At Rest Vs Active Soil Pressure... Bottom of Wall moving.
At Rest Vs Active Soil Pressure... Bottom of Wall moving.
(OP)
Hello,
I'm trying to design a retaining wall that has no footing but it's attached at the top to a concrete slab. That is, a slab on grade that has a very deep >10' exterior beam
All the books I've been going through say that if the top of the wall moves then you use the Active soil pressure, but if the top and bottom is constrained such as in the case of a basement wall then use the at-rest pressure, which is much higher than the active soil pressure.
I have not been able to find a place that explains what soil pressure to use if the bottom of the wall moves outwardly away from the retained soil. Can I use the Active soil pressure in this case too?
Anyone know?
Thanks.
I'm trying to design a retaining wall that has no footing but it's attached at the top to a concrete slab. That is, a slab on grade that has a very deep >10' exterior beam
All the books I've been going through say that if the top of the wall moves then you use the Active soil pressure, but if the top and bottom is constrained such as in the case of a basement wall then use the at-rest pressure, which is much higher than the active soil pressure.
I have not been able to find a place that explains what soil pressure to use if the bottom of the wall moves outwardly away from the retained soil. Can I use the Active soil pressure in this case too?
Anyone know?
Thanks.





RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
Dik
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
I wasn't planning to transfer the moment in the slab. I was hoping for the bottom of the wall to move enough to *hopefully* relieve the soil pressure and hence use the active pressure, which in turn, with enough beam depth I can rely on the passive soil pressure going the opposite direction and I can design the wall as a pin-pin (with the stirrups taking the moment).
Again, I don't know if this would be correct.
Being on the safe side, I tried to design the wall pinned-pinned with the At-rest pressure pushing instead of the active, and I had to go many feet deep to develop enough counteracting passive pressure. So, I'm looking for ways to design this wall without doing a footing at the bottom like a normal retaining wall.
This is for a residential project... talking to other residential engineers, I seem to be the only one concerned about soil pushing on a deep beam because of the bagged soil behind the wall. This sounds like BS to me.
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
However, if the bottom of the wall moves enough then you will have active pressures.
The structural connection at the top is out side my normal area, but it would seem that there would be a lot of moment in the wall that has to go somewhere.
Good luck.
Mike Lambert
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
This being said, I think I have found the answer. The model I'm trying to mimic is similar to a sheet pile wall with an anchor at the top. I understand some of your concerns about the wall moving and supporting a house but, it only needs to move 0.002H for the Active pressure to occur, that's less than 1/4". See Attachment. Using the Active pressure instead of At-rest makes a huge difference.
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
StructuralEngineerTX,
"My issue is that nowhere have I found something that says the bags won't eventually burst or disintegrate and hence I'm being careful by assuming eventually there will be lateral pressure on the wall."
If you use geotextile bags, it will take more than a hundred years before they burst or disintegrate. In that way, you can design your fill to work as a gabion wall, and you will be able to also design a relatively slim concrete wall as a facade.
Regards
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
As for embedment depth, according to Tschebotarioff, anchored walls can have a toe depth of 0.43H for stability. It may be smaller embedment depth if your passive soils are cohesive. If passive soils are all Clay, you would have rectangular passive resistance consisting only of cohesion. If you have soils with cohesion & friction, you would have a trapezoidal passive resistance distribution. In your drawing, however, the passive slope seems to be descending, so you would have a lower calculated Kp value. Look for Blum's Equivalent Beam method in your Foundation Design books.
http://www.soilstructure.com/
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
http://www.starsandgarters.com/.shared/image.html?...
http://www.jwkhomeinspections.com/blog/joe-kereszt...
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
Then how do you hold the outside form? On the inside down there will you run the bags full depth also? How do you set and fix the vertical re-bars with only the bags there? Not simple. Deserves plenty of thought. Will your locals also recommend bags outside down there? I'd not do that, but compact the fill. Set re-bar cages before or after forms? Pre-cast walls instead?
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
The way the form the beam is they put a wood form and several diagonal braces, it's a little shady. It is not uncommon for the bottom of the beam to sway out of plane just from the weight of the concrete. When you go look at some of these beams after they've cured it's hard not to notice the huge bulge sticking out. By the way, I'm not talking just about small mom & pop home builders, these are big publicly traded companies too.
That's right, The bags are not compacted. I've walked on a few of these pads, the heel of my shoe (that's a men's shoe) sinks easily into the fill. I've gone around this problem by placing piers when the fill exceeds 5ft so the slab and the beams are expected to act as suspended. But that's another thread....
When it comes to residential construction, Texas is the wild west.
RE: At Rest Vs Active Soil Pressure... Bottom of Wall moving.
My only question here is, who is the contractor's bagman?
Mike McCann
MMC Engineering