Civil engineer guide to fighting positions, shelters, obstacles

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AFH 10-222 Volume 14 1 August 2008 28 2.5.1.5. Fighting Position Revetments. In wet, sandy, or poorly compacted soils, revetments are used to reinforce the sides of fighting positions and trenches to prevent collapse (Figure 2.5). Revetting is also necessary when positions are dug in soils with large amounts of clay or high plasticity as these soils may liquefy, displace, or shear from ground shock. Without revetments, the walls of a position in sandy soils would have to be sloped to achieve a certain depth. However, this is not recommended since the top opening of the position would be too wide, making it difficult to incorporate overhead cover. Revet fighting positions using material held in place with either braced or tied pickets. Pickets can be lumber, wooden poles, standard concertina wire pickets, metal poles, angular posts, or trees. Wooden pickets from trees or saplings should be at least 3 inches in diameter. The pickets are used to hold matting material (i.e., brushwood mats, lumber, wire mesh, geotextile fabric, cloth mat, etc.) against the soil face. More rigid facing materials include plywood, landing mats, corrugated metal, and timbers. Use the field expedient procedures outlined in Attachment 3 to classify soils and determine whether revetments are needed. Figure 2.5. Fighting Position Revetment.
AFH 10-222 Volume 14 1 August 2008 29 2.5.1.6. Crawl Trenches. If time is available and the overall defensive posture needs to be enhanced, crawl trenches can be constructed to connect fighting positions and prevent personnel from being exposed to enemy fire as they move about the area. The depth of these trenches is based on the availability of time and resources. Engineers will have to assist others with excavation and trench reinforcement (Figure 2.6). Crawl trenches should be dug in a zigzagged pattern to prevent the enemy from being able to fire down a long section if the trench is breached. This pattern will also help to reduce the effects of fragmentation if the crawl trench is hit with explosive munitions. 2.5.1.7. Ground Shock Intercept Trenches. Since fighting positions might be needed in areas subject to high levels of ground shock based on certain soil or geological conditions, civil engineers must have some general knowledge of geology and different soil types. Ground shock sufficient enough to collapse fighting positions can occur when these positions are constructed over water tables too close to the bottom of the positions or in areas where the soil is located within 25 feet of and/or over an underlying rock formation, moist clay, or soils with high water content. Ground shock is intensified by reduced resistance through the soil or reflection from the underlying soil and rock formations. One way to reduce the effect of ground shock on fighting positions involves constructing ground shock intercept trenches. Ground shock intercept trenches are meant to collapse from ground shock and thereby absorb enough of the ground shock to prevent fighting positions from collapsing. These types of trenches are dug approximately 12 inches wide to a depth that is at least 2 feet deeper than the fighting positions they are intended to protect. They should also be at least 6 feet longer than the width of the fighting position. Locate the trenches away from fighting positions at a distance approximately 1 1/2 to 2 times the depth of the trench, as illustrated in Figure 2.7. Due to their narrow width, these trenches can easily be camouflaged. Table 2.2 can be used as a checklist to ensure revetments, crawl trenches, and ground shock intercept trenches are being properly constructed.
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