Mining Attack on a Fortress

The French mining attack on the Mastbastion at Sebastopol between February and April 1855, launched from the third parallel about 190m from the Russian ditch. Todleben’s countermines surround the Mastbastion. (From Zschokke, Handbuch der militärischen Sprengtechnik (1911).

The French right attack gallery in the above plan, 0.8m high, has been driven through a clay layer beneath hard chalk at a depth of about 6m but has been broken by heavily overcharged Russian blows.

The attack of a fortress by mining is recorded in the ninth century BC. A tunnel was driven beneath the walls and the soil replaced by timber props, which were then destroyed by burning, causing the walls to collapse. Mining was usually resorted to when artillery had failed and was a slower, but ultimately more certain, method of reducing a fortress. In the first century AD the Roman writer Vitruvius described methods of attacking fortress walls. At ground level covered protection, such as a ‘testudo’, or tortoise, was used against projectiles thrown from above to enable the walls to be attacked with hand tools or a battering ram. Where mining was employed he described the burnt-prop method to bring down walls and also the use of a tunnel to emerge inside the fortress or walled town, from which attacking soldiers issued to surprise the garrison. This technique was used by the Romans to end their nine-year siege of Veii in 396 BC. Sometimes the knowledge that the walls of a fortress were undermined was sufficient for the garrison to capitulate, as at Marqab in 1285 when the Knights of St John surrendered after being shown the extent to which Egyptian miners had tunnelled beneath their great tower. Defences against mining incorporated into fortresses included ready-dug countermines and a deep and wide water-filled ditch. A breach in the walls was so often decisive in breaking a siege that in medieval times it became a convention that the garrison of a castle or fortress might surrender with honour once their walls were breached, whereas if they continued to resist quarter would not be shown and the castle could be sacked.

In fifteenth-century Italy there occurred the only major technological change in military mining from antiquity until 1914, when gunpowder replaced the burning of props to bring down walls. This greatly increased the power and potential of mining, as walls would now not just collapse, but be hurled into the air along with the defenders. Gunpowder also enabled miners to engage in warfare beneath the ground, attacking their opponents’ tunnels by exploding charges, called camouflets, to collapse them, which did not break the surface of the ground. The increased danger to the user entailed by gunpowder saw the rise of regulations to cover mining and, during the seventeenth century, highly sophisticated and standardized forms of fortifications and the means of assault were developed. The French emerged as the masters of siege craft, with the engineer Vauban the dominant figure. The usual method of approach by the besieger was to dig a trench, known as the first parallel, 600 to 700m from the fortification. This was at a distance far enough away for the trench not to be enfiladed (i.e. fired along the length of) by the defenders and earthworks were then thrown up in front for siege artillery to begin firing. Under cover of these guns engineers began to dig approach trenches, known as saps (hence the term ‘Sapper’ for a military engineer), towards the fortress. These were in a zigzag pattern to reduce the effect of enfilade fire. At about 300m from the fortress, a second parallel was dug and new artillery emplacements prepared. From this range the guns could begin to batter a breach in the walls. The defenders might attempt sorties to spike the attackers’ guns. If the artillery assault was not successful, the besiegers continued sapping forwards, by now under small-arms fire, to within a few metres of the walls, or a ditch or moat surrounding the fortress, and constructed a third parallel. If the artillery was still unable to smash gaps in the walls, mining would begin.

A well-designed fortress incorporated a system of tunnels surrounding its walls designed to detect the mines of the attackers, known as countermines (the term ‘mine’ being used for both the explosive charge and the tunnel from which it was laid). Camouflets would be used to destroy the attackers’ mines, but the defenders were restricted in the size of charge that they could use, for fear of destroying their own defences. The distance at which a charge was likely to damage an opponent’s tunnel was known as the ‘radius of rupture’. Mines that were powerful enough to break the surface of the ground to form a crater were known as ‘common mines’. The distance of a mine from the surface, used to calculate whether it would break surface, was the line of least resistance (LLR). To prevent the blast of a mine being directed down the tunnel in which it was laid, the tunnel would be extensively backfilled in a process called ‘tamping’. In the late seventeenth century formulae for the sizes of charges were developed by Vauban and Mesgrigny, followed by Belidor, who carried out trials in 1725. His calculations were not accepted in France, but were taken up by Prussia and used at the Siege of Schweidnitz in 1762, where the Prussians blew mines of up to 2,500kg. The Russians gained much experience during the Russo-Turkish War in 1828 and at Brailov fired two mines of 4,000kg, although with only partial success, as the huge quantity of debris thrown up buried the junction box, preventing the next set of mines from being blown and also ultimately hampering the Russian advance. It was thus not just size that mattered: mines also had to be coordinated with the attack. The Russians used this experience during the most significant mining of the nineteenth century, in the Crimean War during the Siege of Sebastopol. Against a Franco-British attack the Russian chief engineer, General Todleben, organized a system of countermines and some twenty mines were blown, varying in size from 550kg to 2,000kg. Mines were driven through a layer of clay beneath hard chalk. Todleben discovered a second layer at about 15m depth, which he used for a deep level system of countermines. He laid a charge of 4,000kg, which was discovered after the fall of Sebastopol; at that depth it would not have been great enough to break the surface.

Protracted siege warfare and mining did not, however, play a major part in the Franco-Prussian War of 1870-71, during which the French fortresses were forced to surrender through containment or powerful bombardment. After 1870, the opinion of most artillery and engineer officers in the great military powers was that long-range, large-calibre artillery, especially mortars and howitzers with plunging fire, would always defeat fortresses that previously could be breached only by mining.

By the 1880s it seemed that fortresses had become obsolete, along with the ancient means of assaulting them. There were, however, opposing trends. The Russo-Turkish War had involved a five-month siege of Pleva in 1877, conducted by Todleben. During the American Civil War mining was used against field works rather than a fortress at the siege of Petersburg. A 511ft gallery was driven by the 48th Pennsylvania Infantry, which was both commanded by and composed mainly of coal miners. The unit laid a charge of 3,600kg at 6m depth beneath a Confederate earthwork known as Elliott’s Salient, which was blown on 30 July 1864. The mine killed 250 to 350 Confederate soldiers, but in the resulting Battle of the Crater the Union attack was badly coordinated and many of the attackers were trapped in the crater by a counterattack.

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