PURPOSE AND REQUIREMENTS OF FIELD FORTIFICATIONS
Upon completion of this lesson you should be able to
1. Basic Requirement of Field Fortifications. Describe the basic requirements for field fortifications to include efficient employment of weapons, protection qualities, and progressive development.
2. Protective Measures Against Nuclear Weapons. Describe protective measures against nuclear weapons to include before, during and after explosion actions and procedures.
3. Individual Emplacements. Describe construction methods for individual emplacements to include skirmisher's trench, improve emplacement, one-man fox hole etc.
4. Crew Served Infantry Weapons Emplacements. Describe construction of infantry weapons emplacements to include machine gun emplacements, emplacements for recoilless weapons, etc.
5. Vehicle and Artillery Emplacements. Describe construction of vehicle and artillery emplacements to include vehicle pit, towed artillery weapons emplacements, and self-propelled and tank-mounted weapons emplacements.
6. Firebase Construction. Describe construction of a firebase to include layout, artillery requirements, and construction tasks in Phases I, II, and III.
7. Deliberate Shelters and Bunkers. Describe construction of deliberate shelters to include general construction requirements, sectional shelters, bunkers, overhead cover, and standoff.
8. Prefabricated Shelters and Bunkers. Describe construction of prefabricated shelters and bunkers to include design considerations of the WES concrete arch bunker, and WES concrete arch shelter.
9. Protective Shelters for Frozen Environment. Discuss the barriers and shelters which can be constructed of snow or ice and the qualities of snow and ice as protective materials.
Section I. Purpose and Protective Requirements of Field Fortifications
1-1. USE OF FIELD FORTIFICATIONS
a. On the offense. During offensive operations periodic halts may be required to regroup, resupply, or consolidate positions gained. Where the enemy threat is known to include a counterattack capability (or probability), offensive units should seek available cover should dig hasty emplacements.
b. On the defense. A defensive position is built round a series of organized and occupied tactical positions. Positions are selected for their natural defensive strength and the observation afforded. Fortification measures include clearing fields of fire, digging weapons emplacements and positions for personnel, strengthening natural obstacles, installing artificial obstacles, and providing camouflage.
c. Fortification plans. Plans for fortification not only provide for the desired degree of protection but also for bringing the enemy under the maximum volume of effective fire as early as possible. Fortification plans are usually based on progressive construction, that is, proceeding from open to covered emplacements and shelters, to the ultimate protection permissible under the circumstances. Characteristics of personnel and individual weapons emplacements are shown in table 1-1.
d. Dispersion. The separation of units and individuals is a primary means of protection, particularly from the effects of nuclear weapons. If the area occupied by a unit is doubled, it is less vulnerable to shell fire or the effects of nuclear weapons. Proper dispersion can greatly reduce the requirements for high level protection from field fortifications. The amount that a unit spreads out depends on the mission, terrain, and the enemy situation. Fortifications, properly employed, can be used in lieu of, or to supplement, dispersion, but fortifications are particularly important for units that cannot disperse sufficiently to obtain adequate protection.
e. Alternate and dummy positions. When time and the situation permit, dummy and alternate positions should be constructed to deceive the enemy and to allow flexibility in the defense.
Field fortifications are constructed by personnel of all arms and services. Hasty shelters and emplacements are normally constructed by the combat units occupying the position. Some engineer equipment and supervisory assistance are frequently required to assist the combat units. Fortifications of a more complex character may require construction by engineer troops. Actually, engineers at all echelons of command assist in the preparation of plans and orders and furnish technical advice and assistance in the construction of field fortifications.
1-3. BASIC REQUIREMENTS FOR FORTIFICATIONS
a. Employment of weapons. Emplacements must permit effective use of the weapons for which they are designed. This requirement may limit the protection which can be provided and may influence the design and depth of adjacent shelters.
b. Protection. As far as possible, protection should be provided against hazards except a direct hit or a close nuclear explosion. To obtain maximum protection, excavations should be as small as possible, thereby limiting the effective target area for high trajectory weapons and airbursts.
c. Simplicity and economy. The emplacement or shelter should be strong and simple, require as little digging as possible and be constructed with materials that are immediately available.
d. Progressive development. Plans for defensive works should allow for progressive development to improve the usefulness of the fortification. Development fortifications can be accomplished in three steps
(1) Digging in quickly where speed is the principal consideration and no special tools or materials are required.
(2) Improvising with available materials.
(3) Refining, using stock materials.
e. Camouflage and concealment. Fortifications should be built so that the completed work can be camouflaged. It may not be practical to conceal a defensive position completely, but it should be camouflaged enough to prevent the enemy from spotting the position by ground observation. If possible, dummy positions should be constructed the same time as the actual position.
f. Ingenuity. A high degree of imagination and ingenuity is essential to assure the best use of available materials as well as the best choice and use of the fortifications constructed.
1-4. PROTECTION FROM CONVENTIONAL WEAPONS
a. Digging in. Protection against conventional weapons is best provided by constructing a thickness of earth and other materials. This is done by digging into the ground so that personnel and equipment offer the smallest target possible to the line of sight of weapon. This means of protection is effective against direct fire of small arms and horizontally impelled shell fragments. Digging in also provides some protection against artillery, infantry heavy weapons, bombs, and other serial weapons. Advantage should be taken of all available natural cover. Improvement of the position continues until the unit leaves the area.
b. Overhead cover. Overhead protection is important particularly in the forward areas where the threat includes airburst shelling in addition to the possibility of nuclear attack. Covered firing positions should be built for individual riflemen. Small readily accessible shelters adjacent to weapons emplacements are also necessary. A minimum of 15-20 centimeters (6-8 inches) of logs, 45 centimeters (18 inches) sandbags, rocks, and dirt, in that order, is required for overhead protection. Any available material may be used but cover should be kept low. However, cover of this type will not protect personnel against direct shell hits. Overhead cover should be strengthened and improved as long as the position is occupied. Only part of the firing position should be covered. Sandbags are placed over the logs to prevent dirt from falling on the occupants.
1-5. PROTECTION FROM CHEMICAL AND BIOLOGICAL WEAPONS
Open or partially open emplacements afford no protection from chemical or biological attack. Personnel in open emplacements should use the poncho for protection against liquid contamination and the protective mask to provide protection from chemical vapors and biological aerosols. Overhead cover will delay penetration of chemical vapors and biological aerosols, thereby providing additional masking time and protection against direct liquid contamination. Covered emplacements with relatively small apertures and entrance areas which can be closed, provide protection from napalm and flame-flame throwers.
Section II. Nuclear Weapons Protective Methods
Since the threat of nuclear weapons is present in modern warfare, it is necessary to understand the effects of these weapons on personnel and equipment. Well trained and well disciplined soldiers can protect themselves and their equipment against this threat and continue their mission, even though a nuclear weapon has more destructive power than any other device.
1-7. WEAPONS EFFECTS
a. Radiation. The presence of radiation and the high intensity of the blast or earth shock following nuclear explosions distinguishes them from the effects of conventional bombs or other explosives.
(1) Thermal radiation. A nuclear explosion causes extreme heat and light that are comparable in intensity to the surface of the sun. Heat from nuclear explosions causes varying degrees of burns from the equivalent of a mid sunburn to more severe injuries. The intense heat may also set fire to buildings, forests, and equipment. Light from a nuclear explosion may daze personnel for a short time during daylight, and at nighttime the effects are even more severe, lasting about 10 minutes. Night vision may be impaired for an extended period of time, and permanent injury will result if the eyes are focused in the direction of the burst.
(2) Nuclear radiation. Initial and residual radiation effects are associated with nuclear explosions. Initial radiation which is emitted within a minute of the burst travels in straight lines at about the speed of light and has a high penetrating effect. Residual or lingering radiation comes from the radioactive materials originally in a nuclear weapon or from normally nonradioactive materials (such as soil or equipment) which have been made radioactive by the nuclear reaction. Substances, including soil or equipment that have become contaminated and remain radioactive emit induced radiation, one form of residual radiation. Another form of residual radiation, commonly referred to as fallout, is induced when a nuclear explosion occurs under, at, or near the surface of the earth and large quantities of dirt and debris are thrown up, mixing with the fireball. These radioactive particles in the atmosphere gradually fall to earth. Injuries from nuclear radiation are caused by the penetrating rays of the initial and residual radiation. The degree of injury depends on: (a) amount of body exposure; (b) length of time exposed; (c) previous radiation damage to the body tissue; (d) other injuries received which may contribute to disability; (e) general physical condition.
b. Blast and earth shock. Blast and associated earth shock effects are caused by violent changes in pressure that move out in all directions from the center of the explosion, like a very strong wind. Most direct injuries from the blast effects result when personnel are thrown to the ground by the blast. Indirect blast injuries are sustained from flying debris and the collapse of emplacements, shelters, trenches, or buildings.
1-8. PROTECTIVE MEASURES
Individual protective measures against nuclear weapons should be taken before, during, and after the explosion. All personnel should follow the unit standing operating procedure (SOP) which covers such items as the use of protective equipment, warning signals, first aid, firefighting, reorganization, marking of contaminated areas, and decontamination. The following general procedures are also applicable:
a. Before the explosion. If there is warning of a nuclear explosion, and available time and the tactical situation permit
(1) Positions should be improved dug deeper and covered. Even a shelter half, covering the top of a foxhole, provides some protection. A poncho should not be used because it may get too hot, melt, and cause burns. The position should be revetted if possible.
(2) If there is not enough time to prepare a good position, a shallow trench should be dug deep enough so the body is below the surface of the ground and covered with a shelter half.
(3) Helmet should be worn, and personnel should keep their faces down.
b. During the explosion.
(1) Personnel should crouch low in their foxholes with their heads down, or lie flat in their trench with shelter halves over them.
(2) If they are in the open, soldiers should try to get into a nearby ditch or behind a wall, but they should not try to get to a shelter if it is more than a few yards away.
(3) If no shelter is available nearby, personnel should turn their backs to the explosion while dropping to a prone position.
(4) The brilliant flash will cause temporary blindness and may use permanent eye damage if looked at directly.
(5) Personnel should stay where they are until the blast wave passes. By this time, the greatest danger from heat, initial radiation, blast, or shock will be over.
c. After the explosion.
(1) Fallout will usually be present so personnel should
(a) Keep under cover until fallout has stopped.
(b) Brush the dust from their clothing. Scrape up and throw out any dirt or other material which has fallen into the foxholes. Dig out dirt and pile it several inches deep for at last 1 meter around the hole.
(c) Clean equipment as well as available material permits.
(d) Help others as much as possible.
(2) Be prepared to continue the mission. The enemy can be expected to follow up a nuclear explosion to take advantage of any resulting damage and confusion. Stay in position to repel an attack.
1-9. OTHER MEASURES
a. Friendly nuclear weapons. Nuclear weapons may be used close to our own areas. If used close enough to be dangerous there will be warning and instructions on precautions to take. The individual protective measures discussed above are important in a situation of this kind.
b. Contaminated areas. If required to occupy a contaminated (radioactive) area, the following actions should be taken:
(1) Dig foxholes quickly.
(2) Scrape dirt from around the edge of the foxholes for at least 1 meter, and scatter dirt around the foxholes for 10 meters.
c. Contaminated equipment. Trained personnel in each unit will use instruments to test equipment and supplies for contamination. If instruments are lacking, the urgency of the situation dictates whether equipment is used without being tested. Generally, equipment not damaged by the explosion is safe to use. Washing or brushing will usually make contaminated equipment safe to operate.
Section III. Principles and Methods of Construction
a. Natural. Full use is made of all available natural materials such trees, logs, and brush in constructing and camouflaging emplacements, shelters and overhead cover. Usually, enough natural material can be found to meet the requirements for hasty or expedient fortifications. Snow and ice may be used in the construction of emplacement and shelters in cold regions.
b. Other materials.
(1) Manufactured materials, such as pickets, barbed wire, cement, lumber, sandbags, corrugated metal, and other materials for revetting, camouflage, shelter, and concrete construction are supplied by support organizations.
(2) Captured enemy supplies, locally procured material, and demolished buildings are other sources of fortifications construction materials.
1-11. METHODS OF EXCAVATING
a. Handtools. The individual soldier is equipped with an intrenching tool and, if necessary, he can use his bayonet to assist in digging. Pick mattocks, shovels, and other tools are also useful, and frequently available for this purpose (fig 1-1). In addition, captured enemy equipment may be available. The relative value of each tool depends on the soil and terrain. In arctic areas, a larger quantity of picks and pick mattocks are required to aid in the preparation of emplacements in frozen ground.
b. Equipment. Relatively narrow cuts with steep or nearly vertical sides required for most emplacements or shelters can be excavated more accurately by hand. However, intrenching machines, backhoes, bulldozers, bucket loaders, and scrapers may be where the situation will permit the use of heavy equipment. Usually, these machines cannot dig out the exact shape desired or will dig more earth than necessary, requiring completion of the excavation by hand. Additional revetment material is usually required when machines are used. Distinctive scars on the ground resulting from the use of heavy equipment require more effort for effective camouflage than fortification work performed by hand.
c. Explosives. Many fortification tasks are made easier and accomplished more quickly by using explosive in any type of soil. Special explosive digging aids available include the M2A3 and M3 shaped demolition charges, and the field expedient det cord wick.
Figure 1-1. Intrenching equipment.
SECTION IV. Construction of Individual Emplacements
1-12. TYPES OF EMPLACEMENTS
a. Hasty emplacements. Hasty emplacements are dug by troops in contact with the enemy, when time and materials are limited. Hasty positions should be supplemented with overhead cover and strengthened as conditions permit. If the situation permits, the small unit leader will verify the sectors of observation and fire for the individual members of the squad from their designated positions before they dig individual foxholes. When the situation is stabilized, even temporarily, positions are selected so they can be connected by trenches later. The emplacements described below provide protection against flat trajectory fire. They are used when there is no natural cover. Hasty positions (figure 1-2) are good for a short time because they give some protection from direct fire. If the unit remains in the area, they must be developed into well-prepared positions to provide as much protection as possible.
Figure 1-2. Hasty positions in an open field.
(1) Shell crater. A shell or bomb crater of adequate size, 0.6 to 1 meter (2 to 3 ft), offers immediate cover and concealment and can be quickly made into a hasty position (figure 1-3). By digging the crater to a steep face on the side toward the enemy, the occupant can provide himself with a firing position. A small crater can later be developed into a foxhole. Craters, even if developed, are susceptible to being overrun by tracked vehicles.
Figure 1-3. Improved crater.
(2) Skirmisher's trench. This shallow pit type emplacement (fig 1-4) provides a temporary open prone firing position for the individual soldier. When immediate shelter from heavy enemy fire is required and existing defiladed firing positions are no available, each soldier lies prone or on his side, scrapes the soil with his intrenching tool, and piles it in a low parapet between himself and the enemy. In this manner, a shallow body-length pit can be formed quickly in all but the hardest ground. The trench should be oriented so that it is least vulnerable to enfilade fire. A soldier presents a low silhouette in this type of emplacement and is protected to a limited extent from small arms fire. It can be further developed into foxhole or a prone emplacement.
Figure 1-4. Skirmisher's trench.
(3) Prone emplacement. This emplace-ment (fig 1-5) is a further refinement of the skirmisher's trench. The berm dimension of this emplacement, as shown in the parapet detail, is varied to conform to the position and arm length of the occupant. It serves as a good firing position for a rifleman and provides better protection against small arms or direct fire weapons than the improved crater or skirmisher's trench.
(4) Rocks, snow, and ice. Limited protection can be provided by piling up rocks, chunks of ice, or packed snow. Icecrete, formed by mixing dirt and water, is very effective as an arctic building material. A minimum of 30 centimeters (12 inches) of this material will resist penetration of small arms fire.
Figure 1-5. Prone emplacement.
b. Foxholes. Foxholes are the individual rifeman's basic defensive position. They afford good protection against enemy small arms fire and can be developed from well chosen craters, skirmisher's trenches, or prone emplacements. Foxholes should be improved, as time and materials permit, by revetting the sides, adding expedient cover, providing drainage, and excavating a grenade sump to dispose of handgrenades tossed into the hole by the enemy.
(1) One-man foxhole. The overall dimen-sions and layout of the one-man foxhole are shown in figure 1-6.
(2) Construction details.
(a) Fire step. The depth of the fire step will vary depending on the height of a comfortable firing position for the occupant, usually 1 meter to 1.5 meters (3 1/2 to 5 ft). The occupant, crouched in a sitting position on the fire step, must have at least 60 cm (2 ft) of overhead clearance if a tank overruns the foxhole. This will normally provide protection against the crushing action of tanks; however, in loose unstable soils it will be necessary to revet the walls of the foxhole in order to provide this protection.
Figure 1-6. One-man foxhole.
(b) Water sump. A water sump, 45 cm (18 in.) by 60 cm (2 ft.) and 45 cm (18 inches) deep below the fire step, is dug at one end of the foxhole to collect water and to accommodate the feet of a seated occupant. One or two layers of large stones are then placed at the bottom of the hole with smaller stones on top up to the level of the ground (fig 1-6). The sump may simply provide a collecting basin from which water can be bailed.
(c) Grenade sump. A circular grenade sump large enough to accept the largest known enemy grenade and sloped downward at an angle of 30° is excavated under the fire step beginning at the lower part of the fire step riser. Handgrenades thrown into the foxhole are exploded in this sump, and their fragmentation is restricted to the unoccupied end of the foxhole. For good drainage and to assist in disposing of grenades, the fire step is sloped toward the water sump, and the bottom of the water sump is funneled downward to the grenade sump.
(d) Parapet. If excavated spoil is used as a parapet (fig 1-6), it should be placed as a layer about 1 meter (3 feet) wide and 30 cm (6 inches) high all around the foxhole leaving an elbow rest (berm) of original earth about 60 cm (1 foot) wide next to the foxhole. If sod or topsoil is used to camouflage the parapet, the sod or topsoil should be removed from the foxhole and parapet area, set aside until the parapet is complete, and then placed on top in a natural manner.
(e) Camouflage. Whether or not a parapet is constructed in wooded or brushy type terrain, a foxhole can be camouflaged effectively with natural materials, as shown in figure 1-7. In open or cultivated areas, it may be preferable to omit the parapet, remove the excavated soil to an inconspicuous place, and improvise a camouflage cover for the foxhole. This can be a light, open frame of branches garnished with grass or other natural foliage to match the surroundings. As an alternate method, the foxhole can be covered with a shelterhalf, poncho, or other expedient material, and further covered with snow or some other material, according to local terrain conditions (fig 1-7). The occupant raises one side of the cover for observation or firing.
(f) Overhead cover. A half-cover (fig 1-8) over a one-man foxhole provides good protection for the occupant and permits full use of the weapon. Logs, 10 to 15 cm (4 to 6 in.) in diameter of 14 cm (6 in.) timbers approximately 1.2 meters (4 ft.) in length, support the earth cover. They should be long enough to extend at least 30 cm (1 ft.) on each side of the foxhole to provide a good bearing surface. Dirt should be removed on each side of the foxhole so that the supporting logs or timbers are even with the ground surface. If the ground is soft and tends to break away, a bearing surface of planks or timbers should be provided for cover supports. Logs or timbers of this size will support an earth cover 30 to 45 cm (1 ft to 1 1/2 ft) thick. The walls of the foxhole should be stabilized with revetment material (fig 1-9) at least under the overhead cover to prevent a cave-in from the added weight of the cover.
Figure 1-7. Camouflaged one-man hole.
(g) Revetment material. Use of different types of revetting material are shown in figure 1-9. Expedient material, such as brushwood, saplings, sheet metal, or dimensioned lumber should be thin and tough so that it will support the sides of the emplacement when properly staked and tied. Revetment stakes, either metal or wood 1.8 meters (6 feet) in length, should be spaced not more than 60 centimeters (2 feet) apart and driven into the ground 30 to 45 centimeters (12 to 18 inches). The revetment stakes are held firmly in place by anchor wires of barbed wire or 14 gage wire attached to anchor stakes (fig 1-10). Five or six strands of wire should be stretched between the revetment and anchor stakes at ground level and tightened by twisting. The distance between the revetment and anchor stakes should be approximately twice the depth of the excavation. The wire between the stakes should not pass over the parapet in any case.
Figure 1-8. One-man foxhole with half cover.
Figure 1-9. Types of revetting material.
Figure 1-10. Supporting and anchoring revetment.
(3) Open two-man foxhole. In a defensive position, the two-man foxhole (fig. 1-11) is generally preferred to the one-man emplacement.
Figure 1-11. Open two-man foxhole.
(a) Advantages. One man can provide protection while the other is digging. It affords relief and rest, for the occupants as one man rests while the other observes. In this manner, firing positions can be effectively manned for longer periods of time. If one soldier becomes a casualty, the position is still occupied. The psychological effect of two men together permits positions to be occupied for longer intervals.
(b) Disadvantages. If a direct hit occurs, two men will become casualties instead of one. Also, the area that can be occupied may be reduced significantly.
(c) Construction. The two-man foxhole is constructed the same as the one-man foxhole except for the location of the grenade sump which is dug into the face of the foxhole towards the enemy.
c. Full frontal berm overhead rifle positions.
(1) Large positions are weak positions. Each rifle position should begin with a rectangular hole as long as the shoulder-to-shoulder length of the men who will fight in it, as wide as a man plus equipment. Spoil should be placed forward and to the sides to form a sloped, progressively packed berm.
Figure 1-12. Two-man foxhole with offset.
Figure 1-13. Two-man foxhole with offset constructed of timber and culvert.
(2) The next step at the hole is to cut firing apertures at 45 degrees to the direction of the enemy and deepen the hole, tailoring the depth to each man, and carving elbow rests in the parapet for each rifleman to insure solid elbow-under-the piece firing positions. Apertures should be provided with grenade sumps and dug narrow and tapered, just sufficient to command the assigned sector of fire (fig 1-14). Automatic rifles on bipods should have bipod rest slot cu forward to the parapet to allow the bipod to be withdrawn easily and to rest the gun muzzle low. The fitting for firing should be undertaken carefully to counter the natural tendency to shoot high at night. This is accomplished by digging each man down so that standing in his hole in firing position his piece is level at height for graze, firmly seated and supported, and as close to the ground as his mission permits.
Figure 1-14. Rifle position.
(3) Overhead cover is now added. Cover can be fabricated from large logs plus sandbags or dirt, or sapling/bamboo mats in three crosslaid laminations plus sandbags or dirt, but should be sturdy enough to take the full weight jumping of a large combat-loaded soldier. Full overhead cover is constructed over all positions; full cover cuts vulnerability to airbursts and grenades, and lessens prospects of flooding in the event of rain. Care must be exercised to hold the silhouette of the position as low as possible, and the apertures as small as sector of fire permits. The height of the cover is determined by placing the firers in the hole, and the cover adjusted a full inch above their helmets while they take up night firing positions -head high over the sights. Berms and apertures should be extended and sloped forward to cut vertical surfacing, and to hide muzzle flash from the front. Construction of revetted, walls and overhead cover should follow principles outlined in b above.
(4) Camouflage is now added. Preferably this should be rooted plants and grass sod, calculated to grow naturally in place on the position, and to blend fully with the surrounding vegetation. While digging the fighting position, maximum care should be taken to prevent the destruction of natural camouflage growing near the hole, scaling off the sod to a depth that will maintain the roots, and setting it aside to be used for camouflage upon completion of the overhead cover.
(5) A rear entrance is now dug. This rear entry should be an auxiliary, fully open, individual firing position that can be used for throwing grenades and M79 fires. It should be, at minimum designed to allow entry into the hole from the rear, covered with a poncho or similar screen to cut down backlighting the firing apertures. Full consideration should be given to protecting defenders from friendly direct fire weapons located to the rear of the position (including artillery beehive ammunition), and to emergency resupply. The sleeping position should be directly behind the righting position. A completed rifle position (except for camouflage) is shown in (3), figure 1-14.
Figure 1-14. Rifle position (Continued)
(6) A machinegun position is constructed generally following the same procedure as for the rifle position, except that the hole must be designed around the gun. The first step is to emplace the gun on its final protective line and walk the latter to check the site. The hole is then traced to place a sturdy firing table with working room for the loader on the left.
d. Overhead cover for foxhole (fabric). When available Overhead Cover for Foxholes (OCF) is an effective protection for 1-and 2-man foxholes (fig 1-15).
(1) Description. The foxhole cover is a woven dacron fabric laminated to polyester film. It is 1.5 meters (5 feet 4 inches) long by 1.8 meters (6 feet) wide and weighs slightly less than 2 pounds. Connected to each side of the width are tubular sections, 15 centimeters (6 inches) in diameter and 1.5 meters (5 feet 4 inches) long. It will support 45 centimeters (18 inches) of soil over any emplacement, while simultaneously withstanding the blast effects of a nuclear weapon. The unit will cover 1- and 2-man foxholes and is capable of combining in multiple to cover shelter portions of crew-served weapons emplacements. It will function over emplacements 25 percent larger than standard.
(a) After completing excavation of the foxhole (trench), unfold the cover (OCF) and place it over the foxhole, locating the pouch at each side of the cover in a position parallel to the edge of the foxhole. Center the cover so that the pouches are approximately the same distance from the edge of the foxhole.
Figure 1-15. Overhead cover for foxhole (fabric).
(b) Allow a minimum of 60 centimeters (2 feet) of open space at one end to permit entrance into the foxhole. Be sure that the cover extends at least 10 centimeters (4 inches) beyond the closed end of the foxhole , fig. 1-16). This will insure that oil does not slide down the end wall of the foxhole.
(c) Mark the inside (nearest foxhole) edge of the pouches in the soil on both sides of the trench. Using the marked lines as the inside edge, dig two (one on each side) shallow trenches approximately 10 centimeters (4 inches) deep, 25 centimeters (10 inches) wide, and 1.67 meters (5 feet 6 inches) long parallel to the length of the foxhole , fig. 1-15).
(d) Using entrenching tool, fill one pouch with soil using both ends as filling points. Fill other pouch in the same manner. Stretch the cover taut between the two pouches because a taut cover does not sag as much as a loose cover , fig. 1-15).
(e) Place soil backfill around the edges of the foxhole to a depth of 45 centimeters (18 inches) minimum. The sloping outside soil edge should cover the soil filled pouch , fig. 1-15).
(f) Complete covering with soil to a uniform depth of 45 centimeters (18 inches) but continue placing soil from the edges toward the center , fig 1-15).
(g) Snap fasteners on each end of the cover provide means of covering a trench by connecting two or more covers , fig 1-15), When connecting two or more units to form a covered trench type structure, it is essential that only steps 1 through 3 should be completed before connecting successive units. Any two units to be connected should be connected with the five middle snaps before the pouches of the second one are filled with soil. After the pouches are filed, the eight remaining snaps should be connected. Step 4 may be completed before all covers are erected over the trench but only on units positioned two or more units back from the last one connected.
1-13. FIELDS OF FIRE
a. Principles. There is little opportunity to clear fields of fire when a unit is in contact with the enemy. Individual riflemen and weapons crews must select the best natural positions available. Usually, there is only time to clear areas in the immediate vicinity of the position. However, in preparing defensive positions for expected contact with the enemy, suitable fields of fire are cleared in front of each position. The following principles are pertinent:
(1) Excessive or careless clearing will disclose firing positions (fig 1-16).
(2) In areas organized for close defense, clearing should start near the position and work forward at least 100 meters (328 ft) or to the maximum effective range of the weapon if time permits.
(3) A thin natural screen of vegetation should be left to hide defensive positions.
(1) Remove the lower branches of large scattered trees in sparsely wooded areas.
(2) In heavy woods, fields of fire may neither be possible nor desirable within the time available. Restrict work to thinning the undergrowth and removing the lower branches of large trees. Clear narrow lanes of fire (fig. 1-17), for automatic weapons.
Figure 1-16. Clearing fields of fire.
(3) Thin or remove dense brush since it is never a suitable obstacle and obstructs the field of fire.
(4) Cut weeds when they obstruct the view from firing positions.
(5) Remove brush, weeds, and limbs that have been cut to areas where they cannot be used to conceal enemy movements or disclose the position.
(6) Do only a limited amount of clearing at one time. Overestimating the capabilities of the unit in this respect may result in a field of fire improperly cleared which would afford the enemy better concealment and cover than the natural state.
(7) Cut or burn grain, hay, and tall weeds.
(8) Whenever possible, check position from the enemy side to be sure that the positions are effectively camouflaged and they are not revealed by clearing fields of fire.
Figure 1-17. Clearing fire lanes.
Section V. Construction of Crew Served Infantry
1-14. PRINCIPAL CONSIDERATIONS
a. Firing positions. While it is desirable to give maximum protection to personnel and equipment, the principal consideration must be the effective use of the weapon. In offensive combat, infantry weapons are sited wherever natural or existing positions are available or where weapons can be emplaced with a minimum of digging. The positions described in this section are designed for use in all types of terrain that will permit excavation.
b. Protection. Protection of crew-served weapons is provided by emplacements which give some protection to the weapon and crew while in firing positions. As the positions are developed, the emplacements are deepened and provided with half overhead cover, if possible. Then, if the positions are occupied for an extended period of time, shelters adjoining the emplacement or close to it should be built. Characteristics of crew served infantry weapons emplacements are shown in table 1-2.
c. Crew shelters. Shelters immediately adjoining and opening into emplacements improve the operational capability of the crew, since the men are not exposed when moving between the shelter and the weapon.
1-15. MACHINEGUN EMPLACEMENTS
a. Pit type. The gun is emplaced initially in a hasty position (fig. 1-18).
b. Horseshoe type. The dimensions and layout of the completed emplacement are shown in figure 1-19. The horseshoe shaped trench, about 60 centimeters (2 feet) wide, is dug along the rear and sides, leaving a chest-high shelf in the center to serve as the gun platform. The spoil from this trench is used to form the parapet, making it at least 1 meter (40 inches) wide and low enough to permit all-round fire. This type emplacement permits easy traverse of the gun through an arc of 180°, but the crew cannot fire to the rear effectively. The firing table must be reverted to prevent the vibrations of the automatic weapons from breaking down the walls of the table.
Figure 1-18. Planview and cross section of machinegun emplacement.
Figure 1-19. Horseshoe type machinegun emplacement.
c. Two one-man foxhole type. This emplacement consists of 2 one-man foxholes close to the gun position as illustrated in figure 1-20. The parapet is low enough for all-round fire and good protection for the crew, A foxhole is dug for the gunner at the rear of the gun and another foxhole is dug for the assistant gunner on the left of the gun and 45 cm (18 in.) in front of the gunner's foxhole. The spoil is piled all around the position to form a this position, fire to the front or rear is most effective since the M60 machinegun is fed from the left side.
Figure 1-20. Two one-man foxhole type machinegun emplacement.
1-16. EMPLACEMENTS FOR RECOILLESS WEAPONS
a. Types. Two types of open emplacements for recoilless weapons are the pit type and the two two-man foxhole type.
(1) Pit type. This emplacement is a circular pit about 1.2 meters (4 feet) in diameter and about 1 meter (40 inches) deep depending on the height of the occupants. A parapet should not be constructed for this emplacement because of the backblast. It is large enough for two men and permits the assistant to turn with the traversing weapon, to avoid being behind it when it is fired. This emplacement is shallow enough to permit the rear end of the weapon to clear the top at maximum elevation, thus insuring that the hot backblast of the rockets is not deflected to the occupants. Since this emplacement offers protection for the crew against d , fig. 1-21).
(2) Two two-man foxhole type. The emplacement shown in figure 1-21 provides limited protection for the crew against nuclear effects and armor except when actually firing.
b. Blast effects. Due to the backblast effects of the recoilless weapon, it should not be fired from a confined space such as a fully covered emplacement. Because the backblast will reveal the firing position, alternate firing positions with the connecting trenches should be constructed if there is sufficient time.
1-17. MORTAR EMPLACEMENTS
a. General. The emplacement illustrate in , figure 1-22 is circular in shape. The emplacement is excavated to the dimensions shown with the sides of the emplacement sloping inward toward the bottom. The floor slopes to the drainage sump located under the open gap in the parapet. An ammunition ready rack or niche, located so that it is convenient for the gunner, is built into the side of the emplacement. The bottom of the ammunition rack is elevated from the floor of the emplacement. Another ready rack may be constructed in one side of the trench leading to the position. The initial emplacement is revetted using sandbags and the improved emplacement is revetted using corrugated metal. Before constructing the parapet, the mortar is laid for direction of fire by the use of an aiming circle or alternate means. The parapet should be not more than 50 cm (20 inches) high and a minimum of 1 meter (3 feet) wide. An exit trench may be constructed leading to personnel shelters and to other mortar positions. Construction of the parapet should be coordinated with the infantry commander.
Figure 1-21. Emplacement for recoilless weapons.
Figure 1-22. Mortar emplacements.
b. The 81-mm mortar. A pit type emplacement for the 81-mm mortar is shown in , figure 1-22.
c. Emplacement for 4.2-inch mortar. The 4.2-inch mortar emplacement is identical to the one parapet, care being taken to pile it so as to permit all-round fire of the weapon. Although 360° fire is possible from described above for the 81-mm mortar except for dimension changes shown in , figure 1-22.
1-18. EMPLACEMENT FOR 106-mm RECOILLESS RIFLE OR TOW MISSILE (JEEP MOUNTED)
These weapons are often fired from their 1/4-ton truck mounts since the weapons should be mobile and moved to new positions after firing a few rounds. In a defensive operation, several open pits should be constructed with concealed routes from these firing positions to a concealed shelter position with overhead cover. The weapons remain in the shelter until needed, then after firing, they are moved to other firing positions or back to their shelters. The firing pit for these weapons should protect the sides and front of the body of the vehicles. The rifle or TOW should be above the parapet level. The rear of the emplacement should be ramped so the vehicle can move out quickly. Emplacements of this type require approximately 30 manhours to construct since alternate positions are required, so the necessity for using heavy equipment is obvious. Figure 1-23 illustrates an emplacement for the 106-mm recoilless rifle which will permit the weapon muzzle to extend over the parapet to preclude damage to the vehicle from the muzzle blast.
Figure 1-23. Emplacement for 106-mm rifle TOW missile.
Section VI. Construction of Vehicle and
1-19. TYPICAL VEHICLE PIT
Digging in should be restricted to essential vehicles. Vehicle pits should be as narrow and as short as the vehicle size permits. They should be oriented randomly. All canvas should be removed and the top of the trucks should be at least 1 foot below the top of the surrounding parapet. The excavations should be as shown in table 1-3 and figure 1-24. Use of soil in construction of the parapet reduces the depth of cut necessary to properly protect a vehicle. The parapet should be streamlined and as well compacted as possible. The majority of vehicles should be concealed or camouflaged, with advantage taken of natural features such as woods, defilade, hegerows, and buildings.
Figure 1-24. Typical vehicle pit.
1-20. TOWED ARTILLERY WEAPONS
a. Purpose. Emplacements for artillery weapons must provide maximum flexibility in the delivery of fire and protect the weapon and its crew against the effects of conventional and nuclear weapons.
b. Emplacement for 105- and 155-mm howitzer. Artillery weapons emplacements are constructed so as to allow for continuous improvement in order to provide additional protection and comfort in the event of prolonged occupation. These emplacements are developed in stages as described in (1) through (4) below.
(1) Stage 1. This stage provides open foxholes for the protection of the crew and open emplacements for infantry weapons used to defend the position. Provision is made for only minimum essential shifting of the gun trail and ammunition is stored in the open. Stage-one emplacement for a 105-mm howitzer is illustrated in figure 1-25.
(2) Stage 2. This stage provides trail logs for all around traverse of the weapon, a low parapet to protect the weapon, and covered emplacements for the crew, defensive weapons, and ammunition. Stage-two emplacement for a 105-mm howitzer is illustrated in , figure 1-25.
(3) Stage 3. In this stage a parapet revetted on the inside which permits all around direction fire is provided. Work is begun on covered shelters for personnel and ammunition. Stage-three emplacement for a 10-mm howitzer Is illustrated in , figure 1-25.
Table 1-3. Dimensions of Typical Vehicle Pits
Figure 1-25. Development of 105-mm Howitzer emplacement.
Figure 1-25. Development of 105-mm Howitzer emplacement. (Continued)
(4) Stage 4. In this stage revetment is provided for the round fighting positions and for the outside and top of the parapet. Overhead cover is also provided for the personnel ready position and the ammunition shelter. Stage-four emplacement for a 106-mm howitzer is illustrated in) , figure 1-25 and , figure 1-26. Dimensions and layout are also shown in figure 1-26. Figure 1-27 shows semipermanent position for 155-mm howitzer.
(5) Use of overhead over. It is usually difficult to provide overhead cover for artillery weapons. The widths and heights involved make such construction impractical under most conditions. Overhead cover would unduly restrict the firing capability of the weapon. In addition, under most conditions, it is not desirable to excavate an emplacement for the weapon much below ground level or to construct a high all-round parapet for the following reasons:
(a) A high all-round parapet restricts the direct fire capability of the weapon.
(b) An emplacement excavated below ground creates difficulty in rapid removal of the weapon from the emplacement.
c. Accessory structures.
(1) Ammunition shelters. Sectional shelters as described previously may be used with overhead cover as ammunition shelters with the types of weapons emplacements discussed above.
(2) Accessory shelters. Ready shelters for personnel and shelters for fire direction centers and switchboards are constructed using standard shelter designs.
a. Self-propelled artillery. Large caliber self-propelled weapons have a limited traverse without turning the vehicle. For this reason it is seldom practical to construct emplacements for this type of weapon. When positions for self-propelled weapons are prepared, a sloped ramp is built to facilitate the vehicle's entry into and withdrawal from the gunpit. In extremely cold weather, gravel, saplings, or similar covering may be necessary for the floor of the pit so that the tracks of the vehicles will not freeze to the ground. The rear of the pit and the sloped ramp should be widened sufficiently to permit driving the vehicle in at an angle in order to compensate for the limited traverse of the weapon.
Figure 1-26. Final stage development, howitzer emplacement.
Figure 1-27. Position for 155-mm howitzer, SP (semipermanent installation).
b. Tanks. A tank is emplaced or protected in the same manner as any other vehicle. Natural defilades such as road cuts or ditches are used where available. In open areas, parapets are provided to protect the sides and front of the hull of the vehicle, and the rear is left open. The simplest form of a dug-in position of this type is shown in figure 1-28. Whenever possible, such positions are constructed and occupied during darkness, with all camouflage being completed before dawn. The emplacement normally includes foxhole protection for relief personnel, preferably connected with the emplacement by a short trench. A dug-in emplacement of this type should have the following:
Figure 1-28. Dug-in emplacement for self-propelled weapons.
(1) An excavation deep enough to afford protection for the tracks and part of the hull of the vehicle with maximum thickness of the parapet at the front of the emplacement and the rear left open for entry and exit of vehicle.
(2) Inside dimensions just large enough to permit entry and exit of vehicle.
(3) An inside depth permitting the weapon to depress to its minimum elevation. Tank emplacements must have sufficient space for the storage of ammunition.
(4) Barrel stops, if necessary, to prevent fire into adjacent units.
(5) Provisions for drainage (if possible) and frostproof flooring to prevent tracks from freezing to the ground.
(6) If it is necessary to deliver fire at elevations higher than permitted by the carriage design, the floor must be sloped up in the direction of fire.
1-22. ARTILLERY EMPLACEMENT IN SOFT GROUND
a. The siting of artillery positions in areas where the ground is soft requires the construction of pads to preclude differential settlement and thus the relaying of the weapon after each round is fired. Wooden pads can be built using laminated construction of radial sleepers (fig. 1-29) and other construction techniques. The wooden pad distributes the load over a large area with no significant settlement and is flexible and strong enough to withstand the turning and movement of self-propelled weapons. The trail logs are anchored just outside the pad for towed weapons. For self-propelled weapons, the recoil spades can be set in compacted material or in a layer of crushed rock just off the pad. Figure 1-30 shows position with pad for the 8-inch or 175-mm gun and figure 1-31 shows another pad for these guns which has non-radial sleepers and laminated flooring. Figure 1-32 shows a concrete gun pad for these weapons. Figure 1-33 shows light and medium artillery battery layouts. Revetments and shelters can be constructed as described in paragraph 1-21.
Figure 1-29. Radial and laminated gun pads.
b. Various synthetic materials, such as fiberglass mats (also used as helicopter landing pads), may be used as gun pads depending on the characteristics of the weapons.
Figure 1-30. Position with 8-inch or 175-mm gun pad with radial sleepers.
Figure 1-31. Gun pad for 8-inch or 175-mm with non-radial sleepers.
Figure 1-31. Gun pad for 8-inch or 175-mm gun with non-radial sleepers. (Continued)
Figure 1-32. Concrete gun pad for 8-inch or 175-mm gun.
Figure 1-33. Light and medium artillery battery layouts.
Section VII. Firebase Construction
The airmobile division engineer battalion is equipped to construct artillery firebases in areas where ground transport is prohibitive. Especially in an unsophisticated environment, such as forest and jungle, these firebases play an integral part in airmobile operations, both as command posts and artillery firebases. The most frequently constructed firebase houses an infantry battalion command element, two infantry companies, a 105-mm howitzer battery and three to six 155-mm howitzers. A firebase housing the above units consists of the following facilities: infantry tactical operations center (TOC), artillery fire direction centers (FDC), ammunition storage pits, garbage sump, command and control helicopter pad, logistics storage area and slingout pad artillery firing positions, helicopter marking area and refuel point, and hardened personnel sleeping positions. Firebases usually are surrounded by a protective berm with perimeter fighting bunkers, two or more bands of tactical wire and a cleared buffer zone to provide adequate fields of fire for perimeter defense. If a local water sources is available, an airmobile engineer water supply point may be established to provide water for the firebase and units in the local area.
Construction of an airmobile firebase may be divided into three phases: combat assault and initial clearing, immediate tactical construction, and final defensive structures.
a. Phase 1. Phase I, combat assault and initial clearing, consists of securing the firebase site and clearing an area large enough to accommodate CH-47 and CH-54 helicopters. The time required to complete this phase depends on the terrain at the firebase site. If the site is free of trees and undergrowth, or if these obstacles have been removed by artillery and tactical air fire preparation, combat engineers can move immediately to Phase II after the initial combat assault on the site. If the site is covered with foliage and trees, the security force and combat engineers may be required to rappel into the site from hovering helicopters. Depending on the density of the foliage on the site, completion of the initial clearing phase by combat engineers with demolitions and chain saws may take up to three hours to accomplish.
b. Phase II. Phase II, immediate tactical construction, commences as soon as the cleared area can accommodate either medium or heavy lift helicopters. Two light airmobile dozers are lifted to the site and are immediately employed clearing brush and stumps to expand the perimeter and to clear and level howitzer positions. Meanwhile, the combat engineers continue to expand the perimeter with chain saws, demolitions, and bangalore torpedoes. If sufficient area is available, a heavy airmobile dozer usually is committed to clearing a logistic storage area and slingout pad, then to expanding the perimeter and fields of fire. The backhoes are committed to excavating positions for the infantry TOC, artillery FDC and, as soon as the perimeter trace is established, perimeter fighting bunkers. The immediate tactical construction phase is characterized by the coordinated effort of infantry, artillery, and engineer forces to produce a tenable tactical position by nightfall on the first day. It is a time of intense helicopter traffic introducing personnel, ammunition, barrier and bunker materials, rations, fuel, water, and artillery pieces onto the site. Aircraft traffic and logistics input must be rigidly controlled to preclude nonessential supplies and aircraft from hampering engineer effort. Therefore, a coordinated site plan and list of priorities for transportation and construction must be prepared and constantly updated. Priorities and the site plan are established by the tactical commander in coordination with the project engineer. As soon as a perimeter trace is established and the site is capable of accepting the logistics and artillery lifts, maximum effort is directed toward the defenses of the firebase. Combat engineers and the heavy dozer continue to push back the undergrowth to permit adequate fields of fire. The two light airmobile dozers may be committed to construction of a 1.2 meter (4 ft) berm around the perimeter to protect against direct fire. Infantry troops are committed to constructing perimeter fighting bunkers at sites previously excavated by the backhoes and, assisted by combat engineers, begin erection of the first band of tactical wire, usually triple standard concertina. Artillery troops not committed immediately to fire missions prepare ammunition storage bunkers and parapet around each howitzer.
c. Phase III. Phase III, final defensive structures, is initiated as construction forces complete the immediate defensive structures. Combat engineers who are not placing the tactical wire or clearing fields of fire commence construction of the infantry TOC and artillery FDC. Infantry and artillery troops are committed to the second band of tactical wire and to erecting personnel sleeping positions with overhead cover. Culvert half sections lend themselves to rapid construction of these positions. Phase III is usually a continuous process, involving constant Improvement and maintenance; however, the majority of protective structures, including sandbag protection of the TOC and personnel bunkers, usually are completed by the end of the fourth day. The controlling parameter in construction of a firebase is time; since the firebase is the first phase of the occupation of a hostile area, the battalion command center and the artillery pieces must be operational as soon as possible and protection against direct and indirect fire requires immediate attention. Several techniques have been developed to increase the efficiency and speed with which construction can progress. Among these are precut TOC structures and the use of culvert as molds for protective bunkers. However, the most effective technique yet adopted is a closely coordinated and controlled plan, outlining the location, priority, and construction force for each phase of the mission.
1-25. FIREBASE LAYOUT
a. The primary purpose of the base is to provide positions for artillery. Thus, physical layout of the fire support base (FSB) will give best possible fields of fire to guns.
b. The base TOC will be located as near FDC as the base terrain allows. The TOC should be fairly centrally located and in a position to control base defense by visual means, if necessary.
c. The helicopter logistical pad should be within the base perimeter but be situated so that incoming and outgoing aircraft do not fly through the primary or most likely sector of fire of the artillery.
d. An admin/VIP helipad large enough for two UH-1 type aircraft should be located in the vicinity of the TOC.
e. If a helicopter rearming/refueling facility is located on the FSB, it should not be so near to either the gun positions or artillery ammunition storage pits that a fire or explosion of one will damage the others.
f. A landing zone for troop lift operations may be located outside the FSB but in the immediate vicinity of the perimeter positions.
g. Typical battery layouts are shown in figure 1-33.
1-26. ARTILLERY POSITION REQUIREMENTS
Engineer construction requirements for an artillery position within a firebase are generally as follows:
a. Platforms which allow the howitzers or guns to fire in all directions and be capable of supporting repeated firing shock in any type of soil condition.
b. Shelters capable of providing adequate protection for the firing crew.
c. Separate shelters large enough to contain an artillery section basic load of projectiles, fuzes, and propelling charges.
d. A wall or parapet around each howitzer or gun to protect the crew from fragments and small arms. The wall should be low enough to allow direct howitzer fire.
e. FDC/TOC shelters large enough to contain the personnel and equipment necessary for the operation of the fire direction center and tactical operations center.
Section VIII. Hasty Shelters
1-27. BASIC CONSIDERATIONS
a. Protection. Shelters are constructed primarily to protect soldiers, equipment, and supplies from enemy action and the weather. Shelters differ from emplacements because there are usually no provisions for firing weapons from them. However, they are usually constructed near or supplement the fighting positions. When natural shelters such as caves, mines, woods, or tunnels are available, they are used instead of constructing artificial shelters. Caves and tunnels must be carefully inspected by competent persons to determine their suitability and safety. The best shelter is usually the one that will provide the most protection with the least amount of effort. Actually, combat troops that have prepared defensive positions have some shelter in their foxholes or weapon emplacements. Shelters are frequently prepared by troops in support of front unit. Troops making a temporary halt in inclement weather when moving into positions prepare shelters as do units in bivouacs, assembly areas, rest areas, and static positions.
b. Surface shelter. The best observation is from this type of shelter and it is easier to enter or leave than an underground shelter. It also requires the least amount of labor to construct, but it is hard to conceal and requires a large amount of cover and revetting material. It provides the least amount of protection from nuclear weapons of the types of shelters discussed in this manual. Surface shelters are seldom used for personnel in forward combat positions unless they can be concealed in woods, on reverse slopes, or among buildings. It may be necessary to use surface shelters when the water level is close to the surface of the ground or when the surface is so hard that digging an underground shelter is impractical.
c. Underground shelters. Shelters of this type generally provide good protection against radiation because the surrounding earth and overhead cover are effective shields against nuclear radiation.
d. Cut-and cover shelters. These shelters are dug into the ground and backfilled on top with as thick a layer as possible of rocks, logs, sod, and excavated soil. These and cave shelters provide excellent protection from weather and enemy action.
e. Siting. Whenever possible, shelters should be sited on reverse slopes, in woods, or in some form of natural defilade as ravines, valleys, and other hollows or depressions in the terrain. They should not be in the path of natural drainage lines. All shelters must be camouflaged or concealed.
a. Principles. Hasty shelters are constructed with a minimum expenditure of time and labor using available materials. They are ordinarily built above ground or dug in deep snow. Shelters that are completely above ground offer protection against the weather and supplement or replace shelter tents which do not provide room for movement. Hasty shelters are useful in the winter when the ground is frozen, in mountainous country where the ground is too hard for deep digging, in deep snow, and in swampy or marshy ground.
b. Sites for winter shelters. Shelter sites that are near wooded areas are the most desirable in winter because these areas are warmer than open fields. They conceal the glow of fires and provide fuel for cooking and heating. In heavy snow tree branches extending to the ground offer some shelter to small units.
(1) Construction. Work on winter shelters should start immediately after the halt so that the men will keep warm. The relaxation and warmth offered by the shelter is usually worth the effort expended in constructing them. Beds of foliage, moss, straw, boards, skis, parkas, or shelter halves may be used as protection against dampness and cold from the ground. Snow should be removed from clothing and equipment before entering the shelter. The entrance of the shelter is located on the side that is least exposed to the wind, is close to the ground and has an upward incline. Plastering the walls with earth and snow reduces the effect of wind. The shelter itself should be as low as possible. The fire is placed low in fire holes and cooking pits.
(2) Insulating. Snow is windproof, so to keep the occupant's body heat from melting the snow, it in necessary only to place a layer of some insulating material such as a shelter half, blanket, or other material between the body and the snow.
a. Lean-to shelters. This shelter (fig. 1-34) is made of the same material as the wigwam (natural saplings woven together and brush). The saplings are placed against a rock wall, a steep hillside, a deadfall, or some other existing vertical surface, on the leeward side. The ends may be closed with shelter halves or evergreen branches.
b. Two-man mountain shelter. This shelter (fig. 1-35) is useful, particularly in winter or in inclement weather when there is frequent rain or snow. It is basically a hole 2.1 meters (7 feet) long, 1 meter (40 inches) wide and 1 meter (40 inches) deep. This hole is covered with 6 to 8 inch diameter logs; then evergreen branches, a shelter half, and local material such as topsoil, leaves, snow, and twigs are added. The floor may be covered with evergreen twigs, a shelter half, or other expedient material. Entrances are provided at both ends if desired. A fire pit may be dug at one end for a small fire or stove. A low parapet is built around the emplacement to provide more height for the occupants. This shelter is very similar to an enlarged, roofed, prone shelter (figure 1-5).
Figure 1-34. Lean-to shelter.
>Figure 1-35. Two-man mountain shelter.
Section IX. Deliberate Shelters and Bunkers
The most effective shelters are deliberate, underground, cut-and-cover shelters. Shelters should be provided with as deep overhead cover as possible. They should be dispersed and have a maximum capacity of 20 to 25 men. Supply shelters may be of any size, depending on location, time, and materials available. The larger the shelter the greater the necessity for easy entrance and exit. Large shelters should have at least two well camouflaged entrances spaced widely apart. The farther away from the frontlines the larger, deeper, and more substantial shelter may be constructed because of more freedom of movement, easier access to materials and equipment, and more time to spend constructing it.
1-31. CONSTRUCTION REQUIREMENTS
a. Drainage. Drainage is an important problem particularly in cut-and-cover and cave shelters. After the shelter is dug, drainage work usually includes keeping the surface and rain water away from the entrance, preventing the water from seeping into the interior by ditching, and removal of water that has collected inside the shelter. The floors of shelters must have a slope of at least 1 percent toward a sump (fig. 1-36) near the entrance, while the entrance should be sloped more steeply toward a ditch or sump outside the shelter (fig. 1-37).
Figure 1-36. Sump for shelter drainage.
Figure 1-37. Floor and entrance drainage.
b. Ventilation. It is particularly important to ventilate cave shelters, especially if it is necessary to close the entrances during an attack. In surface and cut-and-cover shelters, enough fresh air usually is obtained by keeping entrances open. Vertical shafts bored within cave shelters are desirable if not absolutely essential. A stovepipe through a shaft assists the circulation of air. Shelters that are not provided with good ventilation should be used only by personnel who are to remain inactive while they are inside. Since an inactive man requires about .03 cubic meters (1 cu ft) of air per minute, unventilated shelters are limited in capacity. Initial airspace requirements for shelters for not over 12 men are 10 cubic metes (350 cu ft) per man.
c. Entrance covering. If gasproof curtains are not available, improvised curtains made of blankets hung on light, sloping frames may be used. They should be nailed securely to the sides and top entrance timbers. Curtains for cave shelters should be placed in horizontal entrances or horizontal approaches to inclines. Windows should be covered with single curtains. All crevices should be caulked with clay, old cloths or sandbags. Flooring or steps in front of gas curtains should be kept clear of mud and refuse. Small, baffled entrances and/or right angle turns will reduce the effects of nuclear blasts and will keep debris from being blown in. Baffle walls may be constructed of sods or sandbags. Materials which may be injurious to the occupants should be avoided.
d. Sanitary conveniences. Sanitary con-veniences should be provided in all but air-raid emergency shelters and surface-type shelters, where latrines are available. Disposal is by burial or chemical treatment. When waterborne sewage facilities are available, disposal can be into septic tanks or sanitary sewers.
e. Light security. Blackout curtains should be installed in the entrance to all shelters to prevent light leakage. To be most effective, blackout curtains are hung in pairs so that one shields the other. Blankets, shelter-halves, or similar material may be used for this purpose.
f. Emergency exits. Emergency exits in larger shelters are desirable in case the main exit is blocked. If possible, the emergency exit should be more blast-resistant than the main entrance. This can be done by making it just large enough to crawl through. Corrugated pipe sections or 55-gallon drums with the ends removed are useful in making this type of exit. A simple emergency exit which is blast resistant can be constructed by sloping a section of corrugated pipe from the shelter up to the surface, bracing a cover against the inside, and filling the section of pipe with gravel. When the inside cover is removed, the gravel will fall into the shelter, and the occupants can crawl through the exit without digging.
g. Interior marking of shelter. The entrances or interior walls of shelters whose personnel capacity makes it desirable may be marked by reflective tape or paint to facilitate the entry of troops under darkened conditions. There should be no sacrifice of camouflage discipline.
1-32. LOG SURFACE SHELTER
A log shelter (fig. 1-38) can be constructed in form of a box braced in every direction. The framework must be strong enough to support a minimum of 45 cm (18 in.) of earth cover and to withstand the concussion of a near-miss of a shell or bomb or the shock of a distant nuclear explosion. The size of the logs used is limited by the size of available logs for the roof supports and by the difficulty of transporting large timbers.
Figure 1-38. Log framed shelter.
a. Size. Shelters 2 to 3 meters (6.5 to 10 ft) wide by 4.2 meters (14 ft) long are suitable for normal use.
b. Timbers. All timbers should be the same size, if possible, approximately 15 to 20 cm (6 to 8 in.) in diameter depending on the width of the shelter (table 1-4). The uprights should be approximately 60 cm (2 ft) apart except at the entrance where they may have to be spaced farther apart. The roof supports should be spaced the same as the uprights. Holes should be drilled for driftpins at all joints.
Table 1-4. Size of Roof Supports
c. Bracing. Boards 2.5 by 10 cm (1 in. by 4 in.) in size for the diagonal bracing are nailed to caps, sill, and uprights.
d. Walls. The log shelters should be covered with board or saplings and backfilled with approximately 60 cm (2 ft) of earth, or hollow wall may be constructed around the buildings and filled with dirt.
e. Cover. A roof of planks, sheet metal, or other material is then laid over the roof supports and perpendicular to them to hold a minimum of 46 cm (18 in.) of earth cover which is effective against fragmentation (shrapnel) effects of mortars, artillery, and rockets.
1-33. SECTIONAL SHELTERS (SURFACE OR SUBSURFACE)
Shelters of the type in figure 1-39 are designed so that the 1.8 by 2.4 meters (6 by 8 ft) sections may be assembled for use individually or in combinations of two or more sections to provide the required shelter area. They may be surface or subsurface. The advantages of sectional shelters for the purpose of command post or aid stations are the flexibility of the shelter area that can be provided, the depth of cover the shelter will support, the facet that the design lends itself to prefabrication, and their airtransportability by Huey helicopter, except for the cover. The principal disadvantage is the degree of skill required in constructing the sections from dimensional lumber or logs of comparable strength, necessitating engineers assistance and supervision.
Figure 1-39. Sectional shelters.
Figure 1-39. Sectional shelters. (Continued)
a. Siting. The shelter should be sited on a reverse slope for cut-and-cover construction.
b. Excavation. Assuming that each bent or side unit (fig. 1-39 and table 1-5) is sheathed before installation, the excavated area should be 2.1 meters (7 ft) wide and 3 meters (10 ft) long for one section. The additional length of the excavated area will provide working space to install sheathing on the rear unit. The area for the shelter should be excavated to a depth of 3.6 meters (12 ft) to allow for a heavy overhead cover laminated roof and 3.2 meters (10 ft 6 in.) for heavy overhead cover stringer roof.
c. Assembly. The two bents or side units may be assembled and sheathed before they are placed in the excavated area. In this manner driftpins are installed in the sill, caps and posts before units are placed in the excavated area. Bracing on the side units as well as the bracing and spreaders on the front and rear units are toenailed.
d. Organization of work crews. An engineer squad, or a squad other than engineer under engineer supervision, can be used economically at the worksite to excavate the shelter area, assemble the roofing and cover materials, and construct the overhead cover. Under favorable conditions a trained engineer squad can excavate the area required for the shelter and install the shelter and overhead cover in 18 to 20 hours. However, if a backhoe or bucket loader is available for the excavation, the time can be reduced to approximately 6 hours.
1-34. USE OF STANDOFF
a. Definition. A standoff is a steel or wood curtain or chain link fence erected approximate 3 meters (10 feet) in front of a protective structure to detonate shells and thereby reduce their subsequent penetrating effect. Its use is optional but desirable as additional protection of those protective structures most likely to sustain enemy fire.
Table 1-5. Bill of Materials for One 6' x 8' Sectional Shelter With Post, Cap, and Stringer Construction - Dimensional Timber
b. Construction "condition". A construction "condition" (fig. 1-40 and table 3-3) refers to protective structure with or without a standoff. Condition I means the protective structure has no standoff, condition II - the structure has a steel standoff, and condition III - the structure has a wood standoff (fig. 1-41 and 1-42). A chain link fence standoff is shown in figure 1-43. Table 1-6 shows comparison of relative thicknesses of protective materials needed to withstand penetration of various types of ammunition - with and without standoffs.
1-35. SUBSURFACE SHELTERS
a. Cut-and-cover shelters. The log shelter shown in figure 1-38 is suited to cut-and-cover construction or surface construction. The best location for cut-and-cover shelters is on the reverse slope of a hill, mountain, ridge, or steep bank as shown in figure 1-44. The shelter shown provides 1.8 to 2.1 meters (6 to 7 feet) headroom. The shelter frame is built in the excavation; the spoil is backfilled around and over the frame to ground level, or somewhat above, and camouflaged. The protection offered depends on the type of construction (size of timbers) and the thickness of the overhead cover. As in the case of a surface shelter of similar construction, approximately 45 centimeters (18 inches) of earth cover can be supported.
Figure 1-40. Standoff condition.
Table 1-6. Minimum Thickness of Protective Material Required to Resist Penetration of Rounds1
Figure 1-41. Wooden standoff.
Figure 1-42. Log standoff.
Figure 1-43. Chain link fence standoff.
b. Cave shelters. Caves are dug in deliberate defensive positions, usually by tunneling into hillsides, cliffs, cut, or ridges or excavating into flat ground. Because of the undisturbed overhead cover, a cave is the least conspicuous of all types of shelters if the entrance is covered. One of the best locations for a supply cave entrance is shown in figure 1-45. The disadvantages of cave shelters include limited-observation, congested living conditions, small exits, and difficult drainage and ventilation. Their construction is difficult and time consuming. Exits may be blocked or shoring crushed by a direct hit from a conventional weapon or ground shock from a nuclear explosion.
1-36. SPECIAL USE SHELTERS
a. Observation posts. These are located on terrain features offering as good a view as possible of enemy-held areas (fig. 1-46). The ideal observation post has at least one covered route of approach and cover as well as concealment, while offering an unobstructed view of enemy-held ground.
b. Command posts. Small unit command posts may be located in woods, ravines, in the basements of buildings, or former enemy fortifications. When none of these are available, surface or cut-and-cover shelters previously described may be modified for this purpose.
c. Medical air stations. Cut-and-cover shelters are especially adaptable as aid stations since they are easily cleaned and ventilated. Suitable sites may be found in pits, quarries, under banks, or in small buildings or ruins.
d. Ammunition shelters. Ammunition shelters should be located and constructed so that they protect ammunition against the weather and enemy fire. They should be well concealed, and large enough to hold the desired quantity of ammunition close to the firing position. Figure 1-47 shows an ammunition shelter which may be constructed in an emplacement parapet. If it is necessary to construct ammunition shelters above ground, particularly where the water level is close to the surface, a log crib built up with dirt is suitable.
Figure 1-44. Cut-and-cover shelters.
Figure 1-45. Supply cave in a road cut.
Figure 1-46. Observation post.
Figure 1-47. Ammunition shelter.
Section X. Overhead Cover
1-37. HEAVY OVERHEAD COVER
To provide adequate protection against both penetration and detonation of artillery shells and bombs, a structure would require overhead earth cover so thick as to be impracticable. By combining materials and using them in layers in a logical sequence, the required protection is provided with less excavation and construction effort. Two designs of overhead cover in functional layers which protect against the penetration and explosion from a hit by a 155-mm artillery round are shown in figure 1-48 and describe, below.
Figure 1-48. Heavy overhead cover.
a. Laminated roof construction. In this design either five 5 centimeters (2 inch) or seven 2.5 centimeters (1 inch) layers of lumber are used for laminated roof as shown in , figure 1-48.
(1) Dustproof layer. Tar paper, canvas, or tarpaulins lapped and places above the laminated roof is used to prevent dust and dirt from shaking down on equipment, weapons, and personnel.
(2) Cushion layer. The cushion layer is intended to absorb the shock of detonation or penetration. Untamped earth in the best material for this purpose and should be at least 30 centimeters (12 inches) thick. Materials such as loose gravel transmit excessive shock to the layer below and should not be used in the cushion layer. This layer extends on all sides for a distance equal to the depth of the shelter floor below the ground surface or a minimum of 1.6 metes (5 feet).
(3) Waterproof layer. The waterproof layer is constructed of the same material as the dustproof layer or similar materials. It is intended to keep moisture from the cushion layer in order to retain the cushioning effect of the soft dry earth, and minimize the dead load the structure must carry.
(4) Burster layer. The burster layer is intended to cause detonation of the projectile before it can penetrate into the lower protective layers. This layer is made of 15 to 20 cm (6- to 8-in.) rocks placed in two layers with the joints broken. This layer should be at least 30 cm ((12 in.) thick. Irregular-shaped rocks are more effective for this purpose than flat rocks. If rocks are not available, 20 cm (8-in) logs may be used. They must be wired tightly together in two layers. The burster layer should extend on each side of the shelter a minimum of 1.5 meters (6 ft).
(5) Camouflage layer. The burster layer is covered with about 5 cm (2 in.) of untamped earth or sod, as a camouflage layer. A greater thickness of camouflage material will tend to increase the explosive effect.
(1) A lower cushion layer 30 cm (12 in.) thick on top of the dustproof layer. This layer of untamped earth does not extend beyond the sides of the shelter.
(2) A distribution layer consisting of 20 cm (8 in.) timbers. This layer extends beyond each side of the shelter a minimum of 1.5 meters (5 ft) and rests on undisturbed earth to transmit part of the load of th layers to the undisturbed earth on each side of the shelter.
c. Overhead cover for fight b. Figure 1-49 shows the details for the construction of a fighting bunker with heavy overhead cover. The material requirements for the construction of this bunker are found in table 1-7.
Figure 1-49. Fighting bunker with heavy overhead cover.
d. Heavy overhead cover protection. Heavy overhead cover protects against the following Soviet weapons:
1-38. ENTRENCHMENT COVER SUPPORT
a. Overhead cover is normally supported on the roof of the structure and the resultant load is transmitted through the cape and posts to the foundation on which the structure rests. It may be necessary, in some instances, to support the roof directly on the earth outside a revetted position. When this must be done, the roof timber should not bear directly on the earth outside the excavation. The added load may use the wall to buckle or cave in. Instead, the roof structure is carried on timber sills or foundation logs bedded uniformly in the surface at a safe distance from the cut. This distance should be at least one-fourth the depth of the cut and in no case less than 30 cm (12 in.) to the nearest edge of the sill. Round logs used for this purpose are embedded to at least half their diameter to provide maximum bearing area of log to so. These principles are illustrated in figure 1-50.
Table 1-7. Bill of Materials, Fighting Bunker, (Laminated Construction)
b. Laminated planks or stringers are used to support the roof cover.
(1) Table 1-8 shows the thickness of laminated plank roof required to support various thicknesses of earth cover. The planks should extend from support to support in all layers, and adjoining edged should be staggered from one layer to the next.
Figure 1-50. Support of overhead cover on earth banks.
Table 1-8. Thickness of Laminated Wood Required to Support Various Thicknesses of Earth Cover Over Various Spans
(2) Table 1-9 shows the spacing of stringers required to support 2.5 cm (1-in.) plank roof under various thicknesses of earth over various spans. Stringers are 5 cm x 10 cm (2 in. x 4 in.) unless otherwise indicated.
(3) The roofs shown with the cover indicated are fragment proofs and will give substantial radiation protection, if properly designed entrances are provided.
c. Sandbags are never used to support overhead cover.
1-39. FIGHTING BUNKER WITH LIGHT OVERHEAD COVER
When establishing positions in wooded areas, it is very important to provide overhead cover to protect personnel from the shrapnel of tree bursts. A fighting bunker with light overhead cover is shown and described in figure 1-51. The overhead cover will stop fragments from trees and airburst artillery, and it is strong enough to withstand the effects of a direct hit by an 81-mm mortar. If the side openings are closed with sandbags to prevent the entry of grenades, the fields of fire and observation are limited to the front only. This is a serious disadvantage with this type of position. Chicken wire can be placed over the firing apertures to prevent grenades from entering the bunker. The chicken wire should be sloped with a ditch dug at the base to catch grenades as they roll off the wire.
Table 1-9. Center to Center Spacing in Inches of Wooden Stringers Required to Support a 1-inch Thick Wood Roof with Various Thicknesses of Earth cover over Various Spans
Figure 1-51. Fighting bunker with light overhead cover.
Section XI. Prefabricated Shelters and Bunkers
1-40. DESIGN CONSIDERATIONS
Designs presented are applicable to all levels of operations and geographic areas. They are especially applicable to operations requiring rapid construction of semipermanent emplacements and shelters that provide protection against mortar and artillery fire in forward areas.
a. Shelters. Design considerations for the shelter design are set forth in the following guideline criteria:
(1) Must be constructed from materials available in the theater of operations or constructed of components readily procured and fabricated.
(2) Must be capable of prefabrication by combat engineer units in rear areas, transportation to forward areas by 25-ton tractor traders, and erection by 20-ton crane and combat engineers.
(3) Should be capable of being transported by CH-47 helicopter and be erectable by normally available materiel handling equipment such as motorized crane or combat engineer vehicle (CEV).
(4) Must provide a large reduction in erection time in-forward areas as compared with previous in-place construction of shelters.
(5) Should be capable of forward area assembly (less site preparations, entrances, cover, and antispall) by six men in 1 hour.
(6) Must permit addition of cover with available combat engineer equipment.
(7) Must be a buried structure composed of modular units of 3.6 meters (12 feet) in width that can provide shelter lengths of least 3.6, 7.2 and 10.8 meters (12, 24 and 36 feet).
(8) Facility, with earth cover and add-on-features, must provide protection from the Soviet 152-mm rounds detonating close by. Reinforced concrete structures must provide protection from 155-mm artillery shell detonated from a distance of 9 meters (30 ft) from the bare concrete without earth backfill.
b. Fighting bunkers. Design considerations for the fighting bunker design are set forth in the following guideline criteria:
(1) Must use only those construction materials that are available in the theater of operations or of components readily procured and fabricated.
(2) Must be capable of being prefabricated by combat engineers, transported to forward areas by means of 25-ton tractor-trailers or equal, and emplacement by combat engineers using 20-ton crane or equal.
(3) Should be capable of being transported by CH-47 helicopter and be erectable by normally available materiel-handling equipment such as motorized crane or CEV.
(4) Must have uniform design and be comprised of multi-use modular units.
(5) Should be capable of being assembled in forward area (less site preparation, entrances, and cover) by six men in 1 hour.
(6) Bunker roof must be capable of supporting 1.2 meters (4 feet) of saturated earth cover.
(7) Must accommodate at least four fighting men with sleeping space for two.
(8) Must provide protection from 152-mm rounds detonating close by the completed bunker.
(9) Must provide protection from 82-mm mortar round detonating on the surface of the earth cover.
1-41. WES CONCRETE ARCH SHELTER (FIG 1-52)
a. Description. A basic unit of Waterways Experiment Station (WES) prefabricated concrete arch-type shelter is 3.6 meters (12-feet) wide and 1.2 meters (4-feet) long. A 3.6 meters (12-feet) long structure consists of three 1.2 meter (4-foot) long arch sections and two end wall sections, together with necessary hardware, waterproofing membrane, and earth cover.
(1) The 15 centimeter (6-inch) -thick arch sections have a 1.8 meter (6-foot) inside radius with a 45 centimeter (18-inch) vertical wall extension and 10 centimeter (4-inch) floor. The floor is framed into the vertical wall section through a 45 centimeter (18- inch) -wide by 30 centimeter (12-inch)-thick footing which is chambered 15 centimeter (6 inches) at the top corners. Monolithically cast, each arch section weighs 10,200 pounds.
(2) The 15 cm (6-in) -thick end wall section consists of a 2.1 meter (7-ft) radius semicircle and a 0.6 meter (2-ft) by 4.2 meter (14-ft) rectangle. A door opening 0.71 meters (2-ft, 4-in), by 1.68 meters (5-ft, 6-in) is provided in each end wall section. Monolithically cast, each end wall weighs 7,000 pounds.
b. Design. This concrete arch shelter is designed to meet the essential and desired characteristics outlined in paragraph 1-45 and is subject to the following additional design factor assumptions:
(1) Dead load consists of the weight of the concrete shell and 2.4 meters (8-ft) of saturated earth cover above the crown of the arch.
(2) Safety factor is 1.5.
(3) Material properties:
(a) Concrete compressive strength of 3,000 psi after 28-day cure.
(b) Reinforcing steel tensile strength of 40,000 psi.
(c) Soil unit weight (wet) of 110 pcf.
c. Transportation. Movement of the prefabricated arch sections and end walls to the emplacement site is accomplished by truck, trailer, or helicopter. Weight and dimensions are shown in table 1-10.
d. Site preparation. Location and position of the shelter can be determined by the function of the shelter, the tactical conditions, the topography and other similar considerations which are determined by the field commander. The depth of excavation is determined by the elevation of the ground water table during the wet season. A bulldozer, scooploader, or crane-shovel with attachments is used to excavate a trench to receive the arch sections. A soft bedding or cushion layer of sand should be provided as a base upon which to set the arch sections to avoid structural stress concentrations, to absorb shock blast, effects, and to minimize unequal settlement between the arch sections, and walls, and entrance structures. Footings for each end wall consists of fourteen timbers 4 inches by 12 inches by 3 feet placed side by side and centered normal to the plane of the end wall. Typical trench excavation if shown in figure 1-53.
e. Emplacement. The shelter sections can be emplaced by six men and one truck-mounted crane in 1 hour after site is prepared. The rough terrain crane which is available in the combat engineer battalion can be used effectively for final excavation, lifting the shelter sections into place and backfilling. The sections are firmly seated, alined, and secured with five wire rope tie assemblies. The four joints between the arch sections and end walls are covered with any durable, flexible, waterproof material such as salvaged T-17 membrane. Entrance structures at each end are fabricated from local or manufactured material including timber frame, concrete arch, corrugated metal pipe (CMP), cattle pass, or landing mat. Drainage structures are provided as required. The shelter is then backfilled with sod material. Sandy material, if available, will offer better protection than a clay type sod. The backfill placement should be carried on until a minimum depth of 4 feet and maximum depth of 8 feet insured; the depth of cover includes waterproof membrane, burster course, and camouflage.
Figure 1-52. WES concrete arch shelter design, isometric.
1-42. MULTIPLATE PIPE ARCH SHELTER (FIGURE 1-54)
a. Description. The basic multiplate pipe arch shelter is a 3.6 meter by 3.6 meter (12-ft by 12-ft) prefabricated corrugated steel arch shelter consisting of one 3.6 meter (12-ft) multiplate pipe arch section and two precast reinforced concrete end walls, together with necessary hardware, waterproofing membrane, and earth cover.
(1) The multiplate pipe arch section consists of seven 3.6 meter (12-ft) long corrugated galvanized steel plates of differing curvature which are bolted together along the longitudinal joints. The assembled pipe arch has a span of 3.8 meters (12-ft, 8-in) a rise of 2.4 meters (8-ft, 1-in), and weighs 4, pounds.
(2) The 15 cm (6-in) thick end wall sections are identical to the end wall sections for the WES concrete arch shelter.
Figure 1-54. Multiplate pipe arch shelter.
Table 1-10. Typical trench excavation for shelter.
b. Design. The multiplate pipe arch shelters use commercial pipe arch which is available in a range of spans, rises, and areas and are tabulated in drainage products handbooks and manufacturer' catalogs. This multiplate arch is designed to meet the criteria outlined in paragraph 1-45 and is subject to the following design factor assumptions:
(1) A minimum of 8-gage-thick steel is required for the arch pipe.
(2) Dead load of 8 feet of saturated earth cover is developed at the crown. Soil unit weight (wet) is 110 pcf.
(3) Properties of the 8-gage corrugated steel plate are
(a) Pitch of corrugation is 6-inches.
(b) Depth of corrugation is 2-inches.
(c) Uncoated thickness of steel is (0.1644 inches).
(4) Properties of the reinforced concrete are:
(a) Compressive strength is 3,000 psi after 28-day cure.
(b) Reinforcing steel has tensile strength of 40,000 psi.
c. Prefabrication of multiplate pipe arch section. The two slightly curved floor plates, the two curved (1-ft, 6-in. radius) corner plates, the two haunch plates, and the crown plate are lapped and secured with four 3/4-inch-diameter by 1 1/4-inch-long bolts per linear foot of seam. Bolts are staggered in two rows per seam with one bolt in each valley and each crest of the corrugation. Bolts should not be tightened until all bolts have been installed. Manufacture' catalogs, when available, should be consulted for detailed instructions on assembly technique.
d. Transportation. Movement of the prefabricated arch sections and end walls to the emplacement site is accomplished by truck, trifler, or helicopter. Weights and dimensions are shown in table 1-11.
e. Site preparation. Location and position of the shelter can be determined by the function of the shelter, the tactical conditions, the topography and other similar considerations which are determined by the field commander. The depth of excavation is determined by the elevation to which the ground water table can be expected to rise during the wet seasons. A bulldozer, scoop loader, CEV or crane-shovel with attachments is used to excavate a trench to receive the prefabricated pipe arch section and end walls. A soft bedding or cushion layer of sand should be provided as a base upon which to set the pipe arch section in order to avoid structural stress concentrations, absorb the shock of blast effects, provide drainage, and minimize unequal settlement between the arch section, end walls, and entrance structures. Timber footings under the end wall sections are required where the strength of subgrade is not capable of supporting the 1,000 psf load of end walls.
Table 1-11. Multiplate Arch Shelter (Transportation Data).
f. Emplacement. The shelter sections can be emplaced by six men and one truck-mounted crane in 1 hour after site is prepared. The rough terrain crane which is available in the combat engineer battalion can be used effectively for emplacement, to include final excavation, lifting the shelter sections into place, positioning entrances, and back-filling. Wire rope slings are used to aid the placement of the structure. Entrance structures at each end are fabricated from local or manufactured material such as timber frame, concrete arch, CMP, cattle pass, or landing mat. After excavation has been completed to the proper elevation, a waterproof membrane is installed prior to placing the structure. All bolts should now be checked for tightness and the membrane material wrapped completely around the shelter. Backfill material, dry coarse sand if available, is now placed and compacted. Special attention should be given to the placement and compaction of the soil material under the upturn area of the floor section. Additional backfill material is placed and compacted in 15 cm (6-in.) lifts to an elevation of at least three-fourths o: the height of the structure. These procedures are necessary to provide symmetrical loading and to insure proper setting of the structure. To complete the emplacement, the backfill material is placed to a minimum height of 4 feet or up to a maximum height of 8 feet over the crown of the structure including burster layers and camouflage layers. Drainage is provided as required by the topography.
Section XII. Prefabricated Fighting and Command Post Bunkers (Semipermanent Construction)
1-43. CONCRETE LOG BUNKER
a. Description. The concrete log bunker is a 4-man fighting bunker, 2-3 meters (7 ft, 6 in) square and 1.8 meters (6 ft) high. The bunker is constructed of 90 precast reinforced concrete logs 15 cm (6 in) wide by 20 cm (8 in) deep of various lengths (.6, .9, 1.2, 1.8, 2.4 and 3.0 meters (2, 3, 4, 6, 8, and 10 ft)) that weigh approximately 164 pounds per meter (50 lb per ft). The bottom of the bunker is set 102 cm (3 ft, 4 in) below the ground surface. The roof is made by placing 3 meter (10 ft)-long concrete logs side by side and pinning them together to make a 20 cm (8 in) -thick roof with 30 cm (1 ft) overhang on all sides. The bunker has a 20 by 45 cm (8- by 18-in.) firing port on each of the four sides; the bottom of each firing port is 1.2 meters (4 ft) above the floor level. The logs are joined together with steel pins, 1.9 cm (3/4 in.) in diameter, which are dropped through holes, 3.8 cm (1 1/2 in.) in diameter, alined at 30 cm (1 ft) intervals and cast in the logs. Normal entrance/exit is by means of .9 meter (3 ft) and 1.2 meter (4 ft) diameter corrugated metal pipe (CMP), and an emergency exit is provided by means of two removable logs at the rear face firing port.
Figure 1-55. Concrete log bunker.
Figure 1-56. Concrete log bunker design.
c. Forming, steel placement, and casting. The concrete logs are precast in a staging area and transported to the emplacement site. The forms are constructed in the field on a 3.6 by 4.8 meter (12- by 16-ft) casting bed which permits casting of numerous logs of various lengths at one time. The concrete is mixed, poured, vibrated, and cured in accordance with standard procedures. Two to five days' moist cure is required before the precast logs can be moved safely.
d. Transportation. Movement of the precast concrete logs to the emplacement site is accomplished by truck, trailer, or helicopter.
e. Site preparation. Location of the fighting bunker is determined by the tactical commander. The design depth o excavation is 2.54 meters (8 ft, 4 in.), including 102 cm (3 ft, 4 in.); below grade for bunker floor and 1.5 meters (5 ft) below bunker floor for the entrance; however, the elevation to which the ground water table can be expected to rise during the wet season may require a field modification of this design depth. The emergency exit, located above the normal ground level, allows siting of this bunker in locations where the normal entrance may be subject to temporary flooding. Excavation is accomplished by combat support earthmoving equipment or by hand digging.
f. Emplacement. The concrete log fighting bunker can be assembled by six men in 1 hour after the site is prepared and the CMP entrance structure is in place. For poor soils, a footing will be required for this structure to prevent excessive settlement. Cushion, waterproofing, and burster layers, retained with sandbags, are placed on the roof to develop capability to withstand a direct hit from the equivalent of an 82-mm mortar round. Steel mesh grenade gratings are hinged over the firing ports. Standoff screening against AT rockets, fields of fire, communications, camouflage, firing shelves, and bunks should be provided as required. The modular design of the basic concrete logs permits a wide variety of original designs to suit specific requirements. This flexibility of design may be exploited by testing different arrangements using either full scale or model logs.
1-44. CONCRETE ARCH BUNKER (fig 1-57)
a. Description. The concrete arch bunker is a four-man fighting bunker, semi-circular in plan, 3.6 meters (12 ft) wide at the rear by 2.25 meters (7 ft, 6 in.) deep at centerline of the arch width by 1.8 meters (6 ft.) inside height. The bunker consists of three precast reinforced concrete components: a 1.8 meter (6 ft.)-high arch section, a rectangular back wall section, and a semi-circular roof section. Each of the sections is 15 cm (6 in.) thick and reinforced with No. 4 1.25 cm (0.5-in. diameter) steel rebars. The arch section has a 1.8 meter (6-ft) interior radius plus a 45 cm (1 ft, 6 in.) horizontal extension. The roof section overhangs the arch section providing a 45 cm (18-in.)-wide shield. The back wall section and the roof section each have a 20 cm (8-in.)-thick by 45 cm (18-in.)-wide bulkhead beam. The arch section has four 20- by 45-cm (8- by 18-in.) firing ports, and the back wall has one 20- by 45-cm (8- by 18-in.) firing port and a 20- by 75-cm (8- by 30-in.) emergency exit which can be used for a quick exit or grenade throwing.
c. Forming, steel placement, and casting.
(1) Arch section. Steel forms for the arch section can be fabricated in the field from 16-gage black sheet steel, which is spot welded to 5 cm (2 in.) angles. The form used in the arch shelter can be adapted to casting the arch bunker by omitting the floor portion and adding 0.6 meter (2 ft) of form to the height. The same procedures are used in the steel placement and casting of the 1.8 meter (6 ft) bunker arch as for the 1.2 meter (4 ft) shelter arch, with particular care required in placement, vibrating, and adequate curing before moving to site.
Figure 1-57. Concrete arch fighting bunker.
Figure 1-58. Concrete arch fighting bunker design.
(2) Back wall section and roof section. Forms for casting the back wall and roof are built at the prefabrication site using conventional wood forms. Reinforcing steel details are as described for the WES concrete arch. The bulkhead beams for the roof and back wall sections are reinforced with two No. 5 bars placed with 5 cm (2 in.) of concrete cover from the outside edges of the beam.
d. Transportation. Movement of the three precast sections is accomplished by truck, trailer, or helicopter.
e. Site preparation. Location of the concrete arch fighting bunker is determined by the tactical commander. The design depth of excavation 2.5 meters (5 ft 4 in.) including 1 meter (3 ft, 4 in.) below-grade for the bunker floor and 1.5 meters (5 ft) below the bunker floor for the entrance; however, the elevation to which the ground water table can be expected to rise during the wet season may require a field modification of this depth. The emergency exit, located at the firing port level, allows siting of the arch fighting bunker where the deep entrance may be subject to temporary flooding. Excavation is accomplished by combat support earthmoving equipment or by hand labor.
f. Emplacement. The three sections of the arch fighting bunker can be employed by six men and one heavy crane in one hour after site is prepared and the CMP entrance structure is in place. The rough terrain cane which is available in the combat engineer battalion can be used effectively for emplacement especially for positioning of entrance and lifting and positioning bunker sections. The back wall is bolted to the 20- by 45-cm (8- by 18-in.) bulkhead beam at the rear of the roof section to secure the entire structure against displacement when under attack. Except in very good soil conditions, footings will be required under the modified arch section to prevent excessive settlement. Cushion, waterproof, burster layers, retained with sandbags, are placed on the roof to develop capability to withstand direct hit from the equivalent of an 82-mm Soviet mortar round. Steel mesh gratings are hinged over the firing ports. Standoff screening against antitank (AT) rockets, as well as fields of fire, communications, camouflage, firing shelves, and bunks should be provided as required.
1-45. AIR-TRANSPORTABLE UNDER-GROUND ASSAULT BUNKER (Prefab)
Figure 1-59. Air-transportable underground assault bunker (prefab).
a. Description. This is a prefabricated plywood bunker (fig 1-59) suitable for a commandpost or fire-direction center, which can be moved (completely assembled except for the roof) from site to site as the tactical situation demands. Its sloping walls make for easier pulling from the ground by helicopter for relocation. The bunker can be erected and emplaced by means of handtools only.
b. Construction. The bunker walls and floor may be prefabricated (fig 1-60) in rear areas and then be trucked or flown, assembled or disassembled, to the erection site. Fasteners are provided along the edges of each wall and the floor to allow the Individual members to be locked together into a complete unit. The walls of the bunker should extend below the floor section so that the floor can act as a support for the bottom edge of the walls. The longer side walls are abutted against the shorter end walls. Two large straps, completely around the structure, and placed during construction are used to attach bunker to helicopter lifting hook for bunker pullout and transport The underground site can be excavated by means of explosives and handtools (fig 1-61). The floor area of the excavation should be .6 meter (2 ft.) longer and .6 meter (2 ft.) wider than the area of the bunker floor to allow working space during construction. The roof is concentric to and larger than the floor section and may be fabricated in the rear area or at the erection site. The roof overlaps the walls to be supported on firm (unexcavated) ground - not on the bunker walls. Additional construction recommendations for the bunkers are as follows:
Figure 1-60. Plans of air transportable underground assault bunker (prefab).
(1) Abut longer side walls against shorter end walls because the longer walls must sustain the greatest load. The shorter walls then act as a support. (Miter corners if possible.)
(2) Provide for wall bracing at the top of the bunker. Brace from the center of each wall to the center of each adjacent wall (diamond pattern).
(3) Attach a sheet of plastic or other thin waterproof covering around the outside before backfilling to minimize friction between earth and the walls and increase moisture resistance.
(4) Make the bunker no larger than necessary. It should be no more than 6 1/2 feet high and the floor area should be less than 100 ft unless special effort is made to provide adequate structural members in addition to those used in the test bunker.
Figure 1-61. Installation of airtransportable underground assault bunker (prefab).
(5) Backfilling should be accomplished by hand labor, maintaining a uniform load around the perimeter as backfilling progresses.
(6) Although nails are satisfactory as fasteners for wood members, screws or bolts will offer greater holding strength.
(7) If possible, select fasteners for connecting walls and floors that are simple and adjustable.
(8) Make the bottom of the excavation 2 feet longer and 2 feet wider than the length and width of the structure floor to increase working room during erection and provide adequate clearance for the walls.
(9) Use explosives as extensively as practical during excavation to minimize required hand digging.
(10) Prior to lifting the structure from the installed portion, remove some of the backfill with hand tools to reduce effects of wall friction.
(11) Provide for more than one means of employing lifting devices for removal of the structure. Two large straps completely around the structure, placed during construction, seem to be the best method. U-bolts should be used in the floor through 4- by 4-inch members. To help distribute the load, a metal bearing plate should be placed where the U-bolt bears against the underside of the 4- by 4-inch floor joists.
c. Data. The bunker weighs approximately 1,600 pounds without the roof. It can be pulled from the ground by a lift of 10,000 pounds (CH-47 helicopter). The bunker should be no more than 1.95 meters (6 1/2 ft) high and the floor space should be under 9.3 square meters (100 sq ft). Excavation, erection, backfilling, and construction of roof and entrance can be completed in less than 10 hours.
1-46. PLYWOOD PERIMETER BUNKER
This bunker has plywood revetment (soilbin) walls approximately .6 meter (2 ft) thick. The walls are topped with a plywood cap to prevent entrance of moisture into the soil fill. The bunker may be provided with a column foundation or be constructed directly on the ground. The bunker soilbin revetment walls withstand small arms fire. The bunker walls, by insertion of landing mat, offer additional cushioning effect against heavy caliber rounds that may penetrate the revetment (soilbin).
Figure 1-62. Plywood perimeter bunker.
1-47. BUILDINGS AS SHELTERS OR FIRING POSITIONS
a. Protection. Some protection from enemy fire may be achieved for occupants in a building used as a shelter by strengthening the building, by shoring up ceilings, and bracing walls. Men inside buildings are reasonably well protected against thermal effects and radiation unless they are near doors or windows. The principal danger is from falling masonry and from fire in the building.
b. Basic considerations.
(1) A ground floor or basement is more likely to make a suitable shelter than any other floor. The risk of being trapped must be guarded against. Heavy bars, pieces of pipe, or timbers, should be available in each room that is occupied, for use by the occupants in the event the building is demolished.
(2) Small arms fire will not penetrate the walls if they are 45 cm (18 in.) thick. The walls will not usually splinter from small arms fire if they are 30 cm (12 in.) thick. Additional protection can be obtained by building sandbag walls. If sandbags are used inside the building they reduce the usable space, but last longer and are not conspicuous. Care should be exercised in using sandbags above the first floor due to the weight involved.
(3) Window glass should be removed since it gives no thermal protection and is dangerous when shattered. If it is retained as protection from the weather, it should be screened or boarded.
(4) Several exits are necessary.
(5) Provisions for fighting fire should be made.
(6) Blackout arrangements should be made, if not already provided by thermal screening of doors and windows.
c. Use of weapons. In using a building as a firing position, there are several considerations.
(1) The preparatory work should not disclose the intended use of the building to the enemy.
(2) Weapons must be sited well back from any opening so that neither weapons nor personnel are visible from the outside.
(3) Several firing positions should be available in order to obtain a wide field of fire. The shapes of the openings should not be changed for this purpose.
(4) Any openings other than the normal ones are very conspicuous unless they are close to the ground.
(5) There are no fixed designs for weapons platforms under these circumstances. Platforms must be improvised from materials immediately available. Sandbags should be used sparingly if there is any doubt about the strength of the floor.
Section XIII. Protective Shelters for Frozen Environment
1-48. SNOW HOLE
The snow hole (fig 1-63) is a simple, oneman emergency shelter for protection against a snow storm in open, snowcovered terrain. It can be made quickly, even without tools. Lying down in snow at least 1 meter (3.3 ft.) deep, the soldier pushes with his feet, digs with his hands, and repeatedly turns over, forming a hole the length of his body and as wide as his shoulders. At a depth of at least 1 meter (3.3 ft.), the soldier digs in sideways below the surface, filling the original ditch with the snow that has been dug out until only a small opening remains.
Figure 1-63. Snow hole.
This opening may be entirely closed, depending on the enemy situation and the temperature; the smaller the hole, the warmer the shelter.
1-49. SNOW CAVE
Snow caves (fig 1-64) are made by burrowing into a snowdrift and fashioning a room of desirable size. This type of shelter gives good protection from freezing weather and a maximum amount of concealment. The entrance should slope upward for the best protection against the penetration of cold air. Snow caves may be built large enough for several men if the consistency of the snow is such that it will not cave in. Two entrances can be used while the snow is being taken out of the cave; one entrance is refilled with snow when the cave is completed.
Figure 1-64. Snow cave.
The snowpit (fig 1-65) is dug vertically into the snow with intrenching tools. It is large enough for two or three men. Skis, poles, sticks, branches, shelter halves, and snow are used as roofing. The inside depth of the pit depends upon the depth of the snow, but should be deep enough for kneeling, sitting, and reclining positions. The roof should slope toward one end of the pit. If the snow is not deep enough, the sides of the pit can be made higher by adding snow walls.
Figure 1-65. Snow pit in shallow snow.
The size and roof of a snowhouse are similar to those of a snowpit. The walls consist of snowblocks and may be built to the height of a man. Snow piled on the outside seals the cracks and camouflages the house (fig 1-66).
Figure 1-66. Snowhouse with iceblock walls.
Section XIV. Construction Techniques for
Among engineering materials that have to be handled, modified, removed, used as a base for building on or traveling over, and used as construction materials; snow, ice, and frozen ground are unique in their appearance and disappearance. They are also unique in the rapid changes of their physical properties within short periods of time due to metamorphism, change of temperature, change of state, and of course, relatively large deformations following subjection to load.
Fortifications constructed from snow, ice, and frozen ground have the following merits:
a. The wide distribution and cheapness of the material for constructing the fortifications and the presence the materials at the building site, thus eliminating their transportation.
b. The extensive possibility of substituting snow, ice, and frozen ground during winter for the usual building materials.
c. The relative speed of construction with snow, ice and frozen ground, especially the solidification speed of soils exposed to freezing.
d. The simplicity and speed of repairing structures of snow, ice, and frozen ground if damaged.
e. The great strength of structures made from frozen ground.
f. The complete safety from fire.
1-54. SNOW AS CONSTRUCTION MATERIAL
a. Suitability. Dry fresh fluffy snow is not suitable for expedient construction. Reworked snow, such as piles at road edge after clearing equipment passage, densifies and begins to harden within hours after disturbance even at very low temperatures. Artificial compacting wind compacting, and compacting after a brief thaw make snow even more suitable for expedient shelter and protective structures.
(1) A uniform snow cover with a minimum thickness of 25 cm (10 in.) in a given area is sufficient for shelter and revetment construction. Blocks of uniform size, typically 20 x 30 x 40 cm (8 x 12 x 16 in.) depending upon degree of hardness and density, can be cut from the snow pack with shovels or better with long knives (machetes) or carpenter's saws. Best practice for constructing cold weather shelters is that adopted from natives of polar regions (fig 1-67). It must be remembered that thicker walls render better protection. Systematic overlapping block-over-seam insures stable construction. "Caulking" seams with loose snow insures snug draft-free structures. Igloo shelters in cold regions have been known to survive a whole winter. An Eskimo-style shelter easily withstands above-freezing inside temperatures, thus providing comfortable protection for personnel against wind chill and low temperatures.
(2) Snow fortifications can be built during either freezing or thawing weather, if the thaw is not so long or intense that significant snow melt occurs. Mild thaw or temperatures of 1° or 2°C. are even more favorable than freezing weather, because the snow is then very plastic, conglomerates readily, and assumes any shape without disintegration. Of course, freezing weather is also necessary for snow construction in order to achieve solid freezing and strength. If water is available at low temperatures, expedient protective structures could be built by wetting down snow and shaping it into desired forms with shovels.
1-55. ICE AS CONSTRUCTION MATERIAL
a. Methods. Ice structures can be built in three ways:
(1) Layer-by-layer freezing by repeated watering.
Figure 1-67. Construction of Eskimo style snowhouse.
(2) Freeing of ice fragments into layers by adding water.
(3) Laying of ice blocks.
(1) Layer-by-layer freezing by water produces the strongest ice and is very cheap compared to other methods, but it requires much more time. The main condition for an effective freezing of ice layers by sprinkling is the right application of water according to the weather condition. Under working conditions, it can be assumed that about 0.5 cm (.2 in.) of ice is frozen per day for each degree below 0°C. Layer-by-layer freezing by watering is effective only at air temperatures below -5°C. At higher temperatures, the freezing should be discontinued. The most favorable temperature for this method is -10° to -15°C. with a moderate wind.
(2) Freezing of ice fragments into layers by adding water if very effective and the most frequently used method for building ice structures. The ice fragments are prepared on nearby plots or on the nearest river or water reservoir. The layer should be 20 to 30 cm (8 to 12 in.) thick and packed as densely as possible. The ice fragments should not be crushed as that would cause a weakening of the ice construction. If the weather is favorable (-10° to -15°C. with wind), a 50 cm (20 in.) thick ice layer can be frozen in a day.
(3) Laying of ice blocks is the quickest method but is much more expensive requiring the transportation of the ice blocks from the nearest river or water reservoir to the site. Ice blocks are laid like bricks; the blocks are overlapped. It is desirable to have the ice blocks uniform in size, especially of equal thickness. Before each new layer of ice blocks is laid, the preceding layer must be wet to achieve good adfreezing. Each layer of ice blocks must be allowed to freeze before placing the next.
1-56. FROZEN GROUND
a. Suitability. Frozen ground is three to five times stronger than ice; its strength increases with lower temperature. It has much better resistance to impact and explosion than to steadily acting load, an especially valuable feature for fortification purposes.
b. Methods. Construction using frozen ground is done by
(1) Freezing chunks of frozen ground in layers.
(2) Laying prepared blocks of frozen ground.
(3) Preparing blocks of frozen ground from a mixture of water and aggregate (icecrete). Most suitable is a material consisting of gravel-sand-silt aggregate wetted to saturation and poured similar to Portland cement concrete. After freezing, such a material has the properties of concrete. These methods are analogous to the construction methods using ice.
1-57. PERMANANCE OF STRUCTURES FROM SNOW, ICE, AND FROZEN GROUND
a. Structures in subarctic regions. It is obvious that with the onset of warm weather, structures made of snow, ice, and frozen ground will disintegrate. A snow structure in early spring loses its camouflage. To extend its life, if needed, it should be covered with locally available material such as moss or forest litter. Depending upon the weather and layer thickness, the useful life of such a structure may be extended for more than a week.
b. Structures in arctic regions. In regions of shallow seasonal thaw underlined by permafrost, structures from snow, ice, and especially permafrost may be made permanent. For this purpose, the structure should be covered with the same material as the local permafrost. Typically, in tundra regions, the permafrost is covered by a shallow layer of thick vegetation, the so-called tundra mat which protects the permafrost from melting during the summer. The careful removal of the tundra mat and the construction of the structure on bare ground, and then covering all exposed surfaces with tundra mat material, protects the structure against summer disintegration. Special measures should be taken to minimize disturbance of the area around the structure during construction. Any disturbance of the area around the structure should be repaired. Careful preparation of such an expedient protective structure makes it permanent if needed.
1-58. RESISTANCE TO IMPACT AND EXPLOSION
a. To evaluate snow, ice, and frozen ground as materials for fortification, it is necessary to know the resistance of these materials to impact and explosion.
b. A rifle bullet rapidly loses its penetrating power, depending on the density of the snow. Snow packed in layers deflects the bullet at each layer. Loose snow spread over a defensive position will help smother ricochets.
c. Loose snow greatly reduces the explosive and fragmentation effects of shells. The depth, type of snow, and ammunition type are naturally the main consideration. The use of a delayed action fuse will generally cause the shell to penetrate the snow blanket and explode underneath, smothering and reducing the effect of the fragmentation. One meter (3.3 ft.) of snow will provide some protection against most light artillery fire. A superquick fuse setting will increase the effect of artillery fire, while airburst will cause still more damage to surface targets.
d. Results of resistance tests of the impact of a bullet or explosion on samples of different frozen soils and icecrete at -4° C. are shown in table 1-12. On the basis of these test results the following general conclusions can be made:
(1) Construction from ice, frozen ground, and icecrete have a good resistance to the penetration of bullets and splinters. Because of their friability, which varies with different soils and increases with low temperatures, their resistance to explosion is less. This is especially true for ice, which should not be used for fortifications but only for obstacles.
(2) Fortifications made from frozen ground and icecrete should have an anti-spalling cover. Additionally, it is desirable to reinforce these materials with branches, straw, coniferous needles, and so forth. Wooden sheathing can be used as an anti-spalling cover. This sheathing is necessary during the construction and should be left as an anti-spalling cover after finishing the work. The reinforcement significantly increases the resistance of ice and frozen ground to impact and explosion. This reinforcement should be laid continuously rather than in layers. If reinforcement is made in layers, the impact of explosion will split the material along the layer of reinforcement. Such reinforcement does not strengthen, but weakens the construction.
Table 1-12. Resistance to Impact and Explosion of Frozen Soils and Icecrete
1-59. NATURAL OBSTACLES
a. Snow-covered and icy slopes. A steep slope is an obstacle to troops and vehicles even under normal conditions. When covered by deep snow or ice, it becomes much harder to surmount. The bogging-down action and the loss of traction caused by deep snow frequently create obstacles out of slopes which might be overcome easily, otherwise.
b. Windfalls. Occasionally, strong winds knock down many trees in a wooded area. These fallen trees are known as windfalls. They are very effective obstacles when covered with snow, especially to personnel wearing skis or snowshoes.
c. Avalanches. An avalanche makes an excellent obstacle for blocking passes and roads. Since it occurs in mountainous country where there are few natural avenues of approach, an avalanche can have a far-reaching influence over combat operations. The problem with those avalanches which occur naturally is that, unless their timing and location are just right, they may be of help to the enemy. It is possible to predict in advance where an avalanche can and probably will occur. Then by the use of recoilless rifle or artillery fire, bombs, or explosives, it is possible to induce the avalanche to slide at the desired time. This type of avalanche is an artificial obstacle in the technical sense. Generally, it will be of more value than the natural type.
1-60. ARTIFICIAL OBSTACLES
a. Barbed wire. There are many types of artificial obstacles used under summer conditions which are appropriate for winter use. Barbed wire normally employed makes an effective obstacle in soft, shallow snow. Triple concertina is especially effective since it is easy to install in addition to being difficult to cross. As the snow becomes deeper and more compact, a point is reached where it is possible to cross the barbed wire on top of the snow. One type of barbed wire obstacle built to overcome this problem is known as the Lapland fence.
b. Lapland fence. The Lapland fence uses a floating type of anchor point or one which is not sunk into the ground. Poles are used to form a tripod. The tripod is mounted on a triangular base of wood. Six strands of wire are strung along the enemy side of the fence, four strands along the friendly side, and four strands along the bas. As the snow becomes deeper, the tripods are raised out of the snow with poles or by other means to rest the obstacle on top of newly fallen snow. The base of the tripod and the base wires give enough bearing surface to prevent the fence from sinking into the snow.
c. Knife, rests. Knife rests are portable barbed wire fences, usually constructed prior to the snowfall. The fences are constructed by tying two wood poles at their center, forming an X. A similar X is made out of two other poles and then the two X's are lashed at each end of a 3 meter (10 ft) to 3.5 meter (11.7 ft) pole. This forms a framework to which barbed wire is fastened on a four sides. The obstacles can be stored until needed and easily transported to the desired location.
d. Concertina wire. Concertina wire is another quick way to improve on snow- covered obstacles. The concertina comes in 15 meter (50 ft.) sections which can be anchored quickly to the top of existing obstacles.
e. Abatis. The abatis is similar to a windfall. Trees are felled at an angle of about 45° to the enemy's direction of approach. The trees should be left attached to the stump to retard removal. Along trails, roads, and slopes, abatis can cause much trouble for skiers and vehicles.
f. Iced road grades. A useful obstacle can be made by pouring water on road grades. The ice that forms will seriously hamper vehicular traffic.
g. Ice demolition. In creating water obstacles to the enemy during winter conditions it becomes expedient to place charges below the ice and under water. To place charges under water, make boreholes in the ice with axes, chisels, ice augers, or shaped charges, then place the main charge below the ice. A charge of 36.3 kg (80 lb) of M3 demolition blocks, through 1.37 meters (4 1/2 ft) of ice, produces a crater 12 meters (40 ft) in diameter. To create a minefield in ice, sink boreholes about 3 meters (10 ft) apart in staggered rows. Suspend charge below the ice by means of cords with sticks bridging the top of the holes. The charges should be set least 60 cm (2 ft) below the bottom of the (fig 1-68). The size of the charge depends on the thickness of ice. Activate the firing devices on two or three charges in each underwater minefield, one on each end and one in the middle. The rest of the charges will detonate sympathetically. Blowing a field like this creates an obstacle to enemy vehicles for approximately 24 hours at -24°F.
Figure 1-68. Method of Placing charges under ice.