It was in consequence necessary to institute a system of artillery retaliation, by which a certain number of rounds were fired back at the enemy by howitzers for every trench-mortar round he fired. Howitzers of various calibres up to 6-inch as well as the field-guns took part in these retaliatory shoots, according to the ammunition supply. As this became more liberal, this system tended to check the enemy's trench-mortar activity, particularly when, by careful observation, the positions of his trench-mortar emplacements were located, and the retaliation was brought to bear actually on the offending mortars. The enemy could, however, bombard certain sections of trenches so systematically that he was able to obliterate them. This was particularly the case opposite La Boisselle and in an area known as the Salient. The latter received so much attention as to become untenable, and on 1st December it was evacuated, a retrenchment being dug and occupied behind it. During these trench-mortar bombardments, it had been discovered that the French shelters, having only three or four feet of cover, were not shell-proof. They were, in fact, death-traps, as they gave the men a false sense of security. In consequence, during a bombardment men would crowd into them to take cover, and should a trench-mortar bomb then strike one of them, it was blown in and all its occupants were killed. On 21st October it was therefore decided to provide fresh dug-out accommodation for the whole garrison. The tunnelling company, as has been explained, was strained to its utmost to keep pace with the mining situation. The construction of the dug-outs was therefore entrusted to the 8th Royal Scots, who had joined the Division as the Divisional Pioneer Battalion on 25th August 1915. Fresh calls were made on the infantry for skilled miners, who were attached to the 8th Royal Scots. Work was carried on day and night, and the greatest exertions were made to provide the Division with shell-proof shelters with the minimum of delay. These dug-outs were designed to give 10 to 12 feet of head cover. Charges of 60 lb. and 100 lb. of gun-cotton were exploded on the roofs of dug-outs of this depth, and the shock of the explosion was successfully resisted.[4] Attempts were made to construct dug-outs by digging deep pits and then roofing them in with layers of material calculated to burst shells; but this was found to take more time than was required to construct a dug-out by tunnelling.