Bunker

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. They also provide shelter against the weather. Some bunkers may have partially open tops to allow weapons to be discharged with the muzzle pointing upwards (e.g. mortars and anti-aircraft weapons).[10]

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 metres (11 ft) thick,[11] and an underground bunker was constructed for the V-3 cannon.

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, such as 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.

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.[12] It may also serve the purpose of a safe room.

Munitions storage bunkers are designed to securely store explosive ordnance, and contain any internal explosions. The most common configuration for high explosives storage is the igloo shaped bunker. They are often built into a hillside in order to provide additional containment mass.

A specialized version of the munitions bunker called a Gravel Gertie is designed to contain radioactive debris from an explosive accident while assembling or disassembling nuclear warheads. They are installed at all facilities in the United States and United Kingdom which do warhead assembly and disassembly, the largest being the Pantex plant in Amarillo, Texas, which has 12 Gravel Gerties.

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 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 meter, flexible tent of thin wood is placed in a deep trench, and then covered with cloth or plastic, and then covered with 1–2 meters of tamped earth.

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 to protect inhabitants from the walls and floors.[13]

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.

A bunker on the island of Texel, in the Netherlands.

The doors must be at least as strong as the walls. The usual design is now starting to incorporate vault doors. 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 worn flat rubber tire treads nailed or bolted to frames strong enough to resist the maximum overpressure.[14]