From the Eastern Front comes a report of the German type of winter
field fortifications and shelters, with a description of an effective concrete made
of a frozen sand, or sand with broken stone, and water mixture.
* * *
Construction of field fortifications in winter presents a number of
special difficulties due to cold, frozen ground, ice, and snow which may
occasionally reach a depth of several meters. The men's capacity for work is moreover
lowered by extreme cold. For this reason allowance must be made for a
considerable increase in time and personnel requirements, often amounting to many
times the normal. Special tools and equipment suitable for work under winter
conditions must be obtained well in advance.
The depth to which ground is frozen on the Eastern Front often reaches
1.5 meters (5 ft.).
In snow-covered terrain, special attention should be paid to concealment
against ground and air observation. Paths caused by trampling, ditches, working
sites, etc., can be recognized from the air with particular ease. For this
reason, before beginning work snow should be cleared to one side so that it may
be available for subsequent camouflage, and finished work must again be covered
with snow. Trenches can be covered with planks, beams, pine branches, or
sheet-iron, on which snow should be heaped.
c. Construction of Shelters, Trenches, and Breastworks
(1) Construction of Earth Shelters in Frozen Ground
(a) In the presence of the enemy, for speedy and silent preparation of
shelters in frozen ground, sandbags are used; for this purpose, canvas rather
than paper sacks are to be recommended. Sandbags are filled in the rear, and
carried forward to the point where they are to be used. Freezing sandbags by
pouring water on them improves their protective properties for the duration of
(b) Where the tactical situation permits unimpeded work, the following
practice is adopted. In constructing trenches in ground which is not frozen to
a great depth, in order to avoid the labor of digging through the frozen ground,
the surface is divided up by furrows into the desired sections. These sections
are then undermined, and the frozen crust is caved in and removed. For this
work heavy pickaxes, crowbars, iron wedges, etc., are necessary.
Deeply frozen ground can be broken up by engineers using power drilling
equipment (concrete breakers driven by portable compressors) and explosives.
Holes for explosives can be made in frozen ground by driving in red-hot, pointed
iron rods or crowbars. In excavating trenches in deeply frozen ground, the best
method is to dig holes at an interval of several feet down to the full depth of the
trench; these holes are subsequently connected by tunnels under the frozen
surface, and finally the surface is caved in.
(2) Construction of Shelters in Snow and Earth
If the depth of snow is great, fieldworks must be constructed partly in
snow and partly in the ground. Small shafts are sunk to the full depth planned
and are then connected by trenches dug in the snow. The deepening of these
trenches into the ground can be carried out later. If there has been only a short
frost before the snowfall, the ground will be found to be only slightly frozen,
since the snow acts as a protective layer against hard freezing.
(3) Construction of Shelters in Snow
If heavy snowfall is to be expected, or if time is short, or if equipment
for excavation of frozen ground is not available, breastworks of snow can be
erected on the surface. Snow, if it is to be used as protection against enemy
fire, must be tamped solid. It must also be camouflaged by scattering loose
snow over it. Its effectiveness as a protection is raised by pouring water over
it. The rear side of the breastwork should be revetted with sandbags filled
with snow; canvas rather than paper should be used for this purpose. Alternative
materials are round timber, wire netting, or wooden planks secured to posts,
like a fence.
If it is impossible to drive in or anchor the posts, simple trestles of
triangular cross section should be erected at intervals of 5 to 6 1/2 feet, as shown
in figure 1. The best practice is to carry the trestles ready-made to the site
where they are to be used. After adding the revetment and bearing planks, snow
is shoveled over it and tamped hard. The center of the snow wall can be formed
of any other suitable material: e.g., round timber, stones, gravel, sand, etc.
(4) Protective Qualities of Snow and Ice
The following are the thicknesses of snow and ice which afford protection
against ordinary rifle fire, but NOT against fire concentrated on a single point:
New snow (minimum) 13 ft
Tamped snow (minimum) 8 - 10 ft
Frozen snow (minimum) 6 ft 6 in
Ice (minimum) 3 ft 3 in
(5) Covered Trenches
Trenches can be covered over to protect them from snowing-up, and to
conceal them, as shown in figure 2. The cover of round timbers, sawn timbers,
planks, or beams must be strong enough to carry the maximum weight of snow
that can be expected.
d. Tunneling in Snow
If the snow is sufficiently deep, tunnels can easily be constructed. They
do not provide effective protection against artillery fire, but this disadvantage is
considerably outweighed by the complete concealment they afford. The method
of construction varies according to the condition of the snow, which may be new
(powdery), already frozen, and of varying depths. The following are the methods
(1) Digging in from the surface and covering over with planks and layers of snow;
(2) Digging in from the surface and the construction of sheeting or revetting
with planks, beams, brushwood, or sheet-iron;
(3) Underground tunnelling, construction of wooden sheeting or revetting
with planks, beams, or brushwood (figure 3);
(4) The construction of tunnels without sheeting (figure 4). In long
tunnels, ventilation must be provided by ventilation shafts, as shown in figure 5.
|FIG. 4||FIG. 5|
e. Construction of Shelters, Covered Positions, Positions for AT or
Infantry Guns, and Ammunition or Supply Shelters
The same methods are used as in construction in the ground. The floor
and walls should be constructed with particular care, and the roofing formed of
planks or beams. In addition, roofs and walls must be covered with roofing felt,
which should also be laid under the floor. Inner insulation must also be
provided by mats and straw, layers of wool, or sacking, and cracks should be filled
with moss, sod, or straw. Another effective method of building walls is to use
a double revetment of planks with a heat-insulating space between them. The
revetment of intermediate space is necessary not only as protection against
cold, but also to avoid the melting of the snow by internal heating stoves, etc.
Doors and entrances should be small and well fitting. Even if shelters are
unheated, a snow covering of sufficient thickness will raise the temperature in
shelters of this kind to 3 to 5° C (37 to 41° F). Owing to their slight insulating
properties, sheet-iron side walls are suitable only for excavations which are not
to be occupied by personnel.
When a thaw sets in, special provision must be made for draining away
water, and this should be provided, when the position is first constructed, by
ditches and other methods. Crawl-trenches and tunnels must be built with a
gradient sufficient to drain the water away. Failure to observe these precautions
will quickly result in the flooding of the excavation and the caving-in of the
weakened and undercut walls.
Ice-concrete is a dense, frozen mixture of sand and water, or sand with
gravel or broken stone and water.
Ice-concrete is especially suitable for reinforcement of breastworks and
for the construction of roofs and shelters. An example is shown in figure 6 on
the following page. Ice-concrete can be protected for a considerable period
against effects of rising temperature by being covered with earth.
(3) Strength and Composition
Ice-concrete is many times stronger than normal ice. Regarding its
composition, experience is as follows:
(a) A high proportion of fine sand increases the strength. The strongest
mixture of all is composed of sand alone.
(b) If insufficient sand is available, gravel or broken stone can be used.
The proportion of fine sand should, however, not fall below 10 percent.
(c) A small proportion of topsoil, clay, or mud is not injurious.
(d) Only as much water should be added as the mixture is capable of
absorbing, and as will cause it to become slightly liquid.
(4) Preparation of Ice-Concrete
(a) In preparing the mixture by hand, it is shovelled over, if possible
in a trough, and the water added gradually; or, mixing can be done in a concrete
mixer. The wet mixture is immediately poured into the forms. This operation
is carried out in layers of from 4 to 6 inches, accompanied by tamping, in order
to consolidate the mixture.
If gravel is used, the material is pre-mixed without adding water. The
mixture is then poured into the forms in layers 4 to 6 inches thick, and water is
poured in to complete saturation, accompanied by stirring and tamping.
(b) In both cases successive layers should be added as soon as the
previous layer is beginning to freeze. Freezing takes place more slowly if the water
is added later. In order to hasten the process of freezing, sand and gravel should
be already at a freezing temperature before water is added, and the water itself
should be as cold as possible. If the material is frozen in large lumps, it should
be broken up before mixing.
(c) Ordinary wooden forms should be used, but snow, ice, earth, straw,
or brushwood can also be used for this purpose.
As a protection against warming from inside (i.e., heating by stoves,
etc.), the inner forms are left standing. The outer forms should be removed
as soon as possible in order to hasten freezing.
(d) As a guide, it may be noted that a sheet of ice-concrete, 4 inches
thick, will be completely frozen at a temperature of 13 degrees below zero, F°,
in 4 to 6 hours.