Tunneling
163. Depth.—The depth at which it becomes economical to tunnel depends mainly upon the character of the material to be excavated and on the surface conditions. In soft dry material with unobstructed working space at the surface, open cut may be desirable to depths as great as 35 or 40 feet. Tunnels are cut in rock at depths of 15 feet or less. In some very wet and running quicksand encountered in the construction of sewers for the Sanitary District of Chicago it was found economical to tunnel at depths of 20 feet and less. Crowded conditions on the surface, expensive pavements, or extensive underground structures near the surface may make it advantageous to tunnel at shallower depths than would otherwise be economical. Winter is the best season for tunneling as the workmen are protected from the elements and labor is more plentiful.
164. Shafts.—In sinking a shaft in soft material, the excavation is usually done by hand, the material being thrown into a bucket which is hoisted to the surface and dumped. The size of the shaft is independent of the size of the sewer and depends principally on the machinery which it is necessary to lower into the tunnel. Ordinarily a shaft 6 feet in the clear is satisfactory. A method of timbering a shaft is shown in Fig. 115. Because of the timbering the shaft must be started sufficiently large at the top to finish with the desired dimensions at the bottom. This excess size is sometimes obviated by driving the sheeting at an angle to maintain the same size of shaft from top to bottom.
In timbering a shaft as shown in Fig. 115 the upper frame is staked securely in position at the surface of the ground. This frame is composed of timbers fastened together in the form of a square with the ends of the timbers extending about 12 inches on all sides. The protruding ends are used to hold the frame in position. Excavation is begun inside the frame, and sheeting is driven around the outside of it as excavation progresses. Only two or three men can work advantageously at one time in these small shafts. The second frame is made up of the same size timbers, but all are cut off flush with the outside of the square. The outside dimensions of this frame are such as to allow sheeting to be slipped in between it and the sheeting already driven. The frame is lowered into position and supported from the upper frame by vertical struts nailed to it. The lower end of the sheeting already driven is held out from the lower frame by blocks of the thickness of the next length of sheeting. These blocks are removed as the next length of sheeting is placed and driven. The driving of the sheeting is facilitated by excavating beneath it as it descends.
Fig. 115.—Section of Shaft Timbering.
Abbot, Journal Western Society of Engineers, Vol. 22.
The sizes of sheeting and timbering should be computed on the same basis as that for trench sheeting except that for depths greater than 30 to 35 feet Rankine’s Theory is not applicable and judgment must be relied on for computing the sizes for deep shafts. In stiff dry material the pressures will change very little as the depth increases. Sheeting is needed in shaft excavation in rock only to protect the workmen from falling fragments, but in sand, particularly in quicksand and in wet ground, the pressures increase directly with the depth and the sheeting should be computed accordingly. Care must be taken to prevent the formation of cavities behind the sheeting, to fill them if formed, and to see that all pieces of the sheeting and bracing have a firm bearing. It is difficult to prevent the collapse of the shaft once the movement of earth against the sheeting has commenced.
Shafts are also sunk in soft ground by constructing a concrete or metal shell resting on a cutting shoe on the surface. The material inside is dug out and the shell sinks of its own or added weight. The first section of the shell may be from 5 to 10 feet long. As this section sinks other sections are added. This is called the caisson method. It is advantageous in wet ground and when the shafts are to be left as a permanent manhole. If a permanent shaft is to be left in an excavation being braced with wood, the permanent lining should follow within 20 to 30 feet of the shaft excavation. This is done to avoid the difficulty of maintaining a great length of temporary wood shaft with the danger of collapse, or of blocks or other objects falling on the workers below.
The distance between shafts is controlled by the depth and size of the tunnel, surface conditions, and the character of the material being tunneled. Except where surface conditions are crowded the shallower the cover to the tunnel the more frequent the shafts. The advantage of frequent shafts lies in the possibility of removing excavated material from the tunnel promptly, and in making ventilation of the tunnel easier. The saving made by the construction of numerous shafts must be balanced against the extra cost of the shafts. For the shallowest tunnels the shafts are seldom placed closer than every 500 feet.
165. Timbering.—After the shaft has been excavated to the proper grade the tunnel is struck out either by cutting through the wooden sheeting or by removing portions of the caisson lining. Practically all tunnels except those in solid rock must be framed to some extent. Some of the types of frames used in tunnel construction are shown in Fig. 116. Different combinations of these may be used in different classes of materials. In solid rock which remains firm on exposure no timbering is necessary. Where the roof only need be supported and the sides are strong enough to be used for support, a timber “hitch” or frame supported on the sides of the tunnel may be used. This is suitable for loose rock roofs with solid rock sides. Timbering such as is shown in the lower left hand corner of Fig. 116 becomes necessary in extremely soft, wet, or swelling material, where the bottom and sides as well as the roof tend to push in. The remaining frame in Fig. 116 shows a form frequently used and lying between the two extremes indicated. In wet tunnels a channel may be cut in the bottom below the sill for drainage purposes as shown in this form. The needle beam method of timbering is also shown in Fig. 116. This method of timbering is used mainly near the heading because of the speed and ease with which it can be installed, but it is undesirable because of the space occupied.
The distance between frames is dependent on the size of the tunnel and the character of the material. It is seldom greater than 6 feet and the frames are sometimes placed touching each other. The size of the timbering is a matter of experience and is generally determined by the judgment of the responsible person in charge of the construction as the result of observation during the progress of the work.
The sheeting between frames is called poling boards, or spiling or lagging according as it is sharpened and driven ahead of the excavation or placed after the excavation has progressed. The horizontal strips placed between the frames to keep them apart are called wales.
Fig. 116.—Types of Frames and Timbering for Tunnels.
In cutting out from the shaft in soft materials requiring support, where the width of the tunnel is the same or smaller than that of the shaft, a frame with a maximum width four thicknesses of sheeting less than the width of the tunnel is set up against the lining of the shaft. The vertical side pieces of the tunnel frame rest on the bottom frame of the shaft as a sill and are securely wedged into position. As the lining of the shaft at the top is cut away the top poling boards of the tunnel are slipped in between the cap of the first tunnel frame and the shaft frame immediately above it. The poling boards are driven with an upward pitch so that there may be room to slip the second length of boards between the next tunnel frame and the first length of boards. The placing of the side sheeting follows in a similar manner. Excavation is then started and the poling boards driven to keep pace with it. The next frame is placed in position and the previous sheeting or boards wedged out a sufficient distance to allow the advance lining to be slipped in when the wedges are removed. Waling pieces are nailed firmly between the frames to hold them in position. The various phases in the driving of a 12–foot sewer tunnel in Seattle are shown in Fig. 117.
Fig. 117.—Stages of Sewer Tunneling.
Eng. Record, Vol. 69, 1914, p. 195.
In soft or running material it may be necessary to protect the face of the tunnel by horizontal boards, called breast boards, wedged back to the last frame placed. The excavation is performed by removing one board at a time, excavating behind it and then replacing it in the advance position. The advance is made from the top downwards. This represents the method pursued in the most difficult material where wooden sheeting without a shield is used. The timbering during the advance may be modified in any manner that the character of the material will permit. The timbering may lag behind the excavation a distance of two or more frames, or it may be omitted altogether. Heavier timbering may be necessary in soft, slipping or shattered rock.
Fig. 118.—Shield for Driving Milwaukee Sewer Tunnel.
Eng. News-Record, Vol. 80, 1918, p. 669.
166. Shields.—Shields are used in tunneling in soft wet material and are particularly suitable for work under air pressure. They are used in rock tunnels where water is anticipated or air pressure is used. The shields often save the expense and difficulty of timbering as the masonry of the sewer follows closely behind the shield. Fig. 118 shows the arrangement for a shield for tunneling in soft material in the construction of the Milwaukee sewers. The shield has an exterior diameter of 9 feet 4 inches and an overall length of 9 feet 8⅛ inches. The cutting edge section is 20 inches long. The shell is made of one inch plate to the back of the jack chambers and one-half inch plate in the tail. The shield is driven by ten 60–ton hydraulic jacks. The jacks are shown in position in the figure. These jacks rest against the finished tunnel lining and serve to consolidate it at the same time that they push the shield into the material to be excavated. The face of the tunnel is cut with a pick and shovel while the jacks are removed one at a time and a new ring of lining is put in place. The lining may be temporary timbering to receive the thrust of the jacks, but it is usually desirable that the permanent lining follow immediately behind the shield. Since the shield is larger than the outside of the lining the space left by its passage should be grouted immediately after it has passed.
167. Tunnel Machines.—Tunnel machines have been used successfully on sewer tunnels in soft materials, but not in rock.[[95]] The machines are of different types, but in general consist of a revolving cutting head, equipped with knives, and driven by an electric motor. The bearing on which the shaft for the cutting head rests is supported against the sides of the tunnel. The muck is carried away by means of a conveyor and dumped into muck cars without rehandling. Rapid progress can be made with these machines in suitable conditions.
Fig. 119.—Method of Drilling and Loading Rock Tunnel Face.
Courtesy, Aetna Power Co.
168. Rock Tunnels.—Tunnels in rock are advanced by drilling into the face as shown in diagrammatic form in Fig. 119. The holes near the center are driven in at an angle towards the center and to depths from 6 to 15 feet. The harder the rock the greater the angle with the tunnel. This is called the center cut. Other holes are driven near the outer edge of the tunnel and parallel to its axis. When fired, the wedge of rock between the center cut holes is thrown back into the tunnel and a delayed explosion then throws the sides into the hole thus made. A final delay thrusting shot throws the muck so formed away from the face of the tunnel. For tunnels up to 6 or 8 feet in height the entire bore is cut out in this fashion. For larger tunnels, the upper portion called the heading, is taken out in this way, and the remainder, called the bench, is taken out by drilling and blowing holes normal to the axis of the tunnel. The amount of powder necessary in the bench holes is much less than that required in the heading.
169. Ventilation.—No tunnel more than 50 feet long should be built without ventilation. A fair amount of air for ordinary conditions is 75 cubic feet of free air per minute per person in the tunnel, and double this amount for each animal. Where explosive gases are met, or under conditions where the tunnel is hot, five or six times as much air may be needed in order to cool the tunnel or to dilute the gases. In order that the air may be fresh and cool at the face of the tunnel where work is going on it should be conducted to the tunnel face in a pipe and blown out into the tunnel. Immediately following a blast at the face the current should be reversed so as to draw the poisonous gases out of the tunnel through the duct. The high pressure air line leading to the drills should be opened at the same time to create a current towards the face in order to accelerate the clearing of the air at the heading. The capacity of the air machines should be sufficient to exhaust four times the volume of the gases created by the explosion, in 15 minutes. This will ordinarily call for a capacity of about 4,000 cubic feet of free air per minute. If the same blower is to be used for exhausting the gases as for ventilation while work is going on, it should have a high overload capacity to care for this situation. The air line should be arranged to allow for reversal of flow.
The diameter of the air pipe should be determined by a study of the saving of the cost and operation of the air equipment compared to the increased cost of a larger pipe line. Other factors affecting the size of the pipe line to be used are: the available space in the tunnel, the temporary character of the installation, the use of the exhaust from high-pressure air machines for the purpose of ventilation, etc. Cast-iron, spiral-riveted galvanized sheet iron, and canvas pipes have been used for conducting low-pressure ventilating air.
Ventilation in tunnels working under air pressure is supplied from the compressors, and the air is delivered near the face of the heading, except that being used in the locks. In tunnels using air drills, the air for the drills is conducted through a separate pipe as it is not economical to compress the ventilating air to the pressure necessary to operate the drills.
170. Compressed Air.—Compressed air is used in tunnel work to prevent the entrance of water into the tunnel and to keep the work dry. The pressure of air used is closely that of the pressure of the ground water but in a large tunnel or a tunnel with a weak roof the pressure may be somewhat lower on account of the danger of blowing through the roof. It is evident that the water pressure cannot be balanced at the top and the bottom of the tunnel. To balance it at the bottom makes a blow out near the top more probable. To balance the pressure at the top may leave the bottom wet. Judgment and care must be exercised during construction and if the pressure is balanced at or near the bottom the roof must be carefully guarded by grouting and puddling with clay, or the surface, particularly if under water, may be covered with a clay bank. If the cavities in the tunnel lining are large, sawdust can be mixed with the grout to advantage, the mixture being pumped through holes in the roof by hand or power operated force pumps. “Blows” must be carefully guarded against as they endanger the lives of the workmen and threaten the loss of the tunnel. The pressure and volume of air supplied for some large subaqueous tunnels is shown in Table 61.
Labor under compressed air is arduous and dangerous with the best of safeguards.[[96]] Pressure more than about 43 pounds per square inch cannot be used and at this high pressure men cannot work more than four hours at a time. Little or no distress is noted at pressures less than 15 pounds.
| TABLE 61 | |||||
|---|---|---|---|---|---|
| Volume and Pressure of Compressed Air in Tunnels | |||||
| (American Civil Engineers Pocket Book) | |||||
| Tunnel | Maximum Distance High Water to Invert, Feet | Minimum Cover in Feet | Maximum Air Pressure, Pounds per Square Inch | Average Air Pressure, Pounds per Square Inch | Conditions and Cubic Feet of Free Air per Minute |
| City and South London | 34 | 42 | 15 | In water bearing-sand. 1660 cubic feet per minute per face. When grouted 1000 to 1300 cubic feet per minute per face | |
| Blackwall | 80 | 5 | 37 | 35 | 10,000 cubic feet per minute per face in open ballast for some time |
| Baker St. and Waterloo | 70 | 18 | 35 | 28 | In gravel, 3300 cubic feet of air per minute per face. Parallel tunnel 1650 cubic feet per min. per face |
| Greenwich | 70 | 30 | 28 | 20 | Average 83.5 per man per minute. Never less than 66.7 |
| Battery, East River. N. Y. | 94 | 12 | 42 | 26 | In sand. Two working faces. Maximum 32,000 |
| East River, N. Y., Penn. R.R. | 93 | 8 | 42 | 27 | Maximum for one face 25,000 cubic feet per minute for 24 hours. Capacity of plant for 8 faces, 80,400 cubic feet per minute |
| North River, N. Y., Penn. R.R. | 98 | 20 | 37 | 26 | Maximum in gravel 10,000 cubic feet per man per hour. Generally ranged between 1500 and 5000 |
Entrance and exit to the tunnel are gained through air locks. These are sheet iron cylinders concreted into the lining of the tunnel or shaft. Air-tight iron doors are provided at both ends, which open inwards towards the tunnel. On entering the lock from the outside the door to the tunnel is found tightly closed. The outside door is then closed by hand, the air valve is opened and air is admitted to the lock until the pressure on the lock side of the tunnel door equalizes that on the tunnel side and the tunnel door is swung open by hand. When the lock is open to the tunnel the pressure in the tunnel keeps the outside door closed. In order to leave the tunnel the process is reversed. Materials are passed through the lock by the lock tender or tenders who pass through the lock with the material if the pressure is low, or who manipulate the air outside of the lock if the pressure is high. If pressures of 30 to 40 pounds are being used, two or even three locks may be necessary.
Explosives and Blasting[[97]]
171. Requirements.—The desirable features in an explosive to be used in trenching and tunneling in rock are: (1) stability in make up so as not to deteriorate in strength or to become dangerous during storage, (2) imperviousness to ordinary variations in temperature and moisture, (3) insensibility to ordinary shocks received in transportation and handling, (4) not too difficult of detonation, (5) convenient form for transportation and loading and for making up charges of different weights, (6) the non-formation of poisonous gases when fired, (7) imperviousness to water and usefulness in wet holes, (8) power without bulk, etc.
172. Types of Explosives.—Explosives which fill some or all these requirements can be divided into two classes, deflagrating and detonating. A deflagration is an explosion transmitted progressively from grain to grain. A detonation is a sudden disruption caused by synchronous vibrations of a wave-like character. The deflagrating explosives are represented by gun-powders and contractors’ powders. They must be carefully tamped in the hole to develop their full power and they must be ignited by a fuse or flame. They are valueless in water or moist holes. These powders are used mainly for loosening frozen earth, soft sandstone, cemented gravels and similar materials where a thrusting action rather than a disruption is desired. The detonating explosives are most commonly represented by the dynamites. These are exploded by a shock usually caused by another explosive which has been ignited by a fuse or electric spark, and which is known as the “detonator.” Detonating explosives are more powerful than deflagrating explosives and are used in all but the softest materials.
Gunpowder.—This is a mechanical mixture of sulphur, charcoal, and saltpeter generally in the proportions of 10 parts sulphur, 15 parts charcoal, and 75 parts saltpeter (sodium nitrate). It weighs about 62½ pounds per cubic foot and produces about 280 times its own volume in gas at a pressure of 4.68 tons per square inch at a temperature of 32 degrees F., which amounts to a pressure of approximately 38 tons per square inch at the temperature of explosion of 4,000 degrees F.
Blasting Powder.—This is a mixture of 19 parts sulphur, 15 parts charcoal, and 66 parts saltpeter. These powders are made in different size angular polished grains, from the size of a pin head to sizes just passing a ⅜ to ½ inch hole. The larger the grains the slower the action of the powder.
Nitro-Substitution Compounds.—These compounds are formed by the action of nitric acid on hydrocarbons. Triton, T.N.T., or trinitrotoluene, made famous during the war, is an example of these compounds. It is made by the successive nitration of toluene, a coal tar derivative. It melts at 80 degrees C., is very stable, and is of great explosive strength. It is manufactured in a convenient form, being compressed into blocks about 2 inches square by about 4 inches long with a specific gravity of about 1.5. The blocks are usually copper plated to protect the T.N.T. from moisture. The more dense it is the less its sensitiveness. It is also put up in crystalline form in cartridges like dynamite, in which condition it is practically equal to 40 per cent dynamite. It can be cut with a knife, pounded with a hammer, and will burn freely and slowly in small quantities in the open air without exploding. It is suitable for all but the hardest rocks. It creates poisonous gases on detonation which are quickly dissipated in the open air but which render it unsuitable for use in tunnel work.
Nitro-glycerine.—This is formed by the action of nitric and sulphuric acids on animal compounds such as gelatine or glycerine. Nitro-glycerine is a yellowish, oily, highly unstable explosive liquid with a specific gravity of about 1.6. It will burn quietly when ignited in the open air. It will freeze at 41 degrees F., and will explode at 388 degrees F., or on concussion at a lower temperature. It develops about 1,500 times its volume in gas, which due to the heat of combustion is increased to about 10,000 times its volume. It is a very dangerous explosive to handle, and is unsuitable for use in the liquid form.
Blasting Gelatine.—This is made by soaking guncotton in nitro-glycerine. Gelatine dynamite is a combination of blasting gelatine and an absorbent. Forcite is a gelatine dynamite in which the blasting gelatine, forming 50 per cent of the compound, contains 90 per cent nitro-glycerine and 2 per cent guncotton; and the absorbent, forming the other 50 per cent of the compound, contains 76 per cent of sodium nitrate, 3 per cent sulphur, 20 per cent of wood tar, and 1 per cent of wood pulp.
Blasting gelatine is packed in a jelly-like mass in metal lined wooden boxes. It is less sensitive than straight dynamite and is one of the most powerful explosives known. It can be made up to equal 100 per cent dynamite. It is suitable for use in the hardest rocks and for subaqueous work as it is not affected by moisture. It is suitable for use in tunnels as the amount of carbon monoxide, peroxide of nitrogen, hydrogen sulphide and other dangerous gases is comparatively low when fully detonated. Gelatine dynamite[[98]] is sold as 30 per cent to 70 per cent dynamite, the actual percentage of nitro-glycerine being less than the nominal quantity given.
Dynamite.—The dynamites are made by soaking nitro-glycerine in some absorbent. If the absorbent is some neutral substance such as infusorial earth the combination is known as a true dynamite. The false or active dynamites are those in which the absorbent is also an explosive compound. The false dynamites form the best known contractors’ explosives. Among the materials mixed with the nitro-glycerine are: magnesium carbonate, sulphur, wood meal, wood pulp, wood fiber, wood tar, nut galls, kieselguhr, sawdust, resin, pitch, sugar, charcoal, and guncotton. The strength of dynamites is noted by the per cent of nitro-glycerine and nitro substitutes contained. Dualin and Hercules powder both contain 40 per cent nitro-glycerine. Dualin contains 30 per cent sawdust and 30 per cent potassium nitrate, but the Hercules powder, which is stronger, contains 16 per cent sugar, 3 per cent potassium chlorate, 31 per cent potassium nitrate, and 10 per cent magnesium carbonate.
Dynamite is the most common explosive used on construction work. It is supplied in cylindrical sticks wrapped in paper, the diameter of the sticks varying between ⅞ and 2 inches. They are about 8 inches long. Forty per cent dynamite is the common strength found on the market. It is suitable for ordinary work in all but very hard rocks or very soft material. Direct contact with water separates the nitro-glycerine from the base and is dangerous when the explosive is used in wet places unless it is fired immediately after the hole is loaded. It freezes at about 42 degrees F., or at even higher temperatures and in the frozen state it is highly dangerous, requiring powerful detonators for firing, but exploding spontaneously from a slight jar, or the breaking of the stick. Special low-freezing dynamites are made that will not freeze above 35 degrees F.
Ammonia Compounds.—Ammonia dynamite is a combination of nitro-glycerine, ammonium nitrate and such other ingredients as sodium nitrate, calcium carbonate and combustible material. This form of explosive is advantageous for underground work because, like gelatine dynamite, its explosion does not create large quantities of poisonous gases. It has a low freezing point and is relatively low in cost. It is seriously affected by moisture, however, and can not be used in wet places. Ammonium nitrate explosives which do not contain nitro-glycerine include 70 per cent to 95 per cent ammonium nitrate and some combustible material. Ammonal is a special type of this class formed by a mixture of ammonium nitrate, aluminum, and triton. All of these explosives are deliquescent, insensitive to shock, and are cheaper than the dynamites.
173. Permissible Explosives.—As specified by the United States Bureau of Mines explosives whose rapidity, detonation, and temperature of explosion will not ignite explosive mixtures of pit gases and air are known as permissible explosives. They include nitrate explosives, ammonia dynamite, and others.
Gunpowder, triton, picric acid, blasting gelatine, dynamite, guncotton, etc., are not classed as permissible explosives.
174. Strength.—The relative weights for equal strength of various explosives are given in Table 62.
| TABLE 62 | |
|---|---|
| Relative Weights of Explosives with the Same Strength as a Unit Weight of 40 Per Cent Dynamite | |
| Explosive | Relative Weight |
| Picric acid | 0.86 |
| Gun powder (well tamped) | 3.10 |
| Straight dynamite, 15% | 1.45 |
| Straight dynamite, 20 | 1.33 |
| Straight dynamite, 25 | 1.28 |
| Straight dynamite, 30 | 1.18 |
| Straight dynamite, 35 | 1.07 |
| Straight dynamite, 40 | 1.00 |
| Straight dynamite, 45 | 0.93 |
| Straight dynamite, 50 | 0.86 |
| Straight dynamite, 55 | 0.83 |
| Straight dynamite, 60 | 0.78 |
| Low-freezing dynamites are the same as straight dynamites | |
| Smokeless powder, well tamped | 0.74 |
| Triton | 0.86 |
| Blasting gelatine | 0.43 |
| Gelatine dynamite, 30% | 1.28 |
| Gelatine dynamite, 35 | 1.21 |
| Gelatine dynamite, 40 | 1.14 |
| Gelatine dynamite, 50 | 1.04 |
| Gelatine dynamite, 55 | 0.97 |
| Gelatine dynamite, 60 | 0.90 |
| Gelatine dynamite, 70 | 0.83 |
| Ammonia dynamites are the same as gelatine dynamites. | |
| Chlorates (sprengle) Rack-a-rock | 1.33 |
| Guncotton | 0.72 |
175. Fuses and Detonators.—The explosion of gunpowder and other deflagrating explosives is caused by the direct application of a flame led to the charge by a powder fuse, or they may be fired by a blasting cap which is itself exploded by the heat from a fuse or an electric spark. The powder fuse is a cord made up of a train of powder securely wrapped in a number of thicknesses of woven cotton or linen threads and usually made waterproof. Ordinary fuse burns at about 2 feet per minute but there may be wide variations from this rate due to the quality of the fuse, moisture, temperature, or pressure. Moisture tends to retard the rate, pressure to increase it. Instantaneous fuse will burn at about 120 feet per second. It is distinguished from the ordinary safety fuse both by eye and touch due to the rough red braid with which it is covered. It is used in firing a number of charges simultaneously. Powder fuses are lighted by the application of a flame or smoldering torch to the freshly cut or opened end exposing the powder grains. Cordeau Bickford is lead tubing filled with triton, in which the flame travels at about 17,000 feet per second. This is also used for igniting charges simultaneously.
The detonation of an explosive is caused by the shock or heat of the explosion of a more sensitive substance which has been exploded by a powder fuse or electric spark. The common method of detonating explosive charges is by the firing of a blasting cap. These caps are copper cylinders, closed at one end, about 1½ inches long and ¼ to ⅜ of an inch in diameter, or larger. They contain a mixture of about 85 per cent fulminate of mercury and 15 per cent potassium chlorate held in place by a wad of shellac, collodion, or paper. The strength of detonators is based on the weight of fulminate of mercury and is designated as shown in Table 63.
| TABLE 63 | |
|---|---|
| Strength of Blasting Caps | |
| Blasting Cap, Commercial Grade | Grains Fulminate of Mercury |
| 3X or Triple | 8.3 |
| 4X or Quadruple | 10.0 |
| 5X or Quintuple | 12.3 |
| 6X or Sextuple | 15.4 |
| 7X or Number 20 | 23.1 |
| 8X or Number 30 | 30.9 |
| Single strength | 12.3 |
| Double strength | 15.4 |
| Triple strength | 23.1 |
| Quadruple strength | 30.9 |
The force of the explosion is markedly affected by the strength of the caps, the effect being greater for low-grade powders. For 40 per cent dynamite the explosion caused by a 5X cap is 15 per cent stronger than that caused by a 3X cap. For 60 per cent dynamite the difference is only 6 per cent. The deterioration of the caps will reduce the strength of an explosion noticeably. With straight dynamite, 3X caps are generally used, but with gelatine dynamite 6X or heavier caps must be used. Caps may be tested by exploding them in a confined space and noting the report and the effect on the shell. A full strength cap will tear the shell into minute pieces, while a deteriorated cap will merely tear it into three or four large pieces. An ordinary blasting cap is shown in Fig. 120 together with other equipment for blasting.
Firing by electricity is generally safer and more satisfactory than by the use of ordinary caps and powder fuses. The explosion is more certain and its exact time is under the control of the operator. Fig. 121 shows a section through an electric blasting cap or detonator, commonly called an electric fuse. Delayed action electric detonators are made by inserting a slow-burning substance between the platinum bridge and the detonating substance. The time of delay is controlled by the depth of the slow-burning substance. Delayed action detonators are useful in tunnel work where it is desired to explode the charge in three or four stages in order that the debris from one charge may be out of the way of the following, and that the forces of the explosions may not serve to nullify each other.
Fig. 120.—Blasting Supplies.
Courtesy, Aetna Powder Co.
176. Care in Handling.—Some of the don’ts in the handling of explosives recommended by the U. S. Army Engineer Field Manual are: in the use of nitro-glycerine explosives of all kinds—
(a) Don’t store detonators with explosives. Detonators should be kept by themselves.
(b) Don’t open packages of explosives in a store house.
(c) Don’t open packages of explosives with a nail puller, pick or chisel. Packages should be opened with a hard wood wedge and mallet, outside of the magazine and at some distance from it.
(d) Don’t store explosives in a hot or damp place. All explosives spoil rapidly if so stored.
(e) Don’t store explosives containing nitro-glycerine so that the cartridges stand on end. The nitro-glycerine is more likely to leak from the cartridges when they stand on end than it is when they lie on their sides.
(f) Don’t use explosives that are frozen or partly frozen. The charge may not explode completely and serious accidents may result. If the explosion is not complete the full strength of the charge is not exerted and larger quantities of harmful gases are given off.
Fig. 121.—Electric Fuse.
Full size.
(g) Don’t thaw frozen explosives in front of an open fire, nor in a stove, nor over a lamp, nor near a boiler, nor near steam pipes, nor by placing cartridges in hot water. Use a commercial or improvised thawer.
(h) Don’t put hot water or steam pipes in a magazine for thawing purposes.
(i) Don’t carry detonators and explosives in the same package. Detonators are extremely sensitive to heat, friction, or blows of any kind.
(j) Don’t handle detonators or explosives near an open flame.
(k) Don’t expose detonators or explosives to direct sunlight for any length of time. Such exposure may increase the danger in their use.
(l) Don’t open a package of explosives until ready to use the explosive, then use it promptly.
(m) Don’t handle explosives carelessly. They are all sensitive to blows, friction, and fire.
(n) Don’t crimp a detonator (blasting cap) around a fuse with the teeth. Use a cap crimper, which is supplied for this purpose.
(o) Don’t economize by using a short length of fuse.
(p) Don’t return to a charge for at least one-half hour after a miss fire. Hang fires are likely to happen.
(q) Don’t attempt to draw nor to dig out the charge in case of a miss fire.
Some of the positive rules in connection with the handling of explosives are: build the magazine on an earth foundation remote from any other structures, protect it with earth embankments that will direct the force of the explosion upwards, and build it of materials that will supply as few missiles as possible. Hollow tile brick, double-walled galvanized iron filled with sand, and similar constructions are satisfactory. The magazine may be heated by steam or hot-water pipes so located that explosives cannot come in contact with them, or by a cluster of incandescent bulbs, but if the explosives become frozen they must not be thawed out by turning on the steam or hot water. If powder or nitro-glycerine is dropped on the floor the magazine should be emptied, washed out with a hose and spots of nitro-glycerine scrubbed with a brush and a mixture of ½ gallon of wood alcohol, ½ gallon of water and 2 pounds of sodium sulphide. Frozen explosives may be thawed by spreading out on special shelves in a warm thaw house—not in the magazine proper, by burying in a manure pile so that the explosive may not become moistened, or more commonly by heating slowly in a water bath. This is a dry kettle in which the explosives are placed and covered. The kettle is then put in another containing water which is heated gently to about 120 degrees F. It should not be boiled.
In case of a miss fire, instead of digging out the old charge put a new charge on top of the old and fire the two simultaneously.
177. Priming, Loading, and Firing.—Priming is the act of placing the cap or detonator in the cartridge of explosive. The primer is either the cap or the cap and cartridge which are to be detonated by the fuse. If a cap and safety fuse are to be used the paper at the upper end of the cartridge is opened, a hole is poked in the explosive with the finger or a piece of wood, the cap and the attached fuse are pushed into the hole and gently embedded in the explosive so that the end of the cap is exposed sufficiently to prevent the fuse from igniting the dynamite directly. The paper is then folded up and tied firmly around the fuse with a piece of string. The result is shown in Fig. 122.
Fig. 122.—Dynamite Cartridge, Safety Fuse, and Cap.
In placing the fuse in the cap the end of the fuse is cut off square, and inserted in the open end of the cap, care being taken not to spill the loose grains of powder or to grind the fuse down on top of the cap. When the fuse is shoved firmly into place the upper portion of the copper cap is pressed or crimped with the cap crimpers shown in Fig. 120.
The number of primers to be used is dependent on the size and location of the charge, but in practically all sewer work only one primer is used to each hole. In bulky charges the primer should be placed near the center of the charge and the fuse so protected that it will not ignite the charge prematurely. In drill holes the primer is put in last with the cap end down.
In loading a hole, it is first pumped and cleaned out. This can be done satisfactorily with the end of a stick frayed out into a broom. Cartridges which very nearly fill the hole are dropped in one at a time and are pressed firmly together, with a light wooden tamping bar. They should not be pounded. After the primer is placed, a wad of clay or similar material is pressed gently into the hole against it and the hole is then filled with well-tamped clay. In tunnel work tamping is not so essential as an overcharge of powder is usually used and the time of tamping, which is worth more than two or three sticks of dynamite, is saved. In handling bulk explosives, such as gunpowder, they are poured into the hole, the fuse is set in the upper portion and the remainder of the hole is tamped with clay as for dynamite cartridges.
Fig. 123.—Methods for Cutting Safety Fuse for Splicing.
If a large number of charges are to be fired simultaneously with a safety fuse, the length of the fuse to each charge should be made equal or a safety fuse used to a common center and approximately equal lengths of instantaneous fuse or Cordeau Bickford used from there to the charge. In splicing the fuses for such connections they are cut diagonally as shown in Fig. 123 and bound together firmly with tape. Electric connections are particularly advantageous under such conditions as they avoid the dangers incidental to spliced fuses and are less expensive. In tunnel work simultaneous electric detonation is not desirable as the holes should be fired progressively: 1st, the cuts; 2nd, the relievers; 3rd, the backs; 4th, the sides; and 5th, the lifters. Different lengths of safety fuse, or delayed action electric fuses can be used for these delay shots.
In igniting a safety fuse an open flame such as that furnished by a match or candle is the most satisfactory. For electric fuses the current is generated by a magneto shown in Fig. 120. Pressing vigorously down on the handle closes the circuit and generates an electric current which heats the platinum bridges and explodes the charges. For the small number of charges used in ordinary construction they are connected in series so that if there is a broken connection anywhere no charge will be exploded. If many charges are to be fired and a line circuit is to be used, the final connection should not be made until just before the charge is to be fired in order to obviate the danger of stray currents firing the charge prematurely. Care should be taken to see that all connections are good and that there are no broken wires on the line.
178. Quantity of Explosive.—The quantity of explosive to be used can be determined satisfactorily only by experience on the job in question, as the factors affecting the necessary quantity are so diverse. The figures in Table 64 indicate the relative amounts needed under different conditions.
| TABLE 64 | ||||||
|---|---|---|---|---|---|---|
| Quantities of Explosives | ||||||
| Kind of Rock | Drift in Feet | Feet[[99]] of Hole | Black[[99]] Powder, Pounds | Dynamite[[99]], Pounds | Grade of Dynamite, Per Cent | Remarks |
| Limestone, Chicago Drainage Canal | 12 | 0.40 | 0.75 | 40 | Gillette | |
| Limestone for crushing | 6 | 1.00 | 0.70 | 40 | Gillette | |
| Limestone for cement | 20 | 0.37 | 50 | Gillette | ||
| Limestone, holes sprung | 15 | 0.40 | 0.26 | 50 | Gillette | |
| Sandstone, side cut | 20 | 0.10 | 1.0 | 0.10 | 40 | Gillette |
| Sandstone, thorough cut | 20 | 0.20 | 2.0 | 0.20 | 40 | Gillette |
| Shale, soft side cut | 24 | 0.08 | 0.7 | 0.03 | 40 | Gillette. Open cut |
| Shale, hard thorough cut | 24 | 0.20 | 1.5 | 0.10 | 40 | Gillette |
| Granite for rubble | 16 | 1.36 | 0.20 | 60 | Gillette | |
| Gneiss, New York City | 12 | 1.33 | 0.60 | 40 | Gillette | |
| Gneiss, New York City | 14 | 0.63 | 0.50 | 40 | Gillette | |
| Syenite, Treadwell Mine | 12 | 1.70 | 0.67 | 40 | Gillette | |
| Magnetic iron ore | 12½ | 0.32 | 0.44 | 52 | Gillette | |
| Trap, seamy | 14 | 0.35 | 0.20 | 75 | Gillette | |
| Trap, massive | 17 | 1.00 | 0.70 | 40 | Gillette | |
| Granite, Grand Trunk | 25 | 0.10 | 0.80 | 50 | 50% dynamite used to spring holes | |
| Clay, rock and Gypsum | Tunnel | 1.00 | ||||
| Hard shale | Tunnel | 2.07 | Grade varied ⅗ at 45%, ⅕ at 60%, some at 100% | |||
| Hard rocky slate | Tunnel | 1.60 | 3.57 | |||
| Hard rocky slate | Tunnel | 1.46 | 3.57 | |||
| Mill Creek sewer, St. Louis | Tunnel | 4.00 | 60 | Mun. Eng’g. Vol. 52, p. 14 | ||