For the tactic of tunneling under an enemy’s works see Mining (military).
Union troops digging a sap with a sap roller on Morris Island, 1863
Sapping is a term used in siege operations. Any trench excavated under defensive musket or artillery fire that was intended to advance a besieging army's position in relation to the works of an attacked fortification was referred to as a sap. Saps of approach were excavated by brigades of trained sappers or instructed troops of the line who 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.
