Geofoam at SRW reinforced zone

[MSEdesigns]

What should be the parameters (internal friction angle and cohesion)for Geofoam. I will like to evaluate using geofoam in a SRW but don't know where to obtain these values. The local EPS supplier does not have any idea. They could only give me the density which was about 1.1 pcf and the strength which was about 1,008 psf at 1% strain.

 

[bushel3]

Why do you want to use geofoam?

That is an interesting idea, but I'm not sure how that would even work. Are you planning to use geofoam in the entire reinforced area?

Geofoam can stand up on its own, so in theory all you really need is something to hold the geofoam in place. Geofoam will not develope an active wedge like and unreinforced soil would, so you wouldn't need reinforcement.

However, the soil that the geofoam was supporting will develope an active wedge because the geofoam will give a little and therefore the soil would need to be reinforced. I would imagine that the geofoam backfilled wall would fail in sliding from the resulting active soil wedge behind the geofoam. That the geogrid would do very little to stop that. Geogrid develops its strength from friction. Friction is based on the weight of the soil/geofoam and geofoam is really light so you'd have almost no friction in the geogrid.

 

[rochplayer]

We evaluated a wall using Geofoam to reduce the stress over a culvert for a highway widening. Basically, the geofoam became part of the facing and the reinforcing wasn't counted until it was within the soil. It made for very long reinforcing.

Geogrid gets only a small part of its strength from friction between the geogrid material and the soil sitting on it. It gets its strength from interlocking with the backfill soil and the effective overburden stress. Geofoam provides no interlocking and virtually no overburden stress.

Geofoam walls were used on I-15 in Salt Lake City for embankments next to existing structures. The geology is compressible Lake Bonneville sediments, so geofoam was used as the embankment fill and concrete facing panels were constructed in front of the walls to protect the geofoam and for aesthetic purposes. They were not, however used as retaining structures, e.g. the geofoam did not have to resist active or at-rest earth pressures. The frankly can't.

When geofoam is used to widen an embankment, generally the existing embankment is sloped and stepped so that the EPS blocks can be placed AND the existing slope is generally stable under its own configuration. Geofoam is a good application if widening using earth materials would cause instability.

 

[PEinc]

Two excellent answers!

 

[MSEdesigns]

Bushel3:
The allowable bearing pressure is limited by the geotechnical engineer to 2500 psf and the wall goes up to 45.33 ft. So you can imagine. My first trials were in the vicinity of 7000 psf

You are right about the wedge behind the geofoam. For that reason we are planning to step the geofoam on a 2h:1v.

To keep costs low we are planning to use A-2-4 and geogrid up to 14 ft. This will have a bearing pressure lower than 2500 psf. From there we will have the geofoam up to 3 ft below the finish grade. On top of the geofoam, a 4 in. concrete slab is proposed for protection and load distribution. The geofoam will start on top of the reinforced soil and start the stepping towards the retained zone on a 2h:1v slope which will start at the end of the reinforced zone.

Let me know your thoughs on this based on the added info here...

 

[bushel3]

MSEdesign,

This is a good problem. I assume that ground modification is not an economical/possible option either?

I'm not sure I understand what the wall will look like. Will the first 14 feet of your wall have A-2-4 with geogrid and the remaining 31 feet be geogrid w/ a concrete cap?

I like what rochplayer said and he probably knows more about this than I do. He said "the geofoam became part of the facing and the reinforcing wasn't counted until it was within the soil." The best thing I can see is to use long geogrid to stabilize the embankment.

If you are not planning on running the geogrid into the soil embankment you need to do a global slope stability analysis on the soil embankment. (Even if you are using long geogrid a global stability analysis is advised.) The geofoam and blocks, (even if geogrid is used between the geofoam them,) will provide very little horizonal force. For calculations I would assume zero horizonal force. However they will provide erosion protection for your slope. If the slope is stable on its own, you are set. If the slope is not stable on its own, you may have to use a 2.5h-1.0v or something like that.

 

[MSEdesigns]

Just to clarify, the 31 ft will be divided on 27.66 ft of geofoam and 3 ft of soil and a slab of concrete directly over the geofoam. The geofoam will have grids every 32 inches and the grids will extend along the geofoam until it meets the slope. Only purpose for this is to stabilize the geofoam during a seismic event. That's why I was asking for parameters to model the analisis for seismic events.

 

[rochplayer]

MSEdesigns:

In my experience, Geofoam is an extremely expensive (>$125 per cubic yard) option which could be eliminated by use of ground improvment methods.

Some questions to ask your geotechnical engineer -
1. What is their allowable design based on? Settlement or ultimate capacity/SF?

2. If settlement, what type and magnitude of the settlement? Elastic or consolidation? 1-inch? 2-inches? More?

3. Is the settlement tolerance realistic? Is settlement being limited to 1-inch total beneath an MSE wall that is supporting a roadway or other forgiving structure? If so, that is overly conservative for a flexible wall.

4. Can a surcharge program be used to preinduce settlement and/or strength the bearing soil?

5. Can subgrade reinforcement be used to increase your bearing resistance? You are already looking to use geogrid in the retained soil.

6. Ground improvement could also be considered as bushel3 suggests. Geopiers, stone columns, CDF shafts, etc. could be used. These may give you a better "bang for your buck" than Geofoam.

Out of curiousity, what is the foundation subgrade like? Is the site level or on a slope?

As a re-read your post, it seems like there ought to be less expensive options that a 30+ foot geofoam wall.

 

[rochplayer]

It looks like our posts crossed - what is the driving force in the seismic case? There will be virtually no intertial force in the geofoam mass. The 2:1 backfill is like a 2:1 batter so Ka will be very low and delta Kae non-existant. It sounds to me like by going with geofoam the seismic analysis of the entire system needs to be redone.

 

[MSEdesigns]

Well rochplayer, the geotechnical guy will be asked some of those questions, rest assured on that. About the geometry, the wall will go over approximately 20 ft of select fill (A-2-4)and a 2h:1v slope in front of the wall. Wall embedment is kept to 5 ft and a 4.5 ft wide concrete leveling pad was proposed to spread the block facing weight and keep the pressure under 2500 psf. The slope is already built (on the first phase of the project) so no chances to reinforce the slope now.
Seismic acceleration coefficient is required to be 0.3 by the local building code.
About the geofoam cost, it is less expensive here. Actual cost is around $65 per cy for type I geofoam (about 1.1 pcf). We have a local producer here at the island (Puerto Rico).