Sapping is a term used in siege operations to describe any trench excavated near an attacked, defended fortification, under defensive small arms or artillery fire. The trench, referred to as a "sap", is intended to advance a besieging army's position in relation to the works of an attacked fortification. The sap is excavated by brigades of trained soldiers, often called sappers. The sappers dig the trenches or specifically instruct the troops of the line to do so.
By using the sap, the besiegers could move closer to the walls of a fortress, without exposing the sappers to direct fire from the defending force. To protect the sappers, trenches were usually dug at an angle in zig-zagged pattern (to protect against enfilading fire from the defenders) and at the head of the sap a defensive shield made of gabions (or a mantlet) could be deployed.
Once the saps were close enough, siege engines or cannon could be moved through the trenches to get closer to—and enable firing at—the fortification. The goal of firing is to batter a breach in the curtain walls, to allow attacking infantry to get past the walls. Prior to the invention of large pieces of siege artillery, miners could start to tunnel from the head of a sap to undermine the walls. A fire or gunpowder would then be used to create a crater into which a section of the fortifications would fall, creating a breach.
Sapping trenches, cannons and gunpowder explosives were a potent force against fortifications. However, the Siege of Godesberg of 1583 during the Cologne War showed that fortresses could still withstand sapping and explosives to a point. After the attacking force of Ferdinand of Bavaria fired large caliber cannons on the fortress, this had little impact on the walls. The cannons were firing heavy shells, but the height of the fortress significantly reduced the power with which they could hit the walls. Although the fortress dated from the 14th century, its construction caused the cannonballs to "bounce" from the walls, having little impact. To breach the walls, Ferdinand ordered his soldiers to dig into the feldspar supporting the side of the mountain and place an explosive charge. Even when the powder was ignited and a substantial portion of the wall, the gate, and the inner walls were blown into the air, the defenders still held out for three days.
Trace Italienne forts
Sapping became necessary as a response to the development and spread of trace Italienne in defensive architecture in the 1500s. The Italian style star fort bastion made siege warfare and sapping the modus operandi of military operations in the late medieval and first decades of the early modern period of warfare. Fortresses with abutments with gentler angles were difficult to breach; cannonballs and mortar shells often had little impact on the walls, or impact that could be readily repaired after night fell. Towers no longer protruded at right angles from the wall; rather, they blended with the wall. These created a two-fold advantage. First, defenders in the towers had a field of fire of 280 degrees or more. This range of fire and the towers' positioning allowed defenders to fire upon the attackers' flank as they advanced, a deadly fire called enfilade. Consequently, a hostile force which ranged their cannons was less effective, as the "hostile cannon [had] to fire from longer range" and defenders could better enfilade attackers.
During the English Civil War, there was a siege of Newark-on-Trent which took place from 6 March 1645 – 8 May 1646. A detailed map of the Cavaliers defences of Newark and the lines of circumvallation and contravallation along with the besiegers redoubts and fortified camps was drawn up by R Clampe, the besieging Roundheads' chief engineer. It includes a zig-zag sap emerging from a bastion of the circumvallation. The zig-zags are at such angles and positions that the defenders were unable to bring enfilade fire to bear. Once the sap was completed four cannons were placed much closer to a gateway than those in bastions of the circumvallation.
American Civil War
In the American Civil War, troops advanced their sap under cover of a sap roller or mantlet by forming a parapet on the engaged side of the trench one gabion at a time and filling it with earth taken from the trench.
The term Russian sap refers to a tunnel that is dug at a shallow depth under no man's land towards an enemy position. It allows the attacking infantry to approach the enemy position beneath the surface of the earth, without being detected and safe from enemy fire. For the attack, the tunnel is opened and the infantry attacks the enemy position at comparatively short range. Russian saps were widely used in the First World War, for example during the Battle of the Somme, when four of them were further equipped with Livens Large Gallery Flame Projectors.
- (in German) Ernst Weyden. Godesberg, das Siebengebirge, und ihre Umgebung. Bonn: T. Habicht Verlag, 1864, p. 43.
- Charles Townshend (editor). The Oxford history of modern war. Oxford: Oxford University Press. 2000, p. 28.
- Townshend, pp. 28-29. ("Such projections from the wall both forced the hostile cannon to fire from longer range and enabled the defenders to enfilade attackers.")
- Campell, E.S.N (1844). A Dictionary Of The Military Science:Containing an Explanation Of The Principal Terms Used In Mathematics, Artillery, and Fortification, And Comprising The Substance Of The Latest Regulations On Courts Martial, Pay, Pension, Allowances, Etc. A Comparative Table Of Ancient And Modern Geography; Achievements Of The British Army; With An Address to Gentlemen Entering The Army (New ed.). London: James Maynard. Retrieved 2007-11-19.
- Townshend (editor)., Charles (2000). The Oxford history of modern war. Oxford: Oxford University Press.
- Weyden, Ernst (1864). Godesberg, das Siebengebirge, und ihre Umgebung (in German). Bonn: T. Habicht Verlag.