Rock Blasting. A practical treatise, etc

Preface

. During the past decade, numerous and great changes have taken place in the system followed and the methods adopted for blasting rocks in industrial operations. The introduction of the machine drill led naturally to these important changes. The system which was suitable to the operations carried on by hand was inefficient under the requirements of machine labour, and the methods which had been adopted as the most appropriate in the former case were found to be more or less unsuitable in the latter. Moreover, the conditions involved in machine boring are such as render necessary stronger ex plosive agents than the common gunpowder hitherto in use, and a more expeditious and effective means of firing them than that afforded by the ordinary fuse. These stronger agents have been found in the nitro-cotton and the nitro-glycerine compounds, and in the ordinary black powder improved in con-

PREFACE.

. VI stitution and fired by detonation ; and this more expeditious and effective means of firing has been discovered in the convenient application of elec tricity. Hence it is that the changes mentioned have been brought about, and hence, also, has arisen a need for a work like the present, in which the subjects are treated of in detail under the new aspects due to the altered conditions. GEO. G. ANDRE. London, 17, King William Stbeet, Strand, January 1st, 1878. CONTENTS.

Chapter I

The Tools, Machines, and other Appliances used in Rock Blasting. PAGH Section I. Hand-boring Tools. — Drills. Hammers. Auxiliary Tools. Sets of Blasting Gear 1 Section II. Machine-boring Tools. — Machine Rock-drills. Borer-bits. Drill Carnages 23 Section III. Appliances for firing Blasting Charges. — Squibs. Safety Fuse. Electric Fuses. Cables. Detonators. Electric Firing-Machines 42

Chapter II

Explosive Agents used in Rock Blasting. Section I. Phenomena accompanying an Explosion. — Nature of an Explosion. Heat liberated by an Explosion. Gases generated by an Explosion. Force developed by an Explosion .. .. 64 Section II. Nature of Explosive Agents. — Mechanical Mixtures. Chemical Compounds 76 Section III. Relative Strength of the common Explosive Agents. — Force developed by Gunpowder. Relative Force developed by Gunpowder, Gun-cotton, and Nitro-Glycerine 88 Section IV. Means of firing the common Explosive Agents. — Action of Heat. Detonation .. .. 92 Section V. Some Properties of the common Explosive Agents. — Gunpowder, Gun-cotton, Dynamite. Firing Temperatures . . 97 Section VI. Some Varieties of the Nitro- Cellulose and the Nitro- Glycerine Compounds. — Nitrated Gun-cotton. Tonite, or Cotton- Powder. Schultze's Powder. Lithofracteur. Brain's Powder. Cellulose-Dynamite CONTENTS. VIII

Chapter III

The Principles of Rock Blasting. rAOE Line of least Resistance. Force required to cause Disruption. Conditions of Disruption. Example of a Heading. Economical Considerations. Tamping 106

Chapter IV

The Operations of Rock Blasting. Hand Boring. — Boring the Shot-holes. Charging the Shot-holes. Firing the Charges 128 Machine Boring. — Boring the Shot-holes. Charging and Firing. Removing the dislodged Rock. Division of Labour 142 Examples of Drivings. — The St. Gothard Tunnel. The Hoosac Tunnel. The Musconetcong Tunnel. Headings at Marihaye, Anzin, and Ronchamp 157

Chapter V

Subaqueous Blasting. Preparation of the Charge. Boring under Water. Submarine Rocks. Obstructions in Water-courses CHAPTEB I. THE TOOLS, MACHINES, AND OTHER APPLIANCES USED IN BLASTING ROCKS. Section I. — Hand Boring. Drills. — The operations of blasting consist in boring suitable holes in the rock to be dislodged, in inserting a charge of some explosive compound into the lower portion of these holes, in filling up, some times, the remaining portion of the holes with suitable material, and in exploding the charge. The subjects which naturally first present themselves for consider ation are : the nature, form, and construction of the tools, machines, and other appliances used. Of these tools, the " drill " or " borer " constitutes the chief. To understand clearly the action of the rock drill, we must consider the nature of the substance which has to be perforated.

He who has examined the mineral constitution of rocks will have recognised the impossi bility of cutting them, using that term in its ordinary acceptation, inasmuch as the rock constituents are frequently harder than the material of the tools Ps employed to penetrate them. As a rock cannot be cut, the only way of removing portions of it is to fracture or to disintegrate it by a blow de livered through the medium of a suitable instru ment. Each blow so delivered may be made to chip off a small fragment, and by this means the rock may be gradually worn away. To effect this chipping, however, the instrument used must present only a small surface to the rock, in order to concentrate the force, and that surface must be bounded by inclined planes or wedge surfaces, to cause a lateral pressure upon the particles of rock in contact with them.

In other words, the instru ment must be provided with an edge similar to that possessed by an ordinary cutting tool. The conditions under which the instrument is worked are obviously such that this edge will be rapidly worn down by attrition from the hard rock material, and by fracture. To withstand these de structive actions, two qualities are requisite in the material of which the instrument is composed, namely, hardness and toughness. Thus there are three im portant conditions concurring to determine the nature and the form of a cutting tool to be used in rock boring — 1, a necessity for a cutting edge ; 2, a necessity for a frequent renewal of that edge ; and 3, a necessity for the qualities of hardness and tough ness in the material of the tool. In very hard rock, a few minutes of work suffice to destroy the cutting edge, and then the tool has to be returned to the smithy to be re-sharpened.

Hence it is manifest that the form of the edge should not be one that is difficult to produce, since, were it so, much time would be consumed in the labour of re-sharpening. Experience has shown that the foregoing conditions are most fully satisfied in the steel rod terminating in a simple chisel edge, now universally adopted. This form of drill is exhibited in Fig. 1, which represents a common " jumper " borer. It consists of a rod terminating at each end in a chisel edge, and having a swell, tech nically described as the "bead," between the extremities to give it weight. The bead divides the jumper into two unequal portions, each of which constitutes a chisel bit, with its shank or " stock." The shorter stock is used while the hole is shallow, and the longer one to continue it to a greater depth.

With the jumper, the blow is obtained from the direct impact of the falling tool. The mode of using the instrument is to lift it with both hands to a height of about a foot, and then to let it drop. In lifting the jumper, care is taken to turn it partially round, that the edge may not fall twice in Fig. 1. the same place. By this means, the edge is made to act most favourably in chipping away the rock, b and the hole is kept fairly circular. So long as the holes are required to be bored vertically down wards, the jumper is a convenient and very efficient tool, and hence in open quarrying operations, it is very commonly employed. But in mining, the shot-holes are more often required to be bored in some other direction, or, as it is termed, " at an angle ; " that is, at an angle with the vertical.

Or it may be that a shot hole is required to be bored vertically upward. It is obvious that in any one of these directions the jumper is useless. To meet the requirements of such cases, recourse is had to the hammer wherewith to deliver the blow, and the drill is con structed to be used with the hammer. We have a suitable form of tool for application in this wise when we cut out the bead of the jumper and leave the ends flat for a striking face, as shown in Figs. 2 and 3. The form of the two chisels thus obtained is that adopted for the ordinary rock drill. It will be understood from these descrip tions that a rock drill consists of the chisel Fig. 2. Fig. 3 edge or bit, the stock, and the striking face. Formerly drills were made of wrought iron and steeled at each end to form the bit and the striking face.

Now they are commonly made of cast steel, which is supplied for that purpose in octagonal bars of the requisite diameter. The advantages offered by steel stocks are numerous. The superior solidity of texture of that material renders it capable of transmitting the force of a blow more effectively than iron. Being stronger than the latter material, a smaller diameter of stock, and, consequently, a less weight, are sufficient. This circumstance also tends to increase the effect of the blow by diminishing the mass through which it is transmitted. On the other hand, a steel stock is more easily broken than one of iron. The cutting edge of a drill demands careful con sideration. To enable the tool to free itself readily in the bore-hole, and also to avoid introducing un necessary weight into the stock, the bit is made wider than the latter ; the difference in width may be as much as 1 inch.

It is evident that in hard rock, the liability of the edge to fracture increases as the difference of width. The edge of the drill may be straight or slightly curved. The straight edge cuts its way somewhat more freely than the curved, but it is weaker at the corners than the latter, a circumstance that renders it less suitable for very hard rock. It is also slightly more difficult to forge. The width of the bit varies, according to the size of the hole required, from 1 inch to 2| inches. Figs. 4, 5, and 6 show the straight and the curved bits, and the angles of the cutting edges for use in rock. The stock is octagonal in section ; it is made in BOCK BLASTING. lengths varying from 20 inches to 42 inches. The shorter the stock the more effectively does it transmit Fig. 4. Fig. 5. Fig. 6. the force of the blow, and therefore it is made as short as possible.

For this reason, several lengths are employed in boring a shot-hole, the shortest being used at the commencement of the hole, a longer one to continue the depth, and a still longer one, sometimes, to complete it. To ensure the longer drills working freely in the hole, the width of the bit should be very slightly reduced in each length. It has already been remarked that the diameter of the stock is less than the width of the bit ; this difference may be greater in coal drills than in rock or "stone" drills; a common difference in the latter is | of an inch for the longer. The following pro portions may be taken as the average adopted : — Width of the Bit. Diameter of the Stock. Width of the Bit. Diameter of the Stock. 1 inch ii U » § inch 3 3 » 7 ■ff » 1 ,, lj inch 2 inches 2i » 2* ,, 1| inch 1 3 H ,, *■*¥ » The striking face of the drill should be flat.

The diameter of the face is less than that of the stock in all but the smallest sizes, the difference being made by drawing in the striking end. The amount of reduction is greater for the largest diameters ; that of the striking face being rarely more than one-eighth of an inch. The making and re-sharpening of rock drills con stitute an extremely important part of the labour of the mine smith. The frequent use of the drill, and its rapid wear, necessitate a daily amount of work of no trifling proportions, and the judgment and skill required in proper tempering render some degree of intelligence in the workman indispensable ; indeed, so much depends upon the smith whose duty it is to repair the miners' tools, that no pains should be spared to obtain a man capable of fulfilling that duty in the most efficient manner possible.

When the borer-steel bars are supplied to the smith, he cuts them up, as required, into the desired lengths. To form the bit, the end of the bar is heated and flattened out by hammering to a width a little greater than the diameter of the hole to be bored. The cutting edge is then hammered up with a light hammer to the requisite angle, and the corners beaten in to give the exact diameter of the bore-hole intended. As the drills are made in sets, the longer stocks will have a bit slightly narrower than the shorter ones, for reasons already given. ROCK BLASTING. The edge is subsequently touched up with a file. In performing these operations, heavy hammering should be avoided, as well as high heats, and care should be taken in making the heat that the steel should be well covered with coal, and far enough removed from the tuyere to be protected from the " raw " air.

Overheated or " burned " steel is liable to fly, and drills so injured are useless until the burned portion has been cut away. Both in making and in re-sharpening drills, great care is required to form the cutting edge evenly, and of the full form and dimensions. If the corners get hammered in, as shown in Fig. 7, they are said to be "nipped," and the tool will not free itself in cutting. When a depression of the straight, or the curved, line forming the edge occurs, as shown Fig. 7. Fig. 8. Fig. 9. in Fig. 8, the bit is said to be " backward," and when one of the corners is too far back, as in Fig. 9, it is spoken of as " odd-cornered." When either of these defects exist — and they are unfortunately common — not only does the bit work less effectively on the rock, but the force of the blow is thrown upon a portion only of the edge, which, being thereby overstrained, is liable to fracture.

The hardening and tempering of steel is a matter requiring careful study and observation. It is a well-known fact that a sudden and great reduction of temperature causes a notable increase of hardness in the metal. The reason of this phenomenon is not understood, but it is certain that it is in some way dependent upon the presence of carbon. The degree of hardness imparted to steel by this means dej)ends upon the amount of the reduction of the temperature, and the proportion of carbon present in the metal, highly carburetted steel being capable of hardening to a higher degree, under the same conditions, than steel containing less carbon. Thus, for steel of the same quality, the wider the range of temperature the higher is the degree of hardness. But here we encounter another condition, which limits the degree of hardness practically attainable.

The change which takes place among the molecules of the metal in consequence of the change of temperature causes internal strains, and thereby puts portions in a state of unequal tension. This state renders the strained parts liable to yield when an additional strain is thrown upon them while the tool is in use ; in other words, the brittleness of the steel increases with its hardness. Here again the proportion of carbon present comes into play, and it must be borne in mind that for equal degrees of hardness ROCK BLASTING. the steel which contains the least carbon will be the most brittle. In hardening borer-steel, which has to combine as far as possible the qualities of hardness and toughness, this matter is one deserving careful attention. It is a remarkable fact, and one of con siderable practical value, that when oil is employed as the cooling medium instead of water, the tough ness of steel is enormously increased.

The tempering of steel, which is a phenomenon of a similar character to that of hardening, also claims careful consideration. When a bright surface of steel is subjected to heat, a series of colours is pro duced, which follow each other in a regular order as the temperature increases. This order is as follows : pale yellow, straw yellow, golden yellow, brown, brown and purple mingled, purple, light blue, full clear blue, and dark blue. Experience has shown that some one of these colours is more suitable than the rest for certain kinds of tools and certain condi tions of working. The selection of the proper colour constitutes a subject for the exercise of judgment and skill on the part of the smith. For rock drills, straw colour is generally the most suitable when the work is in very hard rock, and light blue when the rock is only of moderate hardness.

The processes of hardening and tempering drills are as follows : When the edge of the bit has been formed in the manner already described, from 3 to inches of the end is heated to cherry redness, and dipped in cold water to a depth of about an inch to harden it. While in the water, the bit should be moved slightly up and down, for, were this neglected, the hardness would terminate abruptly, and the bit would be very liable to fracture along the line corresponding with the surface of the water. In cold weather, the water should be slightly warmed, by immersing a piece of hot iron in it, before dipping the steel. When a sufficient degree of hardness has been attained, the remainder of the hot portion is immersed until the heat is reduced sufficiently for tempering. At this stage it is withdrawn, and the colours carefully watched for.

The heat which is left in the stock will pass down to the edge of the bit, and as the temperature increases in that part the colours will appear in regular succession upon the filed surface of the edge. When the proper hue appears, the whole drill is plunged into the water and left there till cold, when the tempering is com plete. When the edge is curved or "bowed," the colours will reach the corners sooner than the middle of the bit. This tendency must be checked by dip ping the corners in the water, for otherwise the edge will not be of equal hardness throughout. As the colour can be best observed in the dark, it is a good plan to darken that portion of the smithy in which tempering is being carried on. The degree of temper required depends upon the quality of the steel and the nature of the work to be performed.

The larger the proportion of carbon present in the metal, the lower must be the temper. Also the state of the blunted edges, whether battered or fractured, will show what degree of hardness it is desirable to produce. From inattention to these matters, good steel is not unfrequently condemned as unsuitable. To form the striking face, the end of the stock is heated to a dull red, and drawn out by a hammer to form a conical head. The extremity is then flattened to form a face from \ inch to 1 inch in diameter. This head is then annealed to a degree that will combine considerable toughness with hardness. The constant blows to which the head is subjected tend to wear it down very rapidly. There is great difference in the lasting qualities of steel in this respect ; some drills will wear away more quickly at the striking than at the bit end.

A smith will, with the assistance of a striker, sharpen and temper about thirty single-hand drills of medium size in an hour, or twenty double-hand drills of medium size in the same time. Of course, much will depend on the degree of bluntness in the cutting edge ; but assuming the drills to be sent up only moderately blunted, this may be taken as a fair average of the work of two men. It will be evident from the foregoing remarks that to enable a drill to stand properly it must be made of good material, be skilfully tempered in the smithy, and provided with a cutting edge having an angle and a shape suited to the character of the rock in which it is used. To these conditions, may be added another, namely, proper handling ; for if the drill be carelessly turned in the hole so as to bring all the work upon a portion only of the cutting edge, or unskilfully struck by the sledge, fracture or blunting will speedily result.

Improper handling often destroys the edge in the first five minutes of using. Drills, as before remarked, are used in sets of different lengths. The sets may be intended for use by one man or by two. In the former case, the sets are described as "single-hand" sets, and they contain a hammer for striking the drills ; in the latter case, the sets are spoken of as "double-handed," and they contain a sledge instead of a hammer for striking. It may appear at first sight that there is a waste of power in employing two men, or, as it is termed, the double set, for that two men cannot bore twice as fast as one. This rate of speed can, however, be obtained, and is due less to the greater effectiveness of the stroke than to the fact that two men can, by repeatedly changing places with each other, keep up almost without intermission a succession of blows for an indefinite length of time ; whereas, with the single set, the man is continually obliged to cease for rest.

Hammers. — To deliver the blow upon a rock drill, hammers and sledges are used. The distinction between a hammer and a sledge is founded on dimensions only : the hammer being intended for use in one hand, is made comparatively light and is furnished with a short handle, while the sledge, being intended for use in both hands, is furnished with a much longer handle and is made heavier. The striking face of the blasting sledge should be flat, to enable the striker to deliver a direct blow with certainty upon the head of the drill ; and to facilitate the directing of the blow, as well as to increase its effect, the mass of metal composing the head should be concentrated within a short length. To cause the sledge to fly off from the head of the drill in the case of a false blow being struck, and thereby to prevent it from striking the hand of the man who holds the drill, the edges of the striking face should be chamfered or bevelled down till the diameter is reduced by nearly one-half.

This requirement is, however, but seldom provided for. The head of a sledge is of iron ; it consists of a pierced central portion called the "eye," and two shanks or " stumps," the steeled ends of which form the striking faces or "panes." The form of the head varies in different localities, but whatever the variations may be, the form may be classed under one of four types or " patterns." A very common HAMMERS. form is that shown in Fig. 10 and known as the " bully " pattern. By varying the width, as shown in Fig. 11, we obtain the " broad bully," the former Fig. 10. Fig. 11. being called for the sake of distinction the "narrow" bully. Another common form is the " pointing " pattern, represented in Fig 12. The form shown Fig. 12. Fig. 13. in Fig. 13 is designated as the "bloat" pattern; and that given in Fig. 14 the " plug " pattern.

Each of these forms possesses peculiar merits which renders it more suitable for certain uses than the others. The same forms are used for hammers. The eye is generally made oval in shape, but some times, especially with the bloat pattern, it is made circular, as shown in Fig. 13. The weight of a sledge head may vary from 5 lb. to 10 lb., but a Fig. 14. common and convenient weight is 7 lb. The length of the helve varies from 20 inches to 30 inches ; a common length for blasting sledges is 24 inches. The average weight of hammer heads is about 3 lb., and the average length ofthe helve 10 inches. Fig. 15 represents a blasting sledge used in South Wales. The stumps are octagonal in section, and Fig. 15. spring from a square block in the centre. The panes or striking faces, however, are circular and flat.

The length of the head is 8| inches, and that of the helve 27 inches, and the weight of the tool complete 7 lb. HAMMERS. 17 Fig. 16 represents a blasting sledge used in North Wales. The central block is an irregular octagon in section, formed by slightly chamfering Fig. 16. the angles of a square section, and the stumps are chamfered down to form a regular octagon at the panes, which are flat. The length of the head is 7| inches, and that of the helve 22 inches, and the weight of the tool complete 6 lb. 7 oz. The sledges used in the north of England have shorter heads, and are lighter than the foregoing. Fig. 17 represents one of these blasting sledges. Fig. 17. The head is nearly square in section at the centre, and the panes are flat. The length of the head is 5 inches, and that of the helve 24^ inches, and the weight of the sledge complete 4 lb. 14 oz.

Auxiliary Tools. — Besides the drill and the hammer, other tools are needed in preparing the hole for the blasting charge. If the bore-hole is inclined down wards, the de'bris or " bore-meal " made by the drill c 7/ remains on the bottom of the hole, where it is con verted into mud or " sludge " by the water there present. This sludge has to be removed as the work progresses, to keep the rock exposed to the action of the drill. The removal of the sludge is effected by a simple tool called a "scraper." It con sists of a rod of iron from { inch to \ inch in diameter, and of sufficient length to reach the bottom of the bore hole. One end of the rod is flattened out on the anvil and made circular in form, and then turned up at right angles to the stem. The disc thus formed must be less in diameter than the bore-hole, to allow it to pass readily down.

When inserted in the hole, the scraper is turned round while it is being pressed to the bottom ; on withdrawing the instrument, the sludge is brought up upon the disc. The operation, two or three times repeated, is sufficient to clear the bore-hole. The other end of the scraper is some times made to terminate in a ring for convenience in handling, as shown in Fig. 18. Instead of the ring, however, at one end, a disc may be made at Fig. 19. Fig. 18 each end, as shown in Fig. 19, the discs in this case being of different diameter, to render the scraper suitable for different size bore-holes. Sometimes the AUXILIARY TOOLS. scraper is made to terminate in a spiral hook or " drag-twist," as represented in Fig. 20. The use of the drag is to thoroughly cleanse the hole before inserting the charge.

A wisp of hay is pushed down the hole, and the drag end of the scraper introduced after it, and turned round till it has become firmly entangled. The withdrawal of the hay by the drag wipes the bore-hole clean. Instead of the twist drag, the " loop " drag is frequently em ployed. This consists of a loop or eye, through which a piece of rag or tow is passed. The rag or tow is used for the same purpose as the hay, namely, to thoroughly cleanse and dry the bore-hole previous to the introduction of the charge. Very frequently the " swab-stick " is used instead of the scraper to clear out the bore-hole. This is simply a deal rod bruised at one end by blows with a hammer until the fibres separate to form a kind of stumpy brush or " swab." When this is pushed down the hole, the sludge passes up around and between the fibres, which are then spread out by being pressed against the bottom of the hole.

On withdrawing the swab, the sludge is brought out with it. When the charge has been placed in the bore-hole, and the fuse laid to it, the hole needs to be tamped, that is, the portion above the charge has to be filled c up with some suitable substance. For this purpose, a " rammer," " stemmer," or " tamping iron," as the instrument is variously called, is required. This instrument is illustrated in Fig. 21. It consists of a metal bar, the tamping end of which is grooved to receive the fuse lying against the side of the bore-hole. The other end is flat, to afford a pressing surface for the hand, or a striking face for the hammer when the latter is needed. To prevent the danger of accidental ignition from sparks caused by the friction of the metal against silicious substances, the employ ment of iron stemmers has been prohi bited by law.

They are usually made of copper or phosphor-bronze, the latter substance being more resisting than the former. Sometimes in wet ground it becomes necessary to shut back the water from the bore-hole before introducing tlie charge of gunpowder. This happens very frequently in shaft sinking. The Fig. 21. Fig. 22. method employed in such cases is to force clay into the interstices through which the water enters. The instrument used for this purpose is the "claying-iron" or " bull," represented in Fig. 22. It consists of a round bar of iron, called the stock or shaft, a little smaller in diameter than the bore-hole, and a thicker AUXILIARY TOOLS. portion, called the head or poll, terminating in a striking face. The lower end of the shaft is pointed, to enable it to penetrate the clay, and the head is pierced by a hole about an inch in diameter to receive a lever.