Bunker

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For other uses, see Bunker (disambiguation).

A bunker is a defensive military fortification. Bunkers are mostly below ground, compared to blockhouses which are mostly above ground. They were used extensively in World War I and World War II. During the Cold War, bunkers became a part of American culture when people built backyard fallout shelters. One famous bunker is NORAD's underground facility at Cheyenne Mountain. The Soviet Union maintained huge bunkers and in Albania Enver Hoxha dotted the country with hundreds of thousands of bunkers. At one time Osama bin Laden was rumored to be hiding in massive 'underground fortresses' in Tora Bora.

1 Bunkers as part of a trench system
2 Pillbox
3 Industrial bunker
4 Design of Blast-resisting Bunkers
5 See also
6 References
7 External links

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Bunkers as part of a trench system

Another type of bunker or blockhouse is a little concrete post, 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). The front bunker of a trench system usually includes machine guns or mortars and form a domainant shooting post. The Rear bunkers are usually used as command posts, for storage and as field hospitals to attend to wounded soldiers.

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Pillbox

Image:Pillbox 800.jpg

Dug-in guard posts (with loopholes for firing through) and made from concrete are also known as "pillboxes". The originally jocular name arose from their perceived similarity to the cylindrical boxes in which medicinal pills were once sold. They are in effect a trench firing step hardened to protect against small-arms fire and grenades and raised a little to improve the field of fire.

Their use seems to have developed during the period of the First World War when defence in depth using the Machine Gun Corps was being perfected. However, most of those seen in Britain, having been left over from the 1940 invasion scare, are designed for use by riflemen rather than for machine gunners. The concrete nature of pillboxes means that they are a feature of prepared positions and their original use is likely to have been in the Hindenburg Line. This is likely to have been the time when they acquired their incongruous English name. The Oxford English Dictionary's earliest record of the use of the word pillbox in connection with a defensive post is from 13 September 1917, after the German withdrawal onto the Hindenburg Line.

Pillboxes are often camouflaged in order to conceal their location and to maximize the element of surprise. They may be part of a trench system, form an interlocking line of defence with other pillboxes by providing covering fire to each other (defence in depth), or they may be placed to guard strategic structures such as bridges and jetties.

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Industrial bunker

Typical industrial bunkers include mining sites, food storage areas, dumps for materials, and sometimes living quarters. Template:Section-stub

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Design of Blast-resisting Bunkers

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 psia of overpressure, bunkers are regularly constructed to survive several hundred psia. This substantially decreases the likelihood that a bomb can harm the structure.

The basic plan is to provide a structure that is very strong in compression. The most common purpose-built structure is a buried, steel-reinforced concrete vault or arch. Most expedient 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 meters of tamped earth.

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

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 fire 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 must have two doors. Normally one door is convenient, and the other is strong. Door shafts may double as ventilation shafts to reduce the digging.

A large ground shock can move the walls of a bunker several centimeters in a few milliseconds. Bunkers designed for large ground shocks must have sprung internal buildings, hammocks, or bean-bag chairs to protect inhabitants from the walls and floors.

Earth is an excellent insulator. In bunkers inhabited for prolonged periods, large amounts of ventilation or air-conditioning must be provided in order to prevent heat prostration. 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 are tire-treads nailed or bolted to frames strong-enough to resist the maximum overpressure.

If a bunker is in a built-up area, it may 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), inexpensive polyethylene film serves quite well, because the overburden protects it from degradation by wind and sunlight.

Experts in preparedness (Such as Cresson Kearny, see below) for war recommend purchasing and stockpiling the materials for an expedient blast or fallout shelter, and then constructing it only if war appears very likely. In real wars, such materials have almost immediately become unavailable as emergency construction depleted stocks. The storage needed is modest, and the materials are inexpensive in peacetime, and easy to inspect and maintain.

When a house is purpose-built with a bunker, the normal location is a reinforced below-grade bathroom with large cabinets.

Some vendors provide true bunkers engineered to provide good protection to individual families at modest cost. One common design approach uses fiber-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. A properly designed, properly installed home shelter does not become a sinkhole in the lawn.

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