Section VI. MINES AND OBSTACLES
6-27. Use of Antitank Mines
a. Antitank mines must be placed on a solid base, otherwise when pressure is applied they will sink into the soft ground or snow and lose much of their effectiveness. In shallow snow a hole may be dug and the mine placed on the frozen ground. In deep snow they must be supported. Additional charges will help overcome the smothering effect of deep snow. The snow may be tamped down or frozen, or the mine may be placed on a plank or something
similar to provide the required firm support (fig. 6-32). In all cases they must be covered with snow or dirt, but not buried too deeply; otherwise the top layer may accept the weight and not detonate the mine. A piece of cardboard over the mine will protect it from moisture which may freeze and hinder the
working parts.
b. In snow-covered terrain, the mines should be painted white to aid in concealment. All minefield must be marked and recorded.

6-28. Antipersonnel Mines
a. Antipersonnel mines are adaptable to northern operations. If using pressure-type igniters, solid support for the mine is necessary. If mines are buried too deeply in snow it is possible that the snow will provide a “bridge” and prevent the mine from detonating. Therefore, when using the pressure-type
igniters, place the mine about 3 cm (l”) beneath the snow.
b. Tripwires should be placed at various levels above the snow when using pull-action igniters. Tripwires placed beneath the surface of the snow often freeze in and fail to function. Time permitting, tripwires should be painted white.
c. Mines can be placed on ski or snowshoe trails (fig. 6-33 ). Tripwire firing systems are the best when using antipersonnel mines in this manner. If pressure-type igniters are used, insure that the mine is placed in such a manner that the maximum weight of the individual will be brought to bear on the mine. Care should be taken to insure that the mine will not be “bridged” by a ski or snowshoe, and fail to detonate.

6-29. Use of Demolitions in Ice
a. In summer, the thousands of lakes, rivers and swamps of the northern regions provide formidable obstacles to armor and personnel. In winter, however, when frozen to sufficient depth, they provide excellent avenues of approach. They also lengthen the frontline of a given sector, requiring more troops and weapons to defend it than in summer. Necessary action must be taken to deny these natural routes to the enemy under winter conditions.
b. Preparing Ice Demolitions.
(1) In order to create water obstacles during winter conditions, explosives are used to blow gaps in lake and river ice to make it impassable to enemy personnel and armor. To install the demolition in ice (fig. 6- 34), holes are sunk 3 meters (10’) apart in staggered rows by use of axes, chisels, ice augers (fig. 6-35 ), steam point drilling equipment, or shaped charges. The shaped charges will not make a hole large enough to pass the charge through but must have the hole widened by other means. Charges are suspended in the water below the ice by means of cords tied to sticks bridging the tops of the holes. The charges should be of an explosive not affected by water. Plastic explosives should be protected from erosion by water currents. Demolitions
laid early in the winter must be placed deep enough so that they will not be encased in the ice as it grows thicker.
(2) The normal thickness of fresh water ice is approximately 120 cm (4') or less. In extremely cold areas 150 cm (5') of ice is not uncommon. At the time the minefield is established, it is difficult to determine how thick the ice will be at the time the ice demolition is detonated. As a rule of thumb, if the ice is expected to be 120 cm (4') thick the charges should be approximately 10 pounds. In the event the depth of the ice is expected to exceed 120 cm (4'), an addition of 2.5 pounds per additional 30 cm (1') of thickness should be emplaced. Electrical firing devices are attached to three charges in each underwater demolition, one in each end charge and one in the middle charge. The rest of the charges may be primed with concussion detonators or electrically primed. The large number of charges does limit the use of electrical means of firing. An ice demolition may consist of several blocks of charges echeloned in width and depth and has at least two rows of mines, each row alternating with the one before it. Blowing a demolition such as this creates an obstacle for enemy armor and vehicles for approximately 24 hours at – 24° F (FM 5-25).
(3) Great care must be exercised when handling electrical firing devices under winter conditions. Because of improper grounding of an individual caused by the snow and ice covering on the ground, the static electricity that builds up might possible detonate the device. Individuals must insure that they are properly grounded prior to handling any type of electrical firing devices. Care should be taken to insure that no radio transmitters are operating in the immediate area. The type of radio signals emitted by this type of equipment can detonate electrical firing devices.
c. Advantages.
(1) Long sectors of the frontline may be cut off at a critical moment from enemy infantry and armor.
(2) Number of personnel and AT weapons needed to defend a given sector is reduced.
(3) Friendly troops may advance or withdraw at any place over the charges without being restricted to the cleared lanes.
(4) Charges laid under thick ice are difficult, and often impossible, to detect by use of mine detectors.
(5) When the holes over the charges have refrozen, the field is very difficult for the enemy to breach.
(6) The charges are not affected by weather or snow conditions.
(7) After a snowfall, detection of the demolitions by the enemy is extremely difficult
d. Disadvantages.
(1) Emplacing the explosives requires considerable time even when ice cutting equipment is available.
(2) The charges can be set off when hit by artillery fire.
(3) The gaps blown in the ice tend to freeze over rapidly in low temperatures.
(4) Continued exposure of the demolition firing system to weather reduces the reliability of the system.
e. Tactical Use. Ice demolitions are used for protection from frontal or flanking attacks. Normally, one or more sets of charges are laid close to the friendly shore and others farther out in the direction of the enemy (fig. 6-36). If desired, the enemy may be allowed to advance past the first set of charges and then both detonated at the same time. The enemy thus will be marooned on an ice floe, unable to continue to advance or retreat, and can be destroyed. The same trapping method may be used against enemy armor, or the charges may be detonated directly under the advancing tanks. Ice demolitions must be kept under observation and secured by friendly fire.

6-30. 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 easily overcome otherwise. Pads of track-laying vehicles should be removed when encountering this type terrain.
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. Lakes and Streams. Not all natural obstacles are equally effective in the winter as in the summer. Normally, bodies of water are considered natural obstacles, but under winter conditions the ice which forms may turn these former obstacles into excellent avenues of approach. This illustrates an important reason for reevaluating defensive positions before cold weather arrives.
d. 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 avalanche is an artificial obstacle in the technical sense. Generally it will be of more value than the natural type. Precautions against avalanche hazard are covered in FM 31-72

6-31. Artificial Obstacles
a. Barbed Wire. There are artificial obstacles used under many types of 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 compacted, 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 (fig. 6-37). Types of wire entanglements and winter obstacles are covered in FM 5-15.
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 base. 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. Abatis. An abatis is similar to a windfall. Trees are felled at an angle of about 415° 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.
d. Iced Road Grades. A useful obstacle can be made by pouring water on road grades. The ice that forms will seriously hamper vehicular traffic.

6-32. Means of Improving Obstacles After Heavy Snowfalls
a. 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 Xs are lashed at either end of a 3 meter (10’) to 3.50 meter (12’) pole. This forms a framework to which barbed wire is fastened on all four sides. The obstacle can be stored until needed and then easily transported to the desired location (fig. 6-38).
b. Concertina Wire. Concertina wire is another quick way to improve on snow-covered obstacles. The concertina comes in 15 meter (50’) sections which can be quickly anchored to the top of existing obstacles.
c. Additional Barbed Wire. The possibility of using additional barbed wire strands should not be overlooked. Frequently, obstacles will have protruding poles to which extra barbed wire strands can be tied. Also, additional strands placed underneath such floating obstacles as Lapland fences and knife rests will help prevent the enemy from tunneling under these obstacles.

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