Introduction:

At Hardened Structures LLC, our Underground Bunkers are individually designed to our client’s protection program and long term survivability requirements. Although the designs and systems are principally determined for nuclear weapons, it can be stated that a well built nuclear bunker also offers good protection against CBRE (Chemical, Biological, Radiological and Explosive) weapons and natural disasters. A Hardened Structures Underground Bunker can be designed and constructed in most locations be it rural, suburban or inner-city.

Underground bunkers are advantageous for several reasons; they provide the added protection of being below the surface thus escaping a major portion of the effects from blasts and radiation, they can be pre-stocked with provisions to sustain their occupants for several months and, most important, they can be located connected to, or adjacent to, a client’s primary residence, thus allowing for quick evacuation.

The bunkers can be designed for either Single Use or Multi Use. A Single Use bunker is used only in case of an emergency and is a stand alone design. The advantage with the Single Use if that may be readily accepted by the local building authority having jurisdiction because it is not required to provide normal every day accommodations for people.

A Multi Use bunker can be used for other purposes (i.e.; store rooms, wine cellars, etc.,) and this ability is sometimes more appealing to our clients. However, it must then be designed to accommodate everyday ingress/egress and building code requirements.

Threat Assessment:

A sudden all out attack by the Soviet Union or other country on the U.S., as envisioned during the Cold War, is now highly unlikely. The recent National Planning Scenarios compiled by the Department of Homeland Security identifies a Nuclear Detonation of a 10-Kiloton Improvised Nuclear Device as the most likely type of nuclear attack by a terrorist group. In this scenario, the bomb is smuggled into the U.S. where the device is carried in a delivery van and detonated in the business district of a major city. The casualties are estimated to be in the hundreds of thousands, there is total devastation from ½ to 3 miles, 350,000 people are instructed to shelter in place and contamination levels will affect up to 3,000 square miles with the recovery timeline estimated as several years. This is the primary scenario for which Hardened Structures basis its nuclear designs for Underground Bunkers. However, it should be noted that a terrorist detonation of a radiological dispersion device (RDD), often called “dirty nuke” or “dirty bomb” is considered far more likely than use of a nuclear device.

Selecting the Location of an Underground Bunker:

Perhaps the most important element in designing an underground bunker is its location or sitting. Although the threat of damage from CBRE (chemical, biological, radiological and explosive) events may be the predominant focus of the evaluation for the bunker location, additional threats may exist from forced entry, tornado, hurricane, flood and seismic events, therefore the evaluation should asses the entire range of threats to the site. It is for this reason that Hardened Structures employs a Multi-Hazard Engineering methodology.

The position of the underground bunker must satisfy the structural requirements from the shelter standpoint along with the evacuation criteria of the client and the protection programming of the mechanical systems. The immediate location and easy accessibility of a bunker has a significant bearing on its life saving ability during a crisis. Therefore, we recommend that all potential users should be able to reach the bunker within 5 minutes, and the bunker door should be secured within 10 minutes.

Generally stated, an underground bunker should be located:

• As deep underground as possible to protect from radiation, flying projectiles and debris.
• Outside of areas known to be flood prone, including areas within the 100 year flood plain.
• The bunker should be placed so that the evacuees have a short route to the entrance.
• Away from any potential debris field and its emergency exits and air inlets can be extended on several sides of the building into zones that are free from debris and fire.
• The bunker should have as much of its external walls against the ground as possible for protection from heat and for support provided by the surrounding soil.
• Away from potential fuel concentrations, flammable materials, vehicles and hazardous materials.
• Away from large objects and multi-story buildings, light poles, antennas, satellite dishes or roof mounted mechanical equipment.
• The bunker should be made easily concealed.

1. Escape Shafts that lead through the bunker shell/wall directly to the outside and then run vertically up the bunker wall.
2. Escape Tunnels run horizontally away from the bunker for a distance of one and one half times the building height to clear any fallen debris.
3. Escape Chimneys that are air and blast proof emergency exits which lead up through the expected pile of debris from a collapsed structure. The chimney must be constructed to a height of at least ¼ of a buildings height when measured from the eves.

Design of an Underground Bunker:

An underground bunker may be as small as a 10’ x 10’ room designed to preserve life with little or no comfort, to a multi-cell shelter housing hundreds of occupants with a storage capacity for years. Access is an important element of bunker design. If obstructions exist along the travel route, or if a bunker is cluttered with non-essential equipment and storage items, access will be impeded and could cause chaos or panic. It is imperative that the route to the bunker remain unencumbered to allow orderly access.

In the scenario described above where a 10 Kiloton Improvised Nuclear Device is detonated, the design for an underground bunker is driven by the need to protect from blast loading, ground shock, nuclear radiation and fire. A detonation involves supersonic combustion of an explosive material and the formation of a shock wave. The three parameters that primarily determine the characteristics and intensity of blast loading are the size of the explosives, the type of explosives and the distance from the point of detonation to the bunker.

A detailed analysis is required to determine the magnitude of pressure and impulse that may load each surface of the bunker relative to the origin and type of the detonation. For these reasons we generally utilize reinforced concrete. Reinforced concrete is a composite material in which the concrete provides the primary resistance to compression and shear and the steel reinforcement provides the resistance to tension and confines the concrete core. In addition to ductile detailing, which allows the reinforced members to sustain large deformations, reinforced concrete also provides inertial resistance as well as the continuity of cast in place construction facilitating designs that are capable of withstanding the high intensity and short durations of blast loading.

When designing for radiation and fallout, it is difficult to predict the intensity of radiation within a specified area. Radioactive fallout distribution depends on such variables as the type of burst, amount of energy released, height of the radioactive cloud, nature of the ground surface and the speed and direction of the wind at different altitudes.

There are three types of radiation in fallout: alpha particles, beta particles and gamma rays. Alpha and beta particles are easily shielded however Gamma rays are not easily attenuated. They are similar to X-rays and are capable of penetrating a considerable thickness of even dense material.

After residual radiation the next most widespread effects are the thermal radiation and related fires. If an underground bunker is located under or adjacent to a wood frame home it must be assumed that the house will catch fire and be totally destroyed. In this scenario a bunker ceiling constructed of 16” of 4,000psi concrete together with walls of 12” and floors of 8” is required.

Protection against airborne chemical, biological and radiological (CBR) agents or contaminants is typically achieved by using particulate and absorption filters in series and providing for positive internal air pressure. There is no single filter which can protect against all CBR materials. All air flow passes through installed filters of the HVAC system. The exterior air intake and exhaust openings in the bunkers are all protected by blast valves. The outer most filters are coarse, low-efficiency (pre-filters) which remove large particles and debris while protecting the mechanical systems. Chemical, radioactive and biological aerosol dispersions (particulates) are efficiently removed by HEPA filters. Sorbent filters are located downstream from the particulate filters. This allows the sorbent to collect vapors generated from liquid aerosols that collect on the particulate filter and reduce the amount of particulate reaching the actual sorbent. Sorbents have different affinities, removal efficiencies and saturation points for different chemical agents and therefore choosing the appropriate sorbent or sorbents for an airborne contaminant is a complex decision.

The Department of Homeland Security, Working Group on Radiological Dispersal Device Preparedness identifies three levels of protection that range from filtration with pressurization (Class 1), filtration with little or no pressurization (Class 2), and passive protection (Class 3). Class 1 is for a large scale release over an extended period of time and is considered primarily for a war-like attack. Class 2 protection is for a terrorist attack or technological accident with little or no warning and is characterized as a short duration small scale release. Class 3 protection is typically applicable to an industrial accident that results in a short duration.

To withstand a CBR event the bunker must be equipped with a filter-rack mechanical system capable of providing the minimum number of air exchanges required by the building code for the shelter’s occupancy classification. This will provide a flushing capability once the CBR hazard has passed and will facilitate use of the bunker for non-CBR events. For Single Use bunkers, 15 cubic feet per person per minute is the minimum air exchange recommended by the International Mechanical Code. At Hardened Structures we recommend a minimum head room of 6’-6” and a minimum of 65 cubic feet of net volume be provided per bunker occupant and we can install a mechanical system to withstand most any class of protection to meet your individual protection goals.

Occupancy Duration:

Occupancy duration (also known as the button-up time) is the length of time that people will be in the bunker with the doors closed ensconced in a protective environment. This time period can last from a few hours to several days. If the occupancy duration of a bunker is less than 24 hours then sleeping areas are typically not required and the occupant load will generally be 20 square feet/person. If the occupancy duration is greater than 24 hours sleeping areas should be incorporated at the rate of 60 square feet/person utilizing single beds or 30 square feet/person using bunk beds.

The total square footage of an underground bunker is determined based on the following:

• Number of persons
• Occupancy duration
• Class of protection
• Storage requirements
• The Client’s particular life sustaining/quality requirements

The duration of occupancy of an underground bunker will vary depending on the intended event for which the bunker has been designed. Protection levels, occupancy duration and specific Client storage/survivability criteria are the most important factors that influence the design process.

Conclusion:

As stated previously, a Hardened Structure Underground Bunker can be constructed in most locations, rural, suburban or inner-city. The fundamental question is the level of protection the Client desires and their resources to achieve that level. A representative from Hardened Structures is available to meet and help determine Asset Assessment, Threat Assessment and Survivability Criteria and will also develop a Feasibility Report for any proposed Bunker location.