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Re: Underground shelter

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        I have a couple of points that you may find helpful.
1.)    Yes to pitching the roof.  Ponding is never a good idea.  A minimum of 1% slope should be O.K.  Anything more than 2% may be excessive.
2.)    For determining saturated soil weight, the dry density of soil solids (for non mineral deposits, of course) is about 2.70 (Applicable to Western Canada, of course.  Local conditions may vary).  If you can get a reasonable estimate for dry weight of soil in place (Again 110 to 120 p.c.f. in Western Canada but your local contractors should have a fair idea) you should be able to estimate the void ratio and, hence, the saturated soil weight.
3.)    If you are concerned and want greater certainty you might consider a few inches of free-draining granular material all around the structure.
H. Daryl Richardson
----- Original Message -----
Sent: Friday, July 09, 2010 12:51 PM
Subject: Underground shelter

I have been ask to do a design for an underground shelter.  For now I am doing some preliminary calculations.  The shelter is basically a large concrete box structure 26?x54? and about 10? high.  It will be constructed in an open excavation and then backfilled.  The depth will be such that there will be 10? of fill over the top of the structure.  There will be an architect and I believe a mechanical engineer involved for the HVAC system.  I have some questions for all those of you out there.      


Dewatering has been discussed.  At this point I would suspect it will take place over the roof of the structure and along the side walls probably at the base.  I will pitch to top of the roof slab to aid in dewatering.  This morning I thought that a saturated soil condition may develop over the top of the structure.  I am not sure how deep a saturated condition would develop, but at this point there is not a geotechnical engineer involved.  I would like to find some ?general? values for various soils in a saturated condition as I don?t have any myself in my library.  Does anyone out there have some general values for saturated soil weights that I might use for some preliminary designs?  Also for lateral pressures?


Trying to save a little time for now.  So thanks,



Joseph R. Grill, PE

Verde Valley Engineering, PLLC

email: VVEng(--nospam--at)