 The gallery had been pushed silently through the clay, avoiding the sandy and chalky layers of the Vimy Ridge but by 9 April 1917 was still short of its target.In the end, two mines were blown before the attack, while three mines and two "Wombat" charges were fired to support the attack, including those forming a northern flank.Trench raiding involved making small-scale surprise attacks on enemy positions, often in the middle of the night for reasons of stealth.