Geofoam backfill 3

[DoubleStud]

I am going to specify geofoam for the first time for a backfill. Currently I have a cmu retaining wall with framed deck right in front of it. The deck spans to the house. The deck is in bad shape and owner wants to replace it with slab on grade patio. I dont want to just backfill the crawlspace below the existing deck because of these reasons:
1. I am not sure the basement/garage foundation wall can handle it
2. There is another cmu wall perpendicular to it. I am not sure it can handle additional 4-5 ft of soil.

I want to just fill the space with geofoam. Is there a resource I can look at to specify this? Also how shall I do the slab? Shall I put enough steel to make sure it can span several feet due to foam movement? Let the slab slides both sides? Is there something I need to put for the gap at the edge between geofoam and the cmu retaining wall? The CMU retaining wall is curved.

 

[wrantler]

I have used geofoam a few times and probably wouldn't again unless I had too.

-The applications I have used it are all above ground. It is foam so it will float.
-It is flammable, they do put some retardant in it tho. But during construction you need to be careful.
-They dont usually all line up perfectly like you have on your dwgs when constructed. For a curved wall I can guarantee the contractor will hate saw cutting custom shapes out of the blocks.

Instead of geofoam I use pumpable lightweight fill like cematrix. It fills complex voids completely and quick. Maybe something to consider.

 

[XR250]

What about a suspended slab?

 

[DoubleStud]

suspended slab idea came up. Contractor would rather do this. One of the span is quite large. He doesnt want to do steel beam or concrete work for footing or stem wall for mid support.

 

[JAE]

I've used geofoam (EPS) before and probably would use it again for the right application.

Here's a Link to one resource, there are many more out there.

When I've used it the blocks are indeed specifically cut into precise blocks that fit tightly. You actually get shop drawings of the block sizes, fit-up.

I wouldn't put the patio slab directly on top of the foam blocks. I'd put down a vapor barrier lining over the blocks and then add a layer of earth/gravel over the top (I've used 3 feet thick but a lesser amount could work). If you are concerned about the weight of the earth/gravel, then excavate down some so the net difference in weight is zero.

Keep in mind that the geofoam dissolves when exposed to gasoline-type liquids (thus the vapor barrer) so any spilled gas or grill lighter fluid could seep down and damage the patio slab support. Not likely but just a warning.

 

[dylansdad]

JAE, (or others) not to hijack this thread, but I am planning a similar construction to extend a patio over a set of "bleachers" for more table/chair seating area. What do you mean by "....then excavate down some so the net difference in weight is zero"? thanks.

 

[JAE]

dylansdad - If you are going to fill up 8 feet, say, and cap it off with a slab, what I would do is first dig down 2 ft. below the existing grade (10 feet below your final new grade)
Then fill in 8 ft. of geofoam, then cap it with a impregnable liner, then 18" of compacted soil. Then 4" of granular base and 4" of slab - or whatever combination of soil/base/slab you need.

What that does is limit the NET additional soil pressure below so you don't initiate any new settlements in the adjacent existing wall...essentially taking away 2 feet of overburden and putting back 2 feet above the geofoam.

 

[cvg]

geofoam and cellular fill both require specialty contractors and therefore, are expensive or difficult to find a qualified contractor.
easier and perhaps cheaper method than foam or lightweight fill would be CLSM. 1 sack or even 1/2 sack mix, placed in lifts will greatly reduce your lateral load and is quick and relatively cheap. you may also be able to get a mix with lightweight aggregate.

 

[JAE]

cvg,
CLSM may be appropriate in some situations.
However, the OP above indicated a need to avoid creating new lateral loads on adjacent walls.

CLSM is essentially a lean concrete mix, usually placed with a very high slump.
Thus, any placement in the OP's space between existing walls will indeed create very high lateral pressures.

It also will be heavy - similar to soil or lightweight concrete, or maybe approaching normal weight concrete.
This can cause downward settlements in the existing soils below which could crack the walls.

You can place CLSM in layers (and it normally is) but there would still be some lateral pressures on the walls and still some new load on the soils below.

I'm not associated with the EPS industry.
When I have specified EPS it didn't require any specialty contractors to install it, only to fabricate it, much like specialty contractors needed for structural steel, masonry, etc.
It's just another product that has quickly developed into a workable solution - more used in highways, etc.

 

[SlideRuleEra]

DoubleStud - Excavate to the elevation of the garage floor and use lightweight material from that elevation.

If this is not done, the weight of the lightweight material + compacted soil on top of lightweight material + concrete slab + live load on the concrete slab become a surcharge load applied to existing soil outside the garage (see my sketch). This could significantly increase the load currently being applied to the wall by existing soil:

 

[MacGruber22]

I have received unit pricing within the last month of $400 cu/yd for 250 cu.ft of EPS-12 (Baltimore). Also, the price is very volatile as I was told by the contractor. Good idea to present conceptual alternates for pricing before taking a design all the way with EPS.

-Mac

 

[bones206]

I’ve had contractors tell me it’s hard to get EPS supplied for small jobs. Same for ultra lightweight foamed aggregate.