Each fort was designed to take direct hits from its neighbor without damage. The taking of Fort Eben Emael in Belgium, where assaulting troops were out of the fields of fire, demonstrated the value of cross fire. In the Maginot Line, forts could be swept of attackers by neighboring forts. The concrete casing was made thick enough to resist three projectiles falling at the same point of impact. During construction, the casing was again tripled to 3.5 meters, and covered by layers of earth.
Major studies were conducted on the effects of the Big Berthas on the Verdun forts to determine the best protective materials for the exposed concrete. Trials with 400mm guns at a distance of 10km revealed that steel-reinforced concrete afforded the best protection for the surface blocks and was superior to the mixed system of masonry, sand, and concrete. The underground galleries were constructed in masonry, which was much cheaper and well-protected by the earth, as long as there was at least 20 meters of earth overhead. Surface concrete was 3.5m thick, double that of Verdun. This was categorized as level 4, maximum protection, for the most important works. Level 1 protection of 1.5m was used for the least important casemates. The rear walls of the casemates were only 1m. The thickness of the steel cupolas was 300mm in most and 350mm in the 75mm, model 1933 turrets.
Each position took advantage of natural cover, well-sighted observation posts, minimum dead ground for fields of fire, maximum area of fire, and location of anti-tank obstacles. Starting at the border, river crossings and crossroads were protected by anti-tank positions and minefields. In the evacuated frontier villages, houses were camouflaged and fortified to provide forward positions. Avant-postes, or forward posts, delayed and gave warning of enemy attack. These consisted of two lines occupied by the Mobile Republican Guard and by elements of the Fortress Infantry Regiments. Their mission was to hold up the enemy for one hour at the frontier blockhouses (maisons fortes), and four hours at the second line. Further back, and about 1200 meters apart, casemates were built in the intervals between the forts. Each was two-story with a firing chamber on the top floor and a crew of 25 - 30 men, equipped with arms, munitions, and stores. Each casemate assured a continuous line of fire and flanked the obstacles with machine-gun and anti-tank weapons. The casemates were further protected by the 75mm guns of the main works. In the front was an anti-personnel ditch. On the roof were armoured cupolas for machine-guns. Each had a diesel generator. Some, known as "coupled" casemates, were joined together by an underground gallery. A double obstacle marked the edge of each RF, an exterior band of anti-tank rails, and an interior network of barbed wire entanglements, which ran the length of the frontier at a distance of 5-10 km, following the relief of the terrain.
At intervals of 5-8 km were the main forts, the ouvrages. These were broken down into large, medium, and small works. Large works were equipped with the higher caliber artillery guns and a garrison of 1000 to 1200 men. The medium forts had 500 men and the smaller works had up to 200 and contained infantry blocks only. It was important that the artillery works were not more than 12 kilometers apart (the maximum range of their 75mm guns) so they could bring down fire on the smaller works. The artillery works were thus the "bastion" of the defensive position and occupied the dominant points. These were the massive underground forts which generated so much fascination and mystery. Beyond the major works were storage depots, concrete shelters for the interval and reserve troops, barracks, and casernes for the troops' families.
The following equipment was used in the fortified regions:
The armament used in the Maginot Line was built specifically for the fortifications. For the infantry there was the 7.5mm Reibel Machine-gun, usually coupled in pairs; The Reibel had a range of 2400m and could fire 500 shots per minute. Anti-tank guns of 37 and 47mm were placed uniquely in casemates and could be interchanged with machine-guns using an overhead rail system. The range of these guns was 800 meters. There were also several positions which used "mixed arms," or a combination of twin machine-guns and one 25mm anti-tank gun with a range of 400 meters.
For close defense of each ouvrage, the following weapons were used:
For defense away from the casemates, and for launching attacks on invaders:
There were no heavy weapons. Studies were made on a 145mm cannon with a range of 29km and the creation of a turret for 340mm naval guns but these projects were never realized.
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