Flowable Fill as Retaining Wall Backfill

[Littlebit]

Just wanted your thoughts on using flowable fill as backfill behind a retaining wall (actually an exterior/end building wall) . If I'm trying to avoid any water behind the wall, does flowable fill help. How would I go about calculating lateral pressures as I think there will be a reduction in pressure with the use of flowable fill (due to self-bonding nature...like concrete) as compared to low-plasticity backfill (even with a clean stone zone). Based on the amount of water added to flowable fill, I would think it is very porous and may end up with hydrostatic pressures anyway, or am I wrong. Also due to expected high lateral pressures when its still plastic (just placed), will placement of say 4 ft thick lifts be ok without have to shore up the wall?

Your thoughts are welcomed

 

[fattdad]

Without commenting on the need for drainage, I'll caution you that the transient time that the flowable fill is uncured, will impart much higher lateral loads then soil. It's like a liquid from the chute. So, you'd likely need to place the flowable wall backfill in lifts, allowing it to cure to some unconfined compressive strength before you then place more. Some engineering calculations would be warranted to determine the lift height with respect to your wall design.

Just some initial thoughts - and don't forget about your drainage!

f-d

¡papá gordo ain’t no madre flaca!

 

[msquared48]

If you choose to use a flowablefill, I would do it in stages, waiting for the previous pour to set fully before pouring the next segment. I would do it uniformly behind the wall in 1' to 2' stages, depending on the advice of the Geotech of course. Kind of the idea of pouring a concrete wall in horizontal segments to limit the lateral pressure on the forms and prevent blowouts.

Mike McCann
McCann Engineering

 

[aeoliantexan]

I'm not sure about the permeability of flowable fill, but I approach basement walls with the assumption that if there is groundwater or infiltrating surface water present, it will get to the wall sooner or later. The pressure of a column of water in a very narrow gap between the wall and the backfill (of whatever kind)is just as great as an ocean. If you don't want water against your wall, provide positive drainage.

Lateral pressure against the wall may be a function of the geometry of the flowable backfill (after it sets, of course). A narrow column of rigid backfill simply transmits the pressure developed by the soil behind it to the wall. A mass of rigid backfill wide enough to act like a gravity retaining wall by itself may exert very little lateral force against the wall after the wall moves a small amount. For an intermediate width, I would look at sliding wedges cut behind the backfill.

 

[civilperson]

The water pressure will load the outside face of the flowable fill and thus the loads will be transfered to the wall. It may have water seepage resistance but no help in hydrostatic pressure. Better option is a perforated drain pipe along the bottom of the footing/wall to pick up all water and make the hydrostatic pressure zero. This cures leakage as well as wall loading.

 

[dirtsqueezer]

I don't think you'd get much in the way of hydrostatic pressure if you're using basically a lean concrete mix, but have you thought about how that lateral load might affect your wall? If there's much pore pressure against the bottom of that wedge, your CDF block is going to want to mobilize, loading the wall from the side, and there you are with a cracked base.

Is there some reason you can't backfill with regular pit run? If it's in a tight spot, you could always use wet sand; the city of Seattle uses it for their utility trenches due to it's ease of placement and lack of big stockpiles and heavy equipment. It's placed with a regular concrete mixer, and self-settles(not completely, but better than nothing). If you're running into water problems, you can just use 2-4" angular rock. Easier to compact, and would drain the side of your wall nicely.

 

[BigH]

I have seen where they use Elastizell (a foamed cement) as backfill in MSE (Reinforced Earth) retwalls. See Elastizell's web site.

 

[DrMurphyDurfy]

If your concerns are based on water proofing, flowable fill will still allow water to seep between the fill and the wall so make sure the waterproofing meets typical standards.

If you are trying to reduce pressures against the wall, consider flyash stabilization of the soil to reduce pressures.

Flowable fill will likely transfer the transient forces to the wall unless poured at a 45 deg slope from the heel of the footing, which may be costly.

There are flowable fill designs that can set up very fast. Depending on the length of the wall, lifts can be 1ft or so. (Wear boots that protect your feet. These mixes are hot). If you have a perimeter drain, you will likely fill it up with flowable fill.

There are so many considerations when using flowable fill, that it may add quite a bit of cost. I recommend using stabilized fill, drainage fill behind the wall and perimeter drain if possible.