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The primary objective of a hardened structure is to withstand the effects of hostile weapons and complete the missions for which it was designed. Depending on the degree of hardening and the nature of this mission, hardened structures may be above or underground. The term “hardened’ applies to facilities intentionally designed to be resistant to conventional explosive effects, nuclear weapons effects, chemical or biological attack, and intruder attack. After establishing the requirements for a hardened facility, concept criteria are developed based on environmental constraints, mission requirements, system configuration, and facility operational modes. In particular, the engagement or operational scenario defines the degree and time of isolation required, the length of warning time the facility commander will have prior to attack, the design weapon effects, and other operational conditions which are necessary for design and operational reliability of the facility.

These facilities must operate in peace, war, and under the threat of war. The installation of equipment and operation of the structure is based on the following operating modes.

A structure is aboveground when all or a portion of the structure projects above the ground. Structures mounded over with slopes steeper than 1:4 are considered aboveground. With respect to the ground surface, a structure is flush or partially buried when its rooftop is flush or buried less than half the structure diameter. Below these levels the structure is deep or shallow-buried depending on whether or not the buried depth enables it to absorb a direct overhead burst. Fortifications and air raid shelters are usually the shallow-buried type and equipped with blast doors, baffles, and labyrinth entrances to provide some blast attenuation.

A deep buried facility so defined is a structure buried deep enough that the direct induced ground motion effects govern design rather than air induced effects. Deep-buried installations can be made almost invulnerable and are generally used for protection of large one-of-a-kind facilities such as command and control centers, which cannot risk relying on redundancy or dispersion to ensure operability. Such important installations are invariable located in hard rock to use the strength of rock for protection and because rock is usually found at the depths of burial necessitated by nuclear weapons of the megaton class.

Deep underground structures are the most costly and present the most operational problems. Deep-underground facilities typically can be several hundred or thousand feet below the surface. Deep-underground facilities must have survivable entrances, exits, communication links, etc., which will be shallow-buried or aboveground facilities.

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