Standard concertinas are readily moved and are well adapted for the temporary closing of gaps or lanes, or for adding rapidly to the obstacle effect of fixed barriers, such as the double-apron fence. Other portable, barbed-wire obstacles are described below.

Spirals of Loose Wire

By filling open spaces in and between wire entanglements with spirals of loose wire, the obstacle effect is substantially increased. Men are tripped, entangled, and temporarily immobilized. Spirals for such use are prepared as follows:

1. Drive four 3.3-foot posts in the ground to form a diamond 3.3 by 1.6 feet.

2. Wind 246 feet of barbed wire tightly around the frame. Start winding at the bottom and wind helically toward the top.

3. Remove the wire from the frame and tie it at quarter points for carrying or hauling to the site where it is to be opened and used. One spiral weighs less than 20 pounds and a man can carry three or more of them by stepping inside the coils and using wire handles of the type furnished with the standard concertina.

4. If spirals are needed in large quantities, mount the diamond-shaped frame on the winch of a truck and use the winch to coil the wire.

Knife Rest

The knife rest (fig. 8-33) is a portable, wooden or metal frame strung with barbed wire. Use it wherever a readily removable barrier is needed; for example, at lanes in wire obstacles or at roadblocks. With a metal

Figure 8-34.-Tanglefoot in barrier system.

Figure 8-35.-Combination bands of wire obstacles.

frame, you can use it as an effective underwater obstacle in beach defense. Knife rests are normally constructed with 9.8 to 16.4 feet between cross members. They should be approximately 3.3 feet high. The cross members must be firmly lashed to the horizontal member with plain wire. When placed in position, knife rests must be fixed securely.

Trip Wires

Immediately after a defensive position is occupied and before an attempt is made to erect protective wire, place trip wires just outside of grenade range, usually 98 to 131 feet. These wires should stretch about 9 inches above the ground and be fastened to pickets at not more than 16.4-foot intervals. Conceal them in long grass or crops on a natural line, such as the side of a path or the edge of a field. Place the trip wires in depth in an irregular pattern.

Tanglefoot

Use tanglefoot (fig. 8-34) where concealment is essential and to prevent the enemy from crawling between fences and in front of emplacements. Use the obstacle in a minimum depth of 32.8 feet. Space the pickets at irregular intervals of 2.5 to 10 feet. The height of the barbed wire should vary between 9 and 30 inches. Site tanglefoot in scrub, if possible, using bushes as supports for part of the wire. In open ground, use short pickets. Control the growth of grass to help prevent the enemy from secretly cutting lanes in, or tunneling under, the entanglement.