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Re: Underground shelter

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I hadn't thought about the equipment used in compacting the soil.  For something this size, the contractor may want to use something heavier than the small walk along soil compactors.  You could temporally shore the lid during this phase.

Joe Venuti wrote:



I think Drew’s info is good.  I tend to be more conservative so 135pcf is what I would use.  If you know any Soils engineers you may talk with him/her about the additional forces from the compactive effort upon the soil.  Are they going to use a D6 and a loaded water truck to achieve compaction or maybe a vibratory piece of equipment.  How does that translate to actual load on roof. For you own protection you need to get a soils engineer involved, even if sub consultant to you.


Joe Venuti,  P.E.

Joven Engineering

La QuintaCA


From: Drew Morris [mailto:dmorris(--nospam--at)]
Sent: Friday, July 09, 2010 2:52 PM
To: seaint(--nospam--at)
Subject: Re: Underground shelter


I use typically use 100 pcf for compacted dry soil.  This gives a porosity of about 40% (100 pcf / 62.4 pcf x specific gravity of 2.6).  Assuming that the voids are completely saturated, the final saturated soil weight would be 100 pcf + 0.4*62.4 pcf = 125 pcf.  For a case of the backfill not being completely backfilled or the soils having a different average specific gravity, the saturated weight would vary.  Any geotechnical book should have further information.

I'm not sure how you plan to allow the water to drain off the structure. It's one thing to design for the weight, its another to design for the long term performance of the water proofing membrane on top of the concrete.  You could put a layer of coarse rocks on the membrane that would allow lateral drainage off the top.  Cover this layer with some some of geotextile to keep them in place and then with some sort of coarse backfill to allow the water to pass down vertically to the concrete cover and then laterally off the structure.  I would cover the walls with the same membrane or something like Xypex.

Along the perimeter, you could put in a french drain and run the outlet laterally to some sort of underground vault for the water to drain out of away from the shelter.

Some of the above can be eliminated if you are in a dry climate.

Some of the above can be eliminated if you are in a dry climate.

Joseph R. Grill wrote:

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)