As a result of the disaster at the Naval Ammunition Depot at Lake Denmark, N. J., in July, 1926, improved storage is being arranged for large quantities of TNT at the Naval Ammunition Depot at Hawthorne, Nev., at the Navy Mine Depot at Yorktown, Va., and at the Naval Ammunition Depot at Oahu Island, Hawaii. Millions of dollars are to be spent, and these projects are probably more extensive than others yet undertaken anywhere to furnish permanent, and particularly safe, peace-time storage for large quantities of military high explosives which are on hand.
At all of these places one will find no pretentious storage buildings. Instead he will find the whole area covered with equidistant groups of small cement “igloos.” Each of these igloos, of identical size, will be shaped like a semi-cylinder with its axis on the ground, with earth piled against its sides as a barricade so as nearly to conceal it from view. There will in each group be seven igloos, i.e., a central one with six others around it as on the comers of a hexagon.
If one is at all curious, one may wonder what line of reasoning led to the adoption of a layout so novel and so essentially different from anything previously built. And one will be quite right in guessing that the ideas evident in this construction were arrived at only after considerable study.
Let us start at the beginning and, assuming the whole area covered with equidistant storage units of identical capacities, first see, with each unit entirely safe from the next one, whether large or small units are the more economical of area.
The American Table of Distances shows for various quantities of explosives detonating in barricaded magazines, that inhabited buildings must be located at the following distances at least in order just to escape “severe structural damage”:
Amount of Explosive Distance = d
200,000 lbs 2,090 ft.
400,000 lbs. 2,530 ft.
600,000 lbs. 2,930 ft.
800,000 lbs. 3,240 ft.
1,000,000 lbs. 3,460 ft.
If spaced equidistantly, units of these sizes will (neglecting borders) each account for an area of .86d2. So the “amount of explosive stored per square foot of ground occupied” would then be:
Size of Unit Lbs. of Explosive Per Sq. Ft. of Area
200,000 lbs. .053
400,000 lbs. .072
600,000 lbs. .081
800,000 lbs. .088
1,000,000 lbs. .097
In such storage, which meets the practically ideal requirements of having each strong and well-protected unit as far from any other as an inhabited building needs to be, economy of ground would clearly indicate the desirability of having each unit of maximum possible capacity. Military considerations, however, would argue strongly against risking over 1,000,000 pounds in the loss of any one building. One-million-pound units would under these conditions, be spaced 3,460 feet apart.
In such ideally safe storage, however, (a) the ground occupied would be more than we would probably be able to afford and (b) the amount of explosive risked in any one unit would still be undesirably large, but (c) if we tried for military reasons to reduce this, there would (while keeping the same general scheme) be even poorer economy of ground.
An escape from such difficulties however, led to splitting up each of these 1,000,000-pound units into a number of equal smaller ones sufficiently separated so that, though possibly they may somewhat damage one another, those of one group will at least not all go off simultaneously. With this arrangement, though a group itself would occupy more area than a single 1,000,000-pound unit, it may on reasonably sized areas still be possible to have between groups nearly the same ideally safe separation described above; since between the nearest buildings of adjacent groups we need then use only the much smaller safety distance indicated for one of these smaller units.
Seven small equal units seem a good number to have in each group, for then we can have a central one with the six others spaced equidistantly around it as on the corners of a hexagon. This would make each small unit hold 143,000 pounds of explosives. Five hundred feet as the spacing distance within such a group would be practically twice the spacing required by the New Jersey laws between barricaded magazines of this capacity, and would seem very conservative. Each group would occupy a circular area one thousand feet in diameter.
The American Table of Distances allows an inhabited building within 1,920 feet of a barricaded magazine containing 143,000 pounds of explosives, this being considered the distance at which it would just escape “severe structural damage.” If we have groups of seven small buildings, each building containing 143,000 (or 1/7 of a million) pounds of explosives, the groups occupying circles 1,000 feet in diameter, with about 1,920 feet between nearest buildings of adjacent groups, the spacing from center to center of the groups would be 2,920 feet. This arrangement covers only 72 per cent of the ground called for by equidistant units of 1,000,000-pound capacity (or only 33 per cent of the ground called for by equidistant units of 143,000-pound capacity). It seems an improvement and it seems quite practicable.
With this layout, the size of each unit is kept down to 143,000 pounds, which does not appear to be an excessively large military risk. The loss of one unit may damage and it may possibly endanger magazines containing not over another 6/7 of a million pounds of explosives. Around the outside of each group which contains a total of 1,000,000 pounds, there will, however, be a space sufficiently wide to protect even inhabited buildings according to the American Table of Distances. The total area occupied by the same total amount of explosives is smaller, with similar protection, than would be possible between any practical sized units if they were spaced equidistantly. The layout as a whole would meet an unprecedented standard of safety.
It was for these reasons that such seven- unit groups were decided upon in this new construction, the relative locations of buildings described above being closely adhered to except where the contour of the terrain, etc., might indicate the desirability of making very minor modifications.