CHAPTER 14

FORTIFICATIONS FOR PARKED ARMY AIRCRAFT

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This chapter provides information to assist in the selection design construction, and maintenance of fortifications to protect parked aircraft from hostile ground fire and the associated damage effects of exploding fuel and ammunition on or near the aircraft. Details of other types of fortifications are in FM 5-103 and TM 5-302-1. This chapter applies to nonnuclear warfare only.

AIRCRAFT FORTIFICATIONS

Aircraft fortifications generally mask the lower parts of the aircraft and provide limited protection to the upper parts. For planning purposes, the protection of aircraft provided by fortifications does not consider overhead protection or structures built to the height required to mask the upper portions of rotary-wing aircraft. However, the fortification plans in this chapter can be adapted to different situations provided large protective structures can be constructed.

Several basic types of fortifications are discussed that satisfy various weather, topographical, and military considerations, including those that can be constructed with hand-tools organic to tactical units. Note that substantial quantities of earth or other protective materials are required to achieve minimum protection against all types of ammunition, including small arms.

FORTIFICATION PLANNING

When planning the construction of aircraft fortifications, engineers must consider the following items:

• Thickness and ballistic data.
• Weather and topography.
• Military considerations.
• Protective materials.

THICKNESS AND BALLISTIC DATA

Tables 14-1 and 14-2, show tabulated thickness data summarized from the ballistic data in the graphs from Appendix P. Table 14-1 shows the thickness of material to defeat the fragmentation from the weapons shown, and Table 14-2 shows the thickness of protective materials required to resist penetration by various types of ammunition. The ballistic data and list of materials are incomplete. However, facilities may be designed to withstand the effects of other ammunition by using this chapter as a guide.

The graphs represented in Figures P-1 through P-11, provide detailed ballistic data for different types of ammunition under Condition I, when no standoff is used. Each graph pertains to 1 of the 11 types of protective material discussed. Figures P-12 through P-22, provide penetration data under Condition II when a shell standoff is used. Figures P-23 through P-33, provide penetration data under Condition III, when a wooden standoff is used. Figure 14-1 shows the three standoff conditions.

WEATHER AND TOPOGRAPHY

The susceptibility of earthwork to the influence of heavy rains or other extreme weather conditions affects the construction of earth revetments. Erosion over an extended period reduces the resistance of penetration. Periods of wet weather produce soil moisture that is generally high and changes the strength of materials. Soils with a high moisture content have very little strength or resistance to penetration. Dampness adversely affects most protective materials, including wood and steel. These materials require treatment or protection against deterioration for prolonged use.

Consider the topography of the area near an airfield when determining the protective requirements for parked aircraft. For example, high ground within 3,500 meters that offers good observation for effective mortar or direct fire may negate the success of fortifications unless an active perimeter defense with effective counterfire is provided. Similarly, wooded areas, villages, or other sites that permit concealment close to parked aircraft enable guerrillas and saboteurs to assemble. In such cases, both active and passive defense fortification measures are required.

MILITARY CONSIDERATIONS

The effectiveness of fortifications and other passive defense measures is substantially increased by an active perimeter defense against infiltration, sabotage, or similar tactics. Therefore, it is best to confine protective construction to an area that can be adequately defended.

Dispersion and Camouflage

If dispersal of aircraft is possible and consistent with active defense measures, varied parking patterns provide fewer lucrative targets for indirect-fire weapons. Prefabricated, hard parking surfaces such as landing mats increase lethal areas of bursting rounds due to induced fragmentation. Effects of other hardened surfaces, such as bituminous materials and concrete, are unknown but probably increase fragment success as well. Reduced damage from indirect-fire attacks should result when parking areas can be adequately maintained on sod or on a surface that does not cause fragment ricochet. Conceptual layouts of airfields that provide random dispersal of parking areas are shown in Figures 14-2 through 14-4.

Fortifications should be considered in their relationship to, and as a means of augmenting, other forms of protection, including dispersion, camouflage, and active defense measures. The use of protective structures for parked aircraft should increase the combat effectiveness of the unit as do similar measures that protect personnel and weapons. The type of fortifications constructed is governed by the tactical situation: enemy capabilities; the availability of materials, construction equipment, and personnel to accomplish the required work; and an available area for construction.

Weapons Capabilities

Fortifications should be capable of resisting penetration by the most effective type of ammunition to which the structure is likely to be exposed. Table 14-2, shows that 120-millimeter mortar and 107-millimeter rifle ammunition generally are most effective against the materials considered. A steel standoff shown in Figure 14-1 reduces the effectiveness of most ammunition by detonating it before it strikes the fortification. Consequently, a lesser thickness of protective material can be used with a standoff than without one. The data in Table 14-1, is based on a penetration resistance factor (PRF) of 1.00. This figure is used for comparative purposes because it provides the minimum thickness of material required to resist penetration by a given type of ammunition at optimum range. The graphs in Appendix P are plotted to show the thickness of material required for PRFs that are smaller or larger than 1.00. The graphs provide a means of estimating the amount of protection afforded when material thicknesses differ to meet local specifications for protection.

NOTE: PRFs of 1.00 or more are effective. PRFs of less than 1.00 are ineffective. For economy of materials, design structures with a PRF slightly larger than 1.00.

PROTECTIVE MATERIALS

The selection of construction materials for fortifications is influenced by availability. The protective qualities of the following materials are described below:

• Soils.
• Soil cement.
• Concrete.
• Timber.
• Steel.
• Asphalt sandwich.
• Plywood.
• Ice, snow, and ice concrete.
• Expedient fortification materials.

The techniques of construction, preservation, and repair or rehabilitation are described later in this chapter.

Soils

Dry soil resists penetration better than wet soil. Table 14-2, indicates that the thickness of wet soil must be approximately double that of dry-soil requirements to resist penetration by a given type of ammunition. It is best to select dry soil for earth revetments and to provide a waterproof cover for the completed earth structures to conserve manpower and materials. An expedient test to determine moisture content in soils is to observe the reaction of a handful of soil when squeezed into a ball. If it retains the shape of a ball, consider it a wet soil. If it fails to adhere, consider it a dry soil. Wet clay is the most susceptible to ballistic penetration and is the least effective fortification material. Dry sand has the most resistance to penetration and is the most desirable soil for fortification.

Soil Cement

Table 14-2 indicates that soil cement is highly resistant to mortar and ball ammunition but considerably less resistant to recoilless rifle ammunition. Prepare soil cement by mixing 1 part by weight of portland cement with 10 parts by weight of dry earth or 6 parts by weight of sand-gravel. When placed in sandbags, the cement sets as the bags take on moisture. This procedure prolongs the useful life of sandbags, which normally deteriorate quite rapidly, particularly in damp climates. Filled sandbags also may be dipped in a thin mixture of cement and water. To produce cement in large quantities, follow the procedures described in FM 5-742.

Concrete

The characteristics, mixture, placement, reinforcement, and curing of concrete and the construction of forms are explained in FM 5-742. Concrete construction should not be undertaken except under qualified supervision to avoid uneconomical use of critical materials.

Timber

Timber can be used as a retaining wall for earth revetments. In addition to support, it contributes to the effective resistance of the fortification. Timber used for this purpose may be either hard or soft but should be free of knots and other imperfections that affect its rigidity or resistance to penetration. When used against earth, treat timber with a preservative such as tar or creosote to prolong its usefulness. Wood such as bamboo may be used for retaining walls if woven into mats and adequately supported, but it has no effective resistance to penetration.

Steel

Steel may be available in forms such as corrugated metal, sheet piling, or pierced landing mat. Consider the thickness of these materials used to retain earth revetments when determining its resistance to penetration. If the material has holes larger than 1/2 inch, disregard its resistance to penetration.

Asphalt Sandwich

This asphalt mix is made into a sandwich between sheets or plates by pouring it into forms. A 2-inch thickness of asphalt mix gives considerable protection from small-arms and conventional-weapons fire. Field-expedient asphalt mixes can be used but are not as effective as hot plant mixes. The fragment-defeating capability is directly related to the aggregate size used in the asphalt mix. The most effective size is coarse aggregate 1/4 inch or larger. To increase the PRF, use layers of asphalt sandwich instead of a greater thickness of asphalt. The asphalt can be left in the forms and installed as protection if the asphalt alone is not sufficient. Asphalt sandwich plates are most effective when at least one plate is left attached to the form on the friendly side. A 2-inch asphalt sandwich of 60 percent aggregate, 30 percent mineral filler, and 10 percent asphalt binder (by weight) with attached 26-gauge steel sheets is 100 percent effective at 30 feet from the detonating point to the wall. These protective walls must be braced and anchored to resist blast effects.

Plywood

One or more layers of plywood make an effective field-expedient protective wall. As more layers of plywood are added, the amount of protection increases. Table 14-3 shows the effectiveness of up to three layers of 3/4-inch fir plywood. Although three layers of plywood stopped a high percentage of fragments from all munitions shown, there is still a large number of lethal fragments perpetrating the plywood. Brace and anchor the plywood to provide stability against blast and aircraft movement. If time allows, form the plywood into a box-type structure and fill it with soil. This will increase protection considerably above that furnished by the plywood layers.

Ice, Snow, and Ice Concrete

Ice concrete is a dense, frozen mixture of sand and water or sand with gravel, crushed rock, and water. At least 10 percent of the mixture should be sand. Add only enough water to make the mixture slightly liquid. A sheet of ice concrete that is 4 inches thick will freeze solid in four to six hours at -13°F. It may be used for overhead cover, parapets, breastworks, or as a sandbag filler. Minimum thicknesses of snow and ice for protection against small arms are as follows:

New snow	13 feet
Tamped snow	8-10 feet
Frozen snow	6.5 feet
Ice	3.25 feet
Ice concrete	1-2 feet

Expedient Fortification Materials

A suitable retaining wall for sandbag or earth revetments is constructed using landing mat supported by wire rope and pickets. Bulkhead-type fortifications are also constructed with these materials.

Corrugated metal, if available in sufficient quantity, is a satisfactory substitute for revetment retaining walls and bulkhead-type fortifications. Additional quantities of wales and vertical supports to withstand the pressure of the earth fill are required to correct its lack of rigidity.

Ammunition boxes filled with earth provide limited protection. They can be used as a retaining wall or bulkhead if they are adequately supported with wales and vertical supports.

A substantial bulkhead fortification is provided by using Conex-type containers filled with moderately dry sand, gravel, or soil. An example of this expedient is shown in Figure 14-5.

Unserviceable 55-gallon drums can be stacked in different configurations and then filled with sand to provide limited protection. Drums can be stacked for extra height but they must be welded together. Run a steel angle or pipe the length of the wall and weld it to each drum for added stability. Weld each level to the level below it.

Sand grid can be used in layers to construct fortifications. Backfill material should be dry and cohesionless. A bituminous coating can be sprayed over the structure to limit water penetration.

Combining Materials

A more effective and substantial fortification usually results if availability and construction skills permit the combined use of two or more materials. For example, the use of timber and soil (dry soil) without standoff may be considered by referring to Table 14-2, or the graphs in Figures P-2 and P-9, for the different materials shown. The 8-inch timbers provide a PRF of 0.3 (8-inch actual thickness divided by the 27 inches of timber given in Table 14-2 as adequate to resist penetration) against 81-millimeter mortar ammunition. The total PRF must equal 1.00, so a sufficient thickness of dry soil must be added for an additional PRF of 0.7 (1.0 - 0.3 = 0.7). Because a single thickness of 60 inches of dry soil will resist penetration by 81-millimeter mortar ammunition, 42 inches (0.7 x 60 = 42) of dry soil must be combined with 8-inch timbers for a PRF of 1.00. This design, featuring a combination of materials, represents a savings in materials and manpower and reduces the areas required for the structure.

FORTIFICATION DESIGN AND CONSTRUCTION

Space is a limiting factor that affects airfield size, type, configuration, and the layout of fortification. Therefore, it is necessary to ensure the airfield area, the anticipated aircraft population, the duration of occupancy, and the area adjacent to the airfield available for dispersal of aircraft are consistent with the tactical situation. Each airfield presents problems in one or more of the above areas for which general guidelines apply. The type of fortification may require modification if there is insufficient space for aircraft dispersal. For example, some areas of the airfield may permit construction of one type of fortification, while other areas may only permit construction of less protective fortification.

Topography such as a deep stream immediately adjacent to the field may minimize the protection required. Maximum protection consistent with mission requirements and available resources should be the guiding consideration in aircraft fortifications.

DESIGN CRITERIA

The size and shape of a revetment are important in determining the total effectiveness of a revetment system. The height of a revetment system is critical. For example, if a fly-in/fly-out capability is necessary for a utility helicopter revetment, the height and effectiveness of the revetment system is limited. Inside dimensions of fortifications necessary to accommodate different types of Army aircraft are listed in Table 14-4. The dimensions given provide limited clearances for aircraft movement and servicing. The area inside the fortification should be the minimum required to tow the aircraft into the fortification and to avoid restricted movement and servicing. The inside area of the fortification also should provide for the largest aircraft in use. Coordinate the inside dimensions of fortifications for Air Force aircraft with the Air Force to satisfy their need.

Designing for Effects of Ammunition

Besides providing protection from hostile ground fire, fortifications should be arranged and spaced to minimize the explosive effects of bulk ammunition stored within the fortifications or on the aircraft. A shell, grenade, or other charge exploded near bulk quantities of ammunition normally sets off a chain reaction that damages or destroys several aircraft. Fortifications can reduce these interacting explosive effects. To determine ammunition effects, estimate the equivalent explosive weight of the ammunition on and near aircraft within a proposed aircraft fortification area. Equivalent explosive weights are found in Table 14-5. Include the explosive weight of the hostile round in the total explosive weight if it is a significant percentage of the total. Apply the computed weight to the graph (Figure 14-6) to determine theoretically safe distances with or without a protective barrier or wall between aircraft, Figure 14-7, shows how to orient fortifications to provide safe distances. Two intervening walls are required between the protected aircraft and the explosive before any reduction in safe distance is obtained.

Designing For Effects of Fuel

The destructive force of exploding fuel is considerably less than the force resulting from exploding ammunition. Protective measures against ammunition with an explosive weight of 100 pounds or more compensate for fuel explosions in the same area. If ammunition or fuel is present, the distance between aircraft should not be less than 85 feet when there are two intervening walls or not less than 150 feet when there are less than two walls. Slope the floor of the fortifications to control the direction of flow of spilled burning fuel. If burning fuel flows under other aircraft, the heat could result in additional explosions.

Estimation Of Weapons' Effects

Intelligence estimates should disclose the types of ammunition against which protection is required. Reconcile the effects of ammunition with soil conditions or moisture content because high-velocity ammunition has more penetration effect against wet or damp soil than it has against dry material. Other factors being equal, provide protection against the type of ammunition having the greatest penetration potential, (See Tables 14-1, 14-2, and Appendix P.)

The essential factors in fortification design are the most effective type of ammunition in common use and the resistance of the protective material available for fortification purposes. The relationship between these two factors has been reduced to the PRF previously defined. A factor of 1.00 provides the theoretical minimum thickness of a given material to resist penetration by the types of ammunition shown in the graphs (Appendix P) under the three construction conditions.

Selection of Materials

If a choice of materials is available, base the selection on the protective characteristics of the different materials or a combination of the materials that will resist penetration by the most effective type of ammunition expected. Other considerations include handling methods, appropriate equipment, labor skills, and the type of fortification being constructed. Use Table 14-6, to estimate the quantities of material required for different types of fortifications. Table 14-6 also states waste factors.

Spacing and Configuration of Fortifications

Fortification spacing should provide an arrangement of individual aircraft protective structures that ensures access to the aircraft for efficient servicing, maintenance, and tactical operations. Anticipated active defense measures for the area are an important consideration in this regard.

Dispersal of aircraft is contingent primarily on the available area. Dispersal should cause the aircraft to be separated sufficiently to minimize the danger of interacting ammunition and fuel explosions. Avoid any consistent pattern that facilitates adjustment of high-angle fire on the aircraft. Conceptual layouts of aircraft dispersal are shown in Figures 14-2 through 14-4.

Fortification Design Example

Fortification for 10 aircraft, type UH-1H, is required to provide maximum protection with available materials. Given--

• Known weapons in use:

-- .50-caliber machine guns
-- 90-millimeter recoilless rifles
-- 60-millimeter mortars

• Available materials:

-- 2-inch sheathing-12,000 square feet (retaining wall)
-- 2-inch x 4-inch lumber-14,000 linear feet (vertical supports)
-- Accessory materials, including wire cable, clamps, nails, and bolts
-- Dry sand for filler

• Aircraft armament:

-- 7.62-millimeter ammunition-100 pounds
-- Rockets-200 pounds

1. Determine a preliminary fortification design for a gravity revetment (condition I-no standoff) to provide full lateral protection. See Figures 14-8 through 14-10, for sketches of revetments.

2. Determine the dimensions of a full protective structure for UH-1H aircraft. from Table 14-4.

Width = 58 feet
Height = 16 feet
Length = 68 feet

3. Estimate required materials using Table 14-6. Material available after computation of waste factor.

Sheathing-20 percent waste factor = 80-percent usable material = 0.80 x 12,000 square feet = 9,600 square feet.

Vertical supports-10 percent waste factor = 90 percent usable material = 0.90 x 14,000 linear feet = 12,600 linear feet.

Linear feet of sheathing required for retaining wall:

(Width (W) + 2L) x number of aircraft = {58 + (2 x 68)} x 10 = 1,940 linear feet or 194 10-foot sections.

Sheathing area for gravity revetments:

10 height (H) per 10-foot section = (10 x 16) x 194 = 31,040 square feet.

NOTE: The amount of sheathing required for this type of protection exceeds the amount available (12, 000 square feet). Therefore, the design must be adjusted accordingly. Partial lateral protection (Figure 14-9) is one possibility. This will protect each side of the aircraft and leave the ends of the fortification open.

4. Revise the fortification design using the improved protection dimensions in Table 14-4.

Width = Not applicable (NA)
Length = 52 feet
Height = 9 feet

5. Repeat the material computations shown for preliminary design-gravity revetment, Condition I.

Linear feet of sheathing required: (Number of aircraft x 2 x L) = 10 x 2 x 52= 1,040 linear feet or 104 10-foot sections.

Sheathing area:

10H per 10-foot section = (10 x 9) x 104 = 9,360 square feet < 12,000 square feet.

Vertical supports (dimensional lumber, using the formula in Table 14-6):

11(H + 2) per 10-foot section = 11(9 + 2) x 104 = 12,584 linear feet < 14,000 feet.

6. Determine the thickness of protective material. Use a PRF of 1.00 (Tables 14-1 and 14-2, or Figure P-4) for 90-millimeter ammunition, which requires 42 inches of dry sand for penetration resistance. Round up to the nearest foot to yield 48 inches (4 feet).

7. Determine the volume of filler material.

= 0.4 (thickness x height (TH) + (H² /2) per 10-foot section.
= 0.4 {4 X 9 + (9² /2)} X 104 = 3,182 cubic yards.

8. Determine spacing of revetments.

Calculate the total equivalent weight of explosives from Table 14-5.

7.62 mm--100 x 0.04	= 40 pounds
Rockets--200 x 0.70	= 140 pounds
Total	= 180 pounds

Enter the vertical scale of the graph in Figure 14-6 opposite 180 pounds, then proceed horizontally to the intersection of the curves and read on the horizontal scale.

108 feet--intervening walls
170 feet--no walls between aircraft

9. The final design type and dimensions follow:

Gravity revetment, dimensioned timber:

Sheathing--

Height-9 feet Length-52 feet

Protective material:

Thickness--48 inches (top); 11-foot base, assuming a 45-degree angle of repose for sand

Fortification spacing:

108 feet--intervening walls

170 feet--no walls between aircraft

DESIGN OF THIN-WALLED REVETMENTS

Thin-walled revetments have been developed for protection of attack, utility, and cargo-type helicopters. These revetments have plywood or corrugated metal walls and contain 12 inches of soil fill. Thin-walled revetments may be post-supported or freestanding. Post-supported revetments use either timber or pipe posts and are designed primarily for protection of cargo-type helicopters. Freestanding revetments are designed for protection of utility and attack helicopters. They provide protection from fragmentation of near misses (10 meters) from mortars and artillery rounds up to 155 millimeters. Thin-walled revetments (12 inches thick) require less fill material, space, equipment, and construction time than thick-walled revetments (4 feet or more). See Table 14-4, for approximate dimensions in accordance with the degree of protection desired. Detailed construction drawings can be found in FM 5-103 and TM 5-302-1.

Post-Supported Revetments

The following points should be kept in mind when constructing post-supported revetments:

Postholes. Holes for both timber and pipe posts should be as large as practical. Alignment will be easier if a truck-mounted hole borer with a 22-inch-diameter bit is used.

Depth of posts. Table 14-7, shows the depths that posts should be sunk in soil and concrete for revetments of various heights.

Use the maximum depths in Table 14-7 for posts in soil or concrete if soil properties are unknown. For tall revetments (9 to 16 feet high), the horizontal bearing area should be increased by attaching 2- x 12-inch lumber (or larger) to both 6-inch dimensions of the 6- x 12-inch timber posts. These attachments should extend from the ground surface to the bottom of the posts. Ideally, the stabilization of the posts will be improved by 25 percent if the horizontal bearing area is increased.

Installation of Walls

Drill holes in the plywood, corrugated-metal wall revetment materials, and horizontal braces on the ground before attaching them to the posts.

Fill Materials

A scoop loader is ideally suited for placing fill material. Tamp periodically while placing the fill material to eliminate air pockets and improve the soil's resistance to fragment penetration.

Waterproofing

To minimize the moisture content, cap each completed revetment with membrane, asphalt, concrete, roofing paper, sandbags filled with a soil-cement mixture, or other waterproofing material.

Freestanding Revetment

Freestanding, plywood revetments and freestanding, corrugated-steel revetments may be anchored with arrowhead anchors, screw-type anchors, steel pickets, or wooden or steel stakes. On concrete surfaces, brace the revetments or secure the footings with weights. Safety precautions limit the height of freestanding revetments to a maximum of 6 feet. Specific designs of plywood and corrugated-steel, freestanding revetments are in QSTAG 306. M8A1 landing mat A-shaped and sand-grid revetment designs are in TM 5-302-1. Other designs are presented later in this chapter.

Prefabricated, Movable Revetments

An example of a prefabricated, movable revetment is the precast concrete unit. Precast concrete units are high in permanency, low in troop cost, easy to repair, and relocatable. Their physical characteristics vary. The height ranges from 4 to 12 feet. The width varies with the height of the unit, but the maximum width at the base of the concrete unit is 5.5 feet. The length of the precast unit is 8 feet for heights up to and including 9 feet. Note that rotor strike may occur on the utility-type helicopter if this type revetment is more than 5.5 feet. Figures 14-11 and 14-12 show two types of relocatable, precast, concrete revetments.

CONSTRUCTION PROCEDURES FOR REVETMENTS

Construction procedures in this section are for the following fortifications:

• Standoff.
• Sandbag revetments.
• Sand-grid revetments.
• Main structure, gravity revetments.
• Main structure, earth revetments.
• Main structure, bulkhead revetments.
• Main structure, freestanding wall.

Construction drawings for these revetments are found in TM 5-302-1. A quality-control checklist for revetment construction is also included.

Make the layout of a fortification with a transit, if available. Otherwise, lay out the fortification using a compass and tape.

Standoffs

The use of a standoff is optional but desirable. See Figures 14-13 through 14-16, for details.

Drainage. Install a temporary drainage system for the area during construction. Incorporate this system into the final drainage plan. See Chapter 6, FM 5-430-00-1/AFPAM 32-8013, Vol 1, for drainage details.

Assembly details. Standoff walls may be constructed using mass production procedures. For dimensional timber, assemble the walls on the ground before erecting them. The framing consists of vertical and horizontal structural members and/or temporary scabbing. Framing is assembled as the first step in retaining-wall construction. The sheathing is attached after the frame is constructed. If logs are used instead of dimensional timber in framing the wall or standoff, the exterior or exposed horizontal members are laid out first. The two rows of vertical logs are attached next. Finally, the interior horizontal members and sheathing are attached, which completes the assembly of the rigid wall for erection.

After the sheathing is attached to the frame, apply the waterproofing agent, mark anchor points on the walls, and adjust the anchorages.

Postholes. Dig postholes and construct anchor points concurrently. Temporary anchorages may be required until construction of the main wall is completed.

Erection. After each section is completed, transport it to the erection site and tilt it into position. Place each section so that a gap of about 1/2 inch is left between it and the adjoining section to facilitate repairs and reduce combat damage.

Anchorages. Loosely attach anchor cables and supports and align the wall. Once aligned, tighten the cables and supports and backfill the postholes, preferably with concrete. Figures 14-17 and 14-18, show anchorage details.

Final drainage. Inspect, repair, or improve the temporary drainage structures and incorporate them into the permanent system.

Sandbag Revetments

Revetments constructed with filled sandbags are a practical expedient for fortifications, particularly when equipment is limited to hand tools or when skilled personnel are not available to supervise the construction of other types of protective structures. Fill the bags at the construction site with sand hauled to the location. The bags also can be filled where the sand is available and hauled to the site; however, this procedure is less preferable because the bags may be damaged during handling.

A disadvantage of sandbag revetments is that the bags deteriorate rapidly, particularly in damp climates. Thus, the filler material may run out, reducing the protective characteristics and endangering the stability of the revetment. Shell hits may require replacement of bags. Figure 14-19 shows the proper construction for a sandbag revetment.

Material. Loose soil is required for effective sandbag revetments. A procedure for stacking bags is shown in Figure 14-19. Stack sandbags without a retaining wall if the sides of the stacks are sloped approximately 1:5. The substitution of a soil-cement mixture described previously partially overcomes the disadvantage of burlap bag deterioration.

Standoffs. A standoff provides substantially more protection when used to augment sandbag revetments. Previously discussed construction details are shown in Figures 14-13 through 14-16.

Drainage. Provide drainage to route water away from the fortification area to reduce settlement and consequent weakening of the revetment.

Sand grid. Sand-grid material can be used to provide an expedient method of constructing field fortifications and revetments. It provides protection up to .50 caliber small-arms fire and blast and fragmentation protection from artillery or mortar rounds and light antitank weapons.

The sand grid should be placed on a firm, level foundation to increase the stability of the fortification constructed. U-pickets are used in each of the four corners of the grid to secure it after the grid has been stretched to the proper length, If the revetment being constructed is longer than one section of grid material, another section of grid can be attached to the first. Small holes should be punched in the top and bottom of the plastic and then secured with wire or rope. Soil, sand, or gravel can be used as fill material. Some compaction of the material is advised, especially if the revetment will be over 6 feet tall. After one layer of grid has been filled, it should be leveled in preparation for the next layer. A sheet of filter material, fabric, or plastic should be placed between each layer of grid to provide support for the next grid section and to help prevent excessive fill leakage. The joints of each successive layer should be staggered to provide additional stability, and total height should not exceed nine layers of grid. Figure 14-20, illustrates the steps of sand-grid construction.

Main Structure, Gravity Revetments

Figures 14-8 through 14-10: Figures 14-13 through 14-18; and Figures 14-21 through 14-26, show construction details for these revetments. The construction procedures are the same as for standoffs except for the following special considerations:

• Complete and waterproof the wall sections. Place them upright, align them, and secure them with temporary anchorages, Then, backfill postholes, preferably with concrete.
• Place backfill against the wall in such a manner as to avoid disturbing the alignment. At the specified heights, construct anchorages and place cable and deadmen in the earth fill. Tighten the anchorage cable just enough to preclude movement after work is complete. Remove any temporary anchorage.
• Install the protective waterproof cover or sod after the fill is placed to the specified height and thickness.

Main Structure, Earth Revetments

See Figure 14-27, for construction details. Apply waterproofing or sod for a protective cover.

Main Structure, Bulkhead Revetments

Refer to Figures 14-8 through 14-10, and Figures 14-28 through 14-32, for construction details. The construction procedure is the same as for standoffs, except for the following specific requirements:

• Place the opposing sections upright after both inner and outer wall sections are completed and waterproofing is applied. Use sturdy spacer blocks to hold the walls apart at the specified distances while the tie cables are tightened. Backfill the postholes, preferably with concrete.
• Carefully deposit the filler material to avoid displacing the spacer blocks or damaging the tie cables.
• Apply a waterproof cover and remove any temporary anchor cables.

Main Structure, Freestanding Walls

See Figures 14-33 and 14-34, for construction details. Install a temporary drainage system for the area during construction. Incorporate this system into the final drainage plan. See Chapter 6 for drainage details and FM 5-742 for the construction method for forms, concrete placement, and form removal.

CONSTRUCTION- CONTROL CHECKLIST

A checklist for the different construction operations follows. Frequent reference to it will assure the supervisor that no important items are overlooked. This list applies to a typical fortification project. Some details may differ from the specific project, but it is a helpful guide when modified for current projects.

1. Preconstruction planning:

2. Layout criteria (Figures 14-8 through 14-10):

3. Standoff criteria (Figures 14-13 through 14-18):

4. Main-structure, gravity-revetment criteria (Figures 14-17 through 14-26):

5. Main-structure, earth-revetment criteria (Figure 14-27):

6. Main-structure, bulkhead criteria (Figures 14-28 through 14-32):

7. Main-structure, freestanding-wall criteria (Figures 14-33 and 14-34):

MAINTENANCE, REPAIRS, AND IMPROVEMENTS

Normal deterioration of construction materials exposed to weather necessitates periodic inspections. Erosion, rot, rust, and poor drainage reduce the protection which fortifications are designed to provide. Timely inspection and repair prevent the need for complete replacement of fortification sections. The following inspections are the minimum required:

• Earth cover, particularly before and during rainy seasons or under other adverse weather conditions.
• Wood members, including walls and horizontal and vertical members, for deterioration.
• Loose vertical supports in the ground, particularly following heavy rains.
• Ditches and pipes, to ensure they are clean and free of debris.
• Headwalls, for settlement or shell damage.

Standoff Repairs

The purpose of a standoff is to detonate shells directed at the fortifications. Therefore, frequent maintenance or replacement of damaged sections is required. Replace damaged sections with prefabricated sections following the construction procedures outlined previously.

Gravity Revetment Repairs

Anchorages shown in construction details provide for deadmen outside the revetment. Complete the following steps to replace a wall section:

1. Remove the damaged retaining-wall section, and disconnect the anchor cable.

2. Remove all soil that has slumped into the interior of the fortification.

3. Construct the replacement-wall sections according to the original specifications.

4. Apply waterproofing material to the wall.

5. Raise the new wall section into place and attach the anchor cable.

6. Backfill new postholes and refill old holes.

7. Fill voids in backfill with suitable soil and apply cover.

8. Repair any damage to the drainage system.

Earth Revetment Repairs

Complete the following steps to repair this type of revetment:

1. Remove earth from the parking area.

2. Replace earth displaced from the revetment.

3. Reshape damaged slopes.

4. Repair waterproof cover or replace sod.

5. Repair any damage to the drainage system.

Bulkhead Revetment Repairs

Complete the following steps to repair this type of revetment:

1. Remove damaged wall sections, disconnect the tie cables, and redig the postholes in the damaged area.

2. Remove all soil that has slumped beyond the wall lines.

3. Construct replacement wall sections as in the original construction.

4. Apply waterproofing.

5. Raise wall sections into place and attach tie cables and spacer blocks.

6. Fill postholes.

7. Fill voids in backfill with suitable soil and apply cover.

8. Repair any damage to the drainage system.

Freestanding, Movable Revetment Repairs

Because these revetments are made of precast concrete, repairs to these structures are described in FM 5-742.

Improvements

Increase the protection provided by aircraft fortifications by adding earth fill and a protective cover to the gravity revetment.

The addition of a steel or wooden standoff to the original construction increases the penetration resistance significantly without disturbing or altering the original construction.

If wet soil is used initially for gravity revetments, the addition of dry soil increases the protective characteristics of the fortification.

Modify bulkhead-type revetments by using the bulkhead as a retaining wall for an earth revetment. This expedient increases the resistance to penetration and increases fortification protection when circumstances preclude the use of a standoff.

Sometimes the serviceability of fortifications must be prolonged. When rapid deterioration of earth-type revetments is apparent, consider constructing permanent-type structures with reinforced concrete or soil-cement walls if the necessary materials, skills, and equipment are available.