The following U.S. military report on WWII Finnish construction of tank traps on ice was originally published in Tactical and Technical Trends, No. 21, March 25, 1943. [DISCLAIMER: The following text is taken from the U.S. War Department publication Tactical and Technical Trends. As with all wartime intelligence information, data may be incomplete or inaccurate. No attempt has been made to update or correct the text. Any views or opinions expressed do not necessarily represent those of the website.]

FINNISH TANK TRAPS OVER FROZEN RIVERS A recent report on the method used by the Finns to open tank-trap channels in the ice over streams and lakes, and to keep them open and hidden, may be useful for purposes of winter operations. Briefly, the Finns saw out a channel in the ice, roof it over, leaving an air space underneath to prevent re-freezing, and replace the snow over the roof to keep the air space warm and to hide the trap. The work is not difficult. After the outline of the trap has been traced, the snow over the ice to be cut is scraped back into windrows (as in fig. 1, stage 1). Then, a channel 13 feet wide is sawn out, with the cut on the down-stream side sloping outward and downward from the center, so that the ice cakes can be pushed down into the current and the channel is left clear, (stage 2). Over the cut is then laid a light framework roof with either a curved, or cigar-shaped cross section of the king-post type (stage 3), made of light material. The arch provides the air space over the water. Then the snow is shoveled back (stage 4). The best time of year for trap making is early winter, while the ice is still thin, although there must be snow enough for insulation and concealment. Ice less than 8 inches thick will not support a roof. The mats which roof over the cut are made in 15- to 16-foot lengths, 4 to 5 feet wide and rolled for transportation (see fig. 3). They must be supported by frames, of which two types are shown in figure 2. Suitable materials for the mats are roofing felt, shingles, cloth of all sorts, and corrugated paper or stiff paper such as cement sacks. In a pinch, brush will probably do. To place the frames in position, the top of each ice bank is notched back a distance of from 9 inches to 2 feet, and the frames are set in the notches and packed with snow or chunks of ice. Their construction is shown in figure 2. In cold weather they freeze in place in a few minutes. These frames are set at intervals of from 1 foot 6 inches to 2 feet. The snow blanket over the mats should be at least 4 to 6 inches thick, and, if blown away, must immediately be replaced or the channel will quickly freeze. The sawing is done with a saw of the design shown in figure 4. At its tip, the saw carries a hinged, paddle-shaped, steel plate which pivots up and down between stops (a) and (b). On the down-stroke, the paddle is forced down against lower stop (b), which serves to keep the blade firmly pressed against the ice, while on the up-stroke, the plate swings up out of the way against stop (a), in alignment with the motion of the saw, thus permitting easy withdrawal. The fitting of the "paddle" attachment should not be beyond the skill of a good army mechanic. The performance of the saw follows: Ice Thickness Cutting Speed (feet) (inches) (feet per hour) 1    0         130 1    8  80 2    4  50 The life of a trap depends on the weather and the care which goes into the making of the trap. Naturally, the degree of insulation against the freezing of the water underneath the mat varies with the thickness of the snow cover and with the water level; variations in either may increase or lessen the space. New ice (up to 4 inches) may be taken out with HE. Finnish experience shows that carefully made traps will remain effective against medium tanks for from 6 weeks to 2 months, up to midwinter.

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