German Winter Field Fortifications and the Use of Ice-Concrete, WWII Tactical and Technical Trends, No. 22: April 8, 1943 (Lone Sentry)

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.

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a. General

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.).

b. Camouflage

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 cold weather.

(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.

[German Winter Field Fortifications, Figure 1]
FIG. 1

(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.

[German Winter Field Fortifications, Figure 2]
FIG. 2

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 employed:

(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);

[German Winter Field Fortifications, Figure 3]
FIG. 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.

[German Winter Field Fortifications, Figure 3]
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.

f. Drainage

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.

g. "Ice-Concrete"

(1) Definition

Ice-concrete is a dense, frozen mixture of sand and water, or sand with gravel or broken stone and water.

(2) Application

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.

[German Winter Field Fortifications, Figure 6]
FIG. 6

(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.

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.