Support of Roof and Sides. — The support of the roof and sides in the mine is an extremely important matter, as about half the total number of fatal accidents that occur in the coal mines of Great Britain are due to falls of ground. No doubt a large proportion of these accidents is unavoidable, but certain of them might be prevented by giving greater consideration to the question of securing the roof. All underground workmen should be impressed with the importance of paying careful attention to the state of the roof and the systematic use of timber to support it.

Roof Pressure. — A casual investigation will soon convince the observer that the pressure exerted on its supports by the roof of any underground working is not that due to the whole weight of strata between the point of support and the surface, as no timber would be able to resist such pressure. Each bed receives a great deal of lateral support, which depends largely on the nature of the material; thus we find hard, strong roofs in many cases at considerable depths, while we may have bad and very unreliable roofs quite near the surface. We thus see that roof pressure is not completely controlled by depth. There are in reality several causes operating in conjunction that affect the amount of timber required to give adequate support to the roof of any seam. These are (1) the nature of the strata forming the roof and floor, (2) the thickness and inclination of the seams worked, (3) the system of working adopted, and (4) the depth from the surface. Where the roof or floor is formed of soft or brittle rock, such as fissile shale, fireclay, or clod, more timber is required than would be under similar circumstances if the roof was of hard sandstone, coal, or fakes. Where the seam worked is a thick one, it requires more timber, other conditions being similar, than a thinner seam does. This is due to the greater length of prop needed, and the greater thickness to give the same resistance to crushing as a thinner but shorter one. Very often where the roof is soft its condition fixes the maximum distance that it is possible to allow between the props, and in such a case it is not necessary to use heavier timber than will just give the requisite resistance.

In certain systems of working, such as pillar and stall, the greater part of the roof weight is carried on the pillars, and the stalls seldom require an elaborate or costly method of support. On the other hand, the working away of the pillars calls for a well-considered system of roof support, and the abundant use of timber, part of which may be recovered and used again. Longwall workings require a large supply of timber to cope with the constantly sinking roof, and in thick seams worked by this system pillars or packs built principally of wood may have to be placed along the roadsides, and even in certain cases at intervals along the face. Where the seams worked lie at a steep inclination, both the roof and rise side of the roads driven level course will require to be lagged. This usually means more timber, but sometimes there is not the same direct pressure upon it, and so some compensation may accrue from the timber used lasting longer than it might do if the inclination was flat.

As a general rule, roof pressure increases with the depth of the seam from the surface, but as already stated the exact nature of the strata immediately overlying the coal-seam determines very largely the system and extent of the timbering required.

Methods of Timbering Working Places. — Different methods of securing the face of the workings are adopted to meet the requirements of different classes of roofs and seams. Two separate operations are involved to secure safety: (1) Setting of props to the roof so as to prevent falls, (2) setting of sprags or gibs to prevent the fall of the coal while it is being undercut. The former is done by (a) single props, (b) single props with lids, (c) props and straps or bars or (d) cogs and chocks. The latter is accomplished by the use of (a) sprags or gibs, (b) cocker megs and cocker sprags or stells, as they are called in certain districts.

Single Props. — In places where the roof is comparatively strong, single props afford all the necessary security. They should be sufficiently close to comply with the timbering rules, which are fixed by the mine manager from experience of the requirements of the seam. Where the seam is flat the props should be set perpendicularly, so that the weight may be transmitted through their centre line. Where roof and floor are both strong they should be set with a lid or cap, and the bottom placed in a shallow foothole to prevent slipping. If the roof is soft and the floor hard, a good plan is to taper the bottom end of the prop and set it up with a good stout lid on the top. This affords a good grip on the roof, and the taper end spreads and allows the prop to shorten without breaking while the roof settles. During the period of roof movement such a prop never ceases to offer resistance to the downcoming roof, but never so much as will cause the roof to break around it.

Should the roof be hard and the floor soft, it is often advisable to set the prop on a sole or foot piece. In certain cases where both roof and floor are soft, the usual cap is placed on the top, while the sole is placed upon a small heap of fine rubbish raised an inch or two above the level of the floor. This tends to prevent either roof or floor from breaking by giving space for the roof to settle before the maximum resistance of the prop is exerted.

Where the seam is highly inclined it is usually better to set the prop with a slight lean to the rise rather than perpendicular between roof and pavement. The normal weight is in the direction of the perpendicular between roof and pavement, but where the roof falls back in the waste a slight downward sliding action may occur.

When the prop has a slight inclination to the rise this action tends to tighten it further, whereas if it is set perpendicular it causes the prop to swing out and allows the roof to come down. In setting props in highly inclined seams it is always advisable to cut out a foothold a few inches down into the floor. If necessary some fine rubbish may be placed in the bottom of this hole to allow the roof to settle down a little before the supporting prop is broken. Single props of the types described are shown in Fig. 49.

Props and Straps. — Where the roof at the face is soft, it is often necessary to secure it by means of straps. These are usually formed by cutting round timber of the required dimensions down the centre, and forming bars with one side flat, the other being half round. The flat side is placed next the roof, and two or more props are driven under it to keep it in position. The straps are usually placed at right angles to the coal face, and when this system of timbering is used in conjunction with coal-cutters, it is a common practice to notch one end of the strap into the coal, the other being supported by a prop. As soon as the coal-cutter passes, another prop is driven under the end next the coal, and close up to it. This method of strapping the roof is shown in Fig. 50.

Chocks or Cogs. — Where this method is adopted the chocks are placed in single or double rows depending upon the nature of the roof and the necessities of the case. The chocks are usually formed from pieces of hard wood cut square, the length varying from 2 to 3 feet, the section being usually 4 inches by 4 inches. These pieces of timber are built up so as to form pillars between the floor and the roof, and are placed at such intervals apart as the condition of the roof demands. The chocks thus formed are usually set upon some loose material which is easily holed out, when it is desired to withdraw the chock. As the face advances the chocks are moved forward in regular order, and packs built along the roadside where the necessary roof material is obtainable for the purpose. Each time the chocks are shifted they are usually moved forward about 6 feet, but the exact distance is fixed by the nature of the roof. The most economical application of this system of securing the working face occurs where the coal is being worked back on the panel system, or by longwall retreating. In each case the roadways are protected by the solid coal, and the whole of the roof behind the workings is allowed to subside, as no roads require to be kept through it. This method of roof support is shown in Fig. 51.

Sprags or Gibs. — These appliances are used to support the coal after it has been undercut either by the miner or by a coal-cutting machine. They consist of short, stumpy pieces of timber of such length and size as allows them to be driven underneath the holed coal. They ought to be given a good grip of the coal, and so placed that they will support it should it become loose or detached. Although a specified maximum distance, which must not be exceeded between any two consecutive gibs, is laid down by the Mines Act, it is advisable to consider the nature of the coal, its liability to break, and the freedom or otherwise from joints or keen partings, before fixing the distance between the gibs. Should the condition of the coal demand close gibbing, then it ought to be done without regard to the legal maximum.

Often when spragging coal that has been undercut by coal-cutters, the spragging may be very effectually done by inserting a short prop lengthwise under the coal, and putting another under the outer end of the first, and at right angles to it, afterwards driving it sufficiently far in to cause the first one to bear hard against the coal. This affords a ready and secure method of spragging, in which the sprags are easily withdrawn at the proper moment with comparatively little trouble. The method of setting sprags is shown in Fig. 52.

Cocker Megs or Stells. — These are placed against the face of the coal, and slightly notched in to prevent them from slipping. A short distance piece is fixed between the end of the stay or stell and the roof, which prevents it slipping up, and thus losing its grip. These stells are used in securing broken parts of the roof or sides where the coal is being removed to prevent the roof materials swinging over the props, as it might do if open at one side. The method of setting stells is shown in Fig. 53.

General Conditions to be observed. — In order to keep down accidents from falls of roof and coal, it is desirable that systematic attention should be given to such points as the following

(1) The regular advance of the working face, as this causes the line of roof-break to extend over the whole face, whereas if certain parts are allowed to drag behind, the weight is concentrated on those parts. This concentration not only crushes the coal, but it causes the roof-breaks to run into each other at that part, and thus the roof at this particular part becomes much more dangerous than it would be under normal conditions.

(2) Shot-firing should be reduced to a minimum, and where shots must be fired they should not be too heavily charged, so that there will be less tendency for them to hurl out material and so dislodge timber. Where there is any risk of this taking place, a few old props may be set in front of the shot, so that they will receive the force of the blast and so prevent it knocking out regular timber, which may possibly be supporting loose parts of the roof.

(3) An ample supply of suitable timber should always be at hand, so that the miner may have no excuse for delay in putting in timber the moment that circumstances suggest that it is required.

(4) Where packing is used as a means of supporting the roof the packs should be put in regularly, and kept as close up to the face as the working conditions will admit. It is always bad policy to allow this work to fall behind, as it undoubtedly creates additional danger by leaving too great an area of roof without adequate support.

(5) Where circumstances are favourable the use of coal cutters removes the danger to which the miner is exposed when lying in front of the coal while holing. No doubt the use of coal-cutters introduces certain other risks, but, on the whole, their employment tends to reduce the number of accidents associated with the actual operations of coal getting.

(6) The method of timbering and the space between the timbers should be definitely fixed by the manager, either from his own experience or that of some responsible person completely conversant with the character of the roof and the nature of the seam. While the miner is compelled to set up props at the fixed distances, whether the roof appears to require it or not, he should be impressed with the fact that additional support should be given if at any time in his own judgment the roof seems to require it. In other words, the law requires a certain minimum of roof support, but does not prevent the miner from using as much additional support as he deems necessary.

(7) Careful inspection and supervision should be given to all parts of the workings at frequent intervals, say twice during each shift, by competent officials, to see that rules are being adhered to, and to advise workmen where circumstances point to advice being necessary.

Testing the Roof. — When the miner enters his working place in the morning, he should make a careful examination of the roof before he starts work. If any part seems doubtful it can be tested by means of a pick, the roof being struck with the flat side and the sound noted. If the roof gives a sharp ringing sound when struck, it is usually safe, whereas, if it is loose, a dull, hollow sound results. To make this test of the utmost value, the miner should place his free hand against the roof close to where he strikes it. If the roof is bad he will feel some vibration, which is more satisfactory evidence than mere sounding, as a large, heavy stone may be quite loose and still ring true. When a loose stone is discovered it should at once be taken down, or a prop set up to it — on no account should it be left until something else is finished; a moment's delay may cause an accident.