Between the two large mines, they charged two chambers in the Glory Hole with 8,000lb each to destroy the German mining system, and dug a communication tunnel for use immediately after the first assault. On the northern flank, Captain Henry Hance, commanding 179 Tunnelling Company, obtained permission to place a mine under a position known as Y Sap, which enfiladed the German line in front of Ovillers. He could not drive directly towards it, owing to the German underground defences, and therefore drove a tunnel diagonally across no man’s land for almost 500ft before making a sharp right turn for another 500ft back to the German lines. Hance later claimed that this was the longest offensive drive made in chalk in the war.²¹ Work, however, was running late and they completed chambering only on 28 June, an estimated 30ft from the nose of the Y Sap and 75ft beneath it. They could hear the Germans in their mine system above them but took fewer precautions over silence, apparently using hand picks throughout. This would have been partly because of the need for speed, but also because they were confident that the Germans could only reach them with a charge that would also destroy the Y Sap. They charged Y Sap with 40,000lbs of ammonal.

The two large mines were heavily overcharged and were designed to throw debris over the adjacent German positions, as well as create lips 15ft high. At the moment of detonation, the dust thrown up by these mines would screen the village from observation and prevent mutual support. The lips would then serve the same purpose, provided the British reached them first.

The Division attacking La Boisselle was the 34th, while to the north the 8th attacked Ovillers. The mines were blown two minutes before zero hour rather than the ten-minute delay of the Hawthorn mine. The initial operation orders issued on 21 June by 34th Division indicated that both La Boisselle mines were to be blown at zero hour. These were subsequently amended so that the Lochnagar mine was to be blown at two minutes before zero and the Y Sap mine at zero, then that both should be blown at two minutes before.²² The timing of the mines was not only concerned with the danger of falling debris, but also with the requirement by 34th Division that the first waves of attackers should rise to the attack no more than 200yds from the German front line, to obtain maximum benefit from the British barrage before it lifted to the next target.