The methods of working, winning, or excavating mineral substances naturally vary, according to the magnitude and direction of the beds, lodes, or seams in which they are contained. With a vein of moderate width, as soon as the preparatory labours have brought the miners to the point of the vein from which the ulterior workings are to ramify, the first object is to divide the mass of ore into solid rectangular compartments by means of oblong galleries, generally pierced ten fathoms below one another, with pits of communication opened up, thirty, forty, or fifty yards asunder, which follow the slope of the vein. These galleries and shafts are usually of the same breadth as the vein, unless when it is very narrow, in which case it is necessary to cut out a portion of the roof or the floor. Such workings serve at once the purposes of mining, by affording a portion of ore, and for the complete investigation of the nature and riches of the vein, a certain extent of which is thus prepared before removing the cubical masses. It is proper to advance first of all in this manner to the greatest distance from the central point which can be mined with economy, and afterwards to remove the rectangular blocks in working back to that point. This latter operation may be carried on in two different ways, by attacking the ore from above or from below. In either case the excavations are disposed in steps, similar to a stair, upon their upper or under side. The first is styled a working in direct or descending steps; the second a working in reverse, or ascending steps. By this method a number of miners are able to proceed simultaneously without interfering with each other.
In rich lodes and in the case of thick masses, the system of working in large chambers or extensive excavations, or by transverse attacks, is employed. Superficial deposits are worked like open quarries.
In British practice, coal is generally worked on the post (or pillar and stall) system, or on the long-wall system.
The pillar and stall principle is carried out in the working away of a certain portion of the coal as a first measure, and in leaving the remainder in pillars, which are either to serve permanently for the support of the roof, or to be at some future time partially or totally removed. In the North of England, where this system has been brought to perfection, it is also named panel-work, because the whole area is divided into quadrangular panels, each panel containing an area of from eight to twelve acres, and round each panel is at first left a solid wall of coal of from forty to fifty yards thick. Through the panel walls roads and air-courses are driven in order to work the coal contained within these walls. Thus all the panels are connected together with the shaft as to roads and ventilation, and each district or panel has a particular name, so that any circumstance relating to the details of the colliery can be readily referred to a specified place.
PART OF A COLLIERY LAID OUT IN FOUR PANELS.
| A. | Engine shaft: divided into three compartments, an engine pit and two coal pits. | F, G. | Two panels completed as to the first work. |
| B, C. | Dip head level. | D. | Panel, with the rooms a a in regular progress to the rise. |
| A, E. | The rise or crop gallery. | H. | Panel fully worked out. |
| K, K. | Panel walls. |
By this plan, of which the above illustration gives a distinct idea, the pillars of a panel may be worked out at any time most suitable for the economy of the mine, and the loss of coal amounts to no more than about a tenth, instead of a third or a half by the old methods.
When the pillars of a panel are to be worked out, the most distant range is first attacked, and as the workmen cut away the furthest pillars, props of wood are placed between the pavement and the roof, within a few feet of each other, as shown by the dots at I. This is continued till an area of one hundred square yards is cleared of pillars. This operation is termed ‘working the goaf.’ The only use of the prop-wood is to prevent the stratum which forms the ceiling over the workmen’s heads from falling down and killing them by its splintery fragments. Experience has proved that before proceeding to take away another set of pillars, it is necessary to allow the last made goaf to fall. The workmen then begin to draw out the props. This is a most hazardous employment. Knocking down the more remote props one after another, they quickly retreat under the protection of the remaining props. Meanwhile the roof stratum begins to break by the sides of the pillars and falls down in immense pieces; while the workmen still persevere, boldly drawing and retreating till every prop is removed. Nay, should any props be so firmly fixed by the top pressure that they will not give way to the blows of heavy mauls, they are cut through with axes, the workmen making it a point of honour to leave not a single prop in the goaf. If any props are left, it causes an irregular subsidence of the strata, and throws more pressure on the adjacent pillars. The miners next proceed to cut away the pillars nearest to the sides of the goaf, setting prop-wood, then drawing it, and retreating as before, until every panel is removed excepting small portions of pillars which require to be left under dangerous stones, to protect the retreat of the workmen. While this operation is going forward and the goaf extending, the superincumbent strata, being exposed without support over a larger area, break progressively higher up; and when strong beds of sandstone are thus giving way, the noise of the rending rocks is very peculiar and terrific; at one time loud and sharp, at another hollow and deep. As the pillars of the panels are taken away, the panel walls are also worked progressively backwards to the pit-bottom, so that only a small portion of the coal is eventually lost.
When mines are fully worked, the main shaft is frequently continued down to other seams of coal, which are excavated in the same way as the first seam. In such cases the workings communicate with each other by shafts called ‘staples,’ which are sunk at intervals between the seams of coal.