Timbering of Drifts. — While drifts and crosscuts are the simplest form of mine workings, the effective timbering of them is sometimes a matter of considerable difficulty. This is particularly the case where these places are being driven across a fault, as tb.e broken ground along the fault plane is frequently very dangerous and requires careful handling.

The conditions met with are different in each particular case, but they may be roughly generalised as follows (1) Cases where the ground is heavy and contains slips and zones of fracture. (2) Running ground where the material is so soft and broken that the whole mass will rush in unless provided with immediate and adequate support. (3) Swelling ground, which, although less frequently met with, proves a source of much trouble. Here the pressure comes from all directions, floor, sides, and roof, and because of this it is more difficult to combat.

The method of dealing with heavy ground containing slips is to support the roof by cross-bars. These are set a few feet apart, and held in position by props or legs placed at each end. Lagging may be used above the crossbars, reaching from set to set. If the sides show a tendency to fall, they may be supported by carrying this lagging down behind the side timbers as well. In very heavy ground it may be necessary to place the timbers close together or, as it is generally termed, skin to skin or corduroyed. For an illustration of these methods of support see Fig. 54.

In dealing with running ground, the method of forepoling or forespiling is that usually adopted. In this method poles are driven forward over the top of the bars and behind the side legs before the excavation of the ground is commenced. As soon as these poles are partly cleared by the advance of the excavation, new sets of timber are put in and another batch of poles driven forward. In driving poles in soft ground it is always advisable to let them slant outwards and upwards at their points. This can be secured by placing a gib between the last driven batch and those being put in. This gib should be close up to the supporting batch. Where the bottom of the drift is soft the timber may be set upon sills running across the roadway, or longitudinally — such sill furnishes the required foothold for the legs.

Swelling ground often gives considerable trouble, and it is often very difficult to keep it open. In such cases sharp pointed legs may often be used with good results. These sink down into the floor without breaking, and as the floor material rises it can be removed, without interfering with the timber in any way.

In repairing drifts the workmen should never hesitate to set up temporary supports when removing old timber, as quite frequently large quantities of loose material may be hanging on what to all appearance is a broken and useless bar. The removal of the bar in such a case would cause an unsuspected fall and the risk of an accident. Regulations are provided to deal with the removal of timber, but here, as in a number of other instances, no rules can replace lack of judgment on the part of the person doing the work. Experience, combined with caution, is of even more importance than a knowledge of what the Mines Act prescribes as a means of lessening danger.

Brickwork and Masonry. — Under certain circumstances timber may not form a suitable means of support, as, for instance, where the ground weathers readily under atmospheric influences. In such cases some form of building material is generally used. Further, it is customary to use masonry for the support of the pit bottom, sidings, air-crossings, and suchlike places where a more permanent support than that afforded by timber is desirable.

Bricks, stone, and concrete are the materials generally used. Good hard-burned bricks are very suitable for this work, and where these can be obtained at a moderate cost they are often used.

Stone is less frequently used as the rough dressing necessary to allow of its being used in mason work costs too much, and it rarely provides greater strength than ordinary brickwork. On the other hand, stone obtained in the mine is largely used for building dry pack walls, but these cannot be considered as masonry since no mortar or cement is used in the building.

Concrete has been used to some extent within recent years, especially in conjunction with iron and steel, in the form of ferro-concrete. Ferro-concrete makes a good support for certain parts in the mine, but is too expensive for extended use.

Brick Arches. — These are used in such places as pit bottoms and sometimes in main roadways. They are costly to erect, but their long life often justifies the high first cost. Where erected no space should be left between the arching and the strata, and if the position is one where roof weight is likely to cause crushing of the arch, it is good practice to put a course of wood blocks in the side walls, these blocks being of the same size as the bricks. When this is done extreme pressure, instead of destroying the arch, simply crushes these wood blocks in the walling without otherwise seriously disturbing the brickwork.

Iron and Steel. — Steel in the form of rolled beams or girders is now largely used as a roof support in main roadways, usually to replace the timber bars or crowntrees. These girders may be carried on side walls built of brick, or they may be supported by legs of timber placed at either end. Where set on brick side-walls a wall-plate of wood should be placed on the top of the wall to rest the girders on. This allows the roof to sink a little before the sidewall supporting the girders is broken, as the girders sink into the wall-plate. Steel props are sometimes used, but steel is not so good for using as props as it is for crownbars.

Steel is rarely used near the face of the workings nor in any part of the mine where it would be likely to be permanently enclosed and lost. It oxidises with the damp atmosphere very readily, and when used should be covered with a coat of good paint before being put in position. The method of using steel beams to support the roof is shown in Fig. 55. It is worth noting that old tramway rails, after having been used up in the lines of a town tramway system, make excellent girders for underground use, and can always be bought at a smaller price per ton than new steel girders.

Classes of Timber used. — In general the so-called soft woods are in greatest demand for mining purposes, although hard wood is also used, but usually for special work. The soft woods include Spruce Fir, Scotch Fir, Red Pine, Pitch Pine and Larch, all of which belong to the same order, the Coniferae. They are straight-fibred, long-grained timbers that resist pressure in the direction of their fibres better than many hard woods, but are not possessed of any great power to resist transverse pressure. The strength in the direction of the grain is usually greatest in timber which has the straightest and most distinct fibres.

It is desirable when possible to have timber which has been cut during the winter, December and January being the two best months, as at that time there is comparatively little sap in the trees. Well-seasoned timber is always preferable to green, as the sap in the latter ferments and causes rapid decay.

Timber for props should be able to resist considerable pressure along the direction of the grain, and should bend to some extent before it breaks. When used for bars or crowns the stress is across the fibres, and here again there should be considerable bending before breakage.

Larch is perhaps the timber that is best adapted for underground use, but in many cases fir makes a capital substitute.

The conditions of the mine have a far-reaching effect on the life of any timber that may be employed for the support of the roof or other purposes. Timber will last longer where the mine is wet than it will if the mine be dry, or alternately wet and dry, as is sometimes the case in shallow mines. Fungus does not grow readily on timber where there is a good supply of pure air. If the circulation of the air is sluggish or the oxygen content low, fungus grows very rapidly, and it is generally quite noticeable that this disease occurs to a much greater extent in return than in intake airways. If green timber be used in the mine it should, if possible, be placed where it will be kept continually wet. If used in a dry position it soon begins to decay.

Preservatives. — To prevent the rapid decay of timber used in the mine and thus reduce cost, the timber, especially that used in the main roads, may be treated with some preservative. Many experiments have been made to ascertain the best means of preservation, but it will be apparent that the value of any process will depend largely on its cost rather than on its efficiency. Various methods of treatment are : (a) Peeling, (b) Seasoning, (c) Treatment with Antiseptics, and (d) Electro-chemical treatment.

(a) Experience shows that in most cases where timber is used underground it lasts longer if peeled. This is due to the sap retained between the bark and the wood fermenting when the timber is unpeeled. This fermentation leads to the growth of wood-destroying fungi, besides the presence of the bark affords refuge for many forms of insects whose ravages aid the process of decay. Larch timber is often peeled when cut, the bark being used for tanning leather.

(b) Seasoning or drying timber, whether peeled or not, adds greatly to its strength and durability. Green timber has little more than half the strength of timber that has been properly seasoned.

(c) Although peeling and seasoning do a great deal to increase the strength and durability of timber, it is possible to obtain a still greater life by some form of preservative treatment. The preservative is usually an antiseptic which will prevent the growth of fungi, and if at the same time it offers resistance to the entrance of moisture, this adds to the resistance to decay. Such substances are Copperas (Sulphate of Iron), Corrosive Sublimate (Bichloride of Mercury), Common Salt (Sodium Chloride), Chloride of Zinc, Sulphate of Copper, and Creosote.

(d) The most perfect system of treatment is to impregnate the timber with certain of the above salts by means of electricity, as by this method the preservative is carried into the interior of the wood. The process is, however, too expensive for general application to mine timber.

Methods of Treatment. — In some cases the preservative is applied by means of a brush, but this method is unsatisfactory, due to the difficulty of completely covering the timber, and also on account of the slight penetration of the liquid. Further, the method involves high labour costs.

A better method is the immersion of the timber in the preservative contained in an open or closed tank. The former is less expensive, and, at the same time, less efficient; the latter gives better results but at a higher cost.

It often happens that the conditions of the mine are fully met by timber treated in open tanks, as such treatment may preserve it from decay until such time as it gets broken by the weight of the strata it has been set up to support. In such a case the least expensive process that will preserve it until this takes place will be the best one to adopt.

Open treatment is usually carried out in a large tank fitted with a double bottom, with coils of steam pipes fitted therein. The steam circulated through these pipes is sufficient to heat the liquid preservative to a temperature above the boiling point of water. The timber, which must have been previously peeled, is placed in the hot liquid in the tank, and after the lapse of an hour or two, the steam is shut off and the liquid allowed to cool. After cooling, the timber is removed, the liquid made up to the desired volume and strength, and afterwards raised to a temperature of 220 degrees Fahrenheit, or thereby. It is then ready to receive a fresh batch, the process being repeated.

During the operation the heat of the liquid expands and expels part of the air and water contained in the wood tissue, and as cooling takes place there is contraction of what air and water remains. This contraction causes the absorption of the preservative liquid, and impregnation takes place as the liquid cools. Air is always more easily expelled than water, hence seasoned wood is more successfully treated than green or partly seasoned wood.

When treated in closed tanks the timber is placed inside a covered tank, which can be heated by steam contained in a jacket which surrounds it. The timber is first subjected to heat, which drives off any moisture it may contain, this moisture being removed largely in the form of vapour by means of a pump. When this has been done, the preservative solution is forced into the tank under pressure, and penetrates deep into the timber. This is a most effective, although an expensive, method, and on account of the circumstances already explained, is not largely used for the treatment of ordinary mine timber.

The use of treated timber underground is so far not very extensive, but is likely to increase as timber is becoming more costly and difficult to get. One feature should never be lost sight of, and that is, the inflammable nature of certain of the preservatives — for instance, Creosote. Fire on the surface is bad, but when it occurs underground the danger is much greater, and from the point of view of safety only, non-inflammable solutions should be used in the treatment of timber which has to be used in the mine.