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Foam Styrene Backfill

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I designed two of the grade separation bridges on the
approaches to the Alex Fraser Bridge in Vancouver, in the
mid - eighties. The subsoil was pretty soft, so most of the
highway used encapsulated hog fuel (plastic wrapped bark
mulch) in the roadbed to save weight and reduce settlement.
At the retaining wall - style abutments for the interchange
bridges, "beadboard" a.k.a. foamed polystyrene blocks, were
used as fill for the last 20 feet or so against the abutment
wall. It was a British Columbia Ministry of Transportation
(Highways Department) choice. I'm not sure whether they were
the innovators, but Styrofoam insulation has been used under
roads in the Arctic for many years. I haven't heard of any
problems with the foam under the bridge approaches. I look
for signs of settlement whenever I drive over the bridges,
and don't feel any bumps.

Dow has lots of technical information on the use of
Styrofoam for foundation insulation.

I also designed a small garage addition where foam
foundations were used, a few years ago. It was a site with
silty, peatey soil, where the owner didn't want to use
piles. There was concern about vibration damage to
neighbouring property  from pile driving. We used a somewhat
dense beadboard, in the form of blocks, under a concrete
slab with stiffening beams. It was a true raft foundation,
in that we excavated enough subsoil material to balance the
weight of the building. Again, the foam manufacturer
provided information on structural properties. Creep was a
concern with low density foams.

As it was an auto service use, we placed polyethylene film
around the perimeter, under the landscaping, to reduce oil
movement under the building. The concrete slab ribs probably
were enough to protect the styrene foam, but the film didn't
cost much. Polyethylene has fairly good resistance to oil,
as I recall.from my enquiries at the time.

To answer the inquiry: plastic foam could probably be used
against a retaining wall to reduce soil pressure and
pprovide insulation. It will be more expensive than pea
gravel.

Jim Warne
Vancouver, British Columbia
jwarne(--nospam--at)direct.ca