removal of the uarried material by mechanical means in the form of cranes and aërial cableways becomes necessary. 155. Tirring or Stripping.-The tirring or stripping may be removed as necessity arises, or, as in the case where the quarries are worked under County Councils directly, it may be accomplished by the roadmen during frost when their services can be spared from the ordinary road work. The earth removed should be deposited at a point not likely to interfere with the future working of the quarry. Attention to this matter will save a considerable amount of trouble and unnecessary expense in lifting it again at some other time. The overburden, when of a tenacious nature, or when it is frozen, may be advantageously broken up by sinking a few holes from 2 to 5 feet in depth, and charging them with some high-grade explosive; on this being exploded the ground will be sufficiently broken up to ensure its easy removal. 156. The simplest form of quarrying is that by which the material can be removed from the parent mass by picks, wedges, and levers. This is, however, only possible in certain quarries, which are by no means numerous, and recourse must usually be had to drilling and blasting. 157. Hand-drilling or Boring.-The boring or drilling of holes by manual labour, or, as it is termed, hand-drilling, is generally effected by either single or double-hand operations. The jumpers, or boring-tools, are for the most part now made from cast steel bars of octagonal section, supplanting those formerly in use made from round iron bars with steeled cutting edges. The cutting edge of the tool may vary in width from 1 to 4 inches, the stocks being of a somewhat less diameter, so as to enable the tool to free itself readily in the bore-hole; this also avoids the introduction of any unnecessary weight into the jumpers. 158. For instance, a boring-tool having a cutting edge of 1 inch in width is made from a steel bar gths of an inch in diameter, while inch bars are selected to make a boring-tool with 11-inch cutting edge, and so on in a similar proportion to tools or jumpers for boring a 2- or 3-inch hole, which are manufactured from bars of 11⁄2 inch in diameter. The cutting edge of the tool may be made straight or slightly curved for haud-boring; the straight cutting edge, although working more freely than those of the curved form, are somewhat weaker at the corners, and consequently liable to fracture. 159. Hand-drills and Jumpers.-The straight and curved cutting edges are shown in figs. 47 and 48, while fig. 49 represents a section of the chisel point, the sides of which should not form a greater angle than 70 degrees. The steel bars from which boring-tools are made are generally cut into varying lengths as may be desired; the tools for boring holes of the smallest diameter are from 18 to 20 inches long, while those for boring holes of greater diameter may vary from 42 inches up to 6 or 7 feet in length. A blow given to a jumper is more effective the shorter the bar, hence it is a common practice to make use of a number of tools, each succeeding one being somewhat longer, while at the same time the cutting edge is slightly reduced in width. The actual diameter of the hole drilled is somewhat greater than that of the drill-bit. The striking face of the steel or stock should be flat, and in the larger sizes the diameter of the head is made less than that of the stock. 160. Hardening and Tempering Hand-drills.-The forging and hardening of boring or drill-bits are operations requiring much skill on the part of the smith, so as to avoid the cutting edge and lower portion of the stock being burnt,' or the formation of water cracks'; and in tempering it is advisable, in order to obtain the best results, to let down the steel to a bright straw yellow. 161. Hammers and Sledges.-Hammers or sledges of cast steel are made of varying weights, according to whether the drilling is to be performed by the single-hand or the double-hand method. These vary considerably in shape according to the ideas and customs of the workmen in the different. localities where quarrying operations are carried on. The weight of hammers for single-hand boring is generally about 4 lbs., and as the expression implies, one man in a sitting position performs the whole operation by manipulating both tool and hammer. For double-hand drilling the sledge may be as heavy as 10 lbs., the shaft or helve, generally 30 inches long, being made of a wood having some elasticity, such as hickory, young oak, or acacia; ash is also much used for the shafts or handles of quarry tools. 162. Starting a Bore-hole. Single- or Double-hand Drilling.-The first step necessary in commencing drilling, after the tirring or stripping has been accomplished, is to fix the position of the bore-hole for blasting in the most advantageous manner, regard being had to the natural veins and fissures of the rock as a means of facilitating its detachment from the parent mass. It is necessary to remove any loose pieces of rock and prepare the surface by chipping, so that it may be nearly at right angles to the direction of the intended bore-hole. Double-hand drilling is sometimes performed by two men; one, sitting on the rock, guides and turns the jumper with one hand, and strikes, by means of a light hammer, alternate blows, the other blows being delivered by a second man using a heavy hammer or sledge. The usual, and at the same time most satisfactory method, is for three men to do the work. One man takes up a sitting position and holds the drill vertically between his knees, keeping it in proper position, and turning it partly round, while the other two men strike the head of the boring-bar with sledges, lightly at first, and increasing the force of the blow after the bit has fairly entered the rock. The degree of turn necessary to be given in drilling is one-eighth of a revolution, which prevents the cutting edge falling twice in the same place; by this means the greatest amount of work is accomplished, and a circular form of bore-hole is ensured. By using water in the bore-hole the drilling is greatly facilitated, the time being reduced by one-third; it also keeps the cutting edge cool, and converts the dust and chippings in the bottom of the hole into sludge, which must be cleaned out by means of a 'scraper' from time to time as it accumulates. In very hard rock a drill may have to be changed frequently, which necessitates the use of several of about the same length; and these should always be in good order and near at hand. The depth to which the bore-holes are drilled by the double-hand method is generally from 5 to 6 feet, sometimes 8 feet; for greater depths up to 10 feet churndrilling is resorted to when the nature of the rock admits of this being done. 163. Churn-drilling.-The drilling-bar or steel for this method of boring has a cutting edge of different width at either end of the stock, the blow given to the rock being obtained by the direct impact of the falling drill, which is raised by two men to a height of about one foot, and then allowed to drop. In long boring-tools a sufficient weight for the purpose is generally FIG. 50.-Long boring-tool. contained in the steel stock, but additional weight may be obtained if necessary by having a bead or swelling fixed on the stock between, but at unequal distances from, the extremities, as shown in fig. 50. 164. The water which is poured into the bore-hole to facilitate the penetration of the bit into the rock is prevented from splashing about, particularly in shallow holes, by placing a perforated leather or india-rubber washer on the shank of the drill at the surface level of the rock. object may also be accomplished by winding straw round the steel, which should be fixed as in the case of the washer. This The sludge formed during the process of drilling must be removed from time to time as the depth of the bore-hole advances, since the oftener this is done the greater will be the amount of drilling performed in a given time. The rock being exposed to the action of the cutting edge of the tool and the full effect of the blows transmitted to it by the jumper, will not be cushioned, as it would be in the case where the sludge is lying between the bottom of the hole and the edge of the boring-tool. The cleaning out of the bore-holes, when not too deep, is effected by a simple tool called a scraper, which consists of a rod of iron varying from to inch in thickness, and of a sufficient length to reach the bottom of the holes. It is provided with a disc attached at each end of the rod of different diameter, so as to make it suitable for the various sizes of holes bored. Instead of having discs at either end, one of these is sometimes formed with a spiral hook or drag-twist, which is very useful, as, by attaching a wisp of hay to it, the bore-hole can be thoroughly wiped dry previous to charging. 165. In the event of the shank of a drilling-tool or jumper being broken by some cause or other while boring operations are in progress, the portion left in the bore-hole may be recovered by various means. The simplest way of accomplishing this is to guide a running noose of cord by the aid of a second drill on to the broken one, and pull the noose tight, when the portion of the drilling-steel left in the bore-hole can be drawn out. Tongs having an arrangement attached for tightening the legs so as to grip the steel firmly are also sometimes employed for this purpose; while another instrument called a beche, made of iron, and having a hole at the lower end of a slightly conical shape, which fits on to the top of the broken steel, is at times made use of for the same purpose. 166. If, as often happens in many quarries from which road material is taken, water finds its way through fissures into the bore-hole, it becomes necessary to line it with clay when the explosive agent employed is blasting powder. This is performed by putting the clay into the bore-hole in a plastic state, inserting an iron tool slightly less in diameter than the hole, and pointed at one end, called a brill, and by repeated blows forcing the clay into the joints and crevices of the rock. This instrument, which at one time formed an important part in a set of blasting gear,' has now become of less importance by the introduction of waterproof cartridges containing gunpowder, and the nitro-glycerine and gelatinous compounds, which are now much used in blasting operations. 167. Amount of Work accomplished by Hand-boring or Manual Labour. The amount of drilling or the depth of holes which can be accomplished in one day will greatly depend on the hardness of the rock and the size of the drilling-tool used. Joints and fissures when lying at an angle to the direction of the bore-hole give considerable trouble, and much time is consequently lost, reducing the amount of work below what could otherwise be accomplished under more favourable circumstances. For this reason it is advisable to select the best possible position for the bore-hole, while at the same time due consideration must be given to its position and direction in relation to the portion of rock which is to be dislodged from the parent mass. The amount of drilling which can be accomplished by hand, in the different classes of rock used for road purposes, varies considerably according to circumstances. The method of calculating and stating the actual amount of drilling performed will be given in cylindrical inches, which is particularly applicable to open quarry work. This system of computing the amount of work done will also be adopted in determining that performed by machine rock drills, which will be described later on. The cylindrical inch has a relative value, and the use of the term is preferable when making a comparison of the amount of work done and the cost of the operation, than by simply stating the number of inches or feet in depth drilled each day. Under the latter circumstances no comparison is possible, as the width of the bit or size of the bore hole may vary considerably according to the depth drilled, while the length of the working day is at times a variable factor in quarrying operations. Therefore, by adopting this numerical figure as a standard for the amount of work performed in a given time, a comparison can be more readily arrived at between drilling by manual labour on the one hand, and that accomplished by a machine rock drill on the other, while the drilling capacity of the different types of the latter can also, by this means, be easily ascertained. In Table XVIII. the average amount of work performed by hand drilling is stated in accordance with the preceding particulars. As the length of a day, however, is a somewhat vague definition of time, more especially when the working hours are interfered with by stress of weather, or shortened during the winter months, the hour is adopted from which a day's work of any specified number of hours can be calculated. The figures are taken from the results of actual experience under varying conditions, time being allowed for cleaning out holes and resting. TABLE XVIII. Rock Drilling by hand: one man manipulating the drill, two men |