April 27, 1917. THE COLLIERY GUARDIAN. 809 ties of a Bureau of Mine Inspection and Surface Support, the duties of which are to investigate the mine workings in their relation to the support of public highways. This bureau has been in existence since August 5, 1913, and has investigated a number of mines and made reports upon such investigations. The reports are not in print. A report on the mine (of the Peoples Coal Company has been made under the provisions of the Davis Mine Cave Act, and, in addi- tion to reviewing the mining conditions in three city wards, contains much information of general appli- cation in the study of the problem of surface subsi- dence. - The subsidence problem in the Pennsylvania anthra- cite field has been further complicated by the glacial deposits which occasionally are localised in 1 pot holes.” These pot holes may extend to a considerable depth below the glacial sheet, making it dangerous to carry on any mining operations near them. When the subsidence of the coal measures extends to a pot hole filled with sand and water, the water and some of the sand may seep into the mine, and if the subsidence has shattered the intervening strata, or if the roof has been thin and weak, a rush of sand may fill the mine workings. Fourteen accidents have been noted, in most of which a large area of the workings has been filled by the rush of glacial material and water, and in several instances extensive surface subsidence resulted. On June 10, 1914, at the Sugar Notch mine, a breast in the Kidney bed broke into the wash. The material entering the mine was largely sand and clay in a semi- fluid state, and its volume was estimated at 20,000 cu. yds. This filled several thousand feet of gangways and tunnels, but no lives were lost. The cave occurred at the face of a breast (No. 15), the face of which was at an elevation of +590 when the break occurred. The elevation of the surface directly above was +657, and it was thought that the bottom of the wash was at +630. It was planned to carry the breast to +600. The bottom of the wash was actually at +600, instead of +630. One of the most serious of the accidents noted was the so-called u Nanticoke disaster” of December 18, 1885, which resulted in the loss of 26 men. The mine workings tapped a pot hole which was subsequently found to be over 200 ft. deep. The hole was 400 ft. away from the present stream, and was covered by a culm bank. The thickness of the rock where the cave occurred has been estimated at from 22 to 48 ft. The subsidence produced a hole in the culm bank 300 ft. across. A very important occurrence of this nature in the anthracite district, as far as amount of material enter- ing the workings is concerned, was the cave at the Prospect Colliery of the Lehigh Valley Coal Company, December 12 and 26, 1915. It was supposed that the rock over the upper bed of coal was 40 to 50 ft. thick, and that the surface soil was thin, these being the conditions at points near the break. It was found, however, that the rock where the break occurred was only about 10 ft. thick, and that the remainder of the cover was loose sand, clay, and gravel, apparently glacial material deposited in an old valley. Probably . the break was due to the collapse of the thin rock at the bottom of a pot hole. Two breaks occurred, both at times when a small stream flowing over the loose material was flooded, and a large amount of this material was washed into the mine. It was estimated that about 140,000 cu. yds. of earth, and 350,000,000 gals, of water entered the mine. No lives were lost, but the financial loss was very considerable, as large expense was incurred in changing the channel of the stream, in addition to the cost of pumping out the water, and to the loss due to interruption of the work of the colliery. In the bituminous fields of Pennsylvania some damage to the surface has resulted from mining operations. Generally the deeds to the coal rights have not required the mining companies to support the surface. In the Connellsville region the surface is of little value as compared with the coal, the topo- graphy is rugged, and, although cracks extend through from the mine workings to the surface, little attention is paid to them unless they are near important struc- tures. Mining 8 ft. of coal at a depth of 600 ft. pro- duces cracks as much as 20 in. wide. The recovery of coal varies from 84 to 90 per cent. When it is neces- sary to protect buildings and railroads, as much as 25 to 50 per cent, of the coal is left in pillars. It has been found that at shallow depths, up to approxi- mately 150 ft., subsidence will amount to 50 per cent, of the thickness of the coal. At greater depths it will be less, approximating 25 per cent, at 300 ft. The attempts to correlate data and to generalise from the data available have not been satisfactory. In dis- cussing the observations made in South-Western Penn- sylvania, the chief engineer of the H. C. Frick Coke Company has reported that the great difference of strata overlying the coal no doubt contributed largely to the great variation noticed throughout the district. In some parts of this region, the stratum immediately above the coal, between it and the sand formation, varies from a few inches to 16 and 20 ft. of shale. Where the sandstone is very close to the top of the coal, the subsidence is considerably greater than it is where the shale thickens; then, too, there is a very heavy percentage of limestone and sandstone in the Leisenring district, while immediately south-east of this, or between Uniontown and Fairchance, the sand- stone measures rapidly thin out, and this, too, con- tributes to the variation in subsidence, or, in a word, where the coal has immediately over it a heavy per- centage of sandstone measures, the subsidence is greater than where a thick stratum of shales appears immediately above the coal. As an illustration, in the territory in the vicinity of Uniontown, where heavy shales appear above the coal, 18 in. of subsidence have been observed where the cover is 300 ft.; in the Leisen- ring district, where heavy sandstone measures appear above the coal, and there is a thin layer of shale imme- diately above the coal, the subsidence is approximately 30 in. Another condition that, no doubt, contributes largely in bringing about this difference in subsidences is the heavy layer of fireclay immediately beneath the seam of coal appearing in the Uniontown district; while very good bottom conditions—“ hard bottom ”— appear in the Leisenring district, and it may be con- cluded that this difference in the condition of the bottom section has more to do with the difference in subsidence than the first two conditions above mentioned. The outcrop of the Pittsburg coal beds extends for many miles in Western Pennsylvania, and above these shallow workings many sink holes have formed. These have attracted very little public attention, as they are considered to be of only a temporary character, and most of the buildings above the mined areas are frame, and the damage to them has also been only temporary, for, if tilted out of line, these buildings have frequently resumed their normal condition after a few months. Observations made by another company show that the surface subsided 2ft., 9 in. after practically all of an 8 ft. seam had been removed at a depth of 400 ft. The overlying rocks consisted of shales, sandstone, and limestone in alternating beds, the thickest limestone bed being 200 ft. from the surface, and reaching a thickness of 50 ft. There are six other beds of lime- stone, varying from 2Q to 30 ft., the total thickness of the seven beds being about 170 ft. The remainder of the column is about equally divided between fireclays, sandstones, and shales. The subsidence took place about 12 months after the mining of the pillars began. In the construction of the Greentree Tunnel of the Wabash Pittsburg Terminal Railway Company in the Pittsburg district, it was found that early mining operations had removed coal from a bed immediately beneath the projected line of the tunnel, and that coal had been removed from another bed overlying. When the heading of the tunnel had proceeded about 500 ft. from the west portal, broken ground wras encountered. The material was fairly solid grey shale, which was easy to drill, but none of it .could be removed without heavy blasting. At first the only indication of dis- turbance was that the rock showed soft pockets, and a little later the strata had separated, so that large pockets could be excavated without blasting. After having proceeded about 20 ft. into the material which had become more or less loosened, the bottom of the heading suddenly broke down about 30 ft. back from the face of the heading, permitting partial collapse of the timbering, as the settlement was about 2 ft. The broken ground encountered in the heading was found to be at the apex of the mass affected by the subsidence in the mine, and the top of the heading Output. 1913. Tons. 1914. Tons. Per- centage above or below 1913. 1915. Tons. Per- centage above or below 1913. 1916. Tons. Per- centage below or above 1913. United Kingdom 287,411,869 265,643,030 - 7 253,179,446 -12 256,348,351 -11 South Wales 56,830,072 53,879,728 - 5 50,452,600 -11 52,080,709 - 8 Scotland 42,456,516 38,847,362 - 8 35,596,856 -16 36,094,631 -15 Northern 58,675,687 52,384,478 -11 47,030,285 -20 47,270,124 -19 York and North Midland 72,951,841 66,807,623 - 8 67,504.130 - 7 68,005,505 - 7 Lancashire and North Wales 28,130,845 26,200,447 ' - 7 24,923,976 -11 24,907,938 -11 Ireland 82,521 92,400 + 12 84,557 + 2 89,833 + 9 Midland and Southern 28,284,387 27,430,992 - 3 27,587,042 - 2 27,899,611 - 1 Number of Persons Employed (including those on Sidings at the Pits, on Private Branch Railways and Tramways, and in Washing and Coking Coal.) For 1914 the number of men shown is the number employed from January 1 to July 31. Total number in 1913. Total num ber in 1914. Per- centage above or below 1913. Total number in 1915. Per- centage above or below 1913. Total number in 1916. Per- centage above or below 1913. United Kingdom 1,127,890 1,133,746 953,642 -15 998,063 -11 South Wales 233,134 234,117 ■— 202,655 -13 214,100 - 8 Scotland 147,519 146,168 - 1 121,854 -17 127,104 -14 Northern 247,011 248,251 184,124 -25 191,338 -22 ' York and North Midland ■ 257,252 260,870 -r 1 229,072 -11 239,331 - 7 . Lancashire and North Wales 123,927 123,909 105,670 -15 111,052 -10 Ireland 770 793 + 3 ' 739 - 4 767 - 3 Midland and Southern 118,247 119,638 + 1 109,528 - 7 114,371 - 3 was approximately 50 ft. above the mine level. Unknown to the engineers at the time, they had been driving the heading for some distance over the broken ground, with a wedge of solid rock between the bottom of the heading and the broken material beneath. On reaching the point where this continually thinning wedge would no longer support the weight, the ledge broke, practically at right angles to the tunnel, and allowed the timbering to drop as before stated. They then moved back in the heading to the solid rock, and drove a shaft on a steep incline until they reached the floor of the mine, about 35 ft. below the bottom of the heading. There it was found that a considerable area of coal had been mined several years earlier, and because of a “ swamp ” and the apparently heavy expense for pumping, the mine was abandoned after having drawn nearly all of the ribs and pillars. After reaching the mine level, two diverging drifts were driven from the foot of the shaft as far as under the two sides of the tunnel, and the two drifts were carried ahead under the prospective tunnel walls. For about 70 ft. the ground was entirely broken between the mine and the grade of the tunnel, and solid brick masonry walls, 18 ft. in height, were built to provide foundation for the regular tunnel side walls. After leaving the space of about 70 ft. already men- tioned, the ribs and pillars had not been withdrawn, and the falling from mine roof was not serious. At some points the roof slate had fallen in to a height about 6 ft. above the normal mine roof, and at other places there had been no breakage whatever. The mine entries and rooms were followed up for a dis- tance of about 600 ft., at which point, on account of the convergence of the tunnel grade toward the mine level, the same elevation was common to the floor of mine and the grade of the tunnel. The alignment of the tunnel making an angle of approximately 30 degs. with the rooms in the mine, the work was more complicated on that account. Solid brick masonry walls were built diagonally across the mine entries and rooms under each of the two tunnel side walls. Where there was a space of several feet between the roof of the mine and the normal founda- tion of tunnel walls, the brick walls were made thick enough to prevent any danger of crushing the slate rock between the brick foundation and the regular tunnel walls. On reaching the point where only 2 or 3 ft. of natural shale would have been left between the roof of mine and bottom of tunnel wall, this was all cut out and the brickwork carried up to the grade of the tunnel. In West Virginia, owing to the character of the topo- graphy and the low value of the surface in the coal districts, very few reliable records are available to show the extent of surface subsidence due to coal mining. Some surface movement has been noted where from 7 to 8 ft. of coal has been mined and the pillars drawn at depths over 500 ft. When the thick- ness of the covers is from 200 to 300 ft., the disturb- ance is greater, and where the cover is light—from 50 to 150 ft.—the cracks are sometimes from 2 to 4 ft. in width, and show a vertical displacement of from 1 to 2 ft. The problem of protecting a seam lying from 70 to 80 ft. above the seam now being worked has confronted some of the coal mining companies. It is proposed to mine the upper seam before the pillars are drawn in the lower seam, as the subsidence which follows the mining of the pillars in the lower seam greatly disturbs the overlying seams, and makes it unprofitable to mine them. (To be continued.) COAL OUTPUT IN 1916. A table of figures compiled under the direction of Mr. Finlay A. Gibson, secretary of the Monmouthshire and South Wales Coal Owners’ Association, specifies the output of coal and number of persons employed per annum, in each inspection division, at mines under the Coal Mines Act from 1913 to 1916 inclusive, and the percentage of increase or decrease in 1914, 1915 and 1916 compared with 1913. Sheffield Mining Students.—Appended is a list of suc- cessful candidates at the examination for colliery firemen’s, examiners’, and deputies’ certificates, recently held in con- nection with the Sheffield Evening Classes at the Salmon Pastures Council School :—Passed in gas testing, air measuring, and hearing: F. S. Jackson, L. Harding, L. Ellis, S. Booker, J. Fletcher, T. Wall, H. Cable, A. Nutt, W. Cotterill, R. S. Taylor, C. H. Liversidge, F. Schofield, A. Carr, B. Ellin, G. K. Thorpe, J. W. Bright, H. Hird. Passed in gas testing and hearing: L. Cherry, J. M. Roberts, W. W. Frisby, J. B. Jackson, L. Cownley, G. A. Bunker, J. J. Jenkinson. Passed in hearing and air measuring : R. Tomlinson, J. T. Copeland. Passed in gas testing: W. Avery. Passed in hearing: A. Gosling, F. Barlow, E. Harding. Passed in air measuring : R. G. Gosling.