Military

a.   Definition. An obstacle is any terrain feature, condition of soil, climate, or man-made object than firepower, that is used to stop, delay, or divert enemy movement.

b.   Purposes. Obstacles should be included in the overall defense plan to restrict the movement of enemy forces, delay them, or require them to regroup.

c.   Tactical obstacles. The following obstacles are commonly referred to a tactical types:

d.   Observation. Tactical obstacles must be under observation and covered by fire for maximum benefit. An obstacle which is not covered by observed fire may be ineffective or at best lead to a false sense of security.

e.   Offensive use of tactical obstacles. Obstacles are used to anchor a flank or flanks of an advancing unit. They may also be used behind enemy lines to delay, disorganize, and harass troop movements and communications, especially when an enemy force is withdrawing. The wide intervals between dispersed units of company size or larger should be blocked by a combination of obstacles and firepower.

f.   Nontactical obstacles. Obstacles falling in this category may be of the same general design as obstacles constructed under tactical conditions, but the same considerations of siting and concealment do not apply. Nontactical obstacles may be used —

a.   Steep slopes. Varying degrees of steepness are required to stoop different types of vehicles. Tanks can negotiate slopes as steep as 60 percent. However, trees, unfavorable soil conditions, large rocks and boulders can make slopes of less than 60 percent impassable, even though this would not be true if the same natural features were encountered on level ground. The movement of infantry is also slowed down by steep slopes since movement is slower and the troops tire more rapidly.

b.   Escarpments. A steep face of rock is a formidable obstacle to both vehicles and personnel if it is over 1 1/2 meters (5 ft) in height.

c.   Ravines, gullies, and ditches. Ravines, gullies, and ditches are generally obstacles to wheeled vehicles. If they are over 5 meters (16 1/2 ft) in width, and approximately 2 meters (6 1/2 ft) in depth and the banks are nearly vertical, they are usually effective against tracked vehicles.

d.   Rivers, streams, and canals. The major obstacle value of rivers, streams, and canals is that they must be crossed by special means, either deepwater fording, surface or aerial. The width, depth, velocity of the water, and bank and bottom conditions determine the ease of crossing a water obstacle by deepwater fording and floating equipment. However, a river over 150 meters (500 ft) wide and over 1 1/2 meters (5 ft) deep is a major obstacle, limited only by the presence of bridges, favorable sites for amphibious vehicles, and fording sites. The obstacle value of fordable rivers, streams, and canals is significant when the stability of the banks and bottoms is considered. Although a few vehicles may be able to ford a water obstacle, the poor condition of the banks and bottom may prevent further use of the ford without time-consuming improvement of the crossing site. Stream velocity may likewise limit the use of a ford and enhance its value as an obstacle.

c.   Frozen streams. Antitank obstacles (fig 3-1) can be improved in frozen streams by cutting an opening about 3 to 4 meters (10 to 13 ft) wide in the ice and forcing the cut blocks of ice under the solid surface so the blocks will be carried downstream by the current. The openings are then closed with a light frame covered with cloth, brush, or tar paper with about 10 cm (4 in.) covering of snow. The effectiveness of this type of obstacle depends on keeping the water in the channel from freezing. A well make trap will be effective for an extended period of time if it is inspected frequently to maintain the snow cover. If the ice freezes solid in the area of the trap, the procedure outlined above must be repeated.

f.   Lakes. Lakes are usually unfordable and if unbridged, must be bypassed unless they are frozen solid enough to support vehicles and personnel.

g.   Swamps and marshes. The principal obstacle value of swamps and marshes is the canalization of vehicular movement onto causeways thereby exposing the columns to air or artillery attacks. Swamps and marshes over 1 meter (3.3 ft) in depth may be better obstacles than rivers, since causeways are usually more difficult to construct than bridges. The physical effort required for foot troops to cross swamps and marshes is an important factor in their usefulness as an obstacle. All roads and causeways through swamps and marshes should be extensively cratered, mined, or blocked by abatis.

h.   Forests. Forests have the effect of canalizing movement, since the roads, trails, and fire breaks through them provide the only means for rapid movement. The obstacle value of a forest is dependent on tree size and density, soil condition, slope, and depth. If the trees are at least 20 cm (8 in.) in diameter and sufficiently close together, they will seriously obstruct or stop the movement of tanks. Even though the trees are seldom close enough together to stop tanks, they may prevent tank movement when they are pushed over and tangles. Much smaller trees (10 cm (4 in.) in diameter) will slow and sometimes stop tanks on 20 percent slopes. Tree stumps that are 45 cm (18 in.) in diameter or larger are obstacles to tank movement. Forest undergrowth in the temperate zone is not usually dense enough to seriously obstruct foot movement, but such movement will be slowed significantly by steep slopes, adverse soil conditions, and fallen trees and branches. The most effective way of increasing the obstacle value of forests is to:

i.   Jungle obstacles. Tropical jungles are important obstacles to the movement of vehicles and personnel. The ground between the trees is usually covered by interwoven vines, bushes, plants, or rotting vegetation. The ground is often swampy or marsh. The tangles undergrowth and overhead foliage limits the visibility and there are few if any paths or trails except those that permit limited foot traffic. Vehicles can seldom operate satisfactorily unless routes are prepared or extensively improved. Foot troops are required to cut trails through the dense undergrowth or move with extreme difficulty. Since the jungle is an effective obstacle to movement, any road or trails that exists should be blocked and the stream fords and amphibian vehicle entry and exit sites should be mines. If the streams and rivers provide the best routes, obstacles should be constructed to slow up or prevent the use of floating equipment. The following obstacles are effective against foot movement in the jungle:

j.   Snow. Snow is considered deep for purposes of foot or vehicle movement when the average depth above ground elevation is 1 meter (3.3 ft). Snow at this depth and even deeper is not unusual in the Arctic and the northernmost regions of the temperate zone. It is found at these depths also in mountainous regions. Deep snow and the accompanying ice and intense cold combine to make obstacles to movement of both foot troops and vehicles. It also blankets terrain features such as boulders, rocky areas, ditches, small streams and fallen trees so as to effectively impede movement. The obstacle value of snow can be increased by-

k.   Deserts. The obstacle value of deserts is that specially equipped vehicles and specially trained personnel are required to operate successfully in this environment. Minefields are comparatively easy to install and relocate in the desert and the prevalent winds quickly cover up the usual signs of mine installation.

l.   Built-up areas. The natural obstacle of built-up areas can be increased by cratering streets, demolishing walls, overturning or derailing street or railroad cars, and constructing roadblocks from steel rails, beams, and rubble. When combines with mines and barbed wire, such obstacles are effective against vehicles and personnel.

a.   Definition. An artificial obstacle is any object constructed to hinder movement. Artificial obstacles include minefields, antitank ditches, contaminated areas, hedgehogs, road craters, demolished bridges, and barbed wire. They may be constructed entirely on land or partially under water as in the case of beach and river line obstacles.

b.   Use. Major types of artificial obstacles are discussed separately in subsequent chapters; however, they are normally used in conjunction with natural obstacles and in combinations of two or more types of artificial obstacles. When artificial obstacles are used in barriers, a variety of them should be used, when practicable, to increase effectiveness and as an aid to surprise and deception. Obstacles can be divided into three groups according to their uses. Seldom does an obstacle fall clearly into one of these three groups. More often than not an obstacle may be used for two or three purposes. The arbitrary classification of obstacles merely clarifies their primary uses.

a.   Coordination with tactical plan. Obstacles should be coordinated with the tactical plan. All obstacles should contribute to the success of this plan, and all units concerned should know the location of and understand the purpose and type of obstacles employed. In addition, all concerned should know when the obstacles are to be executed, and how long they are to be defended. Only by coordination with all elements can an integrated plan be prepared that will use all defensive measures to their best advantage against the enemy.

b.   Covering by observation and fire.

c.   Employment in conjunction with natural and other artificial obstacles. It is fundamental that an obstacle system should usually be as difficult to bypass as it is to breach except when the obstacle is intended to divert or deflect the enemy rather than to delay or stop him. Artificial obstacles must be sited to take full advantage of natural and other artificial obstacles, so as to keep logistic and construction requirements to a minimum. Natural obstacles are improved and exploited to the fullest extent.

d.   Employment in depth. Obstacles do not seriously hamper the enemy's movement until they overload or heavily tax his breaching capabilities. This cannot be accomplished unless obstacles are employed in depth. With the exception of contaminated areas it is usually prohibitive in time and materials to construct a large deep area of continuous obstacles. The same end is accomplished by constructing successive lines of obstacles, one behind the other, as time and conditions permit. These successive lines require the enemy force to continually deploy and regroup, thus dissipating, canalizing, and dividing its effort until friendly forces can destroy it or force its withdrawal.

e.   Camouflage and concealment.

f.   Provision for lanes and gaps. When obstacles are employed around a defensive position or area, lanes or gaps through the system are left and concealed. These lanes are provided so patrols, counter attacks, and friendly troops on other missions may move through the system without difficulty. Under normal circumstances the lanes or gaps necessary to mount a general offensive through the obstacle system are not provided during construction, but prepared later when the need for them arises. It is important there be a sufficient number of lanes to allow for alternate use and that they be concealed and changed periodically to insure they are not discovered by the enemy. Prior plans must exist to insure all lanes or gaps can be blocked quickly when enemy action is expected. Lanes and gaps should be covered by fire to preclude the possibility of the enemy rushing through them before they can be closed.

g.   Affording no advantages to the enemy. Enemy forces may use certain obstacles to an advantage as they are breached or assaulted. Antitank ditches should be constructed so they are useless to the enemy as fighting trenches. Log cribs should be located so the enemy cannot deliver effective fire on defending weapons while using the crib as a breastwork. Obstacles should be located so the enemy cannot use hand grenades against the defenders from cover or concealment provided by the obstacles. Barbed wire, mines, and boobytraps should be used extensively to deny use of any cover or concealment that might be provided to the enemy by natural or reinforcing obstacles. Care should be taken to guard against the inadvertent placing of an obstacle which might later hinder friendly maneuver.

a.   Description. A caltrop (fig 3-4) has four sharpened prongs oriented so that one prong will always be vertical regardless of how the caltrop lands. The prongs are .25 cm (3/32 in.) in diameter and 3.8 cm (1.5 in.) long.

b.   Uses. Caltrops are employed as antipersonnel obstacles either by themselves or in conjunction with barbed wire. When emplaced with a density of 38 per meter (3.3 ft) of barrier front, an effectiveness equivalent to triple standard concertina is achieved. Caltrops are designed to cause injury by penetrating the footgear of a man who steps on one. Serious injury will result if a man quickly falls to the ground to avoid small arms or artillery fire. Caltrops can be dispensed by hand, from the rear of a truck, or from fixed and rotary winged aircraft.

a.   Types (fig 3-5).

b.   Construction procedures.

a.   Use. Crater type obstacles are used for blocking roads, trails, or defiles, preferably at points where the terrain prevents bypassing the obstacle or where terrain suitable for bypassing can be mined and covered by antivehicular fire. Craters should be improved wherever possible by steepening the sides, flooding or mining.

b.   Preparation. As in the case of bridge demolitions, craters are formed by explosive charges placed in advance and prepared for later detonation. The weights of charges, depths, and arrangement are given in detain in FM 5-25. The methods normally employed include—

a.   Hurdles. Log hurdles can be formed of 25- to 45-cm (10- to 18-in.) logs as shown in figure 3-7 may be used to add to the obstacle effect of a crater, or other type of roadblock. The hurdles force vehicles to reduce speed as they approach the obstacles or they may act as an additional means of trapping vehicles in the vicinity of antitank ditches. Each hurdle consists of one 45 cm (18 in.) or three 25 cm (10 in.) logs firmly staked in place on a roadway or on ground suitable for use as a bypass. A hurdle of this size stops or damages most types of wheeled vehicles. Tanks can cross them at reduced speeds on reasonably level ground but are stopped by hurdles on uphill grades which approximate the critical grade of the vehicle. To stop a tank on such a slope, the size and location of the pole or log hurdle must be such that the ground line of the tank will be tilted to a slope of 1 to 1. The poles must be firmly tied between strong stakes at not more than .5 meter (5 ft) intervals. To determine the height of the hurdle required, a stick 3.5 meters (11.5 ft) long is held at an angle of 45° above horizontal, with one end of the ground downhill from the hurdle location. The distance between the downhill end of the stick and the ground is how high to construct the log hurdle. The hurdle should be sited on the steepest part of the slope and as near the top as possible.

b.   Cribs. Rectangular or triangular log cribs (fig 3-8 through 3-10) are used effectively as roadblocks where standing timber is available and where such an obstacle cannot be bypassed readily. Unless substantially built, obstacles of this type are not effective against heavy tracked vehicles. Cribs are strengthened by filling them with earth; and preferably the earth is obtained by digging a shallow ditch in front of the obstacle. Log hurdles in front of a log crib force vehicles to reduce speed and add to the effectiveness of the roadblock. An engineer platoon equipped with platoon tools can build 6 meter (20 ft) of this obstacle in 4 to 8 hours.

a.   Use. Posts are among the most effective antivehicular obstacles because each post presents a breaching problem to the attacker. There is no fast method of breaching a belt of posts. Normally, the attacker will seek to bypass such an obstacle. Post obstacle belts may be constructed using either steel, log, or concrete posts.

b.   Steel posts. These posts usually are sections of rail, heavy pipe or structural members. Due to their small cross-sectional area, steel posts are installed over footings to prevent their being driven into the earth by the weight of a tank.

c.   Log posts. These posts should be hardwood with a minimum diameter of 40 cm (15.8 in.). Footings are used under log posts only where the soil has exceptionally poor load-bearing characteristics. Figure 3-11 depicts a belt of log post obstacles.

d.   Concrete posts. Precast concrete posts may be emplaced either vertically or angled in the direction of the enemy line of approach using lengths, spacing, and arrangements as described for wood or steel post obstacles.

e.   Placing.

a.   Use. Tree felled as shown in figure 3-12 can be used to block a road or defile. To stop tracked vehicles the trees should be at least 60 cm (24 in.) or more in diameter and at least 6 meters (20 ft) tall. To effectively block a road through a heavily wooded area, an abatis, at least 75 meters (250 ft) deep is required.

b.   Construction. Abatis may be constructed using handtools, by the use of explosives alone, or by a combination of notching and explosives as shown in figure 3-13. Using only handtools, one engineer platoon can build 75 meters (250 ft) of abatis in 8 hours. Information on the use of explosives for the construction of abatis is contained in FM 5-25. Bushy-top trees with heavy branches and thick foliage should be used for abatis wherever possible since the branches reduce the movement of the vehicle and the foliage sets up a screen. The trees should be felled as shown in figure 3-12 so that the trunk remains attached to the stump. To insure that the trunk remains attached, no cut is made on the side of the tree toward which it is to fall, the tree is strained to fall in the required direction, and the butt is cut two-thirds through on the opposite side. The effectiveness of an abatis is increased by interlacing barbed wire in the branches of the trees.

a.   Hedgehogs. Steel hedgehogs as shown in figure 3-14 are relatively lightweight for the obstacle effect they provide, and they are quickly installed or removed. They are designed to revolve under wheeled vehicles and puncture them or to belly up tracked vehicles. Unless kept under observation and covered with fire, the enemy can readily move them aside. They are well adapted for use in vegetation high enough to afford complete or partial concealment. Exposed parts should be painted to blend with the background. Hedgehogs are made up in rear areas, using three angles about 10 cm by 10 cm by 1 cm (4 in. by 4 in. by 4 in.), 120 cm (4 ft.) long, and 1 cm (.4 in.) steel plate about 50 cm (20 in.) square. A hedgehog of this size weighs about 75 kg (160 lb.). Hedgehogs are used in rows, with at least 150 hedgehogs to each 100 meters (328 ft.) of front which is to be protected in this manner.

b.   Tetrahedrons. Steel tetrahedrons shown in figure 3-15 are employed in a manner similar to that of hedgehogs. They are usually made of 10 cm by 10 cm by 1.5 cm (4 in by 4 in. by .6 in.) angles, the base and sides in the shape of equilateral triangles, 1.5 meters (5 ft.) on a side. Their finished height is approximately 1.2 meters (4 ft.).

a.   Cubes. Cubes are concrete obstacles of approximately cubical shape, set in irregular rows. A typical size and arrangement is shown in figure 3-16. Because of the weights involved and the simplicity of erecting forms for cubes, these obstacles are best cast in place if the situation permits. A cube of the size shown in figure 3-16 requires about 1.8 cubic meters (2.4 cu yd.) of concrete and weighs slightly less than 4 1/2 metric tons (5 tons).

b.   Cylinders. Concrete obstacles of cylindrical shape are usually smaller than cubes and are light enough to be precast. Their use is similar to that of cubes, and they may be preferable in situations in which precast obstacles are the type required. Cylinders may be precast in forms made of lightweight sheet metal which need not be removed. A cylinder of the size shown in figure 3-17 requires 1 cubic meter (1.3 cu yd.) of concrete and weighs a little less than 3 tons.

c.   Tetrahedrons. Concrete tetrahedrons are pyramids with base and sides of equilateral triangles, 1.5 meters (5 ft.) on a side. They are set in irregular rows as shown in figure 3-18. A tetrahedron of this size has a vertical height of about 1.2 meters (4 ft.), requires 0.9 cubic meter (1.05 cu yd) of concrete, and weighs about 1 metric ton (1.1 tons). They may be precast in trough-shaped forms between triangular divisions, with a lifting ring embedded in the center of the top surface of each tetrahedron.

a.   Roadblocks may be improvised from farm carts, automobiles, and trucks, which are loaded with rock, concrete, or other heavy material. When placed in position their wheels should be damaged or removed, and the vehicles should be anchored firmly.

b.   Vehicles can be moved to close a gap that has been left to keep the road open.

c.   A roadblock which may be effective in some situations is constructed quickly by the method shown in figure 3-19. A heavy tree at one side of a road is cut almost through and its trunk is attached by a wire rope to a tree across the road in such a way that if a passing vehicle strikes the rope the tree will fall and damage the vehicle or pin it in place.

a.   Purpose. Wherever possible, antivehicular obstacles, particularly roadblocks, should be concealed by screens for the following reasons:

b.   Siting. Screens should be sited not more than 3 meters (10 ft.) from the obstacles which they are concealing. If a vehicle goes through a screen at this distance, it will encounter the obstacle before it can halt. Therefore it will not be in position to fire at the obstacle. Screens must not obscure the fields of fire of the defenders.

c.   Construction. A form of screen suitable for concealing a roadblock consists of two horizontal strips of canvas, garnished netting, or blankets, the lower part suspended from wires about 120 cm (4 ft.) from the ground, and the upper part at a height of 2 to 2.5 meters (6 1/2 to 8 ft.). The upper part should overlap the lower part by 15 to 33 cm (6 to 13 in.).

d.   Dummy obstacles. Dummy obstacles should be used extensively to confuse and delay tanks and cause them to waste ammunition. They should be made carefully in order To present a realistic appearance. They can be made of plaster, wood, or asbestos sheets. Wooden obstacles can be used to represent steel obstacles. Antitank and antipersonnel mines should be interspersed extensively between dummy obstacles.

a.   Beach obstacles. Beach obstacles are designed to force landing craft to unload at low tide several hundred yards seaward of the high watermark. Thus, on beaches with gradual slopes assaulting infantry must cross a wide expanse of obstacle-studded beach covered by heavy defensive fire before reaching the high watermark. At high tide, beach and underwater obstacles should be covered by just enough water so they cannot be seen by personnel in landing craft. When landing craft strike the obstacles they are disabled and the assaulting troops are forced to disembark in deep water.

b.   Antiboat obstacles. Antiboat obstacles are constructed at varying heights so they are about 30 to 60 cm (2 to 4 ft.) below the surface of the water at high tide, echeloned in depth in various arrangements of which those shown in figure 3-20 are typical.

a.   Siting. The basic requirements for artificial obstacles and their employment apply equally to beach and river line obstacles. Of particular importance are the requirements that artificial obstacles be used to exploit natural obstacles, that they be inconspicuous, be kept under surveillance, and be capable of being covered by fire. Gaps and lanes are provided and are marked or referenced for the use of friendly troops. Antiboat obstacles selected for use should be of a type which will be effective against boats which can operate in the surf, current, and various wind conditions to be expected. They are sited for maximum obstacle effect at the tide stage at which an assault is probable and for maximum effectiveness against amphibious tracked and wheeled vehicles.

b.   Beach slopes. Due to tide and current action, beaches and river lines tend to fall into two general types-those with steep slopes into deep water, and those with gradually sloping bottoms for a considerable distance offshore. Each type has advantages and disadvantages for the defense. The steep slope prevents debarkation until boats reach the beach, but it renders placing underwater obstacles more difficult. The gentle slope facilitates placing obstacles but it also allows the attacking troops to disembark while still afloat.

c.   Employment in depth. Beach obstacles are typically established in bands in depth, as follows:

a.   Rock-filled cribs and pillars. Rock-filled timber cribs (fig. 3-21) are normally 2 to 3 meters (6 1/2 to 10 ft.) long by 1 meter (3.3 ft.) wide, and have stability at heights up to 2 meters (6 1/2 ft.). The logs are driftpinned at the corners. Cribs may be installed on a beach at low water or may be dragged or lowered into water before completing the rock fill. For lower heights, smaller cribs, triangular in shape and known as pillars (fig. 3-22), are built with less material and effort. Both types may be connected by barbed wire, wire rope, or a combination of both.

b.   Tetrahedrons. Timber tetrahedrons (fig 3-23) are pinned and wired to a triangular bottom frame which is weighted in place with rocks. A post may be driven through the obstacle for improved anchorage. Tetrahedrons are normally spaced at intervals of 5 to 10 meters (16 1/2 to 33 ft.) and may be connected with wire rope or incorporated in a barbed wire fence.

c.   Log scaffolding. In suitable water depths, log scaffolding, as shown in figure 3-24, is effective in impeding small boats. Wooden posts driven into the bottom are reinforced by diagonal braces extending inshore and have horizontal stringers attached to the offshore face.

d.   Braced wooden posts. This obstacle (fig. 3-24) may be built in relatively shallow water in which there is little or no tide range. The posts are driven approximately to water level in two rows. They are staggered so diagonal braces can extend from each rear post to two of the front posts to provide a structure of exceptional rigidity. The bottom ends of the braces may be fastened to the rear posts before the latter are fully driven and before the work is so deep as to require diving equipment. The front posts may be connected with wire rope or barbed wire to further improve the rigidity of the structure and to add to the obstacle effect. The efficiency of this obstacle is further enhanced by the liberal use of barbed wire tangles securely fastened to and between the posts.

e.   Log tripods. Braced log tripods, constructed of logs at least 20 centimeters (8 in.) in diameter, as shown in figure 3-26, are effective antiboat obstacles. The obstacle is positioned with its longest leg facing the direction of expected assault; this leg may be capped with a standard antitank mine or sharpened to a point. Constructed in varying sizes so they are covered by 30 to 60 cm (1 to 2 ft.) of water at high tide, these obstacles are placed on beaches from the low-tide mark back to about halfway to the high-tide line.

f.   Log ramps. Log ramps are constructed as shown in figure 3-27. They are used to tear the bottoms out of assault craft riding up on them, and to upset such craft. They are effective obstacles with or without mines fastened to the high end of the ramp. Ramps may be placed either in an irregular pattern or in a continuous belt spaced at approximately 3-meter (10-ft.) intervals.

g.   Nutcrackers. Nutcrackers are constructed as shown in figure 3-28. The .9- by .9- by .6-meter (3- by 3- by 2-ft.) base has a center well or recess large enough to house one or two antitank mines, depending on whether a one-way or two-way obstacle is desired. It also has a built-in socket for the bottom end of the activating rail or pole. Shearpins, usually of one-half cm soft iron, hold the rail erect and prevent detonation of the mines by wave action. A landing craft striking the pole will break or bend the shearpin sufficiently to detonate the mines. Nutcrackers normally are employed in an irregular pattern interspersed with plain steel and log posts.

a.   Scaffolding. On beaches having considerable tidal range, 5 cm (2 in.) steel pipe may be driven into the bottom and welded together to form a structure of the scaffolding type, as shown in figure 3-29. Floating mines may be attached below the normal water level, to be detonated if scraped by a vessel.

b.   Hedgehogs. Steel hedgehogs of the type shown in figure 3-14 are fabricated in rear areas, shipped knocked down, and quickly assembled with bolted connections. The angles used are usually about 2 meters (6 1/2 ft.) long, making the obstacle about 1 meter (3.3 ft.) high. The hedgehog is emplaced without anchorage so that it revolves under a boat or amphibious vehicle, holes it, and anchors it as it sinks. Normally hedgehogs are installed in several rows, using about 150 hedgehogs to each 100 meters (328 ft.) of beach.

a.   Rock mounds. These consist simply of mounds of rock about 1-meter (3.3 ft.) high and 3.5-meters (11 1/2 ft.) square and staggered at intervals of 3 to 5 meters (10 to 16 1/2 ft.) on the outer edges of reefs or likely landing beaches.

b.   Rocky walls (fig 3-32). Rocky walls are about 1 meter (3.3 ft.) high and 1 meter (3.3 ft.) wide, in sections or continuous lines. They should be mined and topped with concertinas. They should be sited so the top of the wire is just under the surface at high tide.

Lesson 3 Practice Exercise
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