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 fire 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.
[Illustration: Plate 3.--Abatis.]
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 or 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_.
[Illustration: Plate 4.--Slashing.]
WIRE ENTANGLEMENTS
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 post 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.
[Illustration: Plate 4a.--Plan of wire entanglement.]
[Illustration: Plate 5.--High wire entanglement.]
_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._
[Illustration: Plate 5a.--Alarm trap.]
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.
[Illustration: Plate 6.--Portable entanglement. Constructed in the
trenches and rolled into position.]
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
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.
[Illustration: Plate 7.--Plan of barricade for blocking a road.]
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 frustrom 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.
[Illustration: Plate 8.--Fougass.]
[Illustration: Plate 9.--Vertical fougass.]
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.
[Illustration: Plate 9a.--Land mine.]
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.
[Illustration: Plate 10.--Dam construction.]