Use of Mines in Trench Warfare

USE OF MINES IN TRENCH WARFARE.

[From the French St. Cyr School.]

(Secret and Confidential. For Official Use Only. Not to be Taken into First-Line Trenches.)

USE OF MINES.

In sectors where the distance between the two lines is below 150 meters, mine warfare must be used. When the trenches are farther apart, underground warfare is seldom employed. In special cases, however, when there are strong ventilators and the line is stable enough to permit of it, advance may be made underground.

SPECIALLY MENACED POINTS.

The most vulnerable points evidently are the following: The outposts in advance of the line, machine-gun positions approximately located by the enemy, and the junction points of the communicating trenches with the first line.

SURFACE OBSERVATION.

Underground activity, either offensive or defensive, is first observed from those points in our lines nearest to it. All enemy trenches facing a salient of our lines will be the object of particular attention and closest daily observation. This observation of the first-line trenches should disclose the presence of enemy underground works and their approximate location.

One of the difficult questions in mining is the removal of the earth. Expert miners sometimes remove the earth as far as 100 to 200 meters from the entrance to the gallery. They throw it on old ruined shelters, in shell holes, on the reverse of the trenches. But these precautions are not always rigidly observed. When the noncommissioned officer is absent, or the enemy bombards a little strongly, some one in the working party not wanting to work overtime throws several clods of earth on the parapet.

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INDICATIONS REVEALING THE ENEMY GALLERIES.

Freshly placed earth coming from underground is white and less dull in color than that of the parapets, which have been washed by the rain and blackened by explosives. The difference in color for the first day or so is striking. In chalk, large white spots are seen on the reverse of the German trench, daily growing larger. Without doubt they are working near by, and should be watched.

A communication trench comes out at a salient, and runs back from 150 to 200 meters. Patches of chalk, freshly moved and increasing daily, are observed. These are indications of underground work starting from the salient.

Four or five meters of enemy trench without loopholes, but with loopholes close together to the left and right, may mark a gallery entrance.

A miner's working relief reaches the gallery, each man carrying a piece of the frame or a lining plank over his shoulder, the ends of which can be seen over the parapet or through the loopholes. These are indications of a gallery position, especially if the men all move in the same direction and are lost to view at the same place.

From a raised point on the second or third line we see, with field glasses, an abnormal accumulation of sandbags in a well-known area. These sacks may indicate an underground gallery.

Patrols sent out in front of the enemy trenches sometimes bring back valuable information. They may hear the rumble of ventilators, the noise of a truck moving on the rails, men working near a gallery entrance, etc.

The enemy fire with heavy calibers on portions of the first line. Often the same corner is bombarded. This may indicate that the enemy artillery is seeking to facilitate the miner's work by overthrowing the entrances to troublesome galleries in the trench attacked.

Listening for underground noise.—The observation of hostile trenches may give indications which will limit the zone necessary to be watched; but for accurate results we must listen for underground noises.

Positions of listening posts.—The listening post is placed at the head of a gallery, in an angle of a gallery, in a deep dug-out, in a niche under the parapet, or on the bottom of the

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trench. The points nearest to the enemy trench are selected for the listening posts. Below the outposts there is always a niche, allowing a man to place his ear to the ground (hence the name listening posts).

Hours and precautions.—The most favorable hours are morning about 4 or 5 o'clock, at 2 p. m., and at midnight. All the occupants of that part of the trench must keep still, and all work in the trench and the mine must cease for a given time.

What is heard.—The inexperienced ear hears too many things, and is easily mistaken in the noises heard. A relief passing in the enemy trench or in his own trench at 40 meters sounds strangely like the noise of a pick. A man hitting a ground sill or striking it with his heel gives the idea that work is being done. The impact of bullets on the parapet at night, when a fusillade is uninterrupted, also gives the idea of underground work. A man filing a fuse at the foot of his loophole suggests the presence of an enemy revolving borer. A man who snores beside the gallery entrance imitates the noise of a ventilator and may be mistaken for it.

However, to even a partially trained ear the noise of the pickax is characteristic. It is not a harsh sound, like that of a heel striking on chalk ground, nor is it like the shock of bullets piercing the parapet. It is a low, rhythmic sound, with regular cadence. In a gallery the miner works kneeling. When he has struck five or six blows with the pickax, he takes a breath. He repeats this process about 12 times. He stops two or three minutes, and the second miner clears away the earth and fills the truck with sacks. The first miner resumes his work. It is easy to distinguish this regular cadence peculiar to the miner.

In an infantry company there are always several miners by occupation, or several men familiar with engineering. These men are selected specially for the listening service. They can give accurate information to the officers and noncommissioned officers of a sapper company. It is also a good thing for a platoon commander to descend into a gallery and train his ear by exchanging pickax signals with the miner in the adjoining gallery.

Distance at which noise can be heard.—The following indications help in determining the distance of underground work:

Four men work in a gallery. They start the work, then stop. The ear is placed against the side of the wall, the other

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 ear being covered by the hand. If the work is heard under these conditions, it is at a distance of 25 to 30 meters.

If all noise is avoided, and the work is heard without placing the ear against the wall, the distance is 12 to 15 meters.

If there is talking or working going on, and still the underground work is heard in spite of it, the distance is 8 to 10 meters.

At six meters a man can hear all the sounds of the pickax, the chalk crumbling, the pieces rolling down on one another, the sliding and stacking of cases. These noises sound as if they were immediately below.

At four meters a man can hear talking, the scraping of buttons against the wall, the miner turning around.

The humming of a ventilator can be heard at 40 meters without taking precautions to hear it.

An automatic borer can be heard all through the sector.

Directions from which noise comes.—It is easier to determine the direction of noise than the distance. There is always a chance of making observations in the galleries—one on the right, the other on the left of the noise. The exact location of the enemy underground work can be determined by intersections.

The engineer companies have an apparatus for intensifying the sound (strong microphones). They reinforce the sound when the apparatus is in the direction of the source of the sound. Their greatest defect is that they magnify sound too much and too many things are heard. Why hear for a distance of 100 meters when the enemy trench is only 40 meters away? Everything is heard in a mine gallery. It is difficult to distinguish among the many noises that of the enemy miner's pick. The ear is amply sufficient.

The beginner has a tendency to exaggerate the proximity of sounds. He thinks he is close to the enemy when he is still at a distance, and he takes steps to catch the enemy by exploding a camouflet, whose only effect is to retard his own work.