Light weight fill for a retaining wall

[Ho2]

Hi everybody, I am hoping to reduce the pressure at the base of a cantilever retaining wall by using light weight fill behind the wall, Geofoam for example. How do I calculate the earth pressure behind the wall? or is there any? I appreciate if anyone can give me some advise.

 

[BigH]

I am not sure of your site "picture" - but you can actually build your wall out of geofoam. They have do so for many years in Scandanavia - and recently Utah has done some highways too. I was contemplating the suggestion for its use here in India but after thinking of it and the conceptual problems of more accepted and referenced ground improvement techniques, I didn't try. I got some very good information from the internet by hitting on rediff.com and typing in the word styrofoam - for that is what geofoam basically is. Dr. Horvath (some university in NYC area) also has done a lot of work - I can look up info for you from my file at work. In geofoam, I would suggest that there is very little if any pressure behind. I've seen them proposing to use geogrids between the layers of the geofoam with just a protection panel in front.

Other light weight materials - hogfuel (tree bark chippings) although this will settle and you won't be able to support much atop it, flyash - lightweight in dry but with higher moisture levels can become heavier than some expect. Yet another material is Elastizel - a foamed concrete weighing in at about 1/5 the weight of normal concrete. This, too, has been used "neat" with just facing panels. It is out of Michigan (Ann Arbor, I believe).

 

[Focht3]

Another lightweight material is a kiln fired clay product that goes by various names - hadite, etc. It isn't as lightweight as styrofoam, but it is stronger and quite porous. It's dry unit weight is very close to that of water (60 to 65 pcf.)

I personally like the use of styrofoam as long as you don't have a groundwater problem and the fill won't be subjected to traffic loads.

 

[soiset]

I don't see why you wouldn't advise EPS for traffic loads (assuming there is a concrete slab above) and in areas with groundwater problems. The weakest EPS has a compressive strength of over 500 psf, and it is a hydro-phobic material.
As far as calculating loads on retaining walls using EPS as backfill, there shouldn't be any lateral load on the wall if the soil beneath the foam is sloped back at the angle of repose. EPS is great stuff, but the question always comes down to cost. At about a dollar per cf, it ain't cheap. You save money on granular backfill, compaction, and on the retaining wall itself, which, as was mentioned, may not even be necessary with the geofoam. But, even considering all of this, it may still not be the cheaper option. It is most often used when there are constraints that require its properties, and regular fill is just not very practical.

 

[Focht3]

soiset:


EPS can be used in areas subjected to traffic, but requires intermediate materials to distribute the loads. Remember that 500 psf is only about 3.5 psi - and most vehicles have tire pressures (and contact pressures) of 35 to 100 psi. While its' compressive strength can be adequate, it has a low "elastic modulus" (the material isn't particularly linear) and will deflect a lot. Rutting will occur unless the pavement is designed to distribute the traffic loads.

If you don't believe me, run a (dry or "field&quot CBR on EPS. A CBR of 1 means the bearing pressure on the bottom of the CBR piston is 10 psi at a piston deflection of only 0.1 inch. Don't be surprised if the measured CBR is less than 1 - a lot less.

Yes, EPS is hydrophobic. But water will fill the voids around the foam blocks and cause problems with the wall. Given EPS' low density, the stuff will try to float if the water isn't drained. A few EPS walls have failed by flotation!

 

[misterduckie]

Hi--

There is a new material I've seen that is used underneath artificial turf fields. It is composed of small diameter (3-5mm) polypropyelene bead. The beads are fused together at their contact points, which creates a porous but resilient, structural material--it looks like a bunch of tiny golf balls stuck together. Since it is polypropylene instead of styrene, it is elastic and resilient and doesnt crush like sytrene does. I'm not sure where to get it but I can find out--

 

[Focht3]

Sounds interesting - keep us posted!



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