Flowable Fill around utilities

[SiPaul]

Does anyone have any experience using flowable fill around utility crossings where minimum clearances can't be achieved? We have a situation where we can only get about 1" of clearance. I would like top specify using flow ash to insure that the there is a rigid base and settlement does not occur.

Thanks.

 

[Focht3]

The supplier should be able to provide you with strength and shrinkage values for their product, which should allay your fears -

But why worry about movement, when it appears that the gap is only an inch (25 mm)? It would seem to me that simply filling the gap with any flowable fill of even minimal strength and shrinkage properties would be sufficient. I'd be a lot more concerned if the gap was on the order of a foot (300 cm.) Then the shrinkage and strength could be a real issue...



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[jheidt2543]

I have seen a standard, low strength concrete mix used with "pea gravel" (3/8" washed stone)as an aggregate to get in and around pipe ducts. Just another alternative, then you can use the same ready-mix through out the job.

 

[Focht3]

Most of the flowable fill I've seen has no coarse aggregate at all - just sand, cement, fly ash (or other additives) and water. The slump is pretty close to 12 inches i.e. it has no stiffness at all. It's a lot more fluid that even a high slump, small aggregate concrete; usually it's more like a very fluid fine grout.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[PEinc]

I agree with Focht3. The flowable fill is usually very low strength so that it can be excavated later if necessary. Usually the strength is about 50 psi to 200 psi. Also, when placing the flowable fill around the utilities, be careful that you do not pour so much at one time that the utilities float up out of the flowable fill.

 

[bltseattle]

I've also seen polystyrene (styrofoam) used between crossing utilities when the clearance is less than 6 inches; provides no strength but fills voids and it's inexpensive. It especially makes sense if you only have one or two crossings, since preparing a batch of flowable fill would require substantially more work.

 

[Robert168]

I had once used a kind of high flow, high strength,non-shrink grout for the turbine foundation. There is no requirement about the clearance with this material. Not sure whether it suits your case?

 

[twalkengr]

We have used controlled density fill (CDF)around many utilties with mixed results. CDF is a pozzolanic mixture of fly ash, sand and cement. It is high strength and flowable. However, the specified mix is important because many times it hardens like cement and can be very diffcult to "dig" through. Most Utiltiy companies specify it at its lowest strength so that they can dig up the utility later.

 

[jsc123]

Hi SiPaul,
I am with J & S Contracting. We are a Canadian based company. For a number of years I did slabjacking. Have you considered using a hydraulic mud pump? You can use a blend of topsoil, sand and portland cement. This will create a surprisingly strong mix with little to no shrinkage.