Civil engineer guide to fighting positions, shelters, obstacles

AFH 10-222 Volume 14 1 August 2008 57
3.2.3. Fighting Bunker With Overhead Cover. It is impractical to place
enough earth cover on a structure to protect it against all weapons. However,
by combining materials and using them in layers, protection is provided with
less excavation and effort. The designs illustrated in Figure 3.3 through
Figure 3.5 use five 2-inch or seven 1-inch layers of lumber as the base support
for the roof. Layers of materials are placed on the base in sequence to
provide overhead protection. Table 3.1 and Table 3.2 contain a bill of materials
and required stringer thickness based on the thickness of earth cover for
this design. These different layers are described in the following paragraphs.
3.2.3.1. Dust-Proof Layer. Tar paper, canvas, or tarpaulins are lapped and
placed on top of the base of the roof to keep out dust and dirt.
3.2.3.2. Cushion layer. The cushion layer is intended to absorb the shock of
detonation or penetration. Untamped earth is the best material for this purpose
and should be at least 12 inches thick. Materials such as loose gravel
transmit shock to the layer below and should not be used in the cushion layer.
This layer extends on all sides for a distance equal to the depth of the shelter
floor below the ground surface, or to a minimum of 5 feet.
3.2.3.3. Waterproof Layer. The waterproof layer is constructed of the same
or similar materials as the dust-proof layer. It is intended to keep moisture
from the cushion layer in order to retain the cushioning effect of the soft, dry
earth and minimize the dead load the structure must carry.
3.2.3.4. Burster Layer. The burster layer is intended to cause a projectile to
detonate before it can penetrate into the lower layers. This layer is made of 6
to 8 inches of rocks placed in two layers with the joints broken. It should be
at least 12 inches thick. Irregular-shaped rocks are more effective for this
purpose than flat rocks because they can change a projectile’s angle of penetration.
If rocks are not available, 8-inch logs can be used. They must be
wired tightly together in two layers. The burster layer should extend on each
side of the shelter a minimum of 6 feet. Lastly, cover the burster layer with
two inches of untamped earth or sod as a camouflage layer.