Obstacles

The element of the defensive line nearest the enemy is a line or series of lines of obstacles which are designed for the purpose of:

1. Protecting the lines from surprise.

2. Reducing the momentum of the attack, by breaking up the unity of action and cohesion.

3. Holding the enemy under the effective fires of the defenders.

The conditions that obstacles should fulfil are as follows. They must—

1. Be close to the defender's position. As a rule on the western front they are not more than from 50 to 100 yards distant. If they are too close it may be possible to throw hand grenades from the far edge of them into the defender's trenches.

2. As far as practicable, be sheltered and screened from the enemy. Shell fire is the most effective method of destroying obstacles. If they are not concealed they may furnish aiming points for the enemy's fire against the first line fire trench by his being able to estimate its location with reference to the obstacle.

3. Afford no cover or screen to the enemy.

4. Be so placed that the enemy will come upon them as a surprise.

5. Be so constructed as to be difficult of removal under fire and impracticable to negotiate while still reasonably intact.

6. Be arranged so as not to interfere with a counter attack. The obstacles may have occasional gaps left in them which may be mined.

The different classes of obstacles are: Abatis, low wire entanglements, high wire entanglements, barricades, mines, fougasses, crows feet, military pits with wire entanglements, inundations, etc.

Abatis (pronounced abatee) consists of branches of trees lying parallel to each other, butts pointing to the rear, and the branches interlaced with barbed wire. All leaves and small twigs should be removed and the stiff ends of branches pointed. The butts are staked or tied down of, anchored by covering them with earth. When more than one row is used the branches overlap the butts of those in front so as to make the abatis about 5 feet high. An abatis formed by felling trees towards the enemy, leaving the butt hanging to the stump, is called slashing.

Barbed wire is the material most employed in the construction of obstacles. It may be used in the following manner:

1. As a simple trip, for giving the alarm. It is stretched just above the ground and attached to some object that will cause a noise to be made if molested.

2. A simple wire fence, to cause delay and confusion to the enemy in his advance.

3. As an adjunct to tree and brushwood entanglement.

4. As a wire entanglement.

5. As a covering for portable cylinders.

The advantages of the barbed wire entanglement are:

1. It is easily and quickly made,

2. It is difficult to destroy.

3. It is difficult to get through.

4. It offers no obstruction to the view and fire of the defense.

The low wire entanglement is constructed as follows:

1. Drive stakes in the ground until they project about 18 inches. The stakes should be about 6 feet apart, those in each row being opposite the intervals in adjacent rows,

2. The wire is then passed loosely from the head of one stake to another, wound around each and stapled.

3. Where two or more wires cross they should be tied together.

A more useful and efficient modification of the low wire entanglement is made by stapling the wire down the sides of the stakes, allowing five or more feet of slack wire between stakes. Drive the stakes in the ground until the top is flush. This results in a loose network of tangled wires difficult to get through, easily concealed, and difficult to remove.

The high wire entanglement is made by driving stakes so that they protrude from 4 to 6 feet above the ground. They are placed at irregular intervals 5 to 8 feet apart. The head of each stake is connected with the foot of adjoining stakes with the wire loosely drawn, wound around the stakes and stapled fast. Each center post should be stayed by four wires. There should be a trip wire about 9 inches from the ground all the way across the front and another about a foot from the top of the center posts. Barbed wire may then be hung in festoons throughout the entanglement, with no fixed pattern. To increase the entanglement wire may be stapled to the foot of the posts, as indicated in the paragraph above, before they are driven. Large nails should be driven in the tops of the posts with half their length protruding. A number of the wires in the entanglement should be fastened together where they cross. The wire should be passed through paint, if practicable, to take away the bright color. The posts should be painted the color of the surrounding country. Under the conditions encountered on the western front this work has to be done hastily. It is best, therefore, to limit the first stage of construction to just so many strands as will form a nucleus for the whole entanglement, in order that the area may be covered by an obstacle before interruption occurs.

Tight wires help the enemy's advance by forming supports for hurdles. It must be constantly borne in mind that the wires must not be stretched taut.

A portable wire entanglement is constructed by stretching wire loosely around a wooden framework, either circular or square or made on a knife rest, and rolling it into position to close up gaps that may have been made in the entanglement. The illustration shows the wooden framework.

The ordinary repairs to entanglements are made under cover of darkness by working parties detailed for the purpose. Iron posts that can be quickly placed in position are advantageous, their disadvantage being that they may retard bullets that would go through the ordinary wooden posts, thus furnishing just that much cover and protection to attacking parties.

In the construction and repair of entanglements care must be taken to see that they are firmly fastened into the ground with numerous stay posts or "deadmen." This is to prevent the enemy from pulling them to pieces with grappling hooks connected to ropes that lead to his trenches and are attached to powerful windlasses or capstans.

Barricades are employed for the defense of streets, roads, bridges, etc. They may be made out of any available material such as furniture, vehicles (overturned or with wheels removed), carts filled with stones, bales of goods, etc.

Where trees grow along the roadside they may be felled across the road. If necessary, barbed wire may be run through the branches to make the passage more difficult.

Barricades should not as a rule close the road entirely to traffic. Passages are required to allow the defenders to pass through when it is necessary to do so. Hence they should be made in two parts, one overlapping the other, as shown in the illustration.

A fougass is a mine so arranged that upon explosion a large mass of stones is projected against the enemy. An excavation is made in the shape of a frustum of a cone, inclining the axis in the direction of the enemy so as to make an angle with the horizon of about 45 degrees. The sides splay outward slightly. A box of powder is placed in a recess at the bottom. This is covered with a platform of wood several inches thick, on which the stones are piled.

The fuse is placed in a groove cut at the back of the excavation, or the mine may be exploded by means of electricity.

The line of least resistance for the charge must be arranged so that the powder will act in the direction of the axis and not vertically. This is accomplished by throwing the excavated earth on the crest towards the defender's side and ramming it well.

To ascertain the powder charge for any fougass, divide the number of pounds of stone in the charge by 150. This gives the number of pounds of powder in the powder charge. Thus a fougass charged with about 70 pounds of powder will throw about 5 tons of stone over a surface about 160 yards long and 120 yards wide.

When broken up a cubic foot of stone weighs about 100 pounds.

A vertical type of fougass is also shown. A charge of 25 pounds of powder should scatter a cubic yard of stones over an area about 200 by 100 yards.

Small Land Mines

Land mines are placed in the line of the advance of the enemy and exploded either by electricity or fuse from the defense. They are made by digging holes from 2 to 3 yards deep, either by excavation or by boring. In the former case the charge is placed in a recess which extends into the solid earth at the side of the hole, which is then refilled and tamped. In the latter case the charge is placed in the bottom of the hole, which is then refilled and solidly tamped. In common earth the powder charge for a 2-yard hole is 25 pounds. That for a 3-yard hole is 80 pounds. The diameter of the crater formed will be about twice the depth of the charge.

The mines may be arranged in one or more rows. The intervals between mines should be such that the craters will nearly but not quite join. The position of the mines should be concealed as much as possible and further sophisticated by disturbing the ground slightly at points where there are no mines and so situated as to suggest a systematic arrangement.

Inundation

Backing up the water of a stream so that it overflows a considerable area forms a good obstacle, even though of fordable depth. If shallow, the difficulty of fording may be increased by irregular holes or ditches dug before the water comes up, or by constructing wire entanglements in the water. It may be employed with advantage when the drainage of a considerable area passes through a restricted opening, as a natural gorge, culvert or bridge.

Open cribs filled with stones, or tighter ones filled with gravel, may form the basis of the obstruction to the flow of the water. The usual method of tightening spaces or cracks between cribs is by throwing in earth or alternate layers of straw, hay, grass, earth, or sacks of clay. A continuous construction, as shown in the illustration, may be employed. The ends of the dam must be carried well into the solid earth to prevent the water from cutting around them. This type of dam is easily destroyed by artillery fire, and cannot be depended upon.

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