Using backfill to resist wall overturning

[Lingwist]

Hello, I have a question about whether and how you can count on retained soil to resist loads. In the sketch below, the solid walls show the basement walls of a house, with most of the basement wall below grade. The dashed lines above are the first floor framing. The basement walls are essentially pinned top and bottom, so the only way they resist backfill pressure is by being braced by the first floor diaphragm. That works fine, but at the heavy solid line, there is a shear wall above that sits on a beam (beam is at top of basement wall). The beam brings some load, only 3.5 kips according to the EOR, and dumps it as an out-of-plane force on the top of the basement wall. The only thing I see holding the wall from pushing out and falling over is the soil it's retaining. Does it sound reasonable to rely on soil pressure to resist a load like this?

The only hint I found in the ASCE 7-16 load code is something like "when loads due to lateral earth pressure resist the principal load effect, they have a load factor of 0.9 where the load is permanent or 0 where not permanent."

 

[phamENG]

That's an interesting way to do it. I'd probably try to prove that the first floor diaphragm can redistribute it to the parallel basement walls, but this seems plausible - and in reality a combination of the two is probably more likely. Using this soil to resist the load is essentially the same as using the passive resistance of soil against a shallow foundation to resist sliding. I wouldn't have a problem with it so long as the soil is not expansive clay and is properly compacted as it's back filled. Expansive clays can shrink and pull away from the wall, and improper compaction will drastically reduce the passive resistance of the soil.

 

[1503-44]

Is it not SOP to remove expansive clays behind a retaining wall so as to be replaced with a drained sand and gravel.

 

[Lingwist]

phamENG: Yes that sounds cleaner to me, I'm trying to decide if I need to ask the EOR to work it out through the diaphragm. But there may be a reason it's defined this way - maybe the building dimension perpendicular to the shear wall is too wide (my sketch is not to scale) or one end wall has a lot of openings.

1503-44: Probably, but there are no expansive clays here. The backfill will be a drained sand and gravel, or something to that effect. The load here is from wind.

 

[r13]

I am confused. Where is the 3.5 kips load acting on, and what is the retained soil to resist? Is part of the basement above the soil?

 

[phamENG]

r13 - basement walls on 4 sides. One beam in the middle of the building with the shear wall on top.

If you post the full plan, we'd be able to give some more informed thoughts and opinions. Aspect ratio and openings could certainly have an effect.

 

[Lingwist]

Thanks guys, I went looking for a full plan and realized I was wrong about the soil height. Usually these walls are basement walls, but in this case they are almost totally above grade, so there is no chance of counting on soil pressure to resist the load. I don't really see this working...

The red lines are me showing where the EOR specs an out-of-plane load on the precast wall.

 

[r13]

Thanks for the feedback. Now I understand what you were thinking about. Yes, in that case, the retained soil helps, as the building moves onto soil, but the wall in the middle between those transverse walls will subject to increased soil pressure (from active to passive).

 

[phamENG]

Hmmm...the one at the front of the building is plausible, but I agree that the others are unlikely to work if the walls are precast. I haven't done a lot of precast, so maybe there's some need detail out there that would do it, but my impression has always been that precast concrete wall panels have pin connections. They work fine as shear walls - pins at either end act as a couple to resist the moment - but depend on a diaphragm to brace them out of plane and the connections can't take any out of plane moment.

I take it you're the precaster's engineer?

 

[Lingwist]

Thank you, yes I am, but I work in a different division so I don't design this type of residential project - I was just asked to look at this condition. Thanks for your help everyone.