and passes through the shafts without interfering with the cages, or even with the movement of the tubs in the hanging-on place, as the tubs gravitate from b to c. The application is remarkably simple and efficient, a noteworthy point being that the direction of motion of the tubs is never changed, except at the working face. The plan and section explain this; the pit bottom is at the lowest point, the dotted lines represent chains, and the arrows the direction of motion.

Bibliography. The following is a list of the more important memoirs, dealing with the subject matter of this chapter:

MIN. INST. SCOT.: Report of Deputation On the Method of Securing Roof and Sides, iii., 51; Propping at Straiton and Pentland, Robt. Martin, xii., 58. N. E. I. The Use of Iron Supports in the Main Roads of Mines instead of Masonry or Timbering, G. Meyer, and W. J. Bird, xxxvii., 135; On the Introduction of Steel Supports for the Maintenance of Main Roads in the Mines of Cleveland, A. L. Steavenson, xxxvii., 221.

BRIT. SOC. MIN. STUD.: "Kips" or Landings at Shafts, M. H. Douglas, i., 443; Timbering in Mines, H. St. J. Durnford, iii., 207; and W. S. Gresley, iii., 229; Preservation of Timber, ix., 76.

Soc. IND. MIN.: Application du fer au soutènement des galeries à la houillère du Creusot, M. de Biauzat (2a Série), iii., 563; Nouveaux systèmes de boisage, H. Daburon (2° Série), ix., 873; Blindage des galeries aux houillères de Rochebelle, M. Gerrard (2o Série), xv., 391; Effets de diverses préparations sur la durée des bois (Comptes Rendus Mensuels), 1890, 223.

:

CHES. INST. Mine Timbering, J. Clark Jefferson, vii., 270; Shaft Timbering, J. Clark Jefferson, viii., 209.

:

MID. INST. The Use of Rolled Steel Girders for Supporting the Roof in Mines, T. R. Smith, x., 222.

FED. INST. The Treatment of Timber for Use in Mines, R. Martin, x., 531; Improved Apparatus for Drawing Timber in Mines, E. B. Wain, xii., 591; Use of Steel Girders and Props in Mines, E. F. Melly, xiii., 277; The Strength of Pit Props, H. Louis, XV., 343, and xvii., 14; The Hepplewhite Tapered Pit Props and Bars, W. H. Hepplewhite, xix., 8.

INST. C.E.: Timbering in the Ampthill Second Tunnel, E. E. Matheson, cxx., 335.

178

CHAPTER VI

METHODS OF WORKING.

The Two Main Systems. - Broadly speaking, there are two systems of mining coal, called "bord and pillar," and "longwall." Outside the North of England and Scotland the former is but little practised in Great Britain, while the latter, which originally took its rise in the Midlands, is very extensively applied. Endless moditications of each system are employed, and the two gradually merge into each other, until it becomes impossible to say to which system some methods belong. The tendency of the present day is to employ longwall more and more, and this method is slowly but surely superseding every other one. There are, however, some seams which it would be impossible to work longwall-that is to say, at any reasonable cost.

Shaft Pillar and Subsidence.-It is necessary that a certain area of coal around the shafts should not be worked, but should remain to afford support, and to prevent any risk of what is known as creep." It is impossible to give any general rule by which the size of shaft pillars for given depths may be determined. Everything depends on the nature of the beds overlying the seam, the inclination of the strata, the nature of the floor and roof, and the stowing of the excavation.

It is hard to prevent creep in seams having a soft floor, especially if water is present. The pressure on the pillars of coal forces up the soft underclay in the roads between the pillars. When once this action commences it is most difficult to stop, or to keep the roads open; everything seems to be on the move. Perhaps the only method of prevention in longwall work, is efficient and close packing; leaving large pillars is not sufficient, as Mr. J. A. Longden* mentions an instance of a Derbyshire colliery, 520 yards deep, where the shaft pillar was 260 yards broad by 800 yards long, the mine being flat, and yet creep came on so seriously that great fears were entertained that the shaft would be lost. The pit bottom arching had to be put in three times, finally with layers of oak and brickwork alternately.

The working of beds of coal always lowers the overlying strata, giving rise to what is known as "subsidence." A certain height is taken out, and, although the excavation may be filled with material, such packing, even at the best, is loose compared with the solid coal originally existing. The gob is compressed, and the overlying strata and the surface sink down. If the area of subsidence was limited to the strata immediately above the area worked, the problem of determining its direction, if not its amount, would be easy; but even in level measures the disturbance extends beyond the limit of the excavation.

* Brit. Soc. Min. Stud., xii., 127.

With inclined seams the fracture of the beds never takes place in a vertical direction, but always in a plane approaching the perpendicular to the inclination of the strata. Mr. Callon advocated the theory, known as that of the "normal," that subsidence takes place at right angles to the planes of stratification, and extends, without sensible diminution in amount, right up to the surface, whatever may be the depth of the beds. He points out that in seams worked by longwall with complete stowing, the maximum subsidence commences at the centre of the excavation, and gradually extends to the boundaries, when the fracture of the bed immediately above the seam takes place at the points where it is supported on the solid strata. The loosened mass then leaves the bed above it and sinks down on to the stowing below, and a similar process takes place with each successive bed right up to the surface. When unconformable strata overlie the lower measures the direction of the lines of fracture will be considerably altered, as each bed will break at right angles to its bedding plane (Fig. 193). In pillar workings without gobbing the

root falls and fills up the excavation, and the amount of subsidence depends on the compressibility of the débris. With very hard rocks, and a moderate depth, pillar working might cause less subsidence than longwall with complete packing. In the case of hard rocks a bell-shaped cavity, narrowing upwards, will be formed by the breaking down of the roof, while, with soft and non-coherent strata, the cavity will be funnel-shaped.

Owing to serious subsidences taking place in the neighbour

hood of Liége, Mr. G. Dumont was commissioned to inquire into the matter, and, after an exhaustive examination of the district, drew up a report covering over 300 pages of a quarto volume † giving unqualified support to the theory of the "normal," except for seams lying at a greater angle than 68°, because, in the latter, the intensity of the pressure is diminished by the friction due to the obliquity of direction which the broken fragments must take. Thus, if a b (Fig. 194) represents the weight of the broken block A B, this force may be resolved into a c and a d. The greater the inclination the less becomes the force a d acting at right angles to the bedding planes, and totally disappears when they are vertical. Experience seemed to demonstrate that when the angle of inclination was 68°, a c was equal to a d.

Unfortunately, the correctness of Mr. Dumont's deductions is questionable, because the difficulties of observation were increased by the presence of old workings and of the workings of several collieries

within a very small area. The Colliery Owners' Association drew up

*Lectures on Mining (English translation), ii., 306.

+ Des affaisements du sol produits par l'exploitation houillère, Liége, 1871.

*

a reply admitting that the "law of the normal" may hold good where the seams are of small inclination, but arguing that the propagation of a fracture following the normal of the stratification of highlyinclined beds is a mechanical impossibility. They considered that the fracture at the lower extremity of the working will take place in inverted steps, and the fracture at the upper extremity will resemble a flight of steps viewed from below, while the average inclination of these steps will fall between the normal and the vertical (Fig. 195), approaching the one or the other according to local circumstances. They also remark that in steep seams † account must be taken of the fracture by crushing, which, according to Coulomb, occurs at an angle of 45°. The combination of this force with that tending to break the bed by bending, induces fracture along a line intermediate between the

two directions, and such line goes further from the normal as the inclination of the strata increases.

The diversity of opinion among engineers led Mr. H. Fayol to review the whole subject, ‡ and to conduct a series of observations both on ingenious models and on actual subsidences due to working seams of coal in cases where such observations could be made free from all complications. He commenced by summarising the contradictory opinions that have been expressed, for example:

(1) Upon the extension of the movements upwards—

(a) The movement is transmitted to the surface whatever may be the depth of the workings.

(b) The surface is not affected when the workings exceed a certain depth.

(2) Upon the amplitude of the movements

(a) Subsidence extends to the surface without sensible diminution. (b) Movements become more and more feeble as they extend upwards. (3) Upon the relative positions of the surface subsidence and the mining excavation—

(a) Subsidence always takes place vertically above the workings.
(b) Subsidence is limited to an area bounded by lines drawn from the
perimeter of the workings, and perpendicular to the beds.

(c) Subsidence cannot be referred to the excavation either by vertical
lines, or to the normal of the beds, but only to lines drawn at
an angle of 45° to the horizon, the angle of repose of the ground,
or some other similar angle.

• Des affaisements du sol attribués à l'exploitation houillère, Liége, 1875. + Op. cit., 108.

"Note sur les mouvements de terrain provoqués par l'exploitation des mines," Soc. Ind. Min. (2o Série), xiv., 805.

(4) Upon the influence of gobbing

(a) The use of packing protects the surface effectually.
(b) Packing simply diminishes the effect of subsidence.
(c) Subsidence is greater with stowing than without it.

A13 A17

Mr. Fayol points out that the theory of the normal is based on the erroneous supposition that beds break at right angles to the planes of stratification and at the perimeter of the excavation, but from actual experiment he found that in 80 per cent. of the observed cases the plane of fracture was an inclined one, and adds that although opinions are greatly divided these differences are more apparent than real. They are the result of generalising from single facts which are only particular cases of the following rule:* In stratified deposits the zone of subsidence is limited by a sort of dome which has for its base the area of excavation; the extent of the movement diminishes the further one goes away from the centre of that area.

Not only were careful observations made of the extent and amount of subsidence produced in working the mines at Commentry, but, in addition, the following experiments were made on models to reproduce on a small scale movements in the overlying strata caused by working seams of coal, in such a manner as to be able to observe the progress of events. On the bottom of a wooden box having a glass front were placed, side by side, small pieces of wood of equal thickness, about an inch wide, and as long as the width of the box; several rows of these small pieces of wood were sometimes placed one above the other. Upon them were laid successive beds of artificial strata, varying from inch to an inch or more in thickness, consisting of earth, sand, clay, plaster, or other materials. To enable the least movement to be

76325217

176541321110987654321

Wet Sand

Dry Sand

Plaster

Figs. 196, 197, and 198.

A

*This paper is the most important one which has been published on the effect of coal-working on the surface, and throws considerable light on what is perhaps the most intricate problem in mining, and about which few facts are known. careful summary of it, and also of Mr. Dumont's memoir, by Mr. H. F. Bulman, is given in Journ. Brit. Soc. Min. Stud., vol. xii., 1890, and by Mr. W. Galloway in So. Wales Inst., vol. xx., 1897.