At Rest Vs Active Soil Pressure... Bottom of Wall moving.

[StructuralEngineerTX]

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.

 

[dik]

If existing then depending on how high the wall is, you may have problems... You have to develop a moment into the slab and/or beam and to transfer the moment into the beam, the beam stirrups may be required to assist with the moment transfer.

Dik

 

[StructuralEngineerTX]

No, it's a new slab.

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.

 

[GeoPaveTraffic]

I would use at rest. Nobody wants a residential wall moving a couple of inches to achieve full active pressure.

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

 

[hokie66]

How would you construct such a wall? Do you brace it for the complete soil pressure, bottom to top, while the backfilling is done?

 

[StructuralEngineerTX]

The soil is bagged behind the wall so no bracing is required. 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.

 

[oldestguy]

You are building a gravity wall made out of "blocks" you call bags, right. Gabions?? Then the wall you mention is merely for looks and carries no pressure, right? Maybe post a cross section to explain. Is this your first wall design? If you eventually will have pressure, then you had better design the wall as a retaining wall with all the features, such as overturning resistance, bending moments, shear, etc. Also, are you going through a design for the "bags" configuration showing they are "self supporting" and are holding back a soil fill.. If so, you also have pressures from soil behind them in the at rest stage, possibly even into the passive, depending on compaction there.

 

[StructuralEngineerTX]

Well, in residential they put bags of soil behind the wall, and they go up to 10ft or so. The point of contention is, I believe I cannot rely on the bags holding up their own weight because there's no document -University, Building Code, etc- that says they can. So, I would like to design the wall for lateral pressures. I have every contractor (and their engineer) in town telling me I'm over designing.

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.

 

[crcivil]

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

 

[crcivil]

You could even reinforce the top of your fill through soil pinning, which employs the same technology as in rock bolting.

 

[FixedEarth]

It can be analyzed as a propped cantilever. I would use a uniform rectangular earth pressure of 0.65*K*Gamma*H (Peck, Hanson, & Thornburn). If we analyze it as a propped beam, the greatest deflection will be near the lower third height of the wall. I would use At Rest Earth Pressure (Ko = 0.5 for SAND), because the top is anchored and the bottom is fixed.

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/

 

[StructuralEngineerTX]

Gentlemen, I see some confusion here. We're not talking about MSE walls here, If I could, I would - it's just plain sand bags, they refuse to do anything different other than stacking the bags together.... I took me forever to find a picture on the internet, see links. I have yet to find out what material they actually use.

http://www.starsandgarters.com/.shared/image.html?/photos/uncategorized/2007/11/30/foundation.jpg

http://www.jwkhomeinspections.com/b...-home-inspector-new-home-slab-foundation.html

 

[StructuralEngineerTX]

UPDATE: OK, turns out, around these parts of the Country, they use good 'ole 6 Mil vapor retarder as their bagging material.... So, pure junk.

 

[oldestguy]

It appears your project is not the standard local slab on grade, no deep footing, etc., but a form of much higher retaining wall. That soil behind the bags still has to be retained somehow, even if it stands for a while. And, yes with the bags you have a form of temporary gabion wall to resist that soil back there. Gabions are not MSE walls, but gravity only, usually. For gabions, the wire strength is assumed to be reasonably permanent, but plastic, who knows. No sunshine there. Since the bags contain granular fill, probably not compacted, use what ever figures you have for ordinary granular fill and active pressures, even though the top is restrained. The bulk of the wall is not fixed in position. The bottom movement resistance then depends on the passive from the soil outside the wall, as your diagram shows. Don't forget the inside active is full height of the wall since it knows nothing about what is outside. Fixed Earth may have some comments now, since pressure theories are many.

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?

 

[StructuralEngineerTX]

Let me start by saying thanks to all the replies.

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.

 

[msquared48]

It looks to me like the bags are merely a form material, and the slab is a structural slab spanning to grade beams. In all likelihood, the interior fill will settle away from the bottom of the slab with time.

My only question here is, who is the contractor's bagman?

Mike McCann
MMC Engineering