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PRODUCTION CLASSIFIED BY MINING METHODS SYSTEM OF MINING AND PERCENTAGE OF EXTRACTION

The long-wall system of mining, which is in general use in England, is little employed in the United States outside of a few districts such as Northern Illinois and Appanoose County, Iowa, where peculiar conditions of roof make it advantageous. Elsewhere the room and pillar system is generally used, either in its simplest form or modified by division of the mine into panels. Extraction of the pillars is fortunately becoming common in the Appalachians but has still made little headway in the interior fields. The engineers of the United States Coal Commission estimated the average loss represented by unrecovered coal as 34 per cent, of which 19 per cent was held to be preventable and 15 per cent unavoidable. 14

The best practice in the Appalachian region now recovers from 85 to 90 per cent of the coal in the ground, but in large areas of the Middle West the average recovery is less than 60 per cent. The conditions forcing this low recovery are chiefly economic and are beyond the control of the individual operator.

WASTE OF COAL IN MINING

The results of the United States Coal Commission's studies of waste in mining in 10 States east of the Mississippi are summarized in the following table. The field work was done in 1923. The tonnage figures relate to the business in 1921, a year of low production. The same percentage of loss applied to the whole country in a year of ordinary activity, in which 500,000,000 tons was produced, would mean an avoidable waste of 150,000,000 tons. The loss that could be avoided if competitive conditions permitted is thus greater than the entire quantity of bituminous coal produced by present-day Germany.15

TABLE 39.-Summary of coal losses in 1921 in States investigated by engineers of United States Coal Commission

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14 U. S. Coal Commission Report, vol 3, pp. 1841-1875, 1925.

15 U. S. Coal Commission, Report on the amount and nature of losses in mining bituminous coal.

METHODS OF DIGGING THE COAL

The term "mining methods" as used in the following tables refers not to the system of mining but to the method by which the individual miner "digs" or breaks down the coal. Coal in the mine is either blasted from a solid face-"shot off the solid"-as in hardrock mining, or is shot loose or otherwise broken down after a cut has been made in the bed.

The cut or kerf may be made either by hand with the pick or by a cutting machine. The term "mined by hand" as here used includes not only the coal undercut by hand but also coal recovered in certain long-wall districts where the pressure of the roof helps to bring down the coal with a minimum of cutting and shooting. The term "mined by machine" refers to the several types of cutting machines, and not to the mechanical loader, the use of which is discussed on page 588.

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FIGURE 39.-Per cent of total bituminous output mined by different methods, 1911-1923. The record for strip mines does not begin until 1915. The large percentage "not specified" prior to 1915 consists partly of strip-mined coal but chiefly of coal shot off the solid or mined by hand

In mining from strip pits, a form of open-pit mining, the overburden is removed by a steam shovel or some equivalent device, and the coal thus exposed is shot and then loaded into cars by hand or more commonly by the steam shovel.

In the term "not specified" are included all operations not furnishing this information. Most of them are small (see Table 40), and their output is undoubtedly either mined by hand or shot off

the solid.

The 13 years during which the Geological Survey has collected statistics on this subject have witnessed a steady growth in the proportion mined by cutting machines, and a corresponding decrease in the percentage shot off the solid and particularly in that mined by hand (fig. 39).

TABLE 40.-Bituminous coal mined by different methods, 1922-23

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The percentage shot from the solid declined from 17 in 1911 to 12.5 in 1923. The influences that have brought about the change are partly humanitarian and partly economic. The heavy charges of powder weaken the roof and pillars and thus increase the danger of falls of roof and coal, the most prolific' source of fatal accidents to coal miners. Another objection to this method, based upon commercial rather than humanitarian considerations, is that the heavy charges of explosives required to blow down coal that has not been undercut or sheared produce a much higher proportion of fine or small coal and make the lump coal so friable that it disintegrates in handling and in transportation. The growing use of mechanical stokers and the developments made in the use of powdered coal have robbed this objection of much of its force, but the danger to the mine and to the workers still continues, and in some States shooting from the solid is forbidden by law.

The decline in the percentage mined by hand from 29.7 in 1911 to 17.6 in 1923 is due almost entirely to economic influences. Machine cutting makes possible a much greater output per man and it has tended to replace cutting by hand wherever the natural conditions permit and the differential between the wage rate for pick mining and the combined rate for cutting and loading is sufficient to cover the cost of the machine. In consequence, the proportion mined by machine rose from less than half in 1911 to two-thirds in 1923.

In rate of growth the proportion mined by stripping has exceeded even that by machine cutting. In 1915, when the first statistics on the subject were collected, the strip pits produced 0.6 per cent of the total; in 1923 their contribution was 2.1 per cent, or more than three times as much. The use of stripping has been stimulated by high wage rates prevailing in the union fields and by the greater output per man per day which it makes possible.

The relative proportions mined by the several methods differ greatly from State to State (Table 40).

Solid shooting remains the dominant method in Kansas, Arkansas, Iowa, and Tennessee, where from 77.6 to 49.7 per cent of the product in 1923 was obtained in this way. In the southern Appalachians and Virginia it is still largely used. In the northern and middle Appalachians it is seldom reported. Operators in the fields of the interior and in the Far West, outside of Michigan, Utah, and Colorado, still employ it extensively. The States from which the smallest percentages were reported in 1923 were Ohio (1.6 per cent), Pennsylvania (2.5 per cent), and West Virginia (3.9 per cent).

Mining by hand remains the dominant method in Maryland, New Mexico, Texas, and Washington, all States where either natural conditions, the age of the workings, or cheap labor make the substitution of solid shooting or machine cutting less advantageous than elsewhere.

In 16 out of the 26 States shown in Table 40 more coal is now mined with cutting machines than by any other method. Michigan and Ohio lead, with 92.7 and 88.5 per cent respectively, followed by Kentucky (84.2), West Virginia (78.7), Virginia (73.5), Illinois (68.3), and Utah (68.3). Kansas, with 2.2 per cent, shows the smallest proportion mined by machine of all the important producing

States.

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