 In all, the French suffered approximately 150,000 casualties in their attempts to gain control of Vimy Ridge and surrounding territory.The British XVII Corps, commanded by Lieutenant-General Sir Julian Byng, relieved the French Tenth Army in the sector in February 1916, permitting the French to expand their operations at Verdun.The British soon discovered that German tunnelling companies had taken advantage of the relative calm on the surface to build an extensive network of tunnels and deep mines from which they would attack French positions by setting off explosive charges underneath their trenches.