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LESSON 3

OBSTACLE EMPLOYMENT

LESSON OBJECTIVES

Upon completion of this lesson you should be able, in the indicated topic areas, to:

1.   Natural Obstacles. Describe the four characteristics of a well situated natural obstacle to include capability of improvement, defilade from enemy observation, and best location for defensive fires.

2.   Tactical Obstacles. Describe the four types of tactical obstacles - antitank, antivehicle, antipersonnel, and beach and river line.

3.   Principles of Employment. Discuss the principles of employment of obstacles to include coordination with the tactical plan, covering by observation and fire, employment with natural and other artificial obstacles, and employment in depth.

4.   Ditches. Describe the types of ditches, their characteristics, and construction.

5.   Log Hurdles and Cribs. Describe purposes and construction of log hurdles and cribs.

6.   Steel and Concrete Obstacles. Describe the purposes and construction of steel and concrete obstacles.

7.   Expedients. Discuss types of expedient roadblocks and their construction.

8.   Beach and River Line Obstacles. Discuss purposes and construction of obstacles to include beach obstacles, antiboat obstacles, and antipersonnel obstacles.

ATTACHED MEMORANDUM

Section I.   Principles

3-1.   BASIC CONSIDERATIONS

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:

(1)   Antitank obstacles intended to impede or stop the movement of tracked vehicles across country or on roads;

(2)   Antipersonnel obstacles constructed to slow up, confuse or divert enemy foot troops when they attempt to overrun or infiltrate a defended position or locality;

(3)   Antivehicle obstacles including roadblocks, crates and other means that are used to stop or delay enemy wheeled vehicles so they can be brought under aimed fire.

(4)   Beach and river line obstacles that delay, obstruct or divert enemy amphibious operations.

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

(1)   For the protection of important installations against infiltration or sabotage.

(2)   In civil policing operations to check the movement of rioters or to isolate a section of a town or city.

(3)   For administrative purposes.

3-2.   CHARACTERISTICS OF NATURAL OBSTACLES

Desirable characteristics of a natural obstacle are ease of conversion into a more effective obstacle with a minimum of effort, materials, and time; defilade from enemy observation; location where observation and defensive fires can prevent enemy breaching; and difficulty of bypassing. The most effective natural obstacles against tanks are steep slopes, unfrozen swamps, and broad, deep streams. Rice paddies, lava fields, and similar areas can also be formidable obstacles. Usually time labor, and materials can be saved by improving natural obstacles rather than constructing artificial ones to swerve the same purpose.

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.

Figure 3-1.  Antitank trap in ice.

Figure 3-1.   Antitank trap in ice.

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:

(1)   Construct abatis or craters.

(2)   Place mines along the roads, trails, and firebreaks.

(3)   Construct log cribs, hurdles, and post obstacles if the necessary materials are available.

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:

(1)   Punji jungle trap. Punji traps (fig 3-2) are most effective when they merge with or resemble natural jungle obstacles. In the defense, they may be used either as barricades around camps or as barriers to impede the advance of an assault. In the offense, they may be constructed behind enemy lines to stop or hinder any retreat. Enemy patrols can be disbanded by skillful use of these trap in connection with covering snipers. A pit 1.5 to 2 meters (5 to 6 1/2 ft) deep, about the same length and one meter (3.3 ft) wide is dug in the middle of a jungle trail or at a stream crossing. A number of long, sharp punjis (bamboo spikes sharpened to a needle point) are placed upright in this pit, with the fire-hardened points slightly below ground level (fig 3-2). The pit is concealed by a flimsy lid consisting of a bamboo lattice covered with a few bamboo creepers and camouflaged with mud or leaves to blend with the surrounding area. Anyone falling into the pit is instantly impelled on the spikes.

Figure 3-2.  Punji jungle trap

Figure 3-2.   Punji jungle trap.

(2)   Slit trench. A slit trench can be so placed that enemy troops will be likely to use it. Like the cover of the punji pit, the bottom of this trench is false, and underneath it are sharp punjis, which will impale anyone jumping into the trench.

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-

(1)   Erecting snow fences or breaks so that the prevailing winds will accelerate the accumulation of snow into drifts to form obstacles of packed snow.

(2)   Building snow walls (fig 3-3) as obstacles against armor. The snow must be packed hard for this purpose. Walls of this type are most effective when they are sited on an upgrade.

Figure 3-3.  Antivehicular obstacle of packed snow.

Figure 3-3.   Antivehicular obstacle of packed snow.

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.

Section II.   Artificial Land Obstacles

3-3.   BASIC CONSIDERATIONS

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.

(1)   Protective. Protective obstacles are those obstacles used to providesecurity. Obstacles of this type are usually artificial and include such items as wire, minefields, and various warning devices. They are intended primarily to prevent the enemy from making a surprise assault from areas close to a position.

(2)   Defensive. Defensive obstacles are obstacles used to delay the enemy force in areas where it can be engaged with heavy, intense defensive fire. They may be either natural or artificial. A defended roadblock or an obstacle in front of a defensive position which stops or delays the enemy force once it is in range of defensive weapons are examples of this type. Defensive obstacles should be covered by appropriate fire, kept under observation, and should be employed in conjunction with protective obstacles.

(3)   Tactical. Tactical obstacles are obstacles used to break up enemy attack formations and canalize the enemy force into areas where it is blocked by defensive obstacles or can be brought under intensive defensive fires. Tactical obstacles delay, harass, or demoralize the enemy by forcing him to employ dangerous to exhaustive breaching measures.

3-4.   PRINCIPLES OF EMPLOYMENT

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.

(1)   Observation. If accurate fire is to be delivered on an obstacle or obstacle system, it must be under observation. The observation and defense of obstacles for close-in defense is the responsibility of the unit occupying the ground. However, when an obstacle system covers a large area, observation is normally the responsibility of roving patrols, an outpost system, aerial observation, of tactical air. Their final defense is a mission for mobile forces that can be brought quickly to any point of the system. At times it is not feasible to have an obstacle under direct observation. When this is the case, warning devices or alarm systems such as tripflares, boobytraps, or electronic sensors in connection with noisemakers should be used.

(2)   Fire. Covering an obstacle by fire usually means the difference between causing the enemy only small delay and annoyance and forcing him into a costly engagement.

(a)   Both antivehicular and antipersonnel obstacles should be covered by both antivehicular and antipersonnel fire. Fire that covers antipersonnel obstacles should not only be capable of discouraging breaching, bypass, or capture by personnel but should also be capable of stopping any vehicles that may be used in the assault. Also, antivehicular obstacles must be covered by fire that will not only destroy vehicles but will prevent troops from breaching the obstacles and clearing a path for the vehicles.

(b)   Obstacles are best covered by direct-fire weapons, but when this is not feasible, observed artillery fire and tactical air should be used. Artillery covering obstacles should be prepared to deliver fire that is effective against both personnel and vehicles. When it is impossible to cover obstacles by fire, they should be contaminated or heavily boobytrapped to cause the enemy to employ dangerous and exhaustive breaching measures.

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.

(1)   Camouflage. Obstacles should be camouflaged or employed in such a way that they come as a surprise to the enemy. When the enemy has no prior knowledge of an obstacle, he has to reduce it without benefit or prior planning. If the obstacle is defended the defender has the advantage of the enemy's first reaction, which is usually confusion, and the enemy may be caught without the men and material to breach the obstacle.

(2)   Siting. Proper siting is often the easiest solution to obstacle camouflage problem. Large obstacle systems cannot be concealed by siting alone, but when proper advantage is taken of the terrain and the obstacles are located in folds of the ground, around blind curves in roads, or just over the tops of hills, they can be made inconspicuous from the enemy's ground observation. To help camouflage obstacles from aerial observation, regular geometric layouts of obstacles and barrier systems should be avoided and phony obstacles used to confuse the enemy as to the exact location and extent of the system.

(3)   Concealment. The best way to conceal an obstacle usually is to postpone its execution or construction as long as possible, without interfering with its readiness when needed. This cannot be done when large barrier systems are involved but is possible when preparing obstacles to block narrow avenues of approach, such as roads or bridges. Obstacles created by demolitions lend themselves readily to this procedure. When their use is contemplated they should be completely prepared for firing at the last minute.

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.

3-5.   MINEFIELDS

Minefields are not only an obstacle to the advance of the enemy, but unlike obstacles of a passive nature, they can also inflict significant casualties; therefore minefields are considered the best form of artificial obstacle. The installation of minefields changes favorable terrain to unfavorable terrain and materially enhances the strength of the defense.

3-6.   CALTROPS

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.

Figure 3-4.  Caltrop

Figure 3-4. C   altrop.

3-7.   BARBED WIRE ENTANGLEMENTS

Obstacles constructed from barbed wire are simple, flexible and effective against personnel. They may also be used to impede the movement of vehicles.

3-8.   ANTIVEHICULAR OBSTACLES

In defensive positions, antivehicular obstacles are used to obstruct gaps between natural obstacles or they can be placed in a continuous line of considerable length in open terrain. Antivehicular obstacles are usually employed in conjunction with wire entanglements, minefields, and other obstacles. Under some conditions they may be continuous in areas just inland of beaches.

Section III.   Antivehicular Obstacles

3-9.   DEFENSE

Antivehicular obstacles should not be continuous across the front of a position, but should have gaps which can be kept under observation and fire and at which flares and other warning devices can be kept in operational condition. Such gaps tend to canalize vehicular movement. With observation and effective covering fire placed on these gaps, an attack with vehicles can be stopped. If enemy forces are equipped with short gap bridging, the effectiveness of antivehicular obstacles under 20 meters in width is materially decreased. A narrow ditch will halt a unit so equipped only until this organic bridging can be brought into use.

3-10.   SITING

Antivehicular obstacles are sited to take advantage of trees, brush, or folds in the ground for concealment and surprise effect. If they can be sited to permit flooding with water, the obstacle becomes more effective and helps to deny its use to the enemy as a protected firing position for infantry. In some situations, antivehicular obstacles may also be sited for close-in protection in front or to the rear of the main line of resistance and as adjuncts to other obstacles. In such locations, vehicles may be separated from their infantry support and are vulnerable to antivehicular weapons.

3-11.   DITCHES

a.   Types (fig 3-5).

Figure 3-5.  Antivehicular ditches

Figure 3-5.   Antivehicular ditches.

(1)   Triangular ditches. These are relatively easy to build, but a vehicle stopped in a ditch of this type can usually back out and try another route.

(2)   Sidehill cuts. Sidehill cuts are variations of the triangular ditch adapted to sidehill locations, and have the same advantages and limitations.

(3)   Trapezoidal ditches. These require about double the construction time of triangular type ditches, but they are more effective obstacles. In a trapezoidal ditch, as the center of gravity of the vehicle crosses the edge, and if the ditch depth exceeds the height of the vehicle wheels or treads, vehicles are trapped. Sections of ditch longer than 100 meters (328 ft) are not normally camouflages. In winter a trapezoidal ditch may be camouflaged by snow to resemble a standard trench (fig 3-6).

Figure 3-6.  Antivehicular ditch camouflaged to resemble a trench.

Figure 3-6.   Antivehicular ditch camouflaged to resemble a trench.

b.   Construction procedures.

(1)   Excavation. Ditches are excavated by earthmoving equipment, by explosives as described in FM 5-25, or by handtools. To be effective, ditches made by explosives must be dressed to true surfaces by excavating equipment or handtools. Triangular and sidehill-cut ditches are constructed rapidly by a combination of explosives and motorized graders and angle-dozers. The actual time required varies widely in different types of soils. If available and if it can be used at the site of the ditching, the standard 1/4-yard (.57 m3) shovel is used in ditch excavation. Estimating factors for construction time in average soil are shown in table 3-1.

(2)   Revetting. The face of the ditch, or both faces in the case of a trapezoidal ditch, should be revetted as soon as possible after it has been dug. Facing type revetting is used almost exclusively, with pole type or brushwood hurdles preferred because of their durability. It is particularly important that the top of the revetment be about 20 cm (8 in.) below the top of the ditch and that the anchor stakes and tieback wires be buries under 30 cm (12 in.) of earth.

3-12.   CRATERS

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

(1)   Placement of charges in a culvert under the road and concealed and wired for detonation from a safe distance.

Table 3-1.   Estimating Data on Ditch Construction (Average Soil)

Table 3-1.  Estimating Data on Ditch Construction (Average Soil)

(2)   If a culvert is not available at the point selected, charges are placed in the bottoms of holes excavated in the road. Truck-mounted earth augers, if available, are used for digging the holes. The charges are placed and wired for detonation at a safe distance. The holes are backfilled in such a way that they are not readily notices. The use of ADM to produce craters in covered in FM 5-26.

3-13.   Log Obstacles and cribs

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.

Figure 3-7.  Types of log hurdles.

Figure 3-7.   Types of log hurdles.

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.

Figure 3-8.  Rectangular log crib used as a roadblock.

Figure 3-8.   Rectangular log crib used as a roadblock.

Figure 3-9.  Details of log crib used as a roadblock.

Figure 3-9.   Details of log crib used as a roadblock.

Figure 3-10.  Triangular log crib used as a roadblock.

Figure 3-10.   Triangular log crib used as a roadblock.

3-14.   POSTS

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.

Figure 3-11.  Belt of log post obstacles.

Figure 3-11.   Belt of log post obstacles.

(1)   Concrete posts should be square in cross section and 3 meters (10 ft.) or more in length. They can be precast readily in horizontal open-top boxes with plank bottoms and removable sides and ends. Two lifting rings are set in the top surface at the quarter points of the length, for loading and unloading, and a similar ring is positioned at the top end for raising it into position. A chisel-shaped point can be formed easily at the bottom end if the concrete posts are to be driven in with pile-driving equipment. Lengthwise reinforcement is provided several centimeters inside the surface near each corner of the square post with a traverse wrapping of wire at each cm (12 in.) of length. Round reinforcing bars of 1.25 cm (l/2 in.) diameter are adequate for the longitudinal reinforcement. Reinforcement can be improvised by using 4 to 6 strands of barbed wire at each corner, attached to the form ends and racked tightly, preferably to almost the breaking strength of the wire. After curing for 1 week or more under wet burlap, such posts are installed in the same manner as described for wood posts or steel posts. If pile-driving equipment is to be used, a steam or air hammer may be required for driving heavy posts of this type depending on the type of soil.

(2)   Round concrete posts may be improvised from corrugated metal pipe of small sizes filled with concrete. Because of the time required to funnel concrete into pipe held vertically and because of the expenditure of the pipe, this method is less efficient than the use of square precast concrete posts.

e.   Placing.

(1)   All posts are buried 1.5 meters (5 ft.) in the ground either vertically or at a slight angle toward the enemy, and project above ground level between 75 and 120 cm (30 and 48 in.). The height should vary from post to post. The minimum acceptable density for posts is 200 per 100 meters (328 ft.) of front. The spacing should be irregular, with at least 1 meters (3.3 ft.) and not more than 2 meters (6.6 ft.) between posts.

(2)   Posts are equally useful whether employed in long belts or in short sections as roadblocks. By predigging holes, lining them with pipe, and covering them for later rapid installation of posts, the road may be kept open for use until the roadblock is needed. The rate of construction of such roadblocks is approximately as follows, based on a 6-meter (20 ft.) road width:

(a)   Using pile-driving equipment, 2 NCOs and 16 men: 4 to 6 hours.

(b)   Using power earth auger or demolitions (shaped charges), 1 NCO and 8 men: 2 to 2 l/2 hours.

(c)   Using handtools, one combat engineer platoon: 3 to 5 hours.

(3)   Use of spirals of wire with posts. The effect of post type obstacles can be improved, and the obstacles made more difficult to breach, by weaving spirals of barbed wire among the posts as shown in figure 3-11. The belt illustrated is an antipersonnel as well as an antivehicular obstacle.

3-15.   ABATIS

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.

Figure 3-12.  Abatis used as a roadblock.

Figure 3-12.   Abatis used as a roadblock.

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.

Figure 3-13.  Preparing explosive charges for abatis construction.

Figure 3-13.   Preparing explosive charges for abatis construction.

3-16.   STEEL OBSTACLES

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.

Figure 3-14.  Steel hedgehog.

Figure 3-14.   Steel hedgehog.

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.).

Figure 3-15.  Steel tetrahedron.

Figure 3-15.   Steel tetrahedron.

3-17.   CONCRETE OBSTACLES

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.

Figure 3-16.  Concrete cubes.

Figure 3-16.   Concrete cubes.

Figure 3-17.  Concrete cylinder.

Figure 3-17.   Concrete cylinder.

Figure 3-18.  Concrete tetrahedron.

Figure 3-18.   Concrete tetrahedron.

3-18.   EXPEDIENTS

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.

Figure 3-19.  Wire-rope roadblock.

Figure 3-19.   Wire-rope roadblock.

3-19.   Use of Screens and Dummy Obstacles

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

(1)   To conceal the true nature of the obstacle.

(2)   To prevent fire from being directed at the most vulnerable part.

(3)   To confuse the crew of the vehicle. Screens should also be erected in front of dummy obstacles and at sites where no obstacle exists, causing delay and expenditure of valuable ammunition. The enemy force will not know with any certainty what form of obstacle or defense opposes it or whether any real obstacle exists. If the force stops to investigate, the defense will have an opportunity to destroy it; if it goes ahead, it runs the risk of running into mines or of being held on an obstacle under fire.

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.

Section IV.   Beach and River Line Obstacles

3-20.   RESPONSIBILITY

In unilateral Army shore-to-shore amphibious operations, Army forces are responsible for the installation and removal of beach and underwater obstacles. In joint Army-Navy amphibious operations, Navy forces are normally responsible for removal of obstacles on a hostile shore seaward from the high waterline. The underwater demolition teams (UDT) of the Navy have the responsibility of removing obstacles from the high waterline to the 3-fathom (5.54 meters (18.2 ft.)) line. Beyond that point Navy minesweepers clear boat and shipping lanes. The responsibility for installation of beach and underwater obstacles in friendly territory is assigned by the commander of the forces involved.

3-21.   OCEAN BEACH DEFENSES

An assault across an ocean normally involves a ship-to-shore assault in which the enemy requires adequate anchorages for assault shipping and shore for beaching large landing craft. Where the overwater distance is short, however, or where the enemy can develop a nearby base in neutral or unoccupied territory, shore-to-shore operations are practicable using smaller craft capable of landing troops and vehicles at almost any point. Against either of these types of operation, antiboat and antipersonnel obstacles at wading depths are desirable in most situations. Antipersonnel obstacles so located, however, are not effective against large landing craft if the latter can beach at the waterline or can sidecarry floating causeways and use them to get ashore.

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.

Figure 3-20.  Antiboat obstacles in beach defense.

Figure 3-20.   Antiboat obstacles in beach defense.

3-22.   RIVER LINE DEFENSES

All possible means of crossing are studied, including assault boats, footbridges, fixed and floating vehicular bridges, and the use or rehabilitation of existing bridges. In addition to antiboat and antipersonnel obstacles, the defender considers the use of obstacles to hamper the enemy's bridging activities and his installation of booms and other protective devices to protect bridges.

3-23.   EFFECTIVE OBSTACLES

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.

(1)   Steep. For beaches with steeply sloping bottoms, provision should be made for stopping landing craft offshore in deep water. The obstacles may include mines of various types anchored just below the waterline, floating log booms anchored or tied to shore, which may have mines attached, and heavy chains or wire rope stretched between pile dolphins. Preferably such obstacles should be submerged so as to be out of sight but tide variations may make this impracticable. In such cases a compromise must be made between minimum visibility and maximum practicable effectiveness. Where possible, provision is made for adjusting the height of log booms and the like, to conform with water level fluctuations.

(2)   Gradual. For beaches with gradually sloping bottoms, the defense attempts to prevent landing craft from reaching the beach or from reaching wading or fording depth for personnel and vehicles. In addition to obstacles of the types described above, in water of wading depth the bottom is covered thoroughly with underwater wire entanglements of all types. These must be anchored in place very securely to prevent damage from surf or currents and so both enemy and friendly fire will tend to form tangles rather than to clear lanes. In such entanglements, channels provided for passage of friendly small boats may be closed rapidly by the use of anchored concertinas or weighted spirals.

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

(1)   Antiboat obstacles. These are located from wading depth at low tide to wading depth at high tide.

(2)   Barbed wire entanglements. These are placed from wading depth at high tide, inshore across the width of the beach.

(3)   Antivehicular and antipersonnel obstacles. These are installed beginning at low waterline and extending inshore across the width of the beach. Mines or other obstacles are normally installed at the beach exit.

(4)   Antivehicular ditches. These are dug beginning at the inshore edge of the beach, where concealment is possible.

(5)   Other obstacles. These are located inshore of the beach area, in the same manner as obstacles for land defense.

3-24.   LOCATION

Some of the types of land obstacles described previously can be used as antiboat obstacles for some types of boats in water depths for which they are adapted and in which they can be sited and anchored. The tide range determines the water depths for which it is practicable to position obstacles on the bottom above the low waterline. Outside this line, heavy obstacles may be sunk from boats or lowered by cranes operating from the beach or afloat in small landing craft. Posts of timber, steel, or concrete are effective antiboat obstacles, readily placed except in rocky or coral bottoms. Posts preferably are emplaced or driven with a slope or batter toward deep water. Wooden obstacles of other types should be filled with rock or otherwise anchored in position. Antiboat obstacles may be connected with wire rope or may have barbed wire or other types of obstacles anchored between them. In rivers or other locations where the water level is constant or the tide range is minor or negligible, standard cased antitank mines tied to posts or other obstacles under the surface provide effective obstacles.

3-25.   TIMBER OBSTACLES

Unpeeled round logs provide the types of antiboat obstacles described and shown, but sawed timbers may be used if more readily available. In addition to the uses of wooden posts described in paragraph 3-14, timber obstacles of the following types are used effectively under various conditions:

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.

Figure 3-21.  Rock-filled cribs.

Figure 3-21.   Rock-filled cribs.

Figure 3-22.  Rock-filled pillars.

Figure 3-22.   Rock-filled pillars.

Figure 3-23.  Timber tetrahedrons.

Figure 3-23.   Timber tetrahedrons.

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.

Figure 3-24.  Log scaffolding.

Figure 3-24.   Log scaffolding.

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.

Figure 3-25.  Braced wooden posts.

Figure 3-25.   Braced wooden posts.

Figure 3-26.  Log tripod.

Figure 3-26.   Log tripod.

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.

Figure 3-27.  Log ramps.

Figure 3-27.   Log ramps.

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.

Figure 3-28.  Nutcracker.

Figure 3-28.   Nutcracker.

3-26.   STEEL OBSTACLES

Steel beams, piles, and rails provide simple and effective antiboat obstacles of the post type. Steel rails can be driven in rocky or coral bottoms in which wood piles would be splintered. Steel obstacles of portable types are advantageous for underwater use because of the high unit weight of steel; they remain in position without anchorage against waves or currents. Steel obstacles intended for field fabrication for antiboat use are described in a and b below.

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.

Figure 3-29.  Steel scaffolding.

Figure 3-29.   Steel scaffolding.

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.

3-27.   CONCRETE OBSTACLES

As with obstacles of other materials, all types of concrete obstacles previously described can be used as beach obstacles under certain conditions. Concrete obstacles of post type particularly are useful if heavy pile-driving equipment is available. Some types are improved for antiboat use by embedding rails in their tops to form horned scullies. The cylinder modified in this manner is shown in figures 3-30 and 3-31). By setting the rails at an angle of about 45 with the vertical, a fast-moving boat is holed and may be sunk as its momentum carries it down over the length of the horn. The horns may be improved by pointing them, using oxyacetylene cutting equipment.

Figure 3-30.  Horned scully bases on concrete cylinder.

Figure 3-30.   Horned scully bases on concrete cylinder.

3-28.   BARBED WIRE BEACH

OBSTACLES

Wire entanglements are used as antipersonnel obstacles but will stop light landing craft. They are placed inshore of scaffolding or sunken obstacles and, if possible, are covered by machine gun fire. Entanglements normally are built at low tide. They require constant maintenance, particularly if placed in surf. Wire also is erected on beaches or riverbanks, often in connection with antitank and antipersonnel manifolds. Almost all of the types of wire obstacles described previously may be used in conjunction with other types of beach and underwater obstacles.

Figure 3-31.  Horned scullies based on small dragon's teeth.

Figure 3-31.   Horned scullies based on small dragon's teeth.

3-29.   EXPEDIENT UNDERWATER OBSTACLES

The obstacles described in a and b below are made with native materials, some supplemented with barbed wire, and are difficult to reduce. Wherever possible, mines should be used with the obstacles to increase their effectiveness and to hinder removal by enemy underwater demolition teams.

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.

Figure 3-32.  Rock walls.

Figure 3-32.   Rock walls.


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