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Sapping, or undermining, was a siege method used since Antiquity against a walled city, fortress or castle. The term has been borrowed in geomorphology to denote the undermining of slopes by groundwater erosion (see groundwater sapping).
Use in antiquity
The Greek historian Polybius, in his Histories, gives a graphic account of mining and counter mining at the Roman siege of Ambracia:
Use in the Middle Ages
In medieval warfare during 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 coupure. The great concentric ringed fortresses like Beaumaris Castle on Anglesey were designed in such a way that the inner walls were ready build coupures which if an attacker succeeded in breaching the outer walls would have left them in a killing field between the lower outer walls and the higher inner walls.
Use in the American Civil War
During the Siege of Vicksburg in 1863 Union troops led by General Ulysses S. Grant tunnelled under the Confederate trenches and detonated the mine beneath the 3rd Louisiana Redan on June 25, 1863. The following assualt led by General John A. Logan gained a foothold in the Confederate trenches where the crater was formed but the attackers were eventually forced to withdraw.
A more famous instance occured during the Siege of Petersburg, Union troops dug a tunnel under the Confederate lines at Elliott's Salient 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 the 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 warfare
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.
Another example is recorded in Louis Trenker's Berge in Flammen. Whole mountain peaks at the Alps were exploded during the mountain war. Col Di Lana, Lagazuoi, Marmolata, were a few of these peaks.
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 United States forces were holding Khe Sanh.
Sometimes sapping efforts were countered by a sapping effort of the besieged party, trying either to penetrate the tunnel to confront the enemy, undermine the enemy tunnel by digging below it, or to blow it up, such as in the First Siege of Vienna.
"Sapping"