Explosives Safety 101 – We’ve Got You (Earth) Covered
If you’ve been around DoD planning or commercial munitions production for very long, chances are that you’ve seen a series of earth mounds with large steel doors on one end. These are known as Earth-Covered Magazines (ECM) or sometimes igloos, and they are used to store explosives materials. They’re not the most exciting looking facilities on an installation, but they serve a unique purpose, one that is commonly misunderstood.
Why do we store explosives in robust structures covered in soil? Having worked with planners and customers across the world, I’ve found most people initially think all of that soil on top is to contain the effects of an explosion from within. Sure, it’s a big heavy structure, and if something is going to go off inside then it would be good to try to hold it all in if possible. But these structures are not designed to contain an explosion. They’re actually pretty bad at it.
So Why Do We Build Them This Way?
The actual functional purpose of these bunkers is not to contain internal forces, but to shield the stored munitions from effects of external forces. Modern ECMs are designed to protect their contents in the event that a neighboring explosives storage or operations location has an accident. The explosives safety criteria we use today to locate and space ECMs has been developed through testing to allow these facilities to be sited as close as possible and still prevent sympathetic detonation from one facility to the next. In ‘ES101-All About Relationships’ we discuss how Intermagazine Distance (IMD) provides a separation that prevents blast wave coalescence, but ECMs are the exception to that rule. ECMs are designed not only to prevent simultaneous detonation, but also to protect the munitions inside.
An ECM is designed to absorb or deflect an incoming blast wave, as well as shield its contents from incoming fragments and debris. The earth cover provides significant mass that absorbs a large amount of the incoming impulse load from the blast. The shape of the earth mound is slanted, with the intent of providing a transition surface for the blast to roll over instead of slamming against a flat wall.
But Can They Actually Contain a Blast?
These robust structures have some ability to contain an internal blast, but it is very limited. Testing by the DoD in 1979 (known as the Flagstaff and Hastings tests) sought to determine the maximum allowable explosive weight that could be stored without creating a hazard arc (blast or fragment) from an ECM. Weights of up to 150 lbs were shown to create no appreciable hazard outside of the magazine. However, the magazine was destroyed in each case. Explosives quantities as small as 12 lbs were shown to cause enough interior structural damage to collapse the magazine or otherwise make it unusable.
Brief History of ECMs
The world hasn’t always tucked their munitions away in earth-covered bunkers. And when we did start using earth cover, it wasn’t just for protection from blast waves. The Department of Defense Explosives Safety Board (DDESB) Technical Paper 15 (credit to my colleague Mr. Eric Deschambault) provides an excellent history for these special structures.
The protective ability of ECMs was first recognized by the US DoD after a massive accident in 1926 at Lake Denmark Naval Ammunition Storage Depot, part of the Picatinny Arsenal in New Jersey. A lightning strike caused a majority of the depot explosives to detonate, leveling hundreds of structures, killing 19 people and injuring dozens more. Notably, some of the structures that survived were a series of these “igloo” structures. The US Army learned of these structures and adopted the concept. The structural properties and benefits had not been thoroughly analyzed or tested, but it was recognized that these structures would be strong enough to “protect its contents from fire, wind pressure, snow loads, and other external forces.”
The ECM quickly became the preferred storage method through the 1920’s and 30’s. As they were being built across the country, other benefits were recorded:
Thermal insulation of the concrete and earth reduced extreme temperatures, which accelerated the deterioration of smokeless powder and other stores.
Earth cover could facilitate camouflage.
ECMs were expected to have a smaller hazard arc, and more of a vertical direction to hazards (i.e., it was thought that the debris would be thrown upward instead of outward).
ECMs could also protect contents, particularly from fragments of nearby events.
With the onset of WWII, thousands of these structures were built across the nation from 1941 to 1945. Major depots were created, utilizing hundreds of ECMs. When constructed, these facilities were spaced significantly further apart from one another than they are today. The full potential of the earth cover was not yet known or tested.
After WWII, the US stockpile of ammunition was so large, that we needed to fit everything closer together. We needed to fit more munitions into less space. Testing commenced to determine if munitions could be stored in the open in the space between ECMs. Tests were also conducted to determine how close ECMs could be located and still prevent propagation. These tests occurred in the 1970’s, 80’s and 90’s. At this point, the focus of using these structures became maximizing land use and reduced the separation distances between structures.
In 1997, the DDESB codified minimum design standards for various levels of ECM headwall strength into 6055.09 STD (today referred to as DESR 6055.09). Three categories were created for headwall strength (i.e., how much external blast pressure the flat front face and door can withstand before failing). They were given names based on “bars”, a metric unit of pressure roughly equivalent to 1 atmosphere. That’s why today you often hear ECMs being designated as “7-bar” magazines. If the design of the magazine is not known, has not been analyzed, or cannot be shown to meet minimum standards today, it is referred to as an “undefined” ECM and requires a little larger spacing between structures.
Additional criteria were added to the 6055.09 Standard in 2000 to codify the design requirements for ECMs with flat roofs (also referred to as box magazines). Box ECMs are preferred over arch ECMs because they can allow for wider, modern doors, and they offer significantly better space utilization along the interior edges due to having vertical walls (as opposed to curved walls in an arch ECM). These structures have the benefit of the mass of the soil cover, but do not have the benefit of the strong arch structure inthe roof providing additional support as the blast wave rolls over the magazine. Large Box ECMs typically require support columns spread throughout the interior. Additional criteria restrictions have been made recently to legacy flat-roof ECMs, but that’s a topic for another blog post.