Pipe Trench Flowable Fill Strength vs. TXMF Foundation Overexcavation Flowable Fill

[structuralsteelhead]

Hello Gents,
I have an RFI from a contractor asking if he can use the same 150psi LSM electrical pipe trench flowable mix also below a large 36K transformer foundation over-excavation flowable fill. My concern is quality control of the mix and set time and well yes strength. The bearing pressure not a concern. However the over-excavation does extend 24" beyond the equipment and 24" below. Should I be concerned with shear strength for the 24" extending past the transformer footing. Am I over thinking this since even this low strength LSM mix is stronger that the subgrade in the first place? I feel compelled to increase the strength for at least to 500psi for no other calculated reason other than a higher chance of reaching a consistent compressive strength.

 

[Tomfh]

Set time is the issue in my experience. It takes a while to consolidate and set, which can affect final levels if things are built on it too quickly, due to it continuing to shrink/consildite (same as any subgrade consoldiaiton can affect things above). If it’s a weak mix you can often still leave footprints in it days later, with it continuing to shrink/consolidate. Extra cement speeds it up a lot, so yeah a bit of extra strength is a good idea.

 

[AskTooMuch]

Possibly overthinking. We use 150 psi flow fill all the time because it's the "cheapest" backfill as opposed to soil that needs to be compaction. And like you said 150 psi had higher bearing strength than subgrade.

Never had that consolidate issue, even if it does consolidate, when you pour the concrete/foundation (which is a few days later because you still have to build your rebars) it should be at correct top of concrete.
If foundation has grout, that's even less of an issue because that is what is for.

 

[Tomfh]

Never had that consolidate issue, even if it does consolidate, when you pour the concrete/foundation (which is a few days later because you still have to build your rebars) it should be at correct top of concrete.

Agreed. Typically, it isnt a major issue. Consolidation is generally accommodated elsewhere, such as during the footing pour or the final slab or pavement stage, meaning you won’t notice anything. We’re only talking about a few millimeters here, which will be taken up by the layers above.

I’ve only observed it when paving is laid quickly on top and flush with adjacent surfaces. A small step can form as the new fill shrinks to its final level. This can and does happen with any new subgrade or footing. My comment was just to point out that it can take time for the flowable fill to fully compact and set, similar to what you see in concrete columns, and the corresponding recommendations to delay pouring formed slabs on top for a day or so. For a footing this is likely less critical as you point out.