Bunker
 |
Bunkers in Albania |
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, massive bunker complexes were built to house both strategic (command & control) infrastructure as well as government personnel and stores for the event of a
nuclear war. During that time, bunkers became a part of
American culture with people building
backyard fallout shelters, though these were not intended to protect against direct attacks as bunkers normally would.
Famous bunkers include
NORAD's underground facility at
Cheyenne Mountain and the Canadian set of so-called
Diefenbunkers. The
Soviet Union maintained huge bunkers (one of the secondary uses of the very deeply dug
Moscow Subway system was as nuclear shelters), and in
Albania,
Enver Hoxha dotted the country with hundreds of thousands of bunkers. Dictators and potentates like
Saddam Hussein often spend massive sums building fortresses beneath their palaces.
Osama bin Laden at one time was also rumored to be hiding in massive 'underground fortresses' in
Tora Bora, though these would only be natural features strengthened and extended to some degree.
Another type of
bunker or
blockhouse is a small concrete box, 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 forms a dominant shooting post. The rear bunkers are usually used as
command posts or
Tactical Operations Center (
TOC), for storage and as field
hospitals to attend to wounded soldiers.
Many
mines in
France were transformed into bunkers by both the Germans and the French in
World War I and
World War II.
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Pillbox on shingle bank North Norfolk England |
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A pillbox on the East coast of England. Part of the defences that were built during World War II (the railings are a modern addition) |
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.
Typical industrial bunkers include mining sites, food storage areas, dumps for materials, data storage, and sometimes living quarters.
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 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 block 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 is 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 or 0.13 mm), 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.
*
Flak tower*
Führerbunker*
bunker buster,
nuclear bunker buster*
fallout shelter*
air raid shelter* World War II concrete bunker fortifications
Maginot Line,
Siegfried Line,
Atlantic Wall,
GHQ Line*
Burlington, a city-sized bunker beneath
Wiltshire*
British hardened field defences of World War II*
Kearny, Cresson, Nuclear War Survival Skills (1990), ISBN 094248701X (online copy)*
Pictures of WW2 Coastal Defences Salthouse, Kelling & Weybourne North Norfolk, UK*
British World War 2 Pillboxes*
UK Heritage: Pillboxes - "miniature modern castles"*
Subterranea Britannica Information on Cold War-era underground structures in Britain
*
British World War 2 Pillboxes in Malta*
German Fortifications Observed in Italy (WWII intelligence report)*
Image of one of the hochbunker of Vienna*
Image of the hochbunker of Trier*
Information on Australian World War 2 Fortifications*
Bunker Pictures: Pictures, locations, information about bunkers from WW2 and The Atlantikwall*
Haagse Bunker Ploeg : Photo site about the atlantikwall in the Netherlands
