1. Purpose of Mine Timbers
When coal is removed from a seam, the rock strata immediately over the mined area becomes unstable due to the withdrawal of support and is eventually filled with cracks and openings. These openings vary in size, which depend on the nature of the strata. The fragments of rock nearest the opening will fall if they are not supported since there is nothing to bind them to the roof strata. Above these will be found other fragments of rock that are so interlocked as to form a natural arch over opening. Sometimes the opening in the seam is so narrow that the rock over the opening does not develop cracks and become unstable for some time. In such case the immediate problem of roof support, after an opening has been made, becomes one of holding the draw slate, if such is present, until it can be removed. The problem may be one of supporting a thin layer of roof coal, which will prevent the mine air from reaching the roof. Once mine air is permitted to work into the rock above it can make the top slack and make it unstable. In either case the problem of support is largely that of holding up the immediate roof. The weight of draw slate, which may vary from a few inches to several feet in thickness, may produce a pressure on mine timbers up to 500 pounds per square foot. To this pressure is added from 5 to 50 feet of strata, which crumbles readily and falls out until some solid rock above is reached. This may cause added pressure on timbers up to 3 tons per square foot. Mine timbers, then, should ordinarily be designed to withstand pressure up to 3 tons per square foot.
2. Choices of Timbers:
Timber for mine roof support should be long-grained so that it possesses elasticity as well as strength. This means that a prop subjected to pressure on its ends will bow outwards considerably without breaking while still offering resistance to the applied pressure. If the pressure were to be removed the prop would regain its original shape. A prop should be strong enough to bend without breaking, giving miners warning of an approaching fall. The very soft woods such as cypress and willow, possess great elasticity but do not have the strength to resist great pressures. Hardwood timbers such as oak and beech are strongly resistant to pressure but do not offer any elasticity and would break after very little bending.
3. Posts or Props:
Posts or Props for use in rooms may be either round or splits. Round Props are the natural logs, cut and used in a green or seasoned state. Splitting is preferred to sawing an individual stick where its size is such that several props can be made from it. Splitting does not destroy the sapwood or cause undue injury to the grain or fiber of the prop. Round props vary in diameter from 6 to 18 inches. The ratio of diameter to length is 1 to 12. that is if a coal seam is 4 feet high it would require a 4 inch diameter prop, although a 6 inch would be safer. The ends of the prop are cut to within two inches of the seam height although some extra length will allow the workman who sets the prop to fit it to any situation. Where the seam height is constant, props of the proper length will do away with cutting on the spot and will eliminate much labor. Props should be straight and have square ends. When set Perpendicularly, the supported weight will act through the axis of the prop. A crooked prop will tend to bend and could pop out of place should a force be put it. An axe cut prop with its uneven bearing surface produces similar results. The entire end of the post is not supporting weight and the advantage of the prop is lost. Square ends permit the entire end-bearing surface of the prop to be used most efficiently.
4. Cap pieces:
The function of a cap piece is to distribute the resistance of the post over a greater surface and to prevent the prop from cutting into the roof as it would otherwise. The cap should be of soft wood and should completely cover the head of the prop. As the weight of the roof comes onto the prop, the end will be forced into the soft wood of the cap, which will bind it together preventing the crushing of the end of the prop. The cap piece as normally used with a prop, is a wedge shaped piece of wood having a length sufficient to extend from 2 to 6 inches in both directions beyond the edge of the prop. Where roof support demands a greater bearing surface than can be furnished by a cap piece, it is customary to use larger pieces of heavier wood which may be wedged in place either by a thin cap piece or by using a post of the correct size to make a tight fit.
5. Setting Props
Props are used for three general purposes. The first is to retain the immediate roof in place and prevent the slate and shale from falling. The second is to give warning of increasing pressure in pillar sections. Props should be set tight so they give a “nipping” or cracking sound as pressure comes on the prop. The third is support the roof where its movement is practically irresistible. In this case the butt end should be pointed to allow it to crush, or the prop would be set on a footboard, which in turn is set on a small mound of dirt after which the prop is wedged in place. This allows the roof to settle without breaking the prop immediately and prolongs its usefulness. Where slate exists over the coal, and is not more than 3 inches thick, the best plan is to bar it down and gob it. Where the slate is from 3 to 6 inches thick, a single row of props along the gob side of the room track may be sufficient. If the roof is poor or the slate is very thick, extra long cap pieces may be used to give greater bearing surface. Systematic placing of the timber is the best practice, regardless of whether the appearance of the roof does or does not indicate the need for support. The system to be adopted must be determined for each mine from the following conditions: The width of the opening, the height of the coal, the nature of the roof and floor, the thickness of the pillars, and the location of the road in the room working place. A system adopted must be followed closely as long as conditions remain unchanged. An example is rooms 10 to 12 feet wide must have a line of props down the middle of the room on 4 ½ foot centers. Pillars in this case are 40 to 80 feet thick. Rooms 20 to 25 feet wide will have several rows of props with the same spacing in each row but with row-to-row distances of 4 to 6 feet, depending on conditions. Props along a roadway may be provided with extra long caps. The last post must be not farther than 6 feet from the hand cut face. In a machine cut face this distance is set by the mine foreman and mine inspector jointly. A great number of roof falls will be prevented when systematic timbering is practiced than if the placing of such timber is left to the judgment of the miner.