Letter Re: Advice on Under-Slab Underground Shelters

Mr Rawles,
I was looking at the construction of different areas of my home and trying to think of the best area for a fallout shelter or bunker. I have a basement with access to the area under my garage and under a slab addition. I figure that it would be too dangerous to tunnel under the garage but was wondering if it would be feasible to tunnel under the addition slab. If this were an option I could really have a large bunker area, especially if there is a way to use the garage slab. Thanks, – ART

JWR Replies: Use extreme caution when digging under slabs. In essence, if a slab was not originally designed to span an open space beneath it, then is shouldn’t be expected to handle doing so. For most typical 8+ inch thick residential re-bar reinforced concrete slabs other than those in garages, poured on clay or loam soil, a maximum 3-foot wide tunnel that doesn’t extend more than half the width of a building would probably be fine, but anything wider or longer, or any excavation in sandy soil would require the close scrutiny of a qualified structural engineer. Garage slabs deserve a special of caution: Unless a slab is specifically designed to support the “live” load of a vehicle atop it and span a void beneath it, then you should not attempt to do any tunneling beneath it! Perhaps an engineer would care to chime in here, with further guidance.