Geofoam inclusion to reduce lateral earth pressures on cantilever retaining walls

[YoungGunner]

The builders in this area are becoming more aware of geofoam and are advertised that it has the ability to reduce lateral earth pressures on cantilever retaining walls. The problem I'm running into - how much is that reduction? The technical papers me and my co-workers have reviewed show very mixed results. Generally they say that a thickness of geofoam backfill to height of retaining wall ratio (t/H) of 0.5 yields approximately a 50% reduction in lateral earth pressures. This sounds great and amazing, but I want to see if anyone else have some good sources or experience using geofoam inclusion.

I recognize there is a way to reduce lateral earth pressures to 0 by stepping the geofoam behind the wall at an angle that matches an acceptable stable slope of the soil, but that is incredibly expensive for excavation and product.

I may be young in the industry, but I'm not willing to accept the standard as the standard without solid argument.

 

[phamENG]

Without stepping I'm not sure how you'd get much of a reduction. You're just moving the point of lateral load application back the thickness of the foam, and unless the foam has some incredible sliding and overturning resistance in its own right that load will just get passed to the retaining wall.

 

[YoungGunner]

phamENG - that's what I thought too, but a peer here has several research papers that indicate some amount of geofoam will reduce lateral earth pressures to an extent. I've seen at least two case studies of it being used without stepping. I'm guessing the compressible geofoam allows some measure of movement in the soil, enough to reduce the pressures even further than active. But whether we should just trust these studies or if there is more experience out there is where I'm stuck.

I may be young in the industry, but I'm not willing to accept the standard as the standard without solid argument.

 

[phamENG]

YG - compression in the geofoam is what I was thinking, but 1) I imagine it would be tough to quantify reliably and 2) how much is there really if this is structural geofoam? They use the stuff under road beds - it can't move all that much.

 

[Celt83]

If your just stacking Geofoam on the heel and not stepping back through the failure wedge, then I would argue that even if it results in some reduction to the lateral soil pressure you've just drastically reduced the weight over the heel, 1 pcf foam vs 120+/- pcf soil, resulting in worse overturning and sliding stability.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[dik]

The compressive strength of geofoams is generally too great to make any difference, and over time the compressibility would only increase the lateral pressures, IMHO.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[wth]

I've only used it wedged between a rock face and a backfilled wall, so no pressure from other backfilling except for a top layer of soil. The geotech gave us an K0 (resting pressure coefficient, no movement in the wall) of 0.1, this was large EPS blocks, at least 1000x500x500, stacked. The geotech didn't justify this factor in his report, but from what I researched at the time it didn't seem overly unconservative.
This was in Europe.

 

[PEinc]

It is hard to answer this question in a typed response. There are too many considerations to cover. What type of cantilevered wall? Verticality or sloping of the retained soil? Heel or no heel w/r/t sliding & overturning? Ko vs. Ka (should be Ka for a cantilevered wall)? Methods available to reduce earth pressure due to sloping retained soil? Durability of geofoam vs. lightweight foamed glass aggregate (

http://www.aeroaggregates.com)?

You should probably call more than person for more information. The info is sure to vary greatly between "experts."

http://www.PeirceEngineering.com

 

[GC_Hopi]

Thinking about the mechanism which reduces the lateral pressure. The concept is that the geofoam weighs less and has a "high angle of respose" so that volume contributes very little to the lateral pressure.

 

[bones206]

For retaining wall pressure reduction projects, I've only ever seen geofoam installed with a stepped slope to fill in the active failure wedge zone.

I'm doing the same thing right now on a project using UL-FGA (AeroAggreate) to replace in-situ gravel fill. The original retaining wall design assumed passive pressure that wasn't actually available at the toe, so the foundation walls are overturning and the building is slowly sliding off the top of a steep embankment. Interestingly, the deep fissure that opened up in the interior of the building perfectly aligns with the theoretical active failure wedge.

 

[PEinc]

I agree with bones206. If the retained soil face is vertical against the geofoam, the geofoam then pushes full(?) earth pressure on the vertical foam which pushes full on the back of the wall. I don't see any practical, significant pressure reduction. That's why the foam needs to be installed on a slope against the soil that is sloped to reduce or eliminate lateral pressure. Then you still need to consider the reduced or eliminated weight of soil on the heel of the wall footing.

http://www.PeirceEngineering.com

 

[GC_Hopi]

That makes sense. Kinda of like the sketches below.

 

[Settingsun]

Hi Bones206, why wasn't the design passive pressure available in your case?

 

[bones206]

The building is perched next to a steep slope. Minimal backfill width against the toe side of the wall. So the wall is just plowing the toe side soil down the hill.

 

[Settingsun]

Thanks, Bones. That sounds unfortunate.