EXPLOSIVES AND BLASTING.
When the holes are once drilled, either by hand or power drills, they are charged with explosives. The principal explosives employed in tunneling are gunpowder, nitroglycerine, and dynamite.
Gunpowder.
—Gunpowder is composed of charcoal, sulphur, and saltpeter in proportions varying according to the quality of the powder. For mining purposes the composition employed is 65% saltpeter, 15% sulphur, and 20% charcoal. It is a black granulated powder having a specific gravity of 1.5; the black color is given by the charcoal; and the grains have an angular form, and vary in size from 1⁄8 in. to 3⁄8 in. Good blasting powder should contain no fine grains, which may be detected by pouring some of the powder upon a sheet of white paper. The force developed by the explosion of gunpowder is not accurately known; it depends upon the space in which it is confined. Different authorities estimate the pressure at from 15,000 lbs. per sq. in. in loose blasts to 200,000 lbs. per sq. in. in gunnery. Authorities also differ in opinion as to the character of the gases developed by the explosion of gunpowder, a matter of vital concern to the tunnel engineer, since they are likely to affect the health and comfort of his workmen. It may be assumed in a general way, however, that the oxygen of the saltpeter converts nearly all of the carbon of the charcoal into carbon dioxide, a portion of which combines with the potash of the saltpeter to form carbonate of potash, the remainder continuing in the form of gas. The sulphur is converted into sulphuric acid, and forms a sulphate of potash, which by reaction is decomposed into hyposulphite and sulphide. The nitrogen of the saltpeter is almost entirely evolved in a free state; and the carbon not having been wholly burnt into carbonic acid, there is a proportion of carbonic oxide.
Nitroglycerine.
—Nitroglycerine is one of the modern explosives used as a substitute for gunpowder. It is a fluid produced by mixing glycerine with nitric and sulphuric acids; it freezes at +41° F., and burns very quietly, developing carbonic acid, nitrogen, oxygen, and water. By percussion or by the explosion of some substances, such as capsules of gunpowder or fulminate of mercury, nitroglycerine produces a sudden explosion in which about 1250 volumes of gases are produced. The pressure of these gases has been calculated at 26,000 atmospheres, or 324,000 lbs. per sq. in. Nitroglycerine explodes very easily by percussion in its normal state, but with great difficulty when frozen; hence, in America, at the beginning of its use, it was transported only in a frozen state. When dirty, nitroglycerine undergoes a spontaneous decomposition accompanied by the development of gases and the evolution of heat, which, reaching 388° F., causes it to explode. Notwithstanding the enormous pressures which nitroglycerine develops, it is very seldom used in its liquid state, but is mixed with a granular absorbent earth composed of the shells of diatoms. The fluid undergoes no chemical change by being absorbed, and explodes, freezes, and burns under the same conditions as in the fluid state.
Dynamite.
—The credit of rendering nitroglycerine available for the purposes of the engineer by mixing it with a granular absorbent is due to Albert Nobel of Stockholm, Sweden, who named the new material dynamite. The nitroglycerine in dynamite loses very little of its original explosive power, but is very much less easily exploded by percussion, and can be employed in horizontal as well as vertical holes, which was, of course, not possible in its liquid state. Dynamite must contain at least 50% of nitroglycerine. Some manufacturers, instead of using diatomaceous earth, use other absorbents which develop gases upon explosion and increase the force of the explosion. These mixtures are classed under the general name of false dynamites. A great many varieties of dynamite are manufactured, and each manufacturer usually makes a number of grades to which he gives special names. Dynamite for railway work, tunneling, and mining contains about 50% of nitroglycerine; for quarrying about 35%, and for blasting soft rocks about 30%. It is sold in cylindrical cartridges covered with paper.
Storage of Explosives.
—In driving tunnels through rock large quantities of explosives must be used, and it is necessary to have some safe place for storing them. In many States there are special laws governing the transportation and storage of explosives; where there is no regulation by law the engineer should take suitable precautions of his own devising. It is best to build a special house or hut in one of the most concealed portions of the work and away from the tunnel, and protect it with a lightning-rod and from fire. Strict orders should be given to the watchman in charge not to allow persons inside with lamps or fire in any form, and smoking should be prohibited. The use of hammers for opening the boxes should be prohibited; and dynamite, gunpowder, and fulminate of mercury should not be stored together in the same room. A quantity of dynamite for two or three days’ consumption may be stored near the entrance of the tunnel in a locked box, the keys of which are kept by the foreman of the work. When dynamite has been frozen the engineer should provide some arrangement by which it may be heated to a temperature not exceeding 120° F., and absolutely forbid it being thawed out on a stove or by an open fire.
Fuses.
—When gunpowder is used in tunneling it is ignited by the Blickford match. This match, or fuse as it is more commonly called, consists of a small rope of yarn or cotton having as a core a small continuous thread of fine gunpowder. To protect the outside of the fuse from moisture it is coated with tar or some other impervious substance. These fuses are so well made that they burn very uniformly at the rate of about 1 ft. in 20 seconds, hence the moment of explosion can be pretty accurately fixed beforehand. Blickford matches have the objection for tunnel work of burning with a bad odor, especially when they are coated with tar, and to remedy this many others have been invented. Those of Rzika and Franzl are the best known of these. The former has many advantages, but it burns too quickly, about 3 ft. per second, and is expensive; the latter consists of a small hollow rope filled with dynamite.
Blickford matches cannot be used to explode dynamite, the use of a cartridge being required. These cartridges are small copper cylinders containing fulminate of mercury. They may be attached to the end of the Blickford match, which being ignited the spark travels along its length until it reaches the copper cylinder, where it explodes the fulminate of mercury, which in turn explodes the dynamite. Blasts may also be fired by electricity, which, in fact, is the most common and the preferable method, because several blasts can be fired simultaneously, and because the current is turned on at a great distance, thus affording greater safety to the workmen.
The method of electric firing generally employed in America is known as the connecting series method, and consists in firing several mines simultaneously. The ends of the wires are scraped bare, and the wire of the first hole of the series is twisted together with the wire of the second hole, and so on; finally the two odd wires of the first and last holes are connected to two wires of a single cable or to two separate cables extending to some safe place to which the men can retreat. Here the two cable wires are connected by binding screws to the poles of a battery, or sometimes to a frictional electric machine. The current passes through the wires, making a spark at each break, and so fires the fulminate of mercury, which explodes the dynamite.
Simultaneous firing by electricity by utilizing the united strength of the blasts at the same instant secures about 10% greater efficiency from the explosives. Another advantage of electric firing is that in case of a missfire of any one of the holes there is slight possibility of explosion afterwards, and the place can be approached at once to discover the cause.
Tamping.
—Tamping is the material placed in the hole above the explosive to prevent the gases of explosion from escaping into the air. Tamping generally consists of clay. When gunpowder is used the clay must be well rammed with a wooden tool, and paper, cotton, or some other dry material must be placed between the moist clay and the powder. When dynamite is used it is not necessary to ram the tamping, since the suddenness of the explosion shatters the rock before the clay can be driven from the hole.
A few experienced men should be appointed to fire the blasts. These men should give ample warning previous to the blast in order that all machinery and tools which might be injured by flying fragments may be removed out of danger, and so that the workmen may seek safety. When all is ready they should fire the blasts, keeping accurate count of the explosions to ensure that no holes have missed fire, and should call the workmen back when all danger is over. In case any hole has missed fire it should be marked by a red lamp or flag.
Nature of Explosions.
—When the explosives are ignited a sudden development of gases results, producing a sudden and violent increase of pressure, usually accompanied by a loud report. The energy of the explosion is exerted in all directions in the form of a sphere having its center at the point of explosion, and the waves of energy lose their force as the distance from this central point increases. The energy of the explosion at any point in the sphere of energy is, therefore, inversely proportional to the distance of this point from the center of explosion. In the vicinity of the center of explosion the gases have sufficient power to destroy the force of cohesion and shatter the rock; further on, as they lose strength, they only destroy the elasticity of the material and produce cracks; and still further away they only produce a shock, and do not affect the material. Within the sphere of energy there are, therefore, three other concentric spheres: the first one being where cohesion is destroyed, the second where elasticity is overcome, and the third where the shock is transmitted by elasticity. When the latter sphere comes below the surface, the gases remain inside the rock; but when the surface intersects either of the other two spheres, the gases blow up the rock, forming a cone or crater, whose apex is at the point of explosion, and which is called the blasting-cone. The larger the blasting-cone is, the greater is the amount of rock broken up; and the object of the engineer should, therefore, always be so to regulate the depth of the hole and the quantity of explosive as to secure the largest possible blasting cone in each case. Experiments are required to determine the most efficient depth of hole, and quantity of explosive to be employed, since these differ in different kinds of rock, with the position of the rock strata, etc.; but in ordinary practice, the depths of the holes are made from 2 to 3 ft. in the heading and upper portion of the tunnel, when drilled by hand; and from 6 to 8 ft. when drilled by power drills. In the lower portion of the profile, the holes are made deeper, from 3 ft. to 4 ft. when drilled by hand, and exceeding 6 ft. when drilled by power. The distance of the holes apart should be about equal to the diameter of the blasting-cone; as a general rule it is assumed that the base of the blasting-cone has a diameter equal to twice the depth of the hole. The following table gives the average number of holes required in each part of the excavation for the St. Gothard tunnel in which the heading was excavated by machine drills while the other parts were excavated by hand drills:
| NO. OF PART. [5] | NAME OF PART. | NO. OF HOLES. | ||
|---|---|---|---|---|
| 1. | Heading | 6 | to | 9 |
| 2. | Right wing of heading | 3 | to | 5 |
| 3. | Left wing of heading | 3 | to | 5 |
| 4. | Shallow trench with core | 2 | ||
| 5. | Deepening of trench to floor | 6 | to | 9 |
| 6. | Narrow mass of core to left | 3 | ||
| 7. | Greater mass of core to left | 6 | to | 9 |
| 8. | Culvert | 1 | ||
| Total section | 30 | to | 43 | |
[5] The location of the parts numbered is shown by [Fig. 14], [p. 36].
The quantity of explosives required for blasting depends upon the quality of the rock, since the force of the explosives must overcome the cohesion of the rock, which varies with its nature, and often differs greatly in rocks of the same kind and composition. The quantity of explosives required to secure the greatest efficiency in blasting any particular rock may be determined experimentally, but in practice it is usually deduced by the following rules: (1) The blasting force is directly proportional to the weight of the explosives used, and (2) the bulk of the blasted rock is proportional to the cube of the depth of the holes. It is usually assumed, also, that the explosive should fill at least one-fourth the depth of the hole.
The following table gives the depth of holes and amount of dynamite used at each advance in the [Fort George Tunnel] illustrated on [page 135].
| Order of Firing. | Kinds of Holes. | Depth. | Charge. | Kind of Dynamite. | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Bench Holes | - | 1st round 2nd round | 4 | grading | 3′ to 5′ | 50 | lbs. | 40% | climax | |
| 5 | bench | 9′ 6″ | 45 | „ | 40% | „ | ||||
| 6 | trimming | 3′ to 9′ | 42 | „ | 40% | „ | ||||
| Heading Holes | - | 3d round | 8 | center cut | 9′ | 56 | „ | 60% | „ | |
| 4th round | 8 | side | 8′ | 48 | „ | 40% | „ | |||
| 5th round | 6 | dry | 8′ | 36 | „ | 40% | „ | |||
CHAPTER IV.
GENERAL METHODS OF EXCAVATION: SHAFTS: CLASSIFICATION OF TUNNELS.
A number of different modes of procedure are followed in excavating tunnels, and each of the more important of these will be considered in a separate chapter. There are, however, certain characteristics common to all of these methods, and these will be noted briefly here.
Fig. 14.—Diagram Showing Sequence of Excavation for St. Gothard Tunnel.
Fig. 15.—Diagram Showing Manner of Determining Correspondence of Excavation to Sectional Profile.
Division of Section.
—It may be asserted at the outset that the whole area of the tunnel section is not ordinarily excavated at one time, but that it is removed in sections, and as each section is excavated it is thoroughly timbered or strutted. The order in which these different sections are excavated varies with the method of excavation, and it is clearly shown for each method in succeeding chapters. As a single example to illustrate the proposition just made, the division of the section and the sequence of excavation adopted at the St. Gothard tunnel is selected ([Fig. 14]). The different parts of the section were excavated in the order numbered; the names given to each part, and the number of holes employed in breaking it down, are given by the [table] on [page 35]. Whatever method is employed, the work always begins by driving a heading, which is the most difficult and expensive part of the excavation. All the other operations required in breaking down the remainder of the tunnel section are usually designated by the general term of enlargement of the profile. The various operations of excavation may, therefore, be classified either as excavation of the heading or enlargement of the profile.
Excavation of the Heading.
—There is considerable confusion among the different authorities regarding the exact definition of the term “heading” as it is used in tunnel work. Some authorities call a small passage driven at the top of the profile a heading, and a similar passage driven at the bottom of the profile a drift; others call any passage driven parallel to the tunnel axis, whether at the top or at the bottom of the profile, a drift; and still others give the name “heading” to all such passages. For the sake of distinctness of terminology it seems preferable to call the passage a heading when it is located at the top of the profile, and a drift when it is located near the bottom.
Headings and drifts are driven in advance of the general excavation for the following purposes: (1) To fix correctly the axis of the tunnel; (2) to allow the work to go on at different points without the gangs of laborers interfering with each other; (3) to detect the nature of material to be dealt with and to be ready in any contingency to overcome any trouble caused by a change in the soil; and (4) to collect the water. The dimensions of headings in actual practice vary according to the nature of the soil through which they are driven. As a general rule they should not be less than 7 ft. in height, so as to allow the men to work standing, and have room left for the roof strutting. The width should not be less than 6 ft., to allow two men to work at the front, and to give room for the material cars without interfering with the wall strutting. Usually headings are made 8 ft. wide. The length of headings in practice varies according to circumstances. In very long tunnels through hard rock the headings are sometimes excavated from 1000 ft. to 2000 ft. in advance, in order that they may meet as soon as possible and the ranging of the center line be verified, and so that as great an area of rock as possible may be attacked at the same time in the work of enlarging the profile. In short tunnels, where the ranging of the center line is less liable to error, shorter headings are employed, and in soft soils they are made shorter and shorter as the cohesion of the soil decreases. When the material has too little cohesion to stand alone, the tops and sides of the heading require to be supported by strutting. To prevent caving at the front of the heading, the face of the excavation is made inclined, the inclination following as near as may be the natural slope of the material.
Enlargement of the Profile.
—The enlargement of the profile is accomplished by excavating in succession several small prisms parallel to the heading, and its full length, which are so located that as each one is taken out the cross-section of the original heading is enlarged. The number, location, and sequence of these prisms vary in different methods of excavation, and are explained in succeeding chapters where these methods are described. To direct the excavation so as to keep it always within the boundaries of the adopted profile, the engineer first marks the center line on the roof of the heading by wooden or metal pegs, or by some other suitable means by which a plumb line may be suspended. He next draws to a large scale a profile of the proposed section; and beginning at the top of the vertical axis he draws horizontal lines at regular intervals, as shown by [Fig. 15], until they intersect the boundary lines of the profile, and designates on each of these lines the distance between the vertical axis and the point where it intersects the profile. It is evident that if the foreman of excavation divides his plumb line in a manner corresponding to the engineer’s drawing, and then measures horizontally and at right angles to the vertical center plane of the tunnel the distance designated on the horizontal lines of the drawing, he will have located points on the profile of the section, or in other words have established the limits of excavation.
Fig. 16.—Polar Protractor for Determining Profile of Excavated Cross-Section.
In the excavation of the Croton Aqueduct for the water supply of New York city, an instrument called a polar protractor was used for determining the location of the sectional profile. It was invented by Mr. Alfred Craven, division engineer of the work. This instrument consists of a circular disk graduated to degrees, and mounted on a tripod in such a manner that it may be leveled up, and also have a vertical motion and a motion about the vertical axis. The construction is shown clearly by [Fig. 16]. In use the device is mounted with its center at the axis of the tunnel. A light wooden measuring-rod tapering to a point, shod with brass and graduated to feet and hundredths of a foot, lies upon the wooden arm or rest, which revolves upon the face of the disk, and slides out to a contact with the surface of the excavation at such points as are to be determined. If the only information desired is whether or not the excavation is sufficient or beyond the established lines, the rod is set to the proper radius, and if it swings clear the fact is determined. If a true copy of the actual cross-section is desired, the rod is brought into contact with the significant points in the cross-section, and the angles and distances are recorded.
The general method of directing the excavation in enlarging the profile by referring all points of the profile to the vertical axis is the one usually employed in tunneling, and gives good results. It is considered better in actual practice to have the excavation exceed the profile somewhat than to have it fall short of it, since the voids can be more easily filled in with riprap than the encroaching rock can be excavated during the building of the masonry. In tunnels where strutting is necessary the excavation must be made enough larger than the finished section to provide the space for it. In soft-ground tunnels it is also usual to enlarge the excavation to allow for the probable slight sinking of the masonry. The proper allowance for strutting is usually left to the judgment of the foreman of excavation, but the allowance for settlement must be fixed by the engineer.