However, he later revised his view:

It transpired a few months afterwards that except in a few cases the uses to which these subways were put were somewhat exaggerated, whilst the monetary cost was high. It is also noteworthy that the tunnels were not made for battle purposes of this kind again.²³

Like the Russian saps on the Somme, the subways were little used where the attack successfully secured the front line and where German mortar and artillery fire was prevented from interfering with the troops coming forward for the next phase of the attack.

The tunnelling work planned for the Battle of Arras east of the town resembled more that of the later stages of the Somme offensive, with Russian saps driven across no man’s land with machine-gun positions or bored mines at the ends. The major difference was the opportunity to incorporate extensive existing underground spaces discovered in Arras and immediately behind the British lines and link these with the Russian saps. The British initially explored the Arras sewers. The sewers at Ypres had been used by the RE Signal Service as a protected means of running telephone cable where they were too small to admit a full-grown man, and a small Belgian boy was paid to take the line through. These sewers, however, were later destroyed by the German bombardment with 430mm howitzers.²⁴ The Crinchon sewer in Arras by contrast was ‘a beautifully bricked tunnel’ 8ft high and 6ft wide, with footpaths on either side of the water course.²⁵ It ran along the course of the old fortifications like a ring road on the eastern side of the old town and the British drove several inclined entrances from the cellars in the Grande Place and other locations in the town to this sewer. Arras was built on hard chalk, and many of the seventeenth-century houses were provided with deep cellars, which were found useful for accommodating troops. Some 13,000 British troops were housed. In addition, in October 1916, the British discovered the first of a series of large disused underground chalk quarries on the eastern and south-eastern outskirts. The New Zealand Tunnelling Company was withdrawn from front line mining to exploit these for a future attack: