Bomb shelters must be able to sustain minimal damage in a near miss or direct hit from exploding ordinance. Soft soil can offer only limited resistance to bomb energy, so building in soft soil requires a deep excavation with highly reinforced construction. In areas with poor drainage or a high water table, the option of building deep will require more engineering than this page will offer. Building in solid rock, unless you have a suitable cavern to use as a superstructure, will require blasting and drilling, which also will exceed the scope of this page. Building into the side of a mountain or steep hill is a good choice for people with the resources to do so, and without the depth requirements, the following steps are fairly typical of what needs to be done to successfully complete your bomb shelter project. Building on stable, flat, well-drained land is the most direct approach to getting the job done, and that will be what we assume the reader plans to do.
How many people will it house? This will give you information needed to design the shelter in respect to size and accommodation. Since bomb shelters typical are very temporary in nature unless a sustained bombardment is anticipated, you can expect to allow at least 72 square feet of space per person. Decide what ordinance your structure will need to resist. A typical 500 pound bomb has a blast energy of about 10,000 feet per second using high explosives, and if fused for a ground burst or impact detonation, you will want a minimum of 15 feet of energy absorbing material between the ground and your structural roof. Bunker busters, on the other hand, can penetrate many feet (the exact distance, for obvious reasons, is classified), of reinforced concrete. Since 20 feet is a reasonable limit to excavate and redress, we will assume our design on that depth, other considerations are best left to experts.
