The size and form of colliery shafts vary in different districts. In the United States and Scotland rectangular pits secured by timber framings are still common, but the tendency is now generally to make them round, 20 ft. being about Size of shafts. the largest diameter employed. In the Midland counties, from 7 to 9 ft. is a very common size, but larger dimensions are adopted where a large production is required. Since the accident at Hartley colliery in 1862, caused by the breaking of the pumping-engine beam, which fell into the shaft and blocked it up, whereby the whole of the men then at work in the mine were starved to death, it has been made compulsory upon mine-owners in the United Kingdom to have two pits for each working, in place of the single one divided by walls or brattices which was formerly thought sufficient. The use of two independent connexions—whether separate pits or sections of the same pit, between the surface and the workings—is necessary for the service of the ventilation, fresh air from the surface being carried down one, known as the “downcast,” while the foul or return air of the mine rises through the other or “upcast” pit back to the surface. In a heavily-watered mine it is often necessary to establish a special engine-pit, with pumps permanently fixed, or a division of one of the pits may be devoted to this purpose. The pumps, placed close to the point where the water accumulates, may be worked by an engine on the surface by means of heavy reciprocating rods which pass down the shaft, or by underground motors driven by steam, compressed air or electricity.
Where the water does not accumulate very rapidly it is a common practice to allow it to collect in a pit or sump below the working bottom of the shaft, and to draw it off in a water tub or “hoppet” by the main engine, when the latter is not employed in raising coal.
The laying out of a colliery, after the coal has been won, by sinkings or levels, may be accomplished in various ways, according to the nature of the coal, its thickness and dip, and the extent of ground to be worked. In the South Laying out workings. Staffordshire and other Midland coalfields, where only shallow pits are required, and the coals are thick, a pair of pits may be sunk for a very few acres, while in the North of England, on the other hand, where sinking is expensive, an area of some thousands of acres may be commanded from the same number of pits. In the latter case, which represents the most approved practice, the sinking is usually placed about the centre of the ground, so that the workings may radiate in every direction from the pit bottom, with the view of employing the greatest number of hands to advantage. Where a large area cannot be commanded, it is best to sink to the lowest point of the field for the convenience of drawing the coal and water which become level-free in regard to the pit. Where properties are much divided, it is always necessary to maintain a thick barrier of unwrought coal between the boundary of the mine and the neighbouring workings, especially if the latter are to the dip. If a prominent line of fault crosses the area it may usually be a convenient division of the fields into sections or districts. The first process in laying out the workings consists in driving a gallery on the level along the course of the coal seam, which is known as a “dip head level,” and a lower parallel one, in which the water collects, known as a “lodgment level.” Galleries driven at right angles to these are known as a “dip” or “rise headings,” according to their position above or below the pit bottom. In Staffordshire the main levels are also known as “gate roads.” To secure the perpendicularity of the shaft, it is necessary to leave a large mass or pillar of the seam untouched around the pit bottom. This pillar is known in Scotland as the “pit bottom stoop.” The junction of the levels with the pit is known as the “pit eye”; it is usually of an enlarged section, and lined with masonry or brick-work, so as to afford room for handling the wagons or trams of coal brought from the working faces. In this portion of the pit are generally placed the furnaces for ventilation, and the boilers required for working steam engines underground, as well as the stables and lamp cabin.
The removal of the coal after the roads have been driven may be effected in many different ways, according to the custom of the district. These may, however, all be considered as modifications of two systems, viz. pillar work Method of working coal. and long-wall work. In the former which is also known as “post and stall” or “bord and pillar” in the north of England, “pillar and stall” in South Wales, and “stoop and room” in Scotland, the field is divided into strips by numerous openings driven parallel to the main rise headings, called “bords” or “bord gates,” which are again divided by cutting through them at intervals, so as to leave a series of Pillar working. pillars arranged chequer-wise over the entire area. These pillars are left for the support of the roof as the workings advance, so as to keep the mine open and free from waste. In the oldest form of this class of working, where the size of the pillar is equal to the width of the stall or excavation, about ¾ of the whole seam will be removed, the remainder being left in the pillars. A portion of this may be got by the process known as robbing the pillars, but the coal so obtained is liable to be very much crushed from the pressure of the superincumbent strata. This crushing may take place either from above or below, producing what are known as “creeps” or “sits.”
| Fig 3.—“Creeps” in Coal-Mines. |
A coal seam with a soft pavement and a hard roof is the most subject to a “creep.” The first indication is a dull hollow sound heard when treading on the pavement or floor, probably occasioned by some of the individual layers parting from each other as shown at a fig. 3; the succeeding stages of creep are shown at b, c, d, f, and g, in the same figure; the last being the final stage, when the coal begins to sustain the pressure from the overlying strata, in common with the disturbed pavement.
| Fig. 4.—“Sits” in Mines. |
“Sits” are the reverse of creeps; in the one case the pavement is forced up, and in the other the roof is forced or falls down, for want of proper support or tenacity in itself. This accident generally arises from an improper size of pillars; some roofs, however, are so difficult to support that sits take place where the half of the coal is left in pillars. Fig. 4 will convey a general idea of the appearance of sits,—k, m, n showing different stages.
| Fig. 5.—Pillar Working. |
The modern method of pillar working is shown in fig. 5. In the Northumberland steam coal district, where it is carried out in the most perfect manner, the bords are 5 to 6 yds. in width, while the pillars are 22 yds. broad and 30 yds. long, which are subsequently got out on coming back. In the same figure is also shown the method of working whole coal and pillars at the same time, a barrier of two or three ranges of pillars or a rib of solid coal being left between the working in the solid and those in the pillars. The space from which the entire quantity of coal has been removed is known in different districts as the “goaf,” “gob,” or “waste.”