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portions of the mine. In such cases, lamp stations are frequently provided at some point on the main intake of the mine near the mouth of the entries or headings leading to these workings. Similar lamp stations, called relighting stations, are likewise often provided at different points on the main intake wherever safety lamps are used, where lights that have been extinguished may be relighted. A lamp station is a simple opening made in rib or pillar coal on the intake airway, where a strong current of pure air is passing, and where safety lamps may be kept or relighted when extinguished.

Shanties.-The various other shanties used in the operation of the mine, such as the mine-boss shanty, tool shanty, oil house, etc., as well as the wash rooms and hospital rooms, are simple openings made in the shaft or entry pillar, the size and arrangement depending on their use. Many mines now have wash rooms and hospital rooms at the shaft bottom, supplied with steam and water pipes, for the convenience of the men and for the care of the injured. The walls of these rooms, as also those of the mine-boss shanty, are often cemented and whitewashed, and the floors are also cemented so that they can be kept clean and comfortable. Tool shanties are often located at convenient points for the distribution of the tools to the company men, and sometimes there are blacksmith shops in the mine for the sharpening of the tools, though this is generally done at the surface.

Manway About the Shaft.-A small manway should encircle at least one end of every hoisting shaft. This manway is sometimes made by enlarging the shaft excavation by widening on the rib, but this is not a good plan. At other times, a narrow heading or passageway is driven in the solid coal from one side of the shaft to the other. A manway in the shaft pillar is objected to by some as endangering the shaft pillar, but allowance can be made for it in laying out the size of the shaft pillar, and it can be well timbered, if necessary, so as to run no risk of weakening the strata near the shaft. No hoisting shaft should be operated without such a manway, in order to avoid the risk to which the cager is exposed if obliged to pass under the moving cages.

SURFACE TRACKS FOR SLOPES AND SHAFTS

The arrangement of the tracks on the surface naturally differs at every mine, owing to the different existing conditions. All surface roads should be so arranged that the loaded cars can be moved with the least possible power, always looking out for the return of the empties with as little expenditure of power as possible. To secure the running of the loaded cars from the mouth of the shaft or slope by gravity, a slight grade is necessary, the amount of which depends on the friction of the cars, which varies greatly. Care should be taken that an excessive grade is not constructed, or there will be trouble in returning the empties from the dump to the head of the shaft or slope.

The tracks connecting the top of the shaft and the tipple may be very short, or of considerable length, depending on the conditions at each mine. Usually from 20 to 60 ft. will be sufficient, although no definite rule can be given for this.

There are two general arrangements of tracks about the head of a shaft: First, where the loaded cars are removed from the cage and the empty cars placed upon it from the same side of the shaft; second, where the loaded cars are removed from one side of the shaft and the empty cars returned to the cages from the opposite side of the shaft.

In either case there are usually several empty cars on the platform ready to be put on the cages when the loaded cars have been removed.

Where the conditions are such that the loaded cars can be run by gravity to the dump, a good plan is to have a short incline, equipped with an endless chain, in the empty track. The empty cars can be run to the foot of this, hoisted by machinery to the top, and thus gain height enough to run them back to the shaft or slope by gravity.

At the Philadelphia and Reading Coal and Iron Co.'s Ellangowan colliery, where the tipple at the head of the breaker is above the level of the head of the shaft, the following plan is used: The loaded cars are taken off the east side of the cages, and run by gravity to the foot of the incline where the axles of the car are grasped by hooks on an endless chain and the car pulled up to the tipple. After being dumped, the car is run back from the tipple to the head of the incline, and is carried to the foot of the empty track of the incline by an endless chain. The foot of the empty track is several feet higher than that of the loaded track, and the cars are run by gravity around to the west side of the cages, and are put on from that side. The empty cars, as they run on the cage, have momentum enough to start the loaded car off the cage and

on toward the foot of the incline. There are a number of hooks attached to both the empty and loaded chain on the incline, and there are often several loaded and several empty cars on different parts of the plane at once. This arrangement permits of the hoisting of from 700 to 800 cars per day out of a shaft 110 yd. deep, with single-deck cages.

Another excellent arrangement for handling coal on the surface is the invention of Mr. Robert Ramsey, and has been adopted by the H. C. Frick Coke Co. and a number of other prominent operators. A description of this arrangement as applied at the H. C. Frick Coke Co.'s standard shaft is as follows: The landing of the shaft is made slightly higher than the level of the tipple, which is north of the shaft. South of the shaft is located a double steam ram, one ram being directly in line with the track on each cage. Directly in front of the rams is a transfer truck, worked east and west by wire rope. The loaded car on the cage is run by gravity to the tipple, where it is dumped by means of a nicely balanced dumping arrangement. As soon as it is empty it rights itself and runs by gravity alongside the shaft to the transfer truck, which carries it up a grade to a point directly in line with the cage that is at the landing, and one of the steam rams pushes it on the cage, and at the same time starts the loaded car off toward the tipple. This second loaded car is then returned by the same means to the opposite cage. The whole mechanism is operated by one man, by means of conveniently arranged levers, each of which is automatically locked, except when the proper time to use it arrives. It is therefore impossible for the topman to work the wrong lever and put an empty car into the wrong compartment of the shaft. Besides the one man at the levers, there is but one other man employed at the tipple, and his work is solely to look after the cars when dumping. All switches are worked automatically, and the average hoisting at this shaft is at the rate of 3 cars per minute. The shaft is about 250 ft. deep, and single-deck cages are used.

The Lehigh and Wilkes-Barre Coal Co. has a system in use at a number of collieries that has also proved very effective. In this system the loaded cars are run by gravity from the cage to the dump, and the empties are hauled from the dump back to a transfer truck by a system of endless-rope haulage. The transfer truck carries the car to a point opposite the back of the cage. The empty car runs by gravity to the cage, and its momentum starts the loaded car on the cage on its way to the dump. This system necessitates the employment of more topmen, but is a very good one. At the Nottingham shaft, which is 475 ft. from landing to landing, from 140 to 150 cars per hour are hoisted on single-deck cages.

METHODS OF OPEN WORK

No definite rules can be given for the selection of a method of mining that will cover all the conditions that may exist at any given mine. The system finally selected is that which will yield the maximum percentage of coal in the best marketable condition at the minimum of cost and danger.

All methods of working may be grouped under one of two heads or classes, viz., open work, or closed work.

Open work applies to the mining of those deposits that are either so thick or lie so near the surface that the material overlying them may be removed and the coal quarried out at a profit.

The advantages of this system are that all the coal may be extracted without any loss in pillars or through squeezes, and in the lumpiest condition; no timber is required; unprofitable underground workings do not have to be kept open and in repair; when required, a simple hoisting plant is used; there is less danger to the workmen from falls of roof and from blasting; there is practically no danger from fire; artificial lights are not required; mining can be done more economically, as larger faces are open, larger blasts can be used, and the amount of work accomplished per miner is greater, and better superintendence can be had; the health of the men is usually much better when working in the open; and, under proper conditions, the output can be increased almost indefinitely.

The chief disadvantage of open work is the possible reduction in output during the winter months owing to snow, the exposure of the men to the weather, etc. Further trouble may arise from flooding during the rainy season, and, unless the seam lies parallel to the surface, the cost of removing the over-burden soon becomes excessive,

The removal of the overburden is known as stripping, and may be carried on with or without the use of excavating machinery. When machinery is not used, the covering is removed with pick and shovel when it is earth, and by hand drilling and blasting when it is rock, This is the original method of stripping, probably first applied in the United States to the thick deposits of the anthracite region of Pennsylvania, and is limited in its application to those seams that are either very near the surface or are abnormally thick. Experience has shown that it will pay to remove, without the aid of machinery, 1 ft. in thickness of overburden for each foot in thickness of the underlying coal. Thus a seam 6 ft. thick will permit of the profitable removal of 6 ft. of cover, and one 25 ft. thick of 25 ft., possibly 35 ft., of cover. Mines of this class are known as strippings in Pennsylvania, and as strip-pits in the middle West.

Steam-Shovel Mines.-Steam-shovel mines are the result of the application of the familiar railroad contractor's steam shovel to stripping. Under favorable conditions, there is probably no cheaper method of mining. It is extensively used in the neighborhood of Park City, Utah, in metal mining and on the iron ranges of the Lake Superior region, where an output of 2,000 T. of ore per da. for a steam shovel and one locomotive has been reached.

The cost of removing 97,854 yd. of material from over a seam of anthracite (Pa.) was $1 a yd. of material stripped and $.516 per T. of coal obtained. The average depth of the stripping was 75 ft., and about two-thirds of the material removed was rock. Recent contracts in the same region have been let for as low as 25 c. a yd. for rock and 5 c. a yd. for earth. Where conditions are very favorable and a shovel of large size can be kept steadily employed, even lower average costs per yard (shale rock and dirt combined) may be had. The volume, in cubic yards, of overburden removed per long ton of coal extracted in recent Pennsylvania practice is 3.3, 3.8, 3.5, 3.6, and 3.0 to 1. In one extreme case, 5.4 cu. yd., and in another but 1.8 cu. yd. of overburden were removed per long ton of coal extracted.

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In the Kansas field, where the surface is level and the seams horizontal, shovels of the largest size are employed to remove the covering to an average depth of 17 ft. (6 ft. to 24 ft.) from a seam that is but little more than 36 in. thick. The upper 6 ft. of cover is dirt, the second 6 ft. is soft shale or soapstone, underlying which is blue shale to the top of the coal bed. Where the seam has about 20 ft. of cover, the average steam shovel will, if employed pretty constantly, strip 12 to 15 A. a yr. at a cost of from 5 to 6 c. per cu. yd. The wage scale on an 8-hr. basis, is very close to $2.50, varying from $1.95 for waterboys to $3.05 for blacksmiths. Most of the workers receive $2.52 to $2.62 per da.

Near Danville, Illinois, where the conditions are very similar to those in Kansas, 38 to 40 ft. of overburden, of which 16 to 24 ft. is shale, is profitably removed from a coal seam 8 ft. thick. This is a fair general average for the district, although a ratio of 6 to 1 has been had.

The disposal of the overburden is frequently a matter of difficulty, particularly when it is thick. If it has to be transported to any great distance, the cost thereof may be prohibitory. If much water or sand occurs in the cover the cost of stripping is likewise increased. Strippings liable to overflow from flooded rivers are costly to operate and the workings should be protected by dams built of the overburden.

After the surface covering has been removed, a track is usually laid along the face of the stripping on the bottom of the workings, and the coal, after being blasted, is loaded into railroad cars by the steam shovel if it is shipped as mine run or into smaller cars for transportation to the tipple if it must be screened into sizes.

METHODS OF CLOSED WORK

INTRODUCTORY

By closed work is meant the mining and removal of the coal without the previous removal of the overburden. In general, the word mine is used to define a series of underground workings, and the words stripping, strip-pit, open-cut, open-pit, and the like, to what are more properly coal quarries of the nature just described.

No definite rules can be given for the selection of a method of mining that will cover all the conditions that may exist at any given mine. The system finally selected will be that which will result in the production of the maximum amount of coal per acre in the best marketable condition and at the minimum cost of extraction with the least danger to the workers.

General Considerations. Some of the items to be considered in selecting a method of working are the thickness of the seam and the amount, location, and nature of its impurities; the use to which the coal is to be put; the character of the roof and floor; the amount of cover over the seam; the dip of the coal; the nature and direction of the cleat or cleavage of the seam; the character of the labor to be employed; the presence or absence of gas, etc.

1. Roof Pressure. Of these items, the roof pressure is the most important, and a number of other causes are directly affected by it. The weight of the overlying cover will give a maximum roof pressure, but this may be so variously modified that the determination of the actual pressure is practically impossible, and estimates of this pressure must be based largely on practical experience; hence, rules for its calculation are of comparatively little value. One very essential point, however, must be borne in mind, i. e., that the direction of pressure is perpendicular to the bedding plane.

2. Strength and Character of Roof and Floor. The strength of roof refers to the power of being self-supporting over smaller or larger areas. A strong roof permits larger openings, but increases the load on the pillars, thereby necessitating larger pillars. A weak roof requires smaller openings, and permits smaller pillars when the floor is good. A strong roof may yield and settle gradually, giving good conditions for longwall work, or it may be hard and brittle, and difficult to manage.

The character of floor influences largely the size of pillars. A soft bottom requires large pillars and narrow openings, especially when the roof is strong. 3. Texture of Coal and Inclination and Thickness of Seam.-Soft, friable coal requires large pillars, while a hard, compact coal requires only small pillars. The inclination and thickness of the deposit increase the size of pillars required, and also influence the haulage, drainage, timbering, method of working, arrangement of breasts, etc.

4. Presence of Gas.-The presence of gas in the seam or in the enclosing strata affects the system of working, as ample air passages must be provided, and provision must frequently be made for ventilating separately the different sections of the mine. Where the gas pressure is strong, and outbursts are of frequent occurrence, narrow openings are necessitated that render the working safe until the gas has escaped.

5. Use to Which Coal is Put.-If the coal is destined to be coked, a method of mining is to be preferred which results in the production of the largest possible amount of slack; whereas, if the coal is screened into sizes in the ordinary way, choice should be given to that system which results in the largest amount of lump coal.

When the longwall method is used, it is particularly important to have a constant market for the output, such as obtains if the mine is shipping fuel coal to a railroad at a fixed tonnage per day, as a few days' idleness may cause serious trouble at the face, even in temporarily closing it if the pressure is great.

6. Character of Labor.-While, in the room-and-pillar system of mining, the temporary or even long-continued stoppage of work in a portion or all of the working places, does not commonly have other effect than reducing the output and, consequently, the profits, under the longwall system the shutting down even for a few days of a comparatively few working places may cause

serious trouble in handling the pressure at the face. Hence the necessity, under the longwall system, of having not only a steady car supply as explained, but also steady men who will not lay off for one trivial excuse or another.

General Systems of Mining. For purposes of classification the various systems of mining coal fall into one of two groups, as follows:

1. Systems in which the tract to be exploited is first penetrated by a series of two or more relatively narrow (8 to 10 ft.) entries (headings or gangways) from some of which entries are (usually) turned relatively wide (15 to 30 ft.) rooms (breasts or chambers) which are separated the one from the other by pillars of coal, and in which rooms the bulk of the output of the mine is obtained. This group may be further subdivided into the room-and-pillar, pillar-and-stall, and panel methods.

2. Systems in which entries are not driven, but in which all the coal is extracted in one operation from a continuous face, the roof being allowed to fall or cave as fast as the coal is removed, haulage roads being maintained through the caved area by means of walls of stone built along their sides. The longwall system, largely used abroad, and but to a slight extent in the United States where the vast bulk of the coal is mined by the room-and-pillar method, is the single example of this second group.

The consideration of any system of mining requires a discussion of the following subjects: The system of mining as a whole, including the direction of driving the entries and rooms, the number and grade of the former, etc.; methods of supporting the roof and sides of excavations, see Timbering; methods and appliances for removing water from the workings, see Hydraulics; methods of bringing down the coal at the working face, see Explosives and Blasting; methods of transporting the coal from the face to the tipple, see Haulage and Hoisting; methods of supplying the working places with a current of fresh air from the surface, see Ventilation.

ROOM-AND-PILLAR SYSTEMS OF MINING

PRELIMINARY CONSIDERATIONS

In the room-and-pillar system of mining, the tract to be worked is divided into small districts, or blocks, by main entries and cross-entries intersecting one another at right angles or nearly so. The coal in each block is mined by turning off from the cross-entries a number of rooms.

That part of the coal which is left between the individual rooms and entries is called a pillar, and the pillars are pierced at more or less regular intervals by cross-cuts or break-throughs in part for the purposes of haulage, but most largely to provide a passage for the air-currents that a better ventilation of the working faces may be secured. The removal of the coal in driving the entries and rooms is called the first working, or more rarely the advance working or working on the advance.

Unless it is necessary to leave in the pillars to support the surface permanently they are commonly removed as soon as the rooms, either in part or all those turned from a single cross-entry, have been driven their full length; and unless the pillars can be removed the room-and-pillar system of mining is very wasteful of coal, as from 30 to 50% of the total amount must be left in the mine. The removal of the pillars is variously known as second working, working on the retreat, robbing, drawing-back the pillars, pillar working, pillar drawing, pulling pillars, etc. Fig. 1 shows a mine laid out on the doubleentry room-and-pillar system of mining.

Number of Entries.-The haulageways in a bituminous mine are known as entries or headings and in an anthracite mine as gangways. There are several methods of arranging these passageways, known as single-entry, doubleentry, triple-entry, etc.

The multiple-entry systems are expensive to drive, and the greater the number of entries the greater the expense, and are only used by companies with ample capital, large acreage of undeveloped coal, and a large and regular market.

In the single-entry system, now used but very rarely if at all, a single entry is driven ahead and rooms are turned from one or both sides of it. This entry, which is also the main haulage road, acts as an intake, the air being conducted along it to the last room, up which it passes to the break-through and back along the working faces to the first room and thence by a small air-course to the upcast. The circulation of air is liable, in this method, to be cut off at any

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