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Re: residential surcharge to Ret. wall

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Jordan,

I agree with all that you have said and thanks for your opinion on the LL issue.  I totally agree with your comments on cantilevered walls and the 1500 psf bearing being a "death knell".  Unfortunately the area in which I am trying to practice forces the issue in that designers and architects seldom ask for, or will agree to a geotechnical study, and contractors almost never when it comes to residential work.  Sliding seems to be a big part of the issue, but again, unfortunately I live in an area where there is a lot of bedrock.  You can hear squeals for miles when a contractor sees he is going to have to chip away the rock for a deeper footing or a keyway.  Usually what happens is the walls are designed for the 1500psf and keyways or deep footings and then when the excavation is underway and bedrock is found they are redesigned with some restraint such as shallower keys or the prefered method around here is to drill and epoxy pins into the bedrock.

Thanks for your input

Joe



On Wed Jan 10 18:22 , 'Jordan Truesdell, PE' sent:

Well, aside from the settlement issues, we generally ignore foundation loading due to local slab effects in residential settings. 40psf LL is pretty minor, and very rarely reached in a residential setting - heck, it's less than the weight of your slab.

1500psf is a very difficult number to work to when you look at tall basements, and the death knell for cantilevered walls (i.e. - full-open floor plans with 4:1 aspect ratios on hill sides).  Once you hit 10', your backfill and footing take nearly all the vertical capacity of the soil. You end up with silly looking all-toe footings. But I digress.

Look closer at your design, as you may be missing something if your wall isn't working. 40psf on a 1500psf soil shouldn't be the issue.  If I use a live load, I usually take the added load as the weight timed Ko or Ka, in other words, add an equivalent volume of backfill on top.  Are you using #57 stone for backfill and taking it as 105pcf x Ko ~ 0.4?  Just make sure that you indicate to shore the wall before the backfill and slab are placed. Many residential contractors (and some commercial ones) forget this little bit of physics, and you get a magnificent bow in the middle of the wall where the wall starts to roll.

Also, go deep for sliding resistance, not wide.
Jordan


jrgrill(--nospam--at)cableone.net wrote:

 

I am doing some foundation walls for a residence.  The floor slab is located over the heel of the wall footing.  I am looking at both a cantilever condition and a condition where the top of the wall is restrained ( the wall will need to be shored until the floor slab is cured to restrain the top of the footing).  I don't have a soils report for this residece.

Our building department allows 1500 psf soil bearing without a soils report.  Using this I am getting some pretty large footings using a 40 psf LL surcharge over the heel of the footing.  Part of the footing size comes from sliding resistance, but also from the surcharge.  Is using the surcharge correct or is it too conservative?

Thanks,

Joe Grill



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