Bunker

A bunker is a defensive military fortification designed to protect the inhabitants from falling bombs or other attacks. Bunkers are mostly below ground, compared to blockhouses which are mostly above ground. They were used extensively in World War I, World War II, and the Cold War for weapons facilities, command and control centers, and storage facilities (for example, in the event of nuclear war). Bunkers can also be used as protection from tornadoes.

Trench bunkers are small concrete structures, partly dug into the ground. Many artillery installations, especially for coastal artillery, have historically been protected by extensive bunker systems. Typical industrial bunkers include mining sites, food storage areas, dumps for materials, data storage, and sometimes living quarters. When a house is purpose-built with a bunker, the normal location is a reinforced below-ground bathroom with fibre-reinforced plastic shells. Bunkers deflect the blast wave from nearby explosions to prevent ear and internal injuries to people sheltering in the bunker. Nuclear bunkers must also cope with the underpressure that lasts for several seconds after the shock wave passes, and block radiation.

A bunker's doors must be at least as strong as the walls. In bunkers inhabited for prolonged periods, large amounts of ventilation or air conditioning must be provided. Bunkers can be destroyed with powerful explosives and bunkerbusting warheads. The crew of a pillbox can be killed with flamethrowers.

Etymology
Bunker is a relatively recent addition to the English language to describe a military structure. According to the Oxford English Dictionary it is a "A military dug-out; a reinforced concrete shelter" and its first use was 13 October 1939 "A Nazi field gun hidden in a cemented 'bunker' on the Western front" (War Pictorial). The word is German in origin and was used by the Germans to describe bombproof shelters both above ground as in Hochbunker and below ground as in the Führerbunker. All the early references to its usage in the Oxford English Dictionary are to German fortifications. By 1947 the word was familiar enough in English that Hugh Trevor-Roper in The Last Days of Hitler was describing Hitler's underground complex near the Reich Chancellery as "Hitler's own bunker" without quotes around the word bunker.

Trench
This type of bunker is a small concrete structure, partly dug into the ground, which is usually a part of a trench system. Such bunkers give the defending soldiers better protection than the open trench and also include top protection against aerial attack (grenades, mortar shells). They also provide shelter against the weather.

The front bunker of a trench system usually includes machine guns or mortars and forms a dominant shooting post. The rear bunkers are usually used as command posts or Tactical Operations Centers (TOCs), for storage and as field hospitals to attend to wounded soldiers.

Artillery
Many artillery installations, especially for coastal artillery, have historically been protected by extensive bunker systems. These usually housed the crews serving the weapons, protected the ammunition against counter-battery fire, and in numerous examples also protected the guns themselves, though this was usually a trade-off reducing their fields of fire. Artillery bunkers are some of the largest individual pre-Cold War bunkers. The walls of the 'Batterie Todt' gun installation in northern France were up to 3.5 m thick, and an underground bunker was constructed for the V-3 cannon.

Converted Mines/Caves for WWII Industrial Bunkers
 * Ebensee
 * Hinterbrühl
 * Lager Rebstock
 * Mittelwerk
 * Saint Leu d'Esserent

Industrial
Typical industrial bunkers include mining sites, food storage areas, dumps for materials, data storage, and sometimes living quarters. They were built mainly by nations like Germany during World War II to protect important industries from aerial bombardment. Industrial bunkers are also built for control rooms of dangerous activities, e.g. tests of rocket engines or explosive experiments. They are also built in order to perform dangerous experiments in them or to store radioactive or explosive goods. Such bunkers also exist on non-military facilities.

Personal
When a house is purpose-built with a bunker, the normal location is a reinforced below-ground bathroom with large cabinets. One common design approach uses fibre-reinforced plastic shells. Compressive protection may be provided by inexpensive earth arching. The overburden is designed to shield from radiation. To prevent the shelter from floating to the surface in high groundwater, some designs have a skirt held-down with the overburden. It may also serve the purpose of a safe room.

Blast protection
Bunkers deflect the blast wave from nearby explosions to prevent ear and internal injuries to people sheltering in the bunker. While frame buildings collapse from as little as 3 psi (0.2 bar) of overpressure, bunkers are regularly constructed to survive several hundred psi (over 10 bar). This substantially decreases the likelihood that a bomb (other than a bunker buster) can harm the structure.

The basic plan is to provide a structure that is very strong in physical compression. The most common purpose-built structure is a buried, steel reinforced concrete vault or arch. Most expedient (makeshift) blast shelters are civil engineering structures that contain large buried tubes or pipes such as sewage or rapid transit tunnels. Improvised purpose-built blast shelters normally use earthen arches or vaults. To form these, a narrow (1-2 metre) flexible tent of thin wood is placed in a deep trench (usually the apex is below grade), and then covered with cloth or plastic, and then covered with 1–2 metres of tamped earth.

A large ground shock can move the walls of a bunker several centimetres in a few milliseconds. Bunkers designed for large ground shocks must have sprung internal buildings to protect inhabitants from the walls and floors.

Nuclear protection
Nuclear bunkers must also cope with the underpressure that lasts for several seconds after the shock wave passes, and block radiation. Usually these features are easy to provide. The overburden (soil) and structure provide substantial radiation shielding, and the negative pressure is usually only 1/3 of the overpressure.

General features
The doors must be at least as strong as the walls. The usual design is a trap-door, to minimize the size and expense. To reduce the weight, the door is normally constructed of steel, with a fitted steel lintel and frame. Very thick wood also serves, and is more resistant to heat because it chars rather than melts. If the door is on the surface and will be exposed to the blast wave, the edge of the door is normally counter-sunk in the frame so that the blast wave or a reflection cannot lift the edge. A bunker should have two doors. Door shafts may double as ventilation shafts to reduce digging.

In bunkers inhabited for prolonged periods, large amounts of ventilation or air conditioning must be provided in order to prevent ill effects of heat. In bunkers designed for war-time use, manually operated ventilators must be provided because supplies of electricity or gas are unreliable. One of the most efficient manual ventilator designs is the Kearny Air Pump. Ventilation openings in a bunker must be protected by blast valves. A blast valve is closed by a shock wave, but otherwise remains open. One form of expedient blast valve is tyre-treads nailed or bolted to frames strong enough to resist the maximum overpressure.

If a bunker is in a built-up area, it may have to include water-cooling or an immersion tub and breathing tubes to protect inhabitants from fire storms.

Bunkers must also protect the inhabitants from normal weather, including rain, summer heat and winter cold. A normal form of rainproofing is to place plastic film on the bunker's main structure before burying it. Thick (5-mil or 0.13 mm), inexpensive polyethylene film serves quite well, because the overburden protects it from degradation by wind and sunlight.

Countermeasures
Bunkers can be destroyed with powerful explosives and bunkerbusting warheads. The crew of a pillbox can be killed with flamethrowers. Complex, well-built and well-protected fortifications are often vulnerable to attacks on access points. If the exits to the surface can be closed off, those manning the facility can be trapped. The fortification can then be bypassed.

Famous installations
Famous bunkers include the post-World War I Maginot Line on the French eastern border and Czechoslovak border fortifications mainly on the northern Czech border facing Germany (but to lesser extent all around), Fort Eben-Emael in Belgium, Alpine Wall on the north of Italy, World War II Führerbunker, the V-weapon installations in Germany (Mittelwerk) & France (e.g. La Coupole, and the Blockhaus d'Éperlecques) and the Cold War installations in the United States (Cheyenne Mountain, Site R, and The Greenbrier), United Kingdom (Burlington), Sweden (Boden Fortress) and Canada (Diefenbunker).

The Soviet Union maintained huge bunkers (one of the secondary uses of the very deeply dug Moscow Subway system was as nuclear shelters). A number of facilities were constructed in China, such as Beijing's Underground City and Underground Project 131 in Hubei; in Albania, Enver Hoxha dotted the country with hundreds of thousands of bunkers.