June 8, 1917. THE COLLIERY GUARDIAN. 1079 "beds of shale, sandstone and limestone, none being massive. The working of the 5 ft. seam at a depth of 360 ft. resulted in subsidence amounting to practically 70 per cent, of the thickness excavated. Similar effects resulted from mining the 3 ft. 6 in. bed. At the greater depth subsidence began about six months after the coal had been mined, and continued for years. A report by C. Menzel showed that since 1885 observations of the rate of settlement had been made at 82 points in the vicinity of the collieries of Zwickau, Saxony. The depth of the coal beds varies from 600 to 2,400 ft. A maximum subsidence of 7‘1 ft. was noted 12 years after three seams had been mined out at a depth of from 600 to 900 ft. At a depth of 1,500 ft. the subsidence was only 0’6 ft. By the use of filling, sub- sidence was greatly reduced, it being noted that on an average the filling was compressed to one-half of its volume when stowed. The ratio of subsidence to thick- ness of seam excavated was found to vary from 1 : 1 to 1 : 7, the average being 1 : 2. Frenzel suggested this latter ratio for shallow seams. COAL MEASURES TOP KARP •> N S 101° ____ -4-L FEET 3930 3&IS Surface __ _______ GE FEET/ Fig. 17.—Angle of Fracture at Shirebrook Colliery. Numerous observations have been made in Germany during the last 30 years. R. Hausse has reported upon the angle of break, angle of draw, and the coefficient of increase of volume. Jicinsky, Goldreich, and others have reported upon subsidence in Austria-Hungary, but in general these data have been secured in districts where the coal measures are covered with heavy beds of marl. From the foregoing statements of observations the following may be presented as representative in so far as general statements can be made to apply to mining operations, each of which is conducted under different geological conditions. Angle of Break and Draw. Dr. Niesz has made many observations upon sub- sidence, particularly on the angle of fracture in various kinds of rock and on the compressibility of filling. He states that the angle of fracture of limestones, con- glomerates, etc., is found to be from 45 to 48 degs.— nearly the angle of repose. In quicksand the angle is greater, while in ebay, slate, and marl it may be 60 degs., and in stone under favourable conditions even 75 degs. Sandstones with siliceous binding material are ranked as non-plastic strata. Initial subsidences in these are followed by others, but at longer intervals than in Vertical Section on A-A £> ill 1 1 / ’ / h / f 1 ! t i Angie of dip n run dip Plan Fig. 18.—Location of Shaft in Dipping Bed (O’Donahue). plastic strata. The angle of fracture is generally not less than 82 degs. Dr. J. S. Dixon reported that in a level seam about 6 ft. thick, by careful levelling on the surface prior to and after working, it was found that the draw or angle of subsidence of the strata was about 76 degs. from the horizontal plane. H. F. Bulman says that in a seam dipping 1 in 10, the lines of break extended over the solid coal forming an angle of 45 degs. with the horizontal on rise workings, 50 degs. in level workings, and 56 degs. on dip workings. In a wide goaf area the average inclination of the planes of fracture was 68 degs. from the horizontal plane ; and at the rise side of a shaft pillar the inclination was roughly 58 degs. from the horizontal plane over the solid coal. S. R. Kay has presented the following formula for Q rj rp / determining the radius of support: r—-------—-------- 0'8 where r =. radius of support in feet, d = depth in feet, t =. thickness excavated in feet. This allows for the angle of break or draw. Joseph Dickinson says that the direction of subsidence may be judged of from the slopes of faults and mineral veins, and he gives these slopes as 1 in 5 for hard rock, 1 in 3*75 for medium rock, and 1 in 2 5 for soft rock. O’Donahue says that the angle of break will be from 5 to 8 degs. beyond the vertical for horizontal beds, and that the maximum draw on dip workings will be 24 degs.; he finds the same angle to be the limit for workings to the rise. O’Donahue offers two formulae to determine the angle of draw: d — 8 + f D, and d' = 8 — | D, in which D = inclination of seam in degrees ; d — angle of draw toward dip workings, and d' = angle of draw toward rise workings. E. H. Roberton gives a rule for shaft pillars (used in Northumberland and Durham) which allows for the angle of break and draw: Radius of shaft pillar in feet = -g- + 2 \/ D t; in which D = depth of shaft in feet, and t — thickness of seam in feet. Hausse estimated that in general the line of fracture will be between the vertical and the normal to the seam. In addition to the line of main fracture, Hausse refers to the secondary break or draw. He says that in case of hori- zontal beds this line of secondary break is situated along the bisector of sliding materials of the supplementary angle of the natural slope. The effect of the dip of the strata has been considered by many authors in their discussion of the simplest cases; in fact, most of the formulse for angle of break consider the dip of the strata. Gonot’s law of the normal and Schulz’s rule, the earliest of the theories, considered the angle of dip. As previously noted, Hausse, following Jicinsky, supports the theory that the angle of break will fall midway between the normal to the seam and the vertical. From a careful study of the subsidence occurring in the Saxon coal field, Hausse determined the direction of the plane of fracture by the following formula:— a = angle of fracture, d = dip of strata, tan a = cos in which, sin d cos d if d = 0°, tan a = co and a = 90°, and if d — 90°, tan a = oo and a = 90°. S. R. Kay suggests that for inclined strata the angle of fracture will be midway between the perpendicular to the seam and the vertical. If the angle between the perpendicular to the seam and the vertical is a, then the pillar necessary to protect a given object on the surface must be shifted, on account of the dip, from a position directly beneath the object by an amount equal to d tan 1/2 a cos a, in which d equals the depth. Goldreich gives the following table, showing the angle of break according to the most important theories :— Angle of Break. Hausse’s rule. m German rule. M >> . > ••• a ..46’0...29’36.. Stowing Bully-Grenay, — France 432 . ..44'4.. 30'0 . — ..England ... — 600 . ..75'0... 4'0 . — — ...O’Donahue 2,400 . ..25'0... 4'0 . — .. ••• 33 650 . ..68'0... 5'5 .. — ..England .. Dixon 748 . ..19'0... 7'5 . .. Stowing ..France ...Fayol 2,600 . ..00'0 . 13'0 Hrmlss. dp th. ••• 1,040 . ..00'0...13'0 ... Do.f .. England ...Gresley 390 . ..40'0... 7'0 .. . 33 %> steam put in gob. 325 . ..87'0...30'0 . — „ ...Grazebrook 1,500 . ..30'0... 5'0 . .. Stowing „ ...Hay ..Germany ...Menzel 600-2,400. ..50'0... — .. — * Without stowing. t With stowing. Attempts have been made to formulate rules by which the amount of subsidence may be predicted in advance. Some of the formulae are based upon the thickness of coal and depth of workings. Most of them include factors for character of rock and filling, but few intro- duce factors for inclination of the beds. The discussion of the relation between the depth of workings and the vertical amount of subsidence has brought to the foreground the question as to whether or not subsidence will result irrespective of depth. According to the formulae of Jicinsky and Menzel there is for each thickness of coal bed a depth beyond which mining will not affect the surface. In 1884 Jicinsky suggested the following:—S = m 4-1—1*01 f = m - '011; in which S = vertical subsidence, m = vertical thickness of coal, and t = thickness of overlying beds. Menzel suggests the formula : S = ; in which S = subsidence in yards, t = depth in yards, and m — thickness of seam in yards. The factor 350 must be increased to 400 for depths greater than 350 yards. This principle that there is a a harmless depth has been supported by Fayol, Banneux, Thiriart, Rziha, Jicinsky, and Menzel. Fayol formulated two rules as follows :— (1) The height of the zone of subsidence where sand- stone predominates and the beds have an inclination less than 40 degs., and where the area is infinite, does not exceed 200 times the height of the excavation. (2) When the area is limited, the height of the dome is about twice the breadth excavated for excavations less than 6 ft. and up to four times the breadth excavated for seams more than 6 ft. In general the Germans say that the “ dead point ” or “ harmless depth ” has not been reached in Westphalia, and question whether or not the term should be used. Callon said that there is no harmless depth, and the majority of the British engineers hold that the removal of all the coal over extensive areas will produce subsidence. Time Factor in Subsidence. In a study of subsidence it is frequently important to know (1) how soon after the movement shows in the mine workings it will manifest itself upon the surface; (2) the period during which the movement is most severe, and (3) the duration of subsidence. Upon all these points there seems to be a great difference of opinion, which is due undoubtedly to the great variety of conditions under which the observations have been made. Fayol wrote that the period during movement of the surface may continue is very uncertain. It is allowed to be 10 or 12 year in Belgium and at Saarbruck. In other places it has been as long as 20 and even 50 years. The committee of the Mining and Metallurgical Society of Ostrau, Moravia, reported in 1881 that the land subsidence manifests itself within one to three months after the collapse observed in the mine. It manifests itself most intensely during the first half year, and then becomes less noticeable. It may be assumed that after two years, or more safely, after three years, there do not occur any measurable land subsidences in consequence of a collapsed working. S. R. Kay reported that, in working a 5-ft. seam at 360 ft., subsidence began about six months after the coal was removed, and continued four years. Elevations taken at the Bent Colliery by J. S. Dixon showed that the greater part of the subsidence took place within the first year, and that the maximum subsidence came within three years. The depth to the seam was approximately 650 ft. In observations made by W. Hay at Shire brook Colliery, in which mining was being conducted at 1,700 ft., the maximum subsidence appeared in two years. G. E. J. McMurtrie reported that the mining of ,8 ft. of coal at a maximum depth of 800 ft. caused subsidence continuing 15 years. In discussing the timbering of roadways in longwall mines in Illinois, S. O. Andros says that permanent timbering can be extended only to that point where the first rapid and violent subsidence has ceased, and it is not usual to extend permanent timbering to any point until the face has been advanced beyond it for at least two years. G. Knox says that when workings advance rapidly the tendency will be for the strata to bend with- out fracturing; whereas if the opposite is the case, the force of the motive zone has time to break through, as is frequently shown on the working face after a prolonged stoppage. (To be continued.) Commenting on the imports of German goods into South Africa before the war, a correspondent of The Times Trade Supplement observes that one of the big lines was the acetylene hand lamp for miners and outdoor use. The British manufacturer knows of the opening that exists for such goods, and has made an attempt to fill it; but so long as the war lasts he will be unable to do much. In the meantime America is meeting requirements up to a point. Surveyors’ Institution and Mining Engineers. — At the annual meeting of the Surveyors’ Institution, held at Great George-street, Westminster, on Monday evening, it was reported by the council that , one additional honorary member had been elected during the year, Sir Richard Redmayne, K.C.B., Chief Inspector of Mines, who had interested himself in the formation of the new mining sub- division, and had accepted nomination as a member of the mining committee. Sir Francis Brain was the first elected chairman of the committee representing the mining sub- division. Although its activities had been necessarily limited by the war, the committee had done useful work in recommending candidates for election with the object, of strengthening this branch of the institution; in advising on the syllabus of the mining examination; on the addi- tions to the library needed to make it representative of the mining side of the profession; and in making arrange- ments for bringing up to date the information on mining royalties collected by the Royal Commission. The latter work was now in process of being carried out. .Mr. A. L. Ryde, of London, succeeds Mr. Stewart as president.