Great works of civil engineering, such as the cutting of canals and the piercing of tunnels, involve the use of large quantities of high explosive. Many different types have been evolved, some of which are easily stored and handled without danger
ONE OF THE BIGGEST CHARGES ever used in Great Britain - twenty tons of gunpowder being detonated at Bonawe, on the shores of Loch Etive, Argyll, in 1931. This dislodged about 1,000,000 tons of granite from the mountainside. Gunpowder, or “black powder”, is generally used for blasting in quarries and mines, and for the making of fuses.
IT is difficult to visualize such works as the cutting of the Panama Canal or the piercing of mountain ranges for railway tracks without the use of explosives. In mining for coalor gold or metallic ores, in quarrying stone and marble, for demolition purposes on land and sea, explosives play a part of paramount importance in the work of the engineer.
An explosion is a chemical change by a solid, liquid or a gas into a gas with a volume greatly exceeding that, of the original substance. This sudden production of gas, with its enormous potential volume, exerts tremendous pressure, which is brought under control and used by the engineer.
There are two main groups of explosives, those that change their composition by combustion and those that detonate or change into a high-pressure gas almost instantaneously. The second group comprises the high explosive used by engineers. Explosives in the first group are known as propellents, and they are used for firing projectiles or bullets from guns. Such explosives produce a steadily increasing pressure ideal for the purpose to which they are put; but, although their action is not instantaneous, it is extremely rapid - a split second from start to finish.
Probably the best-known propellent is gunpowder, or “Black powder”, as it is now termed. Gunpowder was introduced in the thirteenth century, and with the passing of time it was improved. Shortly before the outbreak of the Napoleonic Wars, the composition of this explosive was standardized at 75 parts saltpetre, 15 parts charcoal and 10 parts of sulphur. This is the composition of black powder at the present time and it is made in large quantities for blasting in quarries and mines and for the making of the fuses used for blasting with other explosives. The volume of gas produced by the explosion of black powder is about 700 times that of the material used in its composition.
The principal high explosives used in modern engineering are dynamite, gelignite and blasting gelatine. The military high explosives, which may be used also for blasting purposes, are lyddite (picric acid), trinitrotoluene (T.N.T.) and amatol. T.N.T. is largely used in the preparation of fuses used for blasting in civil engineering work.
High explosives require a violent shock to detonate them, and to produce this shock various chemical detonators are used. Detonators generally comprise a thin copper tube containing fulminate of mercury and chlorate of potash. A sharp blow or the heat of a burning fuse sets off the detonator, which in turn detonates the main charge of high explosive.
High explosives are not necessarily dangerous to store or to handle. It is one of the greatest triumphs of the chemist that materials latent with such devastating power have been brought completely under control. If T.N.T. is compressed into a confined space and detonated with fulminate of mercury, half a street may go sky-high; yet small quantities of T.N.T. can be shovelled on to a fire and will burn without danger to any one. Similarly cordite will burn quietly when ignited in the open air.
One of the commonest of high explosives is dynamite. This consists of a special porous earth called kieselguhr, or of some other absorbent material, impregnated with nitroglycerine. Kieselguhr is capable of absorbing nearly twice its weight of nitroglycerine. There are four grades of dynamite, containing respectively 40, 50, 60 and 75 per cent of nitroglycerine. Ordinary dynamite disintegrates in water, but it can be used submerged if wrapped in waterproof material. Gelatine dynamite, prepared from nitroglycerine without the use of kieselguhr, can be used under water and contains 74 or 80 per cent of nitroglycerine according to its grade.
The most efficient explosive known is blasting gelatine, containing 92 per cent of nitroglycerine absorbed in nitrocellulose. Blasting gelatine is half as powerful again as the highest grade dynamite, but is quite safe to handle and is perfectly waterproof. It gives off the minimum amount of fumes and is of great service in tunnelling through the hardest rock.
Yet another useful explosive based on nitroglycerine is gelignite, which is made in four grades. The composition of gelignite varies between 34 and 62 per cent of nitroglycerine combined with other explosive substances, including nitrated cotton and nitrate of potash. Gelignite is plastic and waterproof and its action is slower but more powerful than that of dynamite. It is therefore a convenient and useful explosive for all purposes.
“Drowning Tanks”
The basis of most of the explosives used for blasting purposes is nitroglycerine. This substance is extremely liable to “go up”, and stringent precautions are taken in its preparation. Because of its instability, too, it is made up into compounds for the sake of safety and convenience. It can be used, however, as an explosive in its raw liquid form. The so-called “soup” of the safe cracksman is nitroglycerine. Nitroglycerine, as its name implies, is a combination of nitric acid and glycerine. The nitration process is carried out in a separate building several hundreds of feet away from other houses and surrounded by earth embankments to localize, as far as possible, the effect of any explosion. The cylindrical mixing vessels, each capable of holding about a ton of nitroglycerine, are equipped with cooling coils through which refrigerated brine is pumped. The vessels are partly filled with a mixture of nitric acid and vitriol (sulphuric acid). The function of the vitriol is to absorb the water formed during nitration.
CLEARING A SITE for Fishguard Harbour Station, Pembrokeshire. Engineers of the Great Western Railway building the line to Fishguard in 1903-06 had to clear a way for the track to a site for the new quay and station through a precipitous cliff of quartzite rock 200 feet high. This photograph shows the effect of the blast seen below.
The temperature of the acid mixture is reduced by the cold brine to a predetermined level and then glycerine is introduced in the form of a fine spray. This spray is produced by forcing the glycerine through a number of extremely small holes by compressed air. This method of introducing the glycerine ensures that all of it shall immediately be acted upon by the mixed acid. If glycerine were allowed to accumulate in an unchanged condition in the nitrator, the subsequent action of the acid would soon become uncontrollable.
The nitrator is provided with a thermometer which is carefully watched by the operator in charge of the glycerine supply. Glycerine is admitted at a rate well within a safe limit of temperature, but a further safeguard is provided in the form of a large “drowning tank” filled with water. Should there appear red nitrous fumes accompanied by a rise in temperature beyond the control of the brine coolers, the whole charge is run out through a valve into the drowning tank.
No valves are used in the ordinary way for removing the nitroglycerine from the nitrator: their use is attended with too much danger because of friction. A special method is therefore used to remove the explosive with the minimum of risk. For this purpose advantage is taken of the fact that when the action in the nitrator is completed the nitroglycerine floats on top of the spent acid. Additional spent acid from a previous operation is introduced into the lower portion of the nitrator and the explosive is displaced into an open gutter near the top of the vessel.
The nitroglycerine flows through the gutter to the washing house, where it is freed from all traces of acid by repeated washing with water containing soda. Even this soda solution is not regarded by the explosives engineer without suspicion. It is allowed to collect in a large pond in which dynamite cartridges are exploded periodically to destroy any small quantities of nitroglycerine that are carried over from the washing.
The whole area used in the preparation of nitroglycerine is kept absolutely clean, no grit or dirt being allowed on the floors of the nitration or wash buildings. The operators wear special clothing and rubber boots, and no metal implements are allowed; there must be no risk of any accidental spark.
In addition to nitroglycerine, the other important basis of many explosives is nitrocellulose, generally in the form of cotton or wood pulp that has been treated with nitric acid. The nitration of cotton or other form of cellulose calls for the utmost care.
The cotton, which may comprise new material or waste from the textile mills, is first dried in machines by hot air. It is then passed through other machines that tear it into small pieces and remove all lumps and traces of dirt and foreign matter. The cotton is passed through large pipes by compressed air from one machine to another, and finally to the nitrating room.
Nitrating is carried out in earthenware pans usually from 4 feet to 6 feet in diameter and about 18 in. deep. The charge of cotton, weighing from 20 lb. to 30 lb., rests on a perforated false bottom in the earthenware vessel containing a mixture of nitric and sulphuric acids.
BLASTING THE CLIFFS for the station site at Fishguard. About 2,000,000 tons of rock had to be removed for this purpose, mainly by the use of explosives. The Fishguard line was built as a link in a new route between England, South Wales and Southern Ireland.
To ensure safety, the acid mixture is cooled to a temperature of 5° centigrade. To absorb fumes given off during the nitration process, the cotton is covered with a perforated plate, on top of which flows a thin stream of water. Nitration takes from twenty to sixty minutes and the acid mixture is then allowed to drain away through a valve in the bottom of the vessel. As the acid mixture runs out, water is continuously flowing in at the top of the pan, an arrangement that assists in the removal of the acid. It is highly important that all traces of acid shall be removed from the cotton - the smallest amount of acid may lead eventually to decomposition and to a violent explosion. For this reason the nitrated cotton is washed with boiling water for many hours. Finally a quantity of chalk is added to destroy any acid that remains.
Having been washed, the cotton is carried by a stream of water to “beating” machines, which work it into a uniform pulp absolutely free from grit and dirt. From the beating machines the pulp is pumped into huge tanks, where it is mixed to ensure uniformity of composition.
The pulp is then passed to presses or centrifugal machines which get rid of some of the water; the product is wet nitrated cotton or guncotton. This substance is not much used raw in engineering, but it is of great importance for naval and military purposes.
Guncotton in a dry state is an extremely dangerous substance and it will detonate violently if subjected to friction or a glancing blow. Wet guncotton, however, is devoid of danger, but it can also be detonated by a “primer” of dry guncotton embedded in it when compressed. Guncotton used in this manner constitutes the high explosive carried in naval torpedoes.
Another military explosive plays an important part in engineering constructional work. This is trinitrotoluene, the tremendously powerful high explosive T.N.T. In blasting, however, T.N.T. is used as a fuse for linking charges of other high explosive. It is set off by an ordinary detonator with a safety fuse attached or by an electric detonator; its action is instantaneous. T.N.T. is prepared by treating toluene, obtained from coal tar, with a mixture of nitric acid, sulphuric acid and water in large cast-iron nitrators equipped with mechanical stirring apparatus. Temperature control is achieved by coils through which steam or water can be passed as required. After various washing and drying processes the T.N.T., in powder form, is packed into linen bags and stored in wooden boxes.
Electric Detonators
The methods of using explosives for engineering vary according to the nature of the work in hand. Tunnelling and shaft sinking through solid rock call for different tactics from those used in removing underwater wreckage. Quarrying with explosives requires a different procedure from that applicable to coal mining. The essential in all blasting, however, is to place the explosive charge where it will be most effective. When a tunnel is to be driven through the solid rock of a mountain range, numerous holes are drilled by compressed air or electricity in the rocky face of the tunnel site. These holes all point forward along the surface of an imaginary cone and almost meet at a common apex within the rock.
After the drilling of the boreholes, several feet in depth and generally having a diameter of from 1 in. to 3 in., the next operation is the charging with explosive. Blasting cartridges, in paper casings, vary in size from ¾ in. by 3 in. to 1½ in. by 8 in. and their weights vary in proportion from 1 oz. to nearly 11½ oz. The last cartridge put into each hole is known as the primer, and in this a hole is made with a wooden stick to receive the detonator.
In addition to the fuse-operated detonator already described, there is a type which is exploded by an electric spark or by the incandescence of a piece of thin wire. Where a fuse is used this is attached to the detonator tube and the trailing end leads away, a safe distance, from the borehole. When electric detonators are used the fuse gives place to a pair of wires that are taken to an exploder or shot-firing machine. Exploders comprise a magneto or dynamo worked by a handle or plunger with rack and pinion gear. The magneto type of exploder produces a spark in the detonator.
When the cartridges, with the primer and detonator, are in place, with the fuse (or electric wires), the borehole is filled in or tamped with damp earth or clay.
In the example of tunnelling we are now considering the central charges are arranged to explode first by the use of electric detonators. Into the cavity formed by this explosion, the next series of charges smashes the ring of rock. These explosives are detonated by one-second time fuses, started electrically at the same instant as the firing of the first charges. The loose rock is then cleared away and drilling, charging and exploding follow in rotation again and yet again as the heading is driven through the solid rock.
TAMPING A CHARGE of explosive in a seam of coal at Seghill Colliery, Northumberland. First a borehole is made for the charge. When this has been inserted, the hole is closed, or tamped, generally with damp earth and clay. About seven tons of coal are generally dislodged by one blast or “shot-firing”.
n addition to the fuse-