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Two main entries are driven north and south from the main shaft and cross entries are turned at intervals of 2,200 feet. Panel or room entries are driven at right angles to the cross entries every 800 feet. Rooms are turned right and left from the panel entries to depths of approximately 350 feet. Rooms are not projected to be driven through to rooms of adjacent entries, and no deviations from this plan were observed. Entries are driven 12 feet wide on 52-foot centers; rooms are driven approximately 28 feet wide on 68-foot centers; and cross-cuts are driven at approximately 60-foot intervals in both rooms and entries.
Cutting is done in the bottom coal to a depth of 812 or 9 feet and the cuttings are loaded out after shooting.
ROOF AND FLOOR
The immediate roof consists of from 10 inches to 3 feet of draw slate or shale, which requires close attention. The maximum cover is 500 feet and slips and rolls are encountered infrequently.
The floor is a rough hard fire clay approximately 11 feet thick.
In entries, the slate immediately above the coal is taken down with the coal. The slate arches itself and entries are not generally timbered. However, props and crossbars are set at some places in the entries where the condition of the roof requires their use. Loose rock that required timbering was observed at several places, notably in the first west, south, and the fifteenth and sixteenth north entries. The entries should be timbered systematically.
In rooms, the company has a standardized method, which is verbal and which provides for the following: two staggered rows of props on 41/2-foot centers are carried on each side of the track to within 10 feet of the face. As soon as the coal is loaded out of the center of the face, two safety posts are set. This system is not adhered to in all cases. The timbering rules should be printed, and copies should be given to all underground workmen. Strict compliance should be demanded. In addition, three sets of temporary crossbars should be kept close to the face to insure protection to loading-machine operators, cutters, and drillers. These should be moved up as the face advances, and should be replaced with props. Wedges are used on props instead of cap pieces. Cap pieces at least 18 inches long, as wide as the post, and not less than 3 inches thick should be used on all props. No timber is recovered.
Miners were observed testing the roof with picks. In the interest of safety, all underground employees should be instructed in the vibration method of roof testing with proper roof-testing bars.
When rock is being barred down, broken up, or there is danger from flying objects from other causes, goggles should always be worn to prevent possible eye injuries.
EXPLOSIVES AND BLASTING
Permissible explosives are used in connection with fuse and No. 6 blasting caps for blasting down the coal and rock. Clay stemming in prepared dummies is used, and tamping is done with a scraper. The steel tamping bars should be replaced with wooden ones, and electric detonators fired electrically should be used instead of fuse.
Blasting is done on the off shift by shot-firers who drill the holes and prepare the charges. Primers are made up at the face after the drilling has been done. From 1 to 1142 pounds of explosives are used in each hole. Snubber shots are fired to break the bottom coal after the day shift goes out, and the top holes are fired by the night shot-firers after the bottom coal has been removed.
From three to four holes are fired at one time in each place to be shot. Ample warning is given by the shot-firers when shooting, and when misfires occur, it is stated that another hole is drilled and shot 1 foot from the misfire. It was also stated that after this shot is fired, a careful search is made for the explosives that were in the missed hole.
The face bosses make out a written report at the end of each shift, as to the amount of explosives used, and that which is carried over.
Explosives are transported from the surface in a mule-hauled, insulated explosives car to underground explosive boxes, located on each section. From 1 to 3 days' supply of explosives is usually stored in these boxes. They are substantially constructed, and are provided with strong locks. They are located in crosscuts 100 feet or more from the working faces. Detonators are carried to the working sections of the mine by one of the night crew on foot, and are stored in locked boxes located a safe distance from the explosives boxes.
Explosives together with detonators are carried from the explosives boxes to the working faces in open containers, and are sometimes left near the working face unattended. Examinations for explosive gas are not made before or after shooting, nor is a fire run made after shooting has been completed.
In the interest of safety in coal mines, not more than a 24-hour's supply of explosives or detonators, including any surplus remaining from the previous day, should be stored underground.
Only permissible explosives and permissible blasting devices should be used, and these should be used in a permissible manner. They should be fired only with electric detonators of proper strength, and fired with a permissible shotfiring device. The roof and face of working places should be tested for explosive gas and unsafe conditions before and after a shot is fired. Shots should be fired singly. Fire runs should be made after all shots are fired.
VENTILATION AND GAS
The mine is ventilated by one continuous air current. The volume of air at the main intake of the mine is 55,510 cubic feet per minute, and at the main return it is 55,610 cubic feet per minute. This is believed to be inadequate, and should be substantially increased. Main intake air courses are adequate in number and size, but are congested with rock falls that should be cleaned op, to permit a freer flow of air. There are approximately 107 men in the mine, and a split ventilation system utilizing air crossings should be used instead of doors for conducting ventilation in all main passageways. As evidence of the inadequacy of the ventilating system, it was found that approximately 15 percent of the total intake air was all that was reaching the first working section.
All main haulageways and hoisting are on return air. In the interest of safety in coal mines, haulageways, traveling ways, and hoisting should be kept on intake air so far as possible. The fighting of mine fires or recovery work following mine explosions may be especially difficult and dangerous if the haulage and hoisting are on return air. Numerous lives have been endangered and lost in coal mines through having haulage or hoisting on return air at the time of disaster.
Table 1.- Air measurements, No. 5 Mine, Centralia Coal Co., Centralia, Il.
of air per Location in mine: Full return at main south overcast
55, 610 Main intake near the fan.
55, 510 Last crosscut 16 and 17 north..
7, 280 Last crosscut 18 and 19 north..
4, 845 Last crosscut 11 and 12 south, 6 west
8, 910 Last crosscut 18 and 19 south, 4 west.
5, 100 Intake 22d south, 4 west...
5, 150 Last open crosscut 22d, south, 4 west
(1) 1 No velocity.
AIR AT WORKING FACES In general, the air is not being properly conducted to the working faces, as room crosscuts are not stopped, nor are line brattices used from the last crosscuts to the working faces. This was observed more particularly in the 22d south off the 4 west section, where the velocity was too low to turn the vanes of the anemometer. This was due to too many leaky doors in this section. Many of these doors are not in use, and should be replaced by substantial stoppings; room crosscuts should be stopped off; and line brattices should be constructed from the last open crosscut to the working faces.
Crosscuts are turned in rooms 60 feet apart as a rule; however, some places are being driven as much as 140 feet past the last open crosscut without ventilation. This is a dangerous practice, and should be prohibited as no place should be driven more than 60 feet without a breakthrough.
Some of the old workings are not securely sealed, are not properly ventilated, contain carbon dioxide, and have a low oxygen content.
All entries, rooms, panels, or sections that cannot be kept well-ventilated through and regularly inspected, and that are not being used for coursing the air, travel, haulage, or the extraction of coal, should be sealed by strong fireproof stoppings and provided with some means of bleeding off the gas into the return, should the gas pressure build up behind the seals.
In every sealed area, one or more stoppings should be fitted with pipe and valves to allow sampling of air behind the seals. Excess pressure relief may be secured by boreholes to the surface.
STOPPINGS Main entry stoppings are of fireproof construction. Side entry stoppings are constructed of plastered wood.
There are 18 single doors of wooden construction used in connection with the haulage. These doors are equipped with latches to hold them open while trips or other equipment are passing through. Mine doors should be self-closing, not provided with latches or other devices to hold them open, and should be installed in pairs to form air locks to assure a continuous flow of air to the working sections.
Doors are deliberately left open by haulage men. In the 18th and 19th north, one door was observed to be blocked open for 8 minutes. This practice should be discontinued immediately.
GAS INSPECTION AND TESTING The mine is being operated as nongassy; however, the company takes the precaution of preshift examinations for which they are to be commended. It is said that explosive gas has not been found in the mine by the fire bosses since the mine was opened. The fire bosses mark the date in each place inspected, and record the results of their findings in a book kept on the surface for that purpose. The fire bosses carry permissible flame safety lamps, and use trolley locomotives while traveling in and out of the mine. The use of trolley locomotives by fire bosses is a dangerous practice, and should be discontinued.
TABLE 2.-Analysis of air samples collected in No. 5 mine, Centralia Coal Co.,
Centralia, Ili., Sept. 2, 1942
The laboratory analyses of the air samples, as shown in table 2 on the preceding page, indicate that a poor quality of air is being directed to the working sections of the mine. This is shown by the low oxygen content of the mine atmosphere, except in the sixteenth north section which is first on air, Air in the working places and traveling ways should contain at least 20 percent oxygen, and not more than 0.5 percent carbon dioxide. Pure air insures healthful working conditions and adds to the efficiency and safety of the individuals. Except for
bottle No. 533, only slight traces of methane gas were found by laboratory analyses. Nevertheless, the fact remains that explosive gas accumulations are found in neighboring mines working the same coal bed.
Air sample No. 498 was taken on the surface from an abandoned oil well, for the purpose of calling to the attention of the management the dangers that may come from one of these oil wells becoming opened into the mine, or into a connecting mine.
"It is, therefore, highly important to continue making preshift examinations for inflammable gas, as is being done at the present time, so that in case methane gas is encountered, it will be discovered before the accumulation has reached dangerous proportions.
GAS IGNITION HAZARDS
It is obvious that there is no gas ignition hazard as long as there is no methane gas in sufficient proportions to ignite, but since there is no way of predicting when methane may be liberated into the mine, consideration should be given to the elimination of possible ignition hazards, so far as possible, by operating and maintaining permissible mining equipment, when it is necessary to operate such equipment in other than pure intake air. Another fire hazard could be eliminated by stopping the practice of smoking and the carrying of smokers articles underground. The carbide lamps now being used in the mine are conducive to ignition and fire hazards, and should be replaced by permissible electric cap lamps.
Available records show that the use of open lights and smoking underground have been responsible for 23.7 percent of all explosions occurring in coal mines over a period of many years; consequently, this mine should be classed as a gassy mine, and operated as such.
Bureau of Mines' tests and experiments have shown that the relative explosibility of dust from various coals is indicated by the ratio of the volatile combustible to the total combustible content of the coal, and 'may be expressed by_the formula :
Volatile matter+Fixed carbon Furthermore, these tests have shown that coal dust with a ratio in excess of 0.12 is explosive, and that the explosibility of the dust becomes greater as this ratio increases.
When the values for volatile matter and fixed carbon, as shown previously in this report in the analysis of coal under "Coal bed," are substituted in the foregoing formula, a ratio of 0.45 is obtained. This indicates that coal dust from the No. 6 coal bed at this mine is highly explosive, and would readily propagate an explosion.
Much fine coal dust is made in the operations of mining, loading, and dumping coal. Since water is not used for allaying coal dust, a considerable amount of very fine dust is raised into suspension by the cutting, drilling, blasting, and mechanical loading operations at the face and at dumping points. This dust is picked up by the ventilating air current, and is deposited along the roof, timbers, ribs, gob, and floor of the rooms and entries and, in addition, there is considerable spillage of coal, and dust is thrown into suspension around the shuttle car dumping points.
From observations made of the dust conditions in this mine, the following suggestions are offered to lessen the coal dust explosion hazard, protect the health of the miners, and decrease the accident hazard by increasing visibility:
1. Water should be applied to the cutter bars of mining machines to keep the machine cuttings wet while cutting is being done.
2. The face region should be wetted before and after blasting. 3. Water should be sprayed on the coal pile while the coal is being loaded.
4. Each working place should be kept free from dust by the effective use of water for a distance of approximately 40 feet from the face.
Ten dust samples were collected at this mine and were analyzed at the laboratory of the United States Bureau of Mines at Pittsburgh, Pa. The results of these analyses are shown on the following page.
TABLE 3.- Analysis of dust samples collected in No. 5 mine, Centralia Coal Co., Centralia, IU., Sept. 2, 1942
Cumulative (100 percent
through 20-mesh) percent through
Location in mine
Kind of sample
48-mesh 100-mesh 200-mesh
16 north at room 33.
Rib and roof
Rib and roof.
South, room 16-
Rib and roof.