Structural Thread on use of Geofoam and Earth Pressure . . .

[BigH]

Just saw (and responded to) a thread on earth pressures on a retaining wall if the backfill is geofoam. . . argued that there will be "some" lateral earth pressure if the geofoam is compressible . . . I just don't get it . . . guess I'll be enlightened . . . doesn't seem to me that, other than surcharge loading and cover soil that there is any real pressure at all - especially given that geofoam comes in big blocks . . . Am I off base???

Reminds me of a time when I was in Indonesia (1983) and we were seeing effective friction angles pf clays reported from triaxial testing with pore water pressure measurements of about 8deg whereas the undrained friction angle was 6deg. I was only a few years out of school and EVERY lab in Indonesia was doing the same including the professors from ITB (Institute of Technology of Bandung). Got to a point where I was wondering "what did I know?" - well, later, when visiting one of the labs I found out that they were measuring porewater pressures but were still failing the samples in 15 to 20 minutes . . . . case closed!

 

[Ron]

The pressure comes from the lateral movement of the geofoam based on the compressive load and its Poisson's ratio. If there is soil behind the geofoam, then there would be lateral pressure from the soil, but resisted by the block-to-block friction of the geofoam. Probably won't be much in any case, but theoretically it's there. I would probably ignore it.

 

[BigH]

Agree Ron that there might be a bit of pressure - but those in the structural thread are like wanting to design the wall as if soil is being used. I agree that it wouldn't be the best backfill for a cantilever wall but I don't believe that it is used as such - from what I remember on I-15 articles, they basically hang a "facade" with the styrofoam. One thing to worry about is it's ignitability - needs to have cover. I had thought of this when I was in India and we were putting 11 m high retaining walls on 6 m of Su<20 kPa clays . . . but it would have been too much of a stretch for them at the time.

 

[BigHarvey]

One solution would be to use a lghtweight concrete. With a density of 300 kg/m3, you have the strength of a "normal" soil. The main advantage is that the material has cohesion. I believe it would be a good solution behind a retaining wall.

 

[BigH]

I've used it before BH - called Elastizel in the States . . . interesting material!

 

[Ron]

BH and BigH...I have also used Elastizell or similar cellular concrete for fill. It works fine. I used it to keep the overburden weight down on a heavy load pavement section going through a South Georgia swamp. It obviously helped with the pavement structure as well. I also used it and created a modified mix design to fill the deteriorating cavities of large artificial rocks (some having several hundred cubic yards of void space) at "a major central Florida theme park" that shall remain unnamed! The concern there was the settlement potential after the rocks were filled as they were structurally tied to existing small buildings on shallow foundations. I had to modify the mix design to get the "dry" weight down to below 30 pcf.

 

[BigH]

In the middle of monthly report and other things but I'll write up my experience with Elasitzel . . . good for the right applications

 

[oldestguy]

In the July/August 2013 issue of Civil Engineering Magazine there is an interesting article about the use of light weight fill. The stuff there is expanded polystyrene(EPS). Weighs 1 to 2 pounds per cubic foot with Poisson's Ratio of 0.08 to 0.10 They sloped nearby earth fill to prevent any lateral pressure from that source. The claim is no problem with the lateral pressure out of the light weight stuff against a building. In fact they say no lateral load trsnsfer. Overlying the EPS is 6 feet of earth and a roadway. Total fill height is 30 feet. Concerts for earthquake loads also is expressed.

I was unable to find a reference to it at the ASCE WEB site.

 

[BigH]

Thanks oldestguy - they've used styrofoam (EPS) here in Malaysia and of course, it was used on Utah's I-15. I did a presentation to the Calcutta Geotechnical Study Group (now I guess Kolkata) and Indian Institute of Technology (Kharagpur) on the use of lightweight fill for embankments - this was back in 2005. I've seen where it has been used for rail bridge abutments in Scandanavia. I'll have to wait until I get back to IN to get the mag as they are all sent to my mum's house.

 

[PEinc]

I thought that, when using geofoam backfill, the soil behind the geofoam is supposed to be sloped back so that there is no earth pressure pushing on the foam. If the rear face of the foam is vertical or near vertical, the soil will push on the foam which will push on the structure. Using Geofoam does not guarantee that there will be no earth pressure. As I understand it, the geofoam is just a lightweight, self-supporting material that fills the gap between the structure and the sloped earth backfill.

http://www.PeirceEngineering.com

 

[oldestguy]

With 6 feet of earth on top, plus traffic and with a Poisson's ration above zero, there will be motion sideways. Various grades of the stuff have differing "compressive strength". Thus, at least some of that vertical load gets applied sideways.

 

[BigH]

. . . and usually when using geofoam, it is quite thick - it is not placed for, say, 1 m against the wall to act as a "cushion".