Blasting, the process of breaking rocks with explosive compounds. It is employed for breaking stone from quarries for building purposes, for removing rocks from the surface of the earth, from the beds of watercourses, and from mines, and for the demolition of fortifications, docks, and other works. It follows, therefore, that the process will vary considerably according to the object to be accomplished, and the differences in the material to be acted upon as to hardness, position, and mode of stratification. Until within a few years the only explosive compound used in blasting was gunpowder. It is not known when this agent was first used for this purpose, but as the Chinese were acquainted with its use as a projectile force in very early times, it is not improbable that they also used it in mining operations, which were carried on by them to a considerable extent long before the Christian era. In Europe the Germans were probably the first to employ it in mining. - In making preparations for blasting, the first step is to examine the rock for the purpose of determining the size, location, and form of the cavity for the explosive material, and the amount of the latter necessary to overcome the resistance.
In ordinary blasting operations, simple drill holes are usually fired, and may be so placed and combined in groups as to effect the displacement of great masses of rock; but in large operations mines are excavated for the introduction of the explosive. In either case one of the principal operations is the boring or drilling of the rock. Drills of various forms are employed - short and light for working by hand, larger and longer when they are to be driven with a sledge. These drills are made by flattening the end of a steel bar, and drawing it to a blunt, outwardly curved edge, which should be from one eighth to one fourth of an inch longer than the diameter of the shaft. The included angle at the edge should be from 70° to 90°. This part of the drill is called the bit. Other drills, called jumpers, are made longer and of a different form, and are intended to be driven by the force of their own gravity. The jumper is made of a bar of steel or iron from 5 to 8 ft. long, with a bulbous enlargement rather nearer one end than the other. The bit, which is of steel, has usually the same form as in the hand drill, but sometimes has two cutting edges, formed at right angles with each other.
In using the jumper from two to four men are employed, who simply raise it to the proper height and let it fall, giving it at the same time a sufficient rotary motion to cause it to cut a chip from a bench left by the preceding stroke. The hole is usually commenced with one end of the drill and finished with the other. Some drills which are propelled by their own weight are made very heavy and raised by steam power. Other drills, the most notable among which are the Burleigh, Ingersoll, Wood, Hotch-kiss, and Gardner (see Boring), are mounted on carriages and driven by steam or compressed air, which is delivered by means of pipes and stout hose capable of sustaining a pressure of from 60 to 80 lbs. to the square inch. By the use of air in place of steam, the drill can be worked in chambers where the heat and moisture produced by the discharge of steam would be unendurable. Revolving tools worked on the principle of augers, with bits of various forms to suit the kind of work to be done, may be advantageously used in soft rock. The American diamond drill is a revolving tool which is driven by steam or compressed air. The bit is armed with black diamonds, which are so adjusted as to cut a free passage for the drill rod.
It is much used in deep boring for artesian wells and for prospecting coal and other mines, but is said to be also well adapted to boring holes for blasting. - Natural fissures in the rock are often taken advantage of to introduce powder, which is covered with dry sand, a communication being retained by means of a fuse. This is called a sand blast. For breaking down the huge blocks of native copper in the mines of Lake Superior, no other known method but shaking them by the sand blast would be effectual. Standing upon their edges in the veins, and entirely enclosed in solid rock, they are first uncovered along one of their sides by excavating a horizontal drift or gallery. Small cavities are then made behind the mass, along its upper edge, by repeated blasts in the tangled rock and copper. As these cavities are enlarged, more powder is introduced, till, if the mass be very large, several hundred pounds are spread in the crevice behind it, and fired at once; and thus it is finally thrown over into the open space previously excavated. - As the great labor in blasting consists in drilling the holes, which after all contain but a small quantity of powder, various plans have been devised for enlarging the cavity at the bottom.
In calcareous rock this has been effected by the use of acids, which dissolve the stone. For other rocks a very ingenious process was invented by Mr. A. Stickney, of Concord, N. H. After the hole (which should be not less than 3 in. in diameter) is bored to the depth of 5 or 6 ft., fragments of the best hard-wood charcoal are thrown into the bottom and ignited. A blast is then blown in from a portable bellows through a wrought-iron tube, to which is added at its lower extremity a tube of platinum not less than a foot in length and half an inch in diameter. The lower extremity of this is closed, but its sides are perforated with numerous small holes. As the blast circulates through these the charcoal burns vividly, producing intense heat and melting away the sides of the cavity. The tubes must be frequently withdrawn to hook out the fragments of cinder which accumulate; and as the size of the chamber increases more charcoal is continually dropped into the hole by the side of the tubes, the hole being left open for the escape of the gases. In the course of a few hours the cavity will be sufficiently large to hold 20 or 30 lbs. of powder.
In granitic rocks the effect of this operation is very remarkable; the ingredients melt down into a liquid slag, and if a bucket of cold water is dashed in upon the highly heated surface, this is scaled off in large flakes by the sudden chill, and by the mechanical action of the high steam which is instantly generated. In hard silicious rocks, as the firm sandstones of the Shawangunk range, the rock crumbles down to sand, and this is blown out of the hole as the process goes on, covering the surface around. In calcareous rocks the stone is burned to quicklime, and a large cavity is rapidly produced. The heat generated in this operation is so great, that wrought-iron pipes have been melted down by coming into too close a contact with the charcoal. The enlarged size of the hole at the bottom is particularly favorable for the explosive force of the powder to be exerted to the best advantage. Huge masses of rock are lifted up, and cracks of great extent are opened to a depth not reached by the ordinary method of blasting.
These cracks afford convenient opportunities for the use of the sand blast, and thus very large quantities of rock are broken up with comparatively small expense for drilling. - Firing a number of charges simultaneously by the galvanic battery is sometimes adopted with great advantage, where large bodies are to be moved. The effect produced by the same quantity of powder is much greater than if the charges were separately exploded. The same mode of firing is also conveniently applied to blasting under water. This method has been said to have been first practised in England in 1839, by Gen. Pasley in removing the wreck of the Royal George, and by Mr. Alan Stevenson in submarine rock blasting. But in vol. xxi. of the "American Journal of Science," for 1831, is a letter of Dr. Hare, describing the operations of Mr. Moses Shaw, who had already applied the electrical machine to this purpose, and then by advice of Dr. Hare was making use of the galvanic, battery;' and in vol. xxvi of the same journal (1834) the apparatus is fully described, with drawings which show that the arrangement was essentially the same with that now in use.
In 1843 three charges of 18,000 lbs. of powder were fired simultaneously by this means at Dover, by Mr. William Cubitt A chalk cliff 400 feet high was thrown down with little report, and the beach was covered with 400,000 cubic yards of chalk rock. It is estimated that the saving to the Southeastern railway company in this operation over the ordinary process was not less than £7,000. Very successful blasting was performed at the Holyhead quarries in England in January, 1867, for supplying stone for the breakwater at that place. The accompanying diagram (fig. 1) exhibits the ground plan of the galleries and return chambers. These latter were placed 3 ft. below the level of the ground line of the face of the quarry, because it had been found by experience that if they were placed above the level, a wall of rock would be left standing, expensive to remove. The method of estimating the total quantities of powder for loading the four chambers was as follows: The cubical content of the mass to be dislodged was divided by 12, the minimum number of cubic feet per ton, and the quotient by 5, it being estimated in this case that one pound of powder was required to dislodge five tons of rock.
The length of the face of the rock being 210 ft., its height 115 ft., and the horizontal depth to be removed 40 ft'., the proper quantity of powder was therefore, in round numbers, 16,000 lbs. The quantities applicable to charges No. 1, 2, 3, and 4, the lines of least resistance being respectively 26, 25, 20, and 27 ft., were 4,200, 4,500, 2,300, and 5,000 lbs. That these estimates were very nearly correct appears from the fact that the force of the powder was mainly expended in displacing and breaking up the rock, but little concussion of air being produced. The report of Col. Servante of the royal engineers, who was sent to witness the explosion, says: "The mass was quietly overthrown down to the level of the quarry ground line, with very little noise, and scarcely a stone was thrown into the air." The quantity of rock detached was found to be 120,000 tons, in blocks of from 3 to 40 tons, averaging 7 1/2 tons of stone to one pound of powder. The operations were conducted by Mr. C. G. Reitheimer, the engineer employed by the Messrs. Rigby, the proprietors of the quarry. The galleries and shaft were tamped with clay, and the tamping was extended through the entrance gallery to the surface of the rock.
The description of the operations performed in the demolition of the Russian docks at Sebastopol by the English and French engineers, which is contained in vol. vi. of the "Professional Papers of the Corps of Royal Engineers" of Great Britain, presents interesting examples of blasting. - The choice of the explosive compound depends upon the nature of the work to be performed. In quarrying, gunpowder of slow igniting power is preferred, because it is desired to avoid pulverization; but in simply clearing away material, a more instantaneous explosive is found to be more effectual. Gun-cotton was used in Europe to some extent soon after its discovery, but has never been employed in any important work in this country, except as an experiment. Nitro-glycerine, or some preparation of it, as giant powder, is the compound now relied upon when rapidity and an approach to accuracy of result are desired; and it is generally preferred when the disengaging of surface portions of rock is the immediate object.
It often happens in some situations, especially in excavating chambers under water, where it is of the greatest importance to keep the water bed as firm and intact as possible, that a seamy structure of rock requires the use of an explosive which will expend its force as much as possible in detaching only a certain superficial mass, upon the same principle that a small hammer, propelled with a sharp quick stroke, is better adapted to drive a nail in an unstable and slight body than a heavy one. When gunpowder is used, the holes are usually drilled deeper than for nitro-glycerine, and when practicable the powder is poured into the cavity instead of being introduced in a cartridge. Therefore the holes are drilled in a downward direction, as nearly perpendicular as the course of lamination and other circumstances will admit. The small hand drill is held and driven by one person, and after each stroke it is turned sufficiently to allow of a chip being cut from a section of the bottom. The degree to which this turning is done at each stroke is a matter of consequence, as upon it depends much of the rapidity and economy of the operation. When the bottom of the cavity becomes obstructed, instruments called scrapers or dippers are used to clear it out.
Some of these tools are merely wires bent at right angles at one end, which is flattened so as to form a shelf upon which the rubbish may be taken; but the flattened end should be slightly depressed on one side, so that by a twisting motion the shelf or pan may be made to pass under. A worm is often formed at the other end for carrying a piece of sponge or other material to the bottom of the cavity to absorb water. It is generally advantageous to pour water into the cavity while drilling for the purpose of softening the rock, and keeping the bit from heating. It often happens that water percolates into the cavity, and in either case some contrivance is required to occasionally remove it. When the hole has reached a sufficient depth it is to be thoroughly cleaned and dried with the scraper and a piece of sponge or cloth attached to a stick or to the worm at one end of the scraper. Then the proper charge of powder is poured in and covered with a tamping, which may consist of dry sand, brick dust, or moist clay. When dry sand is used, it is not tamped down, but brick dust or clay is, the material being introduced in small quantities at a time, and successively compacted with a tamping rod, which is simply a straight bar of copper, brass, or wood.
The end of a fuse, which is made of gutta-percha cylinder, impervious to moisture, filled with a mixture of gunpowder, charcoal, and nitre, is passed into the hole and inserted in the body of the charge before the tamping material is introduced, the other end remaining outside and being of a sufficient length to burn the desired time before producing the explosion. When a fuse is not employed, a priming needle made of copper is passed down one side of the hole, with the point extending into the powder. It has a tapering form, so that its withdrawal will not disturb the tamping, which in this case must be more or less damp. When the needle is withdrawn the canal is filled with fine powder, and its ignition effected with a slow match. When the cavity, in consequence of percolation from surrounding rock, cannot be dried, the powder must be used in the form of a cartridge, the case of which is made of tin or pitched paper. When nitro-glycerine is used, it is placed in cartridges and exploded by means of some kind of fulminate, as fulminate of mercury or chlorate of potash, or both together. The fulminate may be ignited either by a fuse or by a galvanic battery.
The use of nitro-glycerine in its raw state being considered very dangerous, preparations of it have been made, which with careful handling are no more hazardous than gunpowder. Of these, giant powder or dynamite, which is composed of 75 per cent. of nitro-glycerine with 25 per cent, of a certain silicious infusorial earth, holds the first rank. When an explosive compound is fired, the great and almost instantaneous expansion of liberated gases, which in the case of gunpowder is many hundred times its volume, produces an equal pressure in all directions. Those surfaces which offer the least resistance of course give way to the greatest extent; and the slower the explosion and consequent expansion, the more will these surfaces be displaced, receiving by direct action and reaction most of the explosive force, while the firmer material will be left undisturbed. When, however, nitro-glycerine is used, the expansion of gases is so nearly instantaneous, that the tampings, even when they are quite unstable, offer an amount of resistance which is considerable.
Even when it is fired upon the surface of a rock under a depth of only a few feet of water, so great is the reaction produced by the inertia of the water that a sufficient force is exerted against the rock to rend it in some instances to a large extent. Under similar circumstances even gunpowder will explode with considerable effect. Mr. Maillefert in the years 1851 and 1852 succeeded, by the use of gunpowder in surface blasting under water, in removing large portions of several of the obstructions to the navigation of the East river at Hell Gate. Rocks known as Pot rock, the Frying Pan, and Way's reef, were very considerably reduced by simply exploding large canisters of gunpowder, by means of a galvanic battery, upon their surfaces. From Aug. 19, 1851, when the first blast was fired, to March 25, 1862, 284 charges, containing 34,231 lbs. of powder, were exploded upon Pot rock, removing about 10 feet of its depth, as careful soundings have since shown, although it was asserted at the time that more had been removed. On Frying Pan and Way's reef 240 charges, containing about 28,000 lbs., were exploded, increasing the depth of water considerably. Since this pioneer work of Mr. Maillefert nitro-glycerlne has been used in similar operations with much greater and more satisfactory results.
In fact, this compound, or some preparation of it, is now employed by the engineer as though it were a kind of chisel for chipping away projections of rock wherever they present themselves. Surface blasting has, however, been abandoned, except for the removal of superficial or unimportant masses of rock. It has been found that when live rock, as firm, undetached, and undisintegrated rock is called, has been reached, the surface blast, even when made with nitro-glycerine, makes so little comparative impression, that it is more expeditious and economical to drill and introduce the charge into the body of the rock. When, however, it forms so much of an obstruction as to require several feet in depth and a considerable horizontal section to bo removed, it has been found preferable to make large excavations into the body of the rock from beneath, proceeding according to the method of mining, and to remove the shell by the simultaneous explosion of charges introduced into it. Practical applications of this method will be noticed further on. - When it is designed to bore a tunnel into a mountain, a heading, as it is called, is commenced at the floor of the tunnel and driven in the direction of its axis.
If the plane of the floor is not beneath the plane upon which the work is begun, and the surface of the rock is sufficiently perpendicular, the work may be commenced by bringing a carriage, armed with one or more Burleigh or other drills, to the face of the rock, drilling a horizontal line of perforations a short distance above the plane of the floor of the tunnel, driving the drills in nn obliquely downward direction, at an angle of about 45°, charging the holes with gunpowder or nitro-glycerine, and firing them simultaneously by means of the galvanic battery.
Fig. 1 - Original Face of Rock. 210 ft. long, 115 ft. high.
Fig. 2. - Burleigh Drill at Work.
(See fig. 2.) If necessary, this operation is to be repeated until a step, facing downward and of sufficient depth, is formed to afford the most efficient displacement of rock by subsequent blasts. Then another line of perforations is drilled in the step, in a plane parallel with its under surface, at a suitable distance above its edge, which are also charged with the explosive and fired. (See fig. 3.) This process is to be repeated until the arch or crown of the tunnel is reached, and then a new bench is to be formed. This work can be advantageously performed by hand drilling, but when it is convenient to work a power drill its employment will generally afford the greatest progress. When the tunnel is of sufficient height it is usual to drive the heading (H, fig. 4) forward beneath the crown, and to follow with one or more benches (B and B'). The work is always driven against the perpendicular faces of the headings and benches, and in the direction of the axis of the tunnel; but the lamination of the rock may be such as to make it preferable to drill the holes in the upper surface of a bench, as at b, and throw the rock horizontally from the face, instead of commencing at b' and throwing it downward.
Nitro-glycerine may be placed in the drill holes in cartridges, and fired without tamping or with water tamping, its action being so instantaneous that a separation is readily effected in the lateral direction, toward the under surface of the bench. When the floor of the tunnel lies beneath the surface and it cannot so readily be reached otherwise, or where counter tunnelling is desirable, a shaft is sunk to the required plane. The process of excavating a shaft is conducted upon principles similar to those which govern the driving of the tunnel, in so far as the forming of benches and the detaching of the rock in the direction of the line of least resistance is concerned, although a heading, from the nature of the case, could not be driven downward in advance of the rest of the shaft with any advantage. The working will of course be varied according to the structure and composition of the rock, and the position of its strata. It may happen at times that considerable portions can be removed with wedges and levers, and this may be the case in the tunnel as well as in the shaft, but not so frequently. In sinking a shaft a bench is formed, and successive portions are detached, either by blasting or other means, until the whole is removed and a new bench formed.
The progress made in blasting at the Hoosac tunnel in Massachusetts during the month of March, 1872, in the east end, at a distance of 10,046 ft. from the entrance, was 120 ft. of heading 24 ft. wide and 9 ft. high. This heading was attacked by 12 Burleigh drills, mounted on two carriages manned by eight men and a foreman. On Dec. 12 of the same year the last portion of rock that divided the excavations was removed, and it was found that the axes of the two only differed by the remarkably small error of five sixths of an inch laterally, and an inch and a half vertically. (See Tunnel.) - In submarine blasting on a large scale, by the modern method, a coffer dam is erected over the rock and a shaft sunk into it, from which tunnels are excavated in radiating directions, and these connected by concentric galleries, while columns of rock are left as supports to the roof, and to maintain the water bed till the work is completed. A sufficient number of charges of an explosive compound are then introduced into the columns in chambers, and in the shell, and simultaneously fired by means of a galvanic battery.
When the work is not too extensive and the superincumbent pressure of rock and water is not too great, the columns of rock supporting the roof may be replaced by wooden ones, thus allowing of the removal of a larger amount of material before the final explosion takes place. This is an advantage, since its removal in this way is less expensive than by rakes and grappling irons after it is broken up and lying beneath the water. In such excavations many precautions are required which are unnecessary in boring a tunnel through a mountain. Mathematical calculations and estimates, requiring extensive engineering knowledge and sound judgment, must be made in order to ascertain the amount of resistance required in the arches and in the columns of support, composed as they are of rock of varying composition, texture, and degree and direction of stratification. If a breach should be made in the water bed, the works would be flooded, causing serious delay and expense in making repairs, which must be done by sinking rocks and cement into the breach and pumping the water from the caverns. Moreover, the breach might be so extensive as to be irreparable, in which case the remainder of the rock which had been tunnelled would have to be removed by surface blasting.
It frequently happens that small fissures are opened, which under the great pressure of water from above cause serious annoyance, and all the ingenuity and knowledge that can be brought to bear are required to stop the leak. To avoid disturbing the water bed, it is also safer to fire the blasts of nitroglycerine singly with a fuse, and not in numbers simultaneously. It is thus perceived that blasting as now practised is an important branch of the science of civil engineering. With the materials and appliances at hand, in the form of gunpowder, nitro-glycerine, perfect safety fuse, the ready and facile command of galvanic electricity, properly constructed drills, and compressed air engines to propel them, the problems presented to the civil engineer are exceedingly interesting, and offer no obstacles which careful and correct calculation cannot overcome. - The removal of Blossom rock in the harbor of San Francisco is an example of the process of removing submarine rocks by conducting the excavation from within.
It is the only operation of the kind which has been completed, although another and more extensive one, previously commenced, is now (1872) in progress at Hallett's point in the East river, opposite New York. The top of Blossom rock was about 5 ft. below the surface of the water at mean low tide. A horizontal section at the depth of 24 ft. measured 195 x 105 ft. The quantity of rock contained within these boundaries was about 5,000 cubic yards, and consisted of a metamorphic sandstone of irregular stratification. The great mass of it was so soft as not to require blasting. In October, 1868, brevet Brig. Gen. B. S. Alexander, lieutenant colonel of engineers U. S. A., communicated a plan for the removal of this rock to Lieut. Col. R. S. Williamson, major of engineers, who had been placed in charge of its survey. Gen. Alexander's plan is briefly explained in the following extract from his communication: "I propose to enclose a small surface of the rock by a water-tight coffer dam; in this space to sink a rectangular shaft about 4 by 9 ft., which is the size I have seen in coal mines; from the bottom of this shaft to run tunnels and make powder chambers in such positions that when exploded the whole rock down to the level of 24 ft. below the level of the water will be lifted in the air and shivered to pieces." In November following, Mr. A. W. von Schmidt, a civil engineer of San Francisco, sent in a plan for the removal of the rock, and offered to perform the work for $75,000, which plan and offer were in due time accepted.
His plan was similar to Gen. Alexander's, except that instead of the ordinary coffer dam he proposed to sink an iron cylinder 6 ft. in diameter, carrying an india-rubber flap at its lower end, pump out the water, bore into the rock, and slide another cylinder inside of the first down into the ex-cavation and secure it by cement. It was, however, found difficult to place the iron cylinder in position without first resorting to the ordinary cribwork coffer dam. The sinking of the shaft was commenced Dec. 7, 1869. Only one man could work at a time, but in the space of four weeks a depth of 30 ft. below low water was reached. Drifts were then run into the longer and shorter axes of the rock, and steam was used in hoisting. The rubbish was dumped upon one side of the rock, from which most of it was washed by the tide. During the month of January, 1870, eight men found room to work. Most of the rock was removed by picks and sledges, only 10 lbs. of explosive (giant powder) being used in the whole operation. In February 16 men found space to work, and by the 20th of April the dimensions of the cavity were 140 by 60 ft., with a maximum height of 12 ft.
Columns of rock were at first left for support, but they were from time to time replaced with upright timbers from 8 to 10 inches in diameter, with the exception of four, which were left standing near the shaft. Preparations were now made to blow up the shell. The following diagram, copied from the official report, will explain the method of conducting the explosion. Powder was used as the explosive, nitrate of soda taking the place of nitrate of potash in its composition. The quantity used was 43,000 lbs. The vessels for containing it were 38 ale casks of 60 gallons each, and seven old tanks made of boiler iron, holding about 300 lbs. of powder each. The explosion was effected by a galvanic battery stationed in a boat about 800 ft. from the rock. A column of water about 200 feet in diameter was thrown into the air to a height of 200 to 300 ft., and pieces of rock and timber were thrown high above the water column. The rock was found to be effectually demolished, although if the excavations had been carried to a greater depth much after labor in clearing away rubbish and projecting points would have been saved.
The contract was fully carried out by Mr. Von Schmidt, under the immediate inspection of Lieut. W. H. Heuer of the corps of engineers. - At New York, the operations of Mr. Maillefert in surface blasting had greatly improved the navigation of the East river; but no comprehensive plan was projected till the summer of 1866, when brevet Major Gen. John Newton was assigned by the war department to the duty of examining the obstructions, and making estimates of the work necessary to be done. He submitted three plans, each of which included the removal of the rock at Hallet's point. Some work was done on some of the smaller rocks by Mr. S. F. Shelbourne, who tried experiments with a rotating diamond drill, and afterward constructed a percussion drill of larger size, which was destroyed by a collision before it was brought to the test of drilling. In the spring of 1869 congress appropriated $175,000 for improvements at Hell Gate, and Gen. Newton proceeded to complete the plans for the performance of the work. The removal of the submarine rock at Hallett's point was the first work decided upon. This rock, projecting some 300 ft. into the stream, and throwing the tide from Long Island sound against an opposing rock called the Gridiron, makes the navigation at that place very difficult.
The plan of operation was to sink a shaft upon Hallett's point, and from it excavate tunnels in the rock in a radiating direction under the river and connect them with concentric galleries; then, after removing from the interior as much of the rock as possible without danger of letting in the water, to blow up the roof and supporting columns. The work was commenced in July, 1869. A coffer dam in the form of an irregular pentagon, whose greatest diameter was 140 ft., was erected on the shore, and a shaft 105 by 95 ft. in diameter was sunk to a depth of 32 ft. below mean low water. Diverging tunnels were then commenced, and after they were sufficiently advanced concentric galleries were excavated, and as the work proceeded their number increased, until at the present time (November, 1872) there are 19 tunnels, some of which are nearly completed, extending from 190 to 240 ft. beyond the shaft, and connected by seven concentric galleries, from which 28,000 cubic yards of rock have been removed. The rock is a tough hornblende gneiss, and lies in strata of various degrees of inclination, presenting interesting problems.
The work has been in satisfactory progress since the summer of 1869, with the exception of one interval, when the available funds were exhausted; but the appropriations have never been nearly equal to what could have been economically expended. The Burleigh drill has been in constant use, but hand drills are also worked with great advantage, as in the progress of the work it is found expedient to use many small blasts of giant powder. When the excavation is completed it is designed to introduce an explosive compound into the columns and various parts of the roof, and produce a simultaneous explosion with a galvanic current. Topographical surveys are continually made during the progress of the work to determine the direction and extent of the excavation, the usual methods of triangulation and levelling being employed. There have been 21,000 soundings and 8,000 borings of the bed of the river, for the purpose of ascertaining the depth of live rock. No accident has happened with the use of nitro-glycerine, owing to the care with which it is prepared, and the prudence with which it is handled. With regard to the preparations of nitro-glycerine, dynamite or giant powder is considered by those who use it to be a safer explosive than gunpowder.
Dualline, which is a somewhat similar preparation, has also been used with satisfactory results. The danger in using nitro-glycerine arises principally from the collection of vapors liable to take place when it is confined.
Fig. 3. - Mode of Forming Steps ("Stoping").
Fig. 5. - Coffer Dam.
Fig. 6 - Vortical Section of Coffer Dam and Excavation at Blossom Rock.
Fig. 7. - Horizontal Section, showing Charges.
Fig. 8. - Ground Plan of Tunnels and Galleries at Hallett's Point.