Retaining wall with soil on both sides?

[AtlAC]

Due to various reasons I have to drop the bottom of a retaining wall (top of footing) well below grade so the end condition will have soil on both sides of the wall. The top of the wall is 36 feet above the top of the footing but the top of the footing will be 14' below finished grade. So I'm really only retaining 22' of soil. I was wondering what other people do in situations like this when it comes to the design of the wall? Obviously, sequence of construction matters for this case. Do people design for the worst case where the GC pours the wall and then backfills the 36' before placing the 12' of soil over the air side of the retaining wall (thus the wall has to retain 36') or do you just design for the 22' of net retaining or do you design for the 36' of retaining with the 14' of soil on the air side counteracting the soil side forces? Just curious because a retaining wall for 36' will get quite large. I can't have the GC temporarily brace the wall due to various reasons as well.

 

[SlideRuleEra]

This is the type situation where competent, timely, on-site construction management pays for itself. If suitable construction management is not available, there is a work-around solution that we have used. Retain an independent qualified inspection agency just for the wall backfill portion of the project. Require the backfill to be placed essentially equally on both sides of the wall. The wall can be safely designed for 22' (assuming there will never be a need to excavate on the "low" side in the future). Do a simple cost analysis to see if the "inspection agency / 22' design" is less costly than a "no inspection / 36' design".

On the project I have in mind (an underground stormwater detention system), the savings resulting from retaining the inspectors was substantial.

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[Nor Cal SE]

SlideRuleEra's thoughts sum up the best route pretty well, especially since temporary bracing is not an option here as you've mentioned.

If the client sees how large the footing would become for the 36-ft condition, simply to resist overturning and sliding, that might motivate them pretty fast to elect the "inspection agency 22-ft" condition. You might be able to convince them easily, even prior to doing any in-depth reinforcing bar calculations.

 

[atrizzy]

Can't you just tell them, on the drawings and in person, that for this wall the backfill must be done evenly on both sides until the low side is completed, upon which time the high side backfill can continue?

 

[AtlAC]

My next question would be if they can back fill both at the same time can you really only design for 22' safely as the soil below grade won't truly cancel itself out because the soil pressure on the high side will be much larger at the base of the wall than on the low side?

 

[PEinc]

If the wall is designed for 36' of soil on one side only, the wall will be gigantic (and is probably the wrong type of wall to build; consider an MSE wall if it is a fill wall or a permanent tiedback wall if a cut wall). IMHO, it would be overly conservative to ignore the full 14' of passive earth pressure in front of the wall. If the wall is designed for an unbalanced soil height of 22', the wall design will be better but it is still a big, conventional, cantilevered wall. Also, there is a good chance someone in the future will excavate deeper than 22' in front of the wall in order to install or repair utilities. If the excavator digs in front and sees wall stem, he may keep digging and assume the wall can support more than 22' of unbalanced soil. Other posters here are correct in that the backfill should be done concurrently on both sides of the wall stem, with only a few feet maximum difference.

http://www.PeirceEngineering.com

 

[AtlAC]

The retaining wall will be underneath an apartment building and the low side is going to be a new parking garage. There won't be a high chance that excavation will occur in the future.

 

[PEinc]

Will the apartment building already be there when the retaining wall is to be built?

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[JLNJ]

You model the soil pressures to get the moments in the wall. It's a little more complicated that saying it's just a 22' cantilever.

The moment will continue to increase (maybe all the way down to the footing); it just will be somewhat balanced by the fill on the other side. At some point the passive pressure may be enough to reduce the increase from the active pressure, but you will need to think about how that passive pressure activates. Maybe your slab at the lower level can be a constraint.

Be careful about the hydrostatic pressures as well. They can be very large.

 

[WARose]

I don't know that I would count on one side cancelling out the other. Yes, passive pressure could stop sliding failure. But it could overturn before it moves enough to engage the passive pressure. A conservative thing to do would be to design it for all or nothing (for each side). But that might get expensive. You might get the geotech to weigh in here.

 

[AtlAC]

PEinc,

We are designing the apartments so the site is a blank canvas to start with the retaining walls going in first.

 

[PEinc]

Good. That simplifies matters a bit. So, is the wall a fill wall or a cut wall?

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