Ouvrage Four-à-Chaux

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Ouvrage Four-à-Chaux is a large fortification of the Maginot Line in northeast France, located in Lembach in the Bas-Rhin department. It sits near a limestone quarry and lime kiln (four à chaux), and was built to guard the Vosges frontier along with nearby ouvrages Lembach and Hochwald.

Fast facts
- Type: Gros ouvrage (large fortification)
- Part of: Fortified Sector of the Vosges, Langensoultzbach sub-area
- Built: 1930–1938 (designed by CORF)
- Size: 26 hectares with about 4.5 km of underground galleries
- Blocks: 8 in total (6 combat blocks + 2 entries)
- Garrison: 19 officers and 491 enlisted (165th Fortress Infantry Regiment, 5th Army)
- Notable features: an artesian well; internal rail system used by carts (not electrified); significant level changes between entrances and blocks; no second 75 mm turret completed

What it looked like and how it worked
- The fort’s blocks are arranged with a large difference in height between the ammunition entrance and the higher personnel entrance (about 24 meters), connected by an inclined gallery after the ammunition entrance.
- It relied on carts running on non-electrified rails inside the complex.
- A dedicated drainage gallery served as an emergency exit.

History in brief
- Construction and purpose: Built to defend the French border with Germany as part of the Vosges sector; Lembach was linked to Four-à-Chaux and Hochwald as a defensive group.
- Wartime action: In the Battle of France (June 1940), Four-à-Chaux was bombed from the air and with German artillery. The fort fired back with its 135 mm and 75 mm turrets. Block 1 was destroyed by the Germans during the fighting and before their retreat in 1945.
- Surrender: The fort formally surrendered on 1 July 1940.
- Lorraine Campaign: It saw little action during the Lorraine Campaign, which mainly involved Hochwald and Schoenenbourg.
- Postwar: In the 1950s, Four-à-Chaux was repaired and kept ready as part of France’s defensive line against a potential Warsaw Pact invasion (the Môle de Haguenau). By the late 1950s, fixed fortifications lost urgency as France developed nuclear deterrence, and the fort was not kept up. It stopped being manned in the early 1970s.

Today
- Four-à-Chaux is a museum site managed by the SILE association (Syndicat d'Initiative de Lembach et Environs). It offers guided visits to Block 2, the barracks, the main gallery, the usine (factory), and the museum.
- It is open to visitors year-round, but the surface area is not accessible because there is unexploded ordnance on the grounds.