The use of mining to destroy the numerous enemy reinforced concrete shelters is now coming to the fore. Artillery-fire, even of the heaviest character, often fails to destroy these shelters, and engineers are called on to destroy them by tunnelling and firing with high explosives. In most cases, it is usually essential for the success of the work that a sufficient depth of clay is found, in order to proceed without noise. On some occasions they have been destroyed from tunnels in chalk, but this is increasingly difficult with the enemy's improved methods of listening. Smaller galleries must usually be driven. It often happens that very little cover to these galleries can be obtained and the work is more hazardous, but the success of these operations is undoubted if careful measures are adopted.
In June, 1917, the writer's company destroyed a German reinforced concrete machine-gun post by tunnelling from the Hindenburg line for a distance of 200 feet, through an average of 7 feet of clay above the chalk. For most of the distance, only 2 feet of cover was possible, but the operation was completed in a few days, and with a charge of 500 pounds of ammonal, placed right up against the concrete, succeeded in entirely destroying this German defense, and largely assisted the infantry in capturing another 500 yards of the enemy Hindenburg trench.
In the instance referred to, the enemy had a listening instrument set up in the chalk, but had evidently failed to hear the work in the clay above.
Nearly all the German trenches are undermined by a series of subterranean galleries, thirty feet or more in depth. As they retreat, in addition to building blocks or breastworks in the trench on top, they will also blow the tunnels underground. It sometimes happens, however, that these underground galleries are not destroyed behind them, and the enemy have taken advantage of this situation by waiting until the trenches are occupied in force by the pursuing troops, and then blowing big mines under them. Careful investigation of the enemy trenches occupied after an advance is essential, though this obvious precaution is sometimes overlooked. In all mining work of this character, speed and silence are of first importance.
No. 55.
LINE CORPS SUMMARY OF INFORMATION
Received up to 6 p.m., 17th August, 191—
CONFIDENTIAL. This document is not to go beyond Battalion or Battery Commanders who are responsible that no copies risk falling into the enemy's hands.
1. BRITISH OPERATIONS.
1. (a) During last night and to-day on the 11th Divisional front the 6th Lincolns Regiment and the 6th Border Regiment, by a series of enterprising patrols and bombing attacks, succeeded in occupying the whole of CONSTANCE TRENCH up to R. 33. a. 5. 4., near which touch was established with the 34th Brigade on their right, the whole of DANUBE TRENCH was also occupied and bombing posts pushed out to near JOSEPH'S TRENCH in R. 32. b.