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CHAPTER II

SEARCH FOR COAL.

Prospecting. The preliminary operations in searching for coal in new districts, consist in carefully examining surface indications to determine the nature and position of the beds exposed in the area under examination. A knowledge of geology is indispensable for such work. The banks of streams and cuttings should be closely examined, and all outcrops noted and laid down on a rough sketchmap. Rocks and fossils of Carboniferous age afford the best indication of the probable existence of coal, but it is not absolutely necessary that such should be found at the surface, nor is it certain that when they are found coal surely exists beneath. For instance, in England, the greater part of the Somerset coal-field is covered with rocks of newer formations (Lias and New Red Sandstone); while in the north of France and Belgium, thick deposits of the Cretaceous formation are passed through before reaching the Coal Measures. Perhaps the most remarkable instance of the reversal of strata is afforded at Drocourt, in the Pas de Calais, where, after sinking through the Cretaceous, they passed at a depth of 413 feet into the Devonian; and after sinking in this formation to a depth of 958 feet from the surface, met with very disturbed Coal Measures, and beds of coal, which were worked for a considerable period. The shaft was sunk deeper and deeper, until at 1886 feet a fault was reached. On passing through this, the ordinary Coal Measures of the district were met with, and are now being worked. The Devonian and first portion of the Coal Measures met with had evidently been bent completely over before the Cretaceous was deposited.

Boring. Even after the examination above referred to, from which the probable existence of minerals may be reasonably inferred, further proofs have to be obtained. If outcrops of actual seams have been found, a great deal can be done by sinking shallow pits or by driving levels. Indications at small depths are, however, seldom conclusive, especially as regards the quality of the coal seams, and the operation of boring is generally resorted to.

Choice of Site.-For proving considerable areas several holes may be required, the sites of which are determined by the extent, location, and general features of the land to be developed. The preliminary survey decides these general features, but consideration has also to be given to the suitability of the spot for the erection of the drilling apparatus and for carrying on the work.

Various Appliances used in Boring-(a) Bits.-For shallow holes in soft ground the borer consists of some heavy instrument of the "scoop" kind, the general form being a cylinder, the cutting edge having a slit up its side like a gimlet. In soft, loose ground, pipes furnished with a cutting edge can be driven down by blows of a heavy

wood block dropped through a considerable height. A second pipe of smaller diameter is lowered inside the drive-pipe, and through it a strong stream of water is forced. This second pipe follows the cuttingshoe, and stirs up the loose material and washes it to the surface.

This method is largely used in America, up to 300 feet of gravel being easily got through. The pressure of water is sufficient to force up gravel of about inch diameter, but if larger pieces than this are encountered they must be chopped to pieces.

For harder ground bits of chisel-shape have to be employed. These are suspended from rods, which are raised up and dropped down, thereby chipping off small quantities of rock. The rods are rotated after every blow, so that the tool drops in a different place each time.

The general form of chisel employed is that having a straight edge (Fig. 8). The angle enclosed by the cutting edges is variable, depending on the nature of the rock. For hard rocks, a chisel with an acute edge is too likely to break; the angle should not exceed 70°. The size of the chisel should be carefully measured before it is lowered into the hole, as if it is too wide it will jam, while if too small the hole will get too narrow. As with all tools, the chisel was formerly made of wrought iron tipped with steel, but is now universally constructed of steel throughout. For very hard rocks a V-shaped chisel is sometimes employed. Various other shapes have been tried from time to time, but abandoned, owing to the difficulty of sharpening.

(b) Rods.-These may be either of wood or iron, the latter being most common. Their usual size is about 1 inch square, and from 28 feet to 36 feet long. Shorter pieces for making up lengths are also used. As the hole gets deeper the thickness of rods has to be increased. The rods are provided with screwed and socket ends, and as the portion on which the screw is cut should be the same size as the rod itself, the metal is thickened out at the joints, forming a shoulder, to which the tools for supporting the rods during changing operations can be conveniently attached. The common triangular screw thread is generally employed, and the socket made deeper than the screw, in order that the shoulders of two successive rods should bear firmly

against each other, thereby preventing the risk of stripping the thread. All the joints should be identical, made to gauge, and be well fitting, or the liability to accident is great. Splitting the socket is perhaps the commonest failing, and may be minimised by exercising care in preventing the vibration of the rods. The rods should always be rotated in the same direction as they are screwed up together, and when the threads begin to wear they should be broken off and fresh end pieces welded on.

After each blow the rods and chisel are turned through a small angle by the "tiller" (Fig. 9), which is attached at the surface. To enable this operation to be easily carried out, a swivel joint is introduced. As the depth of the hole slowly increases the rods necessarily descend, being allowed to do so by the use of an instrument called the "stirrup" (Fig. 10), which consists of a collar gradually working down a long screw. When the limit of travel of this instrument is reached, it is detached, the screw run back into the position shown in the illustration, and a short length of rod inserted between it and the main length attached to the tool. This operation is repeated until sufficient distance has been bored to allow of the insertion of an ordinary length of rod, the smaller making-up pieces being then removed. For unscrewing, an ordinary spanner key is employed.

(o) Guides. To keep the hole vertical a guide-block is fixed at the surface. This generally consists of a block of wood (a, Fig. 11) about 9 feet long, through the centre of which passes a hole of the same diameter as the bore-hole. It is fixed truly vertical, and secured by four pieces of wood arranged in the form of a square. Its upper end is furnished with two stops (b, b) turning around pins. A piece is cut out of each shutter, leaving an opening central with the hole, and of a size slightly larger than the rods, so that when the latter are in position the space is filled in. This shutter really fills two purposes, as it prevents anything falling down the bore-hole, and also suspends the rods during the operation of unscrewing, the hole through it being large enough to allow the rods to pass, but not a joint.

In deep holes other guides are inserted in the rods at regular intervals. A common form is that shown by Fig. 12, which readily passes through water. Discs and other shapes have been abandoned, as even where water-ways are left through them, they set up eddies in the water filling the bore-hole, wearing away the sides, and causing them to fall in, if the rock is at all soft.

(d) Clearing Instruments.-When a sufficient amount of cutting has been done, the débris which has accumulated at the bottom of the hole is removed by the "sludger" (Fig. 13), which consists of a tube from 4 to 6 feet long, having a valve at the bottom, either of the ball or flap-door type. The removal is usually done with a rope, sometimes a few lengths of rods being added to give weight. When the sludger reaches the bottom, it should be picked up and dropped several times before raising to the surface. For deep and large bore-holes a superior class of sludger is employed, having, in addition to the valve at the bottom, a piston working in the barrel portion. When this piston is drawn up it sucks-in the slime.

(e) Levers. The most general way of working the rods in percussive boring is to attach them to the shorter arm of a lever (Fig. 14), the longer end of which receives an up-and-down motion; as previously

mentioned, the rods are suspended by a swivel, and are turned by the bore-master after each blow. Where manual labour is employed two or more smooth cross-bars are attached to the longer arm of the lever, so that more men are able to work at it. With cross-pieces 8 feet long, six men can work on each side. For deep holes manual labour is quite out of place, and the long end of the lever is depressed at intervals, either by large teeth on a revolving wheel driven by steam, or, preferably, by directly connecting it with the piston-rod of a cylinder.

Fig. 12.

Fig. 13.

Fig. 14.

Fig. 15.

(f) Spring Pole.-In our coal districts the vibratory movement is often given to the rods by the use of the spring bar, which consists of a wooden pole having one end fixed to the ground, a fulcrum placed further on, and the rods attached to the other end. The blow is struck by depressing the beam, the rods being raised by the elasticity of it. The lengths of the parts on each side of fulcrum are usually 13 or 5. For shallow holes the axis may be fixed, but for deeper ones it must be movable. An elaboration of this method consists in the employment of two spring poles. The first is from 60 to 70 feet long, fastened at one end (Fig. 15), and at of its length from the fixed point it rests on an upright. To the other end are fixed two cross-bars which the workmen press down on to a second spring pole, thus producing a dancing movement. Between the upright and the cross beams is attached a hook, from which the boring tools are hung in the usual manner.

(g) Frames. In order to enable the rods to be raised perpendicularly, a frame of three shear legs is erected at the surface, to which a pulley is attached at the top, one of the shear legs having steps on it so that this may be easily reached. For shallow holes a windlass supplies motive power, but in larger holes a steam engine and drum is employed. To save labour in unscrewing the rods, it is advisable that the frame should be made as high as possible, so that a long length of rods may be raised at a time. This is done in the following manner :-The stops (2, Fig. 11) are opened, and a hook (Figs. 16 and 17) at the end of the rope is placed beneath a joint in the rods, these

being then detached from the end of the lever. The rope is then hoisted up until the limit in height is reached, when the stops are closed beneath the nearest joint, and the rods above that joint un

Figs. 16 and 17.

screwed and removed. This operation is repeated until all are withdrawn. The sludger is then lowered, either by the same rope, or, if the boring is a large one, by a second one passing over another pulley lower down the frame. After clearing out the débris the rods are replaced by a reversal of these operations. One point must be specially noted: the rods when not in use should never be stood on end, but always suspended.

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Devices employed to meet Difficulties of Deep Boring.— As the depth of holes increases a large number of difficulties arise, the greatest one being the weight of the rods themselves. Rods 1 inch square weigh about 1 ton for every 200 yards. In deep holes not only does this great weight injure the screw joints, alter the structure of the iron, and break the tool which receives the blow, but it sets up excessive vibration in the rods, injures the sides of the hole, and accumulates a mass of broken material above the tool which often leads to rupture when an attempt is made to withdraw it. To overcome these disadvantages several methods are employed.

(a) Lighter Rods.-Hollow iron rods were first suggested, the weight of these for the same strength as solid ones being in the ratio of 1 to 1*35. They were, however, found to be too expensive. Wooden rods were then introduced. They possess certain advantages over iron, as not only are they specifically lighter, but, when the borehole contains water, as their size is also greater, a larger volume of water is displaced. They also fit the hole tightly, and do not rattle about from side to side like iron ones. When a rupture occurs with iron rods, the large weight dropping down causes other breakages and the bending of rods in the hole, often rendering it a most difficult matter to get them out. On the other hand, when a breakdown occurs with wooden rods, there is generally only one fracture. The great objection, however, to their employment is the large diameter of hole required, owing to the necessity of using rods 2 to 3 inches square for shallow depths. For larger and deeper holes this objection is removed, and such rods have been largely employed in Canada and on the Continent.

(b) Free-falling Cutters.-The greatest advance made in percussive boring, was undoubtedly the introduction of what are known as "free-falling cutters." By their use, vibration and shocks in main rods are practically avoided, the only portion of the apparatus that is really let fall being the boring tool itself, and as much of the apparatus as is necessary to give weight to the blow.

The tool designed by Kind has been largely employed. It consists of two fangs or pincers (a, a', Fig. 18), working about centres b, b'. These fangs are enclosed at their upper extremity by a collar c, connected to a circular disc of leather d, through a rod e; at their lower end they grip, during certain stages of the operation, that part, f, of the rods to which the tool is attached. As shown in the illustration, the rods are making their upward stroke, and the pressure of water in