because they seal off the bolt hole. Just as poor roof conditions previously described create a hazard, good roof, which is relatively easy to support, causes its own unique problems. When a mine or section of a mine experiences good roof conditions for any length of time, the miners can be lulled into a false sense of security. They begin to take shortcuts and chances that they would not even consider if roof conditions were poor. Proper roof evaluation and decision making is always an important part of roof control safety, even under good roof conditions. And although some excellent methods, materials and roof control have been developed to prevent roof falls, it is still a very serious problem, because the majority of fatalities in underground coal mines are still caused by roof and rib falls. The underground miner must always be alert to changing roof conditions. He must learn to recognize the basic types of rock and what to expect from the different types of rock found in his mine area. As he grows in knowledge and experience, today's underground coal miner will grow in the assurance and satisfaction, but he is doing his part to prevent injury or death from the ever-present danger of falls of roof and rib. To this end, MCHA has designed this audio-visual presentation.