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.
- Normal conditions. A normal condition exists when” a structure is continually occupied and prepared for the accomplishments of a mission. Normal conditions will exist prior to button-up. Facility power will normally be provided by a commercial utility, though many facilities switch to emergency power when storms occur because of unreliable commercial power. HVAC systems will be operating with the design outside air passing through CB filters. Bypassing the CB filters will not be allowed unless facility mission is minor, and continuous protection against covert attack is not required by the operational scenario. Air from areas such as toilets, equipment rooms, and power plants will be exhausted to the outside. Waste heat will be rejected to the outside through normal cooling towers or radiators. Heat sinks will normally be filled and maintained at design temperature because the time required to lower the heat sink to its design temperature is greater than most warning periods.
- Alert conditions. An alert condition exists during a real or practice exercise. In the alert mode, steps will be taken to improve the defense posture of the facility. The facility power plant will be put in operation and will either share the load with the public utilities or carry it all as prescribed in the operational scenario. No CB filter bypasses are permitted under any conditions. This must be the case because detectors will only indicate that gases or chemicals have been introduced into the system or broken through the filters, leaving no time to take preventive action. Combustion air will continue to be drawn through primary dust scrubbers. Personnel movement in the unoccupied facility areas unprotected by the CB filters will be curtailed. The button-up period normally commences with the alert alarm and continues until the seal-up period starts. Limited egress and ingress may be permitted. In shallow buried facilities, the prime movers are supplied from unhardened fuel storage, and the unhardened cooling towers remain in operation. All other systems are sealed from the outside except for air supply. Hardened heat rejection equipment will be utilized if attack is imminent and throughout the seal-up period.
- Attack conditions. Attack conditions exists when weapons have been detonated in the area. The atmosphere may be contaminated and weapon effects may have rendered external cooling water equipment inoperative. In the attack mode, the facility is closed to protect filters, personnel, and pertinent equipment from blast pressure. The HVAC system is totally isolated from the outside. Ventilation and exhaust air is recirculated through carbon filters for odor removal. The prime mover combustion air is ducted through the primary dust separator and a scrubber for dust removal and temperature control. Contaminated dust slurry from the scrubber is piped to the outside. Facility operation is independent of commercial power. The seal-up period begins with attack warning and continues until the outside environment is tolerable. Fuel is supplied from hardened tanks, and cooling water is supplied from hardened heat sinks and cooling towers.
- Disaster conditions. Under disaster conditions, the installation is inoperative due to damage or exhaustion of cooling water, fuel, or oxygen. To sustain life it may be necessary to utilize oxygen generation and carbon dioxide absorption equipment.
- Postattack conditions After an all clear signal from an attack has been given, the facility can return to an alert condition The post-attack conditions end when the facility objectives are completed.
- Other conditions. The period from button-up or weapon detonation to attack completion is also known as transattack and may range from minutes to days. Together with the postattack it is collectively referred to as the facility endurance period or simply facility endurance.
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.