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Hypothetical Foundation Question

Hypothetical Foundation Question

Hypothetical Foundation Question

(OP)
Let's say you have an 8 ft. tall, masonry foundation wall. If it is constructed with 6 ft. of backfill on one side and 4 ft. on the other, how is the moment determined (assuming it was backfilled simultaneously in equal lifts on each side)? Do you just assume an opposing active pressure wedge on each side or does passive resistance come into play?

Thanks

RE: Hypothetical Foundation Question

I don't think you can have active pressure on both sides - the required movement to develop the active pressures on each side would oppose each other. I think it would have to be active vs passive pressure or both sides at rest....I'm guessing it would likely both be at rest pressures.

RE: Hypothetical Foundation Question

I would say the same, equal pressure on each side, unless you have a surcharge on one side, then that might skew things a bit.

RE: Hypothetical Foundation Question

Retaining wall and not pin-pin basement wall, right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Hypothetical Foundation Question

(OP)
Correct. But lets argue both for giggles.

RE: Hypothetical Foundation Question

Cantilever:

1) if you can tolerate significant movement, I believe it traditional to go active on the high side and passive on the low. Some engineers will be more conservative with the low side treatment.

2) if you want to limit movement, at rest high and low makes sense. Personally, I'll usually go at rest high and active low to be a bit more conservative.

Pin-pin Basement:

1) Same as #2 above if no SOG for some reason.

2) With SOG, active pressure and ignore everything below SOG.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Hypothetical Foundation Question

What are the odds that it gets backfilled equally on both sides at the same time?

RE: Hypothetical Foundation Question

(OP)
Thanks for all the help!

RE: Hypothetical Foundation Question

(OP)

Quote (Kootk)

if you want to limit movement, at rest high and low makes sense. Personally, I'll usually go at rest high and active low to be a bit more conservative.

Sounds like the way to go.

Quote (JStephen)

What are the odds that it gets backfilled equally on both sides at the same time?

Who knows? I would probably design for 3 ft. diff, specify 1 ft. and call it good.

RE: Hypothetical Foundation Question

(OP)
So I was looking at the tables in the IBC for active and at-rest pressures. For well graded gravels, it is 30 for active and 60 psf/ft for at-rest. It has been said in this forum that at-rest should be used for basement walls. If you look at the IRC for foundation (basement) wall loading, it shows 30 for well graded gravels. So they seem to contradict each other. I have been using the IRC tables for my basement wall designs since the beginning. How much movement is required to mobilize the active pressure?

Thanks.

RE: Hypothetical Foundation Question

I've heard varying things. 0.001H to 0.02H for the movement required at the tops of cantilever retaining walls comes to mind. Some background info here: Link. You don't see regular basement walls talked about much for some reason.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Hypothetical Foundation Question

(OP)
My suspicion is that at-rest pressure only exists in theory and in laboratories. No basement wall (or its connection to the floor diaphragm) would ever survive those pressures.
.01h for a 96" tall wall is 1" of movement - that is a lot and rarely seen on basement walls (unless the connection slips) or retaining walls for that matter.

RE: Hypothetical Foundation Question

I think that foundation wall pressures are generally very conservative in general. I've no particular beef with at rest.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Hypothetical Foundation Question

(OP)
I know in our area, you can make a vertical cut in the clay soils and it will stay like that unless erosion occurs. I guess that means the active pressure is zero :>

RE: Hypothetical Foundation Question

XR250, for sand, the wall movement required to cause active pressure is about 0.001H, not 0.01H. Therefore, for a 96" high wall, the movement would be about 0.1 inch, not 0.96 inch. For clay, the requirement is about 0.004H. For reference, see Winterkorn & Fang's Foundation Engineering Handbook, page 405, Table 12.1.
Clay has cohesion which, at least in the short term, will resist the lateral earth pressure until a certain depth (d = 2c/gamma) is exceeded. Cohesion is usually ignored in designs unless very short term. I recommend that you read any soil mechanics book. There will be discussions on active, passive, and at rest earth pressures in addition to discussion on cohesion.

www.PeirceEngineering.com

RE: Hypothetical Foundation Question

(OP)
Hmm, i misread Kootk's post. Most basements in my area are backfilled with clay so let's use .004H = 3/8". That makes sense as I see a lot of them bowed in just a little. That might have more to do with hydrostatic pressure and some expansive component of the clay however.

Quote (XR250)

I know in our area, you can make a vertical cut in the clay soils and it will stay like that unless erosion occurs. I guess that means the active pressure is zero :>

Obviously I was being facetious but the fact is that I see these vertical cuts all the time and they usually just stay like that.

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