In contrast to the Germans, the British expanded underground accommodation in parts of their front line although, like the Germans, they made lavish use of dugouts and tunnels for support and counterattack troops. On the 1st Army front north of Vimy Ridge, where the opposing mine systems reached an impasse in the summer of 1916, the British enlarged their transversal tunnel, which ran beneath or in front of their front line into a communication subway in which it was possible to walk underground almost continuously for four miles.⁵⁴ The northernmost portion of this line was at Givenchy-lès-la-Bassée and here, during the German attacks in April 1918, the British underground defences were put to a severe test. At Givenchy the subway ran for about 720yds beneath the British front line up to the northernmost point at Bunny Hutch shaft head where it ran back at a right angle to Moat Farm in the support lines. This tunnel, called Bunny Hutch or Caledonian tunnel, was dug by 251 Tunnelling Company to provide an additional access to the mine system in case the Germans should have gained occupation of the British front line and they had to blow the shaft head. The underground defences in Givenchy had been conceived in the summer of 1916 by the 1st Army Controller of Mines, Lt Col G.C. Williams, and the scheme, described by the commander of 251 Company, Major H.J. Humphrys, was:

...that the front line should consist of a number of detached posts, strongly fortified, access to which should be obtained by interconnected underground tunnels, well lighted and connected to dug-outs and forward headquarters, all of which should be constructed by the tunnelling companies.⁵⁵

Off Bunny Hutch tunnel was a complex of dugouts housing troops for counterattack. A separate tunnel linked two key strong points in the support line, Givenchy Keep and Marie Redoubt, 300yds apart, and included dugouts to accommodate two battalions with 40ft of cover and numerous exits. In March 1918, the 55th West Lancashire Division practised daily evacuation of the tunnels and dugouts to man the Givenchy positions as rapidly as possible.⁵⁶ This sector was recognized as of key significance owing to the coalfields in the area and the proximity to the Flanders battlefield, which was the main line protecting the channel ports. The German attack of 9 April, the opening of the Battle of the Lys, fell primarily on a Portuguese division, which gave way, but to the south the 55th Division halted the German advance. On the morning of the attack the Germans penetrated 300yds into the village as far as the church, but isolated posts in the front line held out and strong points in the rear halted the attackers. During the morning British counterattack platoons drove the Germans out of the Givenchy positions. The attack failed because the Germans did not follow the tactics required to deal with underground defences: they did not keep up with their barrage to catch the defenders underground and did not mop up the British in the posts that they bypassed when penetrating.

In a subsequent attack on 18 April the Germans showed more preparation for overcoming the underground defences. The British 1st Division, which had relieved the 55th thirty-six hours previously, was not practised in the defence of the sector and in particular in making a rapid exit from the dugouts and tunnels in the event of attack. The attackers this time kept very close to their barrage (aided apparently by heavy guns firing short) and rushed the front line from just 40yds at the moment that their barrage moved forward. The tunnel exits, probably identified during the previous attack, were targeted by the Germans with gas shells and fire was particularly heavy around the Moat Farm and Givenchy Keep dugouts. The two right companies of the 1st Black Watch were caught before they could emerge from their dugouts and possibly also the enlarged transversal tunnel. Despite the strong points suffering direct hits by heavy shells on both shaft heads they nevertheless resisted the fire. All troops except for a few lookouts remained below and the Adjutant of 251 Tunnelling Company, Captain E.J. Ritchie, described the precautions taken by the miners:

Following the procedure which had been agreed upon, parties of one N.C.O. and a few sappers were detailed to remain at the bottom of each stairway or entrance, with orders to effect any necessary repairs and to blow the stairway or entrance should the enemy gain possession of the trenches.⁵⁷

The left of the Black Watch held out at Givenchy Keep and was reinforced via the tunnel by Loyal North Lancashires from Marie Redoubt. The Moat Farm position, used as a dressing station, was filled with wounded and, Ritchie recounted, the British were caught in Bunny Hutch tunnel:

Unfortunately, at one time the enemy gained a strong footing in the trenches at the dug-out entrances and as they had commenced throwing gas bombs down, the position had to be surrendered on humane grounds, for the sake of the wounded below.⁵⁸

The survivors were ordered out by the Germans, the NCOs first, followed by the other ranks. A wounded tunnelling officer was carried out on a stretcher by his men.⁵⁹ Before dark, however, a British counterattack retook their trenches. Ritchie was in no doubt that the underground defences were the key to repelling the attack:

This battle justified the Controller of Mines policy of deep dug-outs, as without the deep dugouts, there is no infantry in the world that could have held Givenchy Ridge that day, because the intense bombardment in the early morning would have wiped them out, whereas our casualties through shell fire were comparatively light up to the time of the general attack.⁶⁰

However, the battle again showed the vulnerability of deep dugouts and tunnels if troops were not well-trained in rapid evacuation the moment the barrage passed over, and the danger if the attacker gained control of all entrances. It also showed the Germans adopting the methods which the Allies had developed for overcoming deep shelters.

Deep dugouts, caves and subways offered great advantages both to defenders and attackers. In particular the British disaster of 1 July 1916 stands as the most successful use of underground shelters in withstanding prolonged artillery bombardment and enabling the infantry garrison to repel an infantry attack. The Germans used deep underground shelters to great advantage in their defensive battles of 1915 to 1916. In late 1916 the Allied experience in dealing with them caused the Germans to radically alter their defensive theory and abandon their use in the front line. The dissemination of the new doctrine throughout the German army took time, however. The Allies’ lack of experience of the defensive battle was exposed by the German attacks of spring 1918, but the example of Givenchy shows the successful British use of underground defences and the shortfalls of German attack tactics in dealing with linked centres of resistance of the type that they had invented and used so effectively in 1916. Subways and underground accommodation aided attackers at Verdun and Vimy, but could not solve the problem of crossing no man’s land. Caves and subways offered great advantages to attacking infantry in concealment, shelter and rest in the days before an attack during their own bombardment and the increased enemy shelling that it attracted. Such refuges meant that troops were able to advance to the attack direct from their places of rest without an exhausting and dangerous approach march into battle. The use of subways at Arras and Vimy was part of greater British and Canadian experience in running and coordinating operations. Their use in 1917 reduced many of the dangers and difficulties faced by the British at the Somme in 1916 when their trenches were heavily shelled and become congested by wounded men. Accommodation and communication tunnels, however, were highly vulnerable to any kind of fluidity returning to operations. Enemy withdrawal rendered the work useless, while rapid advance placed the garrisons at great risk. By the summer of 1917 deep underground shelters in the front line were largely rendered obsolete by more effective methods of attack.