Concrete, steel, pipe or plastic: bunker construction compared.

At a glance

No method is right for every site. Soil, groundwater, depth, size and what the shelter must protect against decide the choice, and every one of these methods needs an engineer's design for the specific site.

Reinforced concrete, cast in place

Reinforced concrete is a rigid structure designed under the American Concrete Institute's ACI 318 code, which calls for 3 inches of concrete over the reinforcing steel wherever concrete is cast against and permanently exposed to earth. It is also the strongest of the common shielding materials: in Oak Ridge National Laboratory's Nuclear War Survival Skills, about 2.4 inches of concrete halves fallout gamma radiation, against about 3.6 inches of packed earth. The U.S. Army's field manual gives 2.2 and 3.3 inches.

Reinforced concrete and fully grouted reinforced masonry are the assemblies most often used in tested storm shelters. In Texas Tech tests for NIST, 8-inch reinforced concrete block took more than 30 impacts at over 100 mph and stayed intact. The weakness is moisture: Kearny's own example of a shelter that was watertight yet dripping with condensation was a reinforced concrete family shelter. Membrane waterproofing outside and dehumidification inside are not optional.

Precast concrete

Precast box sections and vaults are cast in a factory, trucked to the site and set by crane. Standards such as ASTM C1577 cover single-cell precast box sections designed to AASHTO highway loads, and ASTM C857 sets minimum loads for precast underground structures. FEMA notes that prefabricated shelter units must be listed and labeled to the ICC 500 shelter standard. Precast is fast and consistent; its weak points are the joints between sections, which must be sealed.

Insulating concrete forms

Insulating concrete forms are foam blocks that stay in place after a reinforced concrete core is poured inside them. The ICC evaluation report for one widely used system calls for at least 2,500 psi concrete, waterproofing that will not damage the foam below grade, a gypsum thermal barrier inside, and no backfill against the walls until the floor system is in place unless the wall is designed to stand alone. In Texas Tech impact tests, 4 to 6 inch ICF walls showed no cracking of the concrete core, although the missile went through the outer foam or siding.

Corrugated steel pipe

Corrugated steel pipe is a flexible structure. It carries the earth above it by deflecting slightly and bearing against the soil at its sides, so the backfill is part of the structure. The industry installation manual is blunt: faulty compaction has caused more trouble with pipe installations than all other factors combined. Backfill is placed in layers and compacted to 90 percent of standard Proctor density. Kearny advised earth cover of at least half the span.

Durability depends on the soil. Industry guidance estimates service life from soil resistivity and pH, with the floor of the pipe the most vulnerable point; polymer coatings add decades. The steel wall itself adds little shielding, so protection comes from the earth over it.

Welded plate steel

Welded steel shelters are engineered boxes or cylinders. Steel's strength is not the problem underground; corrosion is. Federal rules for underground storage tanks, 40 CFR 280.20, require buried steel to be coated and cathodically protected, clad in a non-corroding material, or left bare only where a corrosion expert finds the site non-corrosive. The National Bureau of Standards' long burial study found that in poorly aerated soils, the pitting rate of buried steel slows little if at all over time. Your county's NRCS soil survey rates the corrosion risk of the ground for uncoated steel and for concrete, and it is worth reading before choosing.

Fiberglass and HDPE

Fiberglass and polyethylene shells do not corrode, and fiberglass is an accepted non-corroding material for buried tanks. Like corrugated pipe, they are flexible structures that depend on stiff, compacted soil around them; highway design limits deflection of plastic pipe to 5 percent of its diameter, and polyethylene's long-term stiffness is a fraction of its short-term value because it creeps under constant load. In wet ground, a buried shell can float, so it must be anchored with concrete pads or deadmen.

Shipping containers

Containers are built to be stacked through their corner posts. The international container safety convention tests the roof with about 660 pounds over a patch two feet by one foot, not the weight of several feet of earth. The International Building Code requires a structural analysis for any container used as a building. Buried, the thin corrugated walls and roof carry loads they were never designed for, and they corrode and sweat. We explain why in a buried shipping container makes a good bunker, and we do not install them.

Water, storms and the rules

Whatever the material, an underground shelter must be kept out of floodwater. FEMA's P-320 says in-ground shelters must be anchored so rising groundwater cannot push them up, and the ICC 500 standard does not permit below-ground shelters in a mapped special flood hazard area. Shelters rated for tornadoes are tested by firing a 15-pound two-by-four at the walls at 100 mph, which corresponds to a 250 mph wind, and at the roof at 67 mph.

What we use

For living quarters, our preference is reinforced concrete, cast in place, precast or in insulating forms, because it carries earth, stops radiation and lasts. For stand-alone bunkers, we also install engineered manufactured shelters in steel or precast concrete where the soil and the household suit them, fitted to the Styrkholm Spec. We do not bury shipping containers.

This guide is general information, not engineering advice. Every underground structure must be designed by a licensed engineer for its site, soil, groundwater and loads.