Mining (military)

Sapping, or undermining, was a siege method used since Antiquity against a walled city, fortress or castle.

Use in antiquity

The Greek historian Polybius, in his Histories, gives a graphic account of mining and counter mining at the at the Roman siege of Ambracia:

The Aetolians [...] offered a gallant resistance to the assault of the siege artillery and [the Romans], therefore, in despair had recourse to mines and underground tunnels. Having safely secured the central one of their three works, and carefully concealed the shaft with wattle screens, they erected in front of it a covered walk or stoa about two hundred feet long, parallel with the wall; and beginning their digging from that, they carried it on unceasingly day and night, working in relays. For a considerable number of days the besieged did not discover them carrying the earth away through the shaft; but when the heap of earth thus brought out became too high to be concealed from those inside the city, the commanders of the besieged garrison set to work vigorously digging a trench inside, parallel to the wall and to the stoa which faced the towers. When the trench was made to the required depth, they next placed in a row along the side of the trench nearest the wall a number of brazen vessels made very thin; and, as they walked along the bottom of the trench past these, they listened for the noise of the digging outside. Having marked the spot indicated by any of these brazen vessels, which were extraordinarily sensitive and vibrated to the sound outside, they began digging from within, at right angles to the trench, another underground tunnel leading under the wall, so calculated as to exactly hit the enemy's tunnel. This was soon accomplished, for the Romans had not only brought their mine up to the wall, but had under-pinned a considerable length of it on either side of their mine; and thus the two parties found themselves face to face.[1]

Use in the Middle Ages

A mine was a tunnel dug under the walls of a castle. Once under the walls, sappers would build wooden structures to hold up the tunnel that they had made. The tunnel would then be filled with flammable material and set on fire. Later, explosives were used for greater effect. If the sapping was done well the wall above it would fall down creating an entrance for the attacking army.

Peter of les Vaux-de-Cernay recounts how at the battle of Carcassonne, during the Albigensian Crusade, "after the top of the wall had been somewhat weakened by bombardment from petraries, our engineers succeeded with great difficulty in bringing a four-wheeled wagon, covered in oxhides, close to the wall, from which they set to work to sap the wall" (les Vaux-de-Cernay, 53).

As in the siege of Carcassonne, defenders worked to prevent sapping by dumping anything they had down on attackers who tried to dig under the wall. Successful sapping usually ended the battle since either the defenders would no longer be able to defend and surrender, or the attackers would simply charge in and engage the defenders in close combat.

There were several methods to resist under mining. Often the siting of a castle could be such as to make mining difficult. The walls of a castle could be constructed either on solid rock or on sandy or water logged land making it difficult to dig mines. A very deep ditch or moat could be constructed in front of the walls, as was done at Pembroke Castle, or even artificial lakes as was done at Kenilworth Castle. This makes it more difficult to dig a mine and even if a breach is made the ditch or moat makes exploiting the breach difficult. The defenders could also dig counter mines. From these they could then either dig into the attackers tunnels and sortie into them to either kill the sappers or to set fire to the pit-props to collapse the attackers tunnel. Alternatively they could under mine the attackers tunnels and create a camouflet to collapse the attackers tunnels. Finally if the walls were breached they could either place obstacles in the breach for example a chevaux de frise to hinder a forlorn hope, or construct a counterscarp. The great concentric ringed fortresses like Beaumaris Castle on Anglesey were designed in such a way that the inner walls were ready build counterscarps which if an attacker succeeded in breaching the outer walls would have left them in a killing field between the the lower outer walls and the higher inner walls.

Use in the American Civil War

During the Siege of Petersburg, Union troops dug a tunnel under the Confederate lines and packed its end with vast amounts of gunpowder. When set off, the resulting explosion killed about 300 soldiers. It might have been decisive if not for faulty Union tactic of storming into, rather than around, the resulting crater, allowing the defenders to shoot down onto attackers unable to climb the steep crater sides. The combat was accordingly known as the Battle of the Crater.

Use in the Modern Era

Sapping saw a brief resurgence as a military tactic during the First World War when army engineers would attempt to break the stalemate of trench warfare by tunneling under no man's land and laying large quantities of explosives beneath the enemy's trench. As in siege warfare, mining was possible due to the static nature of the fighting.

A notable example was Messines Ridge, under which 450 tonnes of high explosive were placed in 21 mines after about two years of sapping. Approximately 10,000 German troops were killed when 19 of the mines were simultaneously detonated. One of the explosive caches exploded years later. The 21st cache was never found and there are still several tonnes of high explosive buried somewhere in the Belgian countryside.

By World War II troop movements were too fluid, and tunneling too slow, for sapping to be worth the investment of effort. Sappers did see some use during the Vietnam War, such as when U.S. forces were holding Khe Sanh.

Sometimes sapping efforts were countered by a sapping effort of the besieged party, trying either to penetrate the tunnel or to blow it up.