(1.) The extraction of all the coal; whereby a better return is obtained by the lessee for his capital, and proportionably-increased acreage rental secured by the lessor.
(2.) As the "shearing" is confined to one or two main roads, there is considerable saving in that part of the labour, and, therefore, better coal at less cost.
(3.) The superincumbent weight often reduces the labour of "holing," or undermining, and with the same economical results, especially in highly-inclined seams.
(4.) The weight very materially aids in bringing down the mass, after being holed; and thus also labour is lessened, the expense of powder saved, and a larger proportion of round coal obtained than could be got otherwise.
On the other hand, in the same advantageous circumstances, there are the following drawbacks: [p.322/323]
(1.) Unless the pack-walls are exceedingly strong, and well built, the weight will crush them down, and cause great expense in keeping the roadways. Mr. Hedley says: "The packs or stone walls made through the goaf, to maintain roads, should be carefully built, so as to be solid and firm, and not liable to be crushed into the roads by the sinking of the roof. This is much neglected by some persons when introducing this mode of working, and frequently the cause of failure, through the packs and pillars being little better than a heap of loose material. Unless they are firm, both goaf-roads and bank face will be buried."
(2.) Unless the work proceeds very regularly and constantly, the roadways and faces are very difficult to keep up, and the ventilation is hindered. Even one face lagging behind the rest causes much annoyance and expense.
(3.) Dykes and dislocations are much more difficult to deal with than in bord-and-pillar, and cause trouble with the roof that does not otherwise occur.
In speaking of Long-wall, the late Mr. Dunn, inspector of mines, thus puts the case:--
(1.) If the coal be thin, hard and capable of bearing pressure; or if the top and bottom be soft, requiring a considerable quantity of cutting for the necessary tram-height: [p.323/324]
(2.) If band or rubbish be mixed with the coal, requiring to be stowed underground, so as to furnish a cheap and plentiful supply of débris for filling:
(3.) If the difference from the shaft to the boundary line be very limited.
(4.) If the roof be free from water, and the workings clear of buildings, rivers, &c.:
(5.) If the roof contain ironstone, to be worked with the coal, thereby producing much refuse; or if it be so soft and brittle that it will not stand for the ordinary width of excavation:
Under these, and many other circumstances, the Long-wall system may be recommended. There may, however, be objections rendering such working impracticable, viz.:
(1.) If the workings produce a considerable quantity of inflammable gas, either from the seam itself or some superior seam:
(2.) If the roof contains water, the letting down of which would spoil the tramways, or overpower the engine or pumps:
(3.) If the coal be so near the surface that the long-wall working would have the effect of damaging buildings:
(4.) If the cuttings of the roof or floor for height are so soft or friable that they would not be sufficient to support the roads; in which case, the expense of setting additional props, or obtaining other material, may [p.324/325] exceed the value of the coal, or its cost by another system:
(5.) If the seam be deep, and, from its thinness, suitable for long-wall; yet the small quantity ordinarily producible from one establishment may render its working unprofitable; or, in other words, the maintenance of expensive roads, or the number of pits required may not be repaid by the working thereof.
Mr. Hedley, another Government inspector, states: "In the course of my experience I have worked mines by various modes, and I have invariably found that the produce of large or round coal has been the greatest by working long-work, wherever circumstances were favourable. Many are of opinion that deep seams cannot be worked, with advantage, by long-work. The deep seam at Monkwearmouth Colliery, Durham (1800 feet below the surface, and 6 feet thick), is now successfully worked by this plan, and is producing a considerably greater yield of large or round coal than is obtained by bord-and-pillar. A moderately-strong seam, at any depth from the surface, may be got be long-work, if the roof be suitable for building the necessary packs; and less injury is done to upper seams than by other methods of working, owing to large areas of the roof gradually settling. This method of working, is not favourable for a tender seam having a heavy roof, as the weight on the bank crushes the coal." And again: "The [p.325/326] long-wall method of working a mine very materially assists in the getting of coal, as the weight on the face of the banks separates the coal from the bed, without the use of powder, and frequently without the use of wedges after the undermining is finished. The great simplicity of working by long-work, and the ease with which it is ventilated, as compared with other methods of working, is a strong recommendation for its adoption, where circumstances are favourable."
In a very thoughtful paper, read at a meeting of the Institution of Engineers in Scotland, Mr. Alexander, inspector for the western district, treats thus of the same matter:
"To realise the full advantage of this system, there are several points which require to be clearly understood. The nature of the coal must be carefully considered, with the view of taking advantage of the natural rents, 'backs' and 'cracks' which are known to exist in the majority of coal seams. The line of 'back' is comparatively regular in its bearing, and generally the distance between the 'backs' ranges from ten inches to three and a-half feet. The distance between the 'cracks' and the angles at which they lie are very irregular. Before laying out a pit upon this plan, it is of importance, then, to examine carefully the situation of the natural rents; a neglect of which, in some cases, led to a difference of as much as one-sixth in the cost of mining in one district of a mine compared with another." [p.326/327]
Again, he says:
"As a general rule, it may be safe to adopt the following in laying out long-wall works: That the roads be driven at right angles, or nearly so, to the line of 'back' of the coal; that the distance in advance, between each wall, do not exceed the width of a 'back,' and that no following-up wall be driven in advance of the one which should give it 'cut.' The disadvantages attending a following-up wall getting in advance of the one designed to give it 'cut' are - the increased difficulty of working, occasioned by 'shearing,' and diminished pressure upon the seam. Where the backs are thin, it may sometimes be judicious to drive the walls on end; and by such an arrangement there will be an increased per-centage of round coal. The subsidence of the roof will generally cease from 50 to 80 yards back from the working face; and the extent of the subsidence will range from one-half to two-thirds of the thickness of the excavation." As to cost, he adds: "I believe that where the seam exceeds two feet nine inches in thickness, long-wall is dearer than stoop-and-room; but if the seam is only two feet, it is decidedly more applicable and economical than pillar-work."
Mr. William Moore thus states the matter:
"The advantages claimed for this system are, the facilities it affords for working under a bad roof; for, while the pillars in stoop-and-room are unyielding, and cause the soft roof to break over them, in long-wall the pressure is extended over a [p.327/328] greater breadth, and, resting on yielding buildings of soft rubbish, allows the roof to bend without breaking; and, if the excavation be carried systematically forward, the roof may be kept unbroken at the face." To the objections that "the waste cannot be ventilated; great injury is done to the coal by the weight: besides the difficulty and expense of maintaining roads;" he answers that "the air can be coursed and split as may be necessary; that there is little small coal made by the collier, except in holing; that the roads only move for a time, and then are generally more secure than other roads; and that the expense of blasting and building is about seven shillings for each fathom of road."
In an estimate made by Mr. Greenwell, as given in his "Mine Engineering," the proportion of round coal from long-wall is stated at about 14 per cent. above what could be obtained from the same seam by bord-and-pillar working.
These copious extracts from the opinions of those who have written on the subject, and who in so doing stated the results of their own experience, furnish very readable evidence of the merits of long-wall, and the circumstances favourable for its adoption. We may, therefore, safely come to the following conclusions comparing it with bord-and-pillar:--
(1.) That depth is immaterial, provided that buildings will not be injured, or water let down. [p.328/329]
(2.) That it is only adapted for moderate areas and outputs.
(3.) That it is unsuitable for tender seams.
(4.) That the field must not be very much cut up by dykes, &c.
(5.) That the position of the "holing" is not very material, but it is most advantageous when at the bottom of the seam.
(6.) That the cost of "getting" is less, provided the position of the "backs" is attended to, and the work goes on regularly.
(7.) That a larger proportion of round coal will be obtained, ranging from 5 to 14 per cent.
(8.) That a larger tonnage per acre will be extracted, averaging from 10 to 25 per cent.
(9.) That the areas suitable for it can be efficiently ventilated without difficulty, provided the yield of firedamp is not excessive.
(10.) That the limit of its economical effect is not dependent on the thickness of the seam, but on the present and future cost of the buildings; and that this future cost is dependent very much on the original stability of the work, and the nature of the roof immediately above the coal.
It follows, therefore, that in a great many cases, especially in Scotland, the question of its adoption will rest almost entirely on the last of these, or the cost of building. Supposing that in any particular case it does not so rest, there are the following items in the estimate of cost:-- [p.329/330]
(1.) Wood. -- A certain amount of propwood is used in all long-wall workings, but if properly managed it will not be great.
(2.) Building. -- In no case will it pay to bring material from a distance. It must either be in the roof, the seam, or the pavement. The thickness of the walls or packs varies from 4 to 12 feet, and the width of the ordinary roads from 4 to 8 feet. Consequently, the amount of material required is the width of the packs x the thickness of the seam; and the height of the "carry," or roof to be taken down, will be this area divided by about 1½ times the intended width of the roadway. Thus, for a 3-feet seam, with 6-feet packs, and a 6-feet roadway, there would be about 4 feet of cutting required; which, after subsidence, would leave the height about 6 feet. Of course, if there be a "daugh" holing, or "bands" in the seam, or a "falling" on the roof, which comes down all along the "face," there will be so much less cutting needed. The tonnage cost of the packs will also depend on the width between the roads, which may range from 12 to 30 yards the principal drawback to the extreme width being the difficulty of getting the coal filled into hutches or tubs. To lessen this, a temporary tramway is sometimes laid along the face, or a sledge used, but neither plan is very satisfactory. Besides, the greater the width between the roadways the greater the pressure upon the "packs;" and, consequently, the need of greater strength, which necessitates more material [p.330/331] and labour. The cost of these "packs" ranges from 3d. to 6d. per ton of coal. Mr. Moores quotation of 7s. per fathom would be about 4d. for a 3-feet seam; and as a general rule it should never exceed 6d. per ton.
(3.) Repair of roads. As already stated, the cost of this will in a great measure depend on the stability of the work previously done; but it will also depend on the nature of the roof, for if it is soft, and has no band of rock near, it will "cut up" a great height, and necessitate almost continuous "redding." The cost of upkeep is lessened, in almost every case, by "brushing" anew every tenth or twelfth road after complete subsidence, and making them main tramways, into which the coal is brought from the intervening "faces" by means of levels or slopes. By this means, rails and sleepers are saved, ventilation is less difficult, the intermediate roads never need be over 200 yards in length, and therefore require less care in their formation. The cost of this re-arrangement is, of course, a tax upon the faces served. Thus, suppose each main road served twelve faces, or 150 yards of width, and each pair of levels or slopes 200 yards of length, the whole area wrought would be 30,000 cubic yards, or 27,000 tons for a seam 3 feet thick. For this, the length of new road would be 350 yards, which, at 6s. per fathom, would be £52 10s., or a tax of about a halfpenny per ton on the coal obtained. [p.331/332]
Having thus (or in any other way) estimated the cost of the wood, packs and upkeep, you have the following items to meet the account:
1. The extra tonnage per acre.
2. An additional per-centage of the round coal.
3. A reduction on the "getting" price.
4. The cost of "redding" or clearing pillars; and,
5. The expense of prop-wood in first and second working.