MILITARY MINING Mining is one of the oldest forms of siege warfare. The blowing of the trumpets and circumambulation were probably done to cover the final mining of the walls of Jericho. But it was not until 1487 that gunpowder was used. When Shakespeare makes Fluellen say at Henry V.'s siege of Harfleur "th' athversary is dight himself four yards under the counter-mines; I think 'a will plow up all ...," he is anticipating that development of siegecraf t by nearly 10o years.
When the attacker, owing to casualty fears, does not care to ad vance above ground by means of saps and trenches he may resort to underground galleries. Large charges exploded in these galleries either destroy the works attacked, or they can be made to form a series of craters, which, occupied by infantry and joined together by trenches, form a new parallel. For this offensive mining, geolog ical conditions must be favourable, and surprise and speed are essential. Successful mine attacks, therefore, cannot be made in water-logged strata nor in hard rock. As a defence against this form of attack countermines are used. In olden days the object was to break into the attacker's galleries and capture them by hand-to-hand fighting. Now, charges sufficient to destroy them are exploded near by. In order to locate the advancing galleries listen ing is of the first importance.
A protective system therefore consists of a series of listening galleries, usually of the herring-bone type, spaced so that the enemy are unable to drive a gallery between them without being heard. The distance apart depends on the range of hearing in the soil. In average chalk the naked ear can detect "picking" at a dis tance of i 5of t., but in clay only up to soft. The attacker having been heard, it is necessary either to destroy his galleries or to make him blow a mine prematurely. To do this the defender prolongs his listening galleries in the direction of the attack and at a suitable distance fires a charge. But in order not to form a crater, which would afford cover to the attacking infantry, the charge is so calcu lated that the explosion while destroying the enemy's gallery, will not break the surface of the ground. This is known as a "camouflet." (See fig. 16.) The action of a mine may be great enough to break the sur face of the ground. The effect is greatest along O.A. the Line of Least Resistance (L.L.R.). The radius of the crater formed is A.B. The ratio A.B• or Radius of crater is the index of the mine.
O.A. L.L.R.
A mine of index r is known as a "common mine." (Fig. 17.) Those
of larger index "overcharged" and those of smaller index
charged mines." Craters are also
known as one-lined, two-lined,
three-lined, etc., according to
whether their diameter is equal or
twice or three times the L.L.R.
Thus a common mine forms a
two-lined crater. The charge
quired to form a crater of given
radius depends on the nature of
the soil, the explosive and depth
it is laid : where D = the depth of the charge in feet, N = the index (Radius of crater) of the mine S = soil factor, E = explosive L. L. R.
factor. The charge in lbs. = S X D
lI Construction.—Mine galleries are started from the bottom of vertical shafts or inclines, and to prevent collapse of the earth they are lined with timber or iron sheeting. Owing to the volume of the soil excavated and to the difficulty of carrying and concealing it, the galleries are of small dimensions: 4ft. loin. by 2ft. 9in. is a normal size, while 4ft. Sin. by 2ft. Sin. is the minimum conven ient section. Alignment and direction are maintained with the help of the miner's dial, and levels with the Dumpy level.
The method of excavation has altered very little since ancient times. The smallness of the galleries and the necessity for silence makes the employment of machinery difficult ; although various types have been tried, the results have been disappointing, and reli ance is still placed on the miner with pick, shovel and grafting tool. In clay or sand, "kicking" is a rapid and silent means of progress. The miner, lying with his back supported by a board fixed across the gallery, pushes his grafting tool (or push pick) horizontally into the soil. The rate of advance varies with the soil and the fa cilities for removing the excavated soil. 15 f t. in 24 hours is an av erage rate, while Soft. has been reached in blue clay. Ventilation is important. When two entrances can be provided to a gallery, a current of air will always flow along it, but for mine heads forced ventilation by mechanical air-pumps and fans is necessary. Light ing is now provided by electricity and portable electric torches.
Of the various high explosives used, ammonal—which is three times more powerful than black powder —has been found the most satisfactory. The charge having been packed in the mine chamber, the whole gallery is tamped with sandbags well bonded together for a distance equal to one and a half times the radius of rupture. The tamping is improved by mak ing two right angle bends in the gallery leading to the chamber, or air spaces may be left between lengths of tamping. The charge is fired by electricity with electric detonators, using alternative cir cuits in case of failure, or a slow and instantaneous fuse (Cordeau Detonant) may be used.
The presence of mine gas and carbon monoxide, which is colourless and odourless, necessi tates special precautions. Canaries and mice, being more easily affected than man, are used to give warning of poisonous gases. For rescue work in a contaminated mine, Proto and Salvus self contained breathing sets with portable oxygen cylinders are used, and mine stretchers of matting enable a man to be dragged along a gallery. Listening is of such vital importance that every effort is made to aid the human ear. When immersed in a basin of water placed on the ground, the ear can detect sound otherwise unheard. Of modern listening apparatus two types are used—the geophone, which works on the principle of the stethoscope and the seismo phone, which is an electrical detector. With two geophones the di rection of the source of sound can be obtained, while with seismo phones (which do not give direction) placed in various galleries and connected to the switchboard of a central listening chamber, one operator can listen over a wide front.
At Frederick the Great's siege of Schweidnitz in 1762 (mentioned in Carlyle's Frederick the Great) a mine of 5,400 lbs., i 6f t. below the surface, made a crater 4 2 f t. in radius. Mining was extensively employed during the Crimean war. At the siege of Sevastopol, over five miles of galleries were driven by the opposing armies, and the French alone used over 130,000 lbs. of powder. In the American Civil War an interesting mine was fired by the Federals in June 1864 at the siege of Peters burg. The circumstances were all abnormal. No military mines, mining tools or materials were available; and no one had any con fidence in its success, except its originator, Lt.-Col. Pleasants, his regiment, which was recruited from a mining area, and Gen. Burn side, the corps commander. The opposing entrenchments were 13o yd. apart. The mine gallery was started behind the Federal lines and driven a distance of i 7oyd. until it came under a redoubt in the Confederate lines. There two branches were made right and left, each about 38f t. long, and in them eight mines aggregating 8,000 lb. of powder were placed. The first attempt to fire them failed, and an officer and a sergeant volunteered to enter the gallery to seek the cause of failure. A defective splice in two lengths of fuse was thus discovered and repaired. At the second attempt all the mines were successfully fired simultaneously and a crater 17oft. long by 6oft. wide and 3of t. deep was formed. Several hundred men in the redoubt were blown up and mostly killed; but the as sault which followed failed completely from want of organization. Modern high explosives were first used during the Russo-Japanese War, where at the siege of Port Arthur in 1905 underground opera tions played an important part, and mines were used for the attack and defence of trenches.
At the commencement of the World War in 1914, the power of the heavy howitzers, which had so rapidly destroyed the Belgian forts, appeared to indicate that for the re duction of fortresses, mining would be unnecessary. It soon, how ever, became evident that with the advent of position warfare, and the difficulty of making any advance above ground in the face of machine guns and barbed wire, the attack of important localities by mining would be advantageous. Further, the opposing lines approached each other so closely that a bombardment of the hos tile front trenches could not be carried out by either side without damage to their own. Mining was therefore resorted to increas ingly. Mines were effectively used in the opening stages of the bat tle of the Somme in 1916, and some brilliant mine attacks were carried out by the French. It was, however, during the attack on the Messines Ridge in 1917 that mining reached its greatest achievement. At 3:10 A.M. on June 7, on a front of nine miles, and within an interval of 3osecs. 20 mines containing r,000,000 lbs. of explosives, were detonated. The largest of them (at St. Eloi) was 125ft. deep, took months to complete and contained 43 tons of ammonal, while the earth shake was clearly felt at Cassel 16m. away. The use of these great mines, the destructive effects of which could not be definitely foretold, was a matter of anxiety, but they were certainly successful. Ludendorff writes in his memoirs : "We should have succeeded in retaining the position but for the exceptionally powerful mines used by the British which paved the way for the attack.... The moral effect of these explosions was simply staggering." With the introduction of tanks, offensive and consequently de fensive mining declined. It was seldom found to be worth the time, labour and materials expended and was only used as a last resort. Exceptional circumstances will, however, arise when, for the possession of important and stubbornly contested localities, mining will again be used, and for underground protection against the increasing bombardment of artillery and bombing aircraft, sub ways and mined dugouts will be extensively required.
(E. H. K.)
