Loading flowable fill in an excavation with sloped bench

[TheDr1ller]

Hi,

Just wanting some expertise regarding backfilling a sloped benched excavation(in rock) with flowable fill then loading this up(via a structural element), is there considerable design required to ensure the transfer of load and or the adequacy of the fill?

its a unique case with extremely high loading which is business as usual when loaded directly to the rock, however due to construction sequencing a 3m deep 1:1 trench is being excavated below and flowable fill is being proposed prior to loading it up.

Cheers.

 

[1503-44]

Flowable fill has an unconfined compressive strength of 30 to 200psi. At BEST only 10% of lowest strength concrete. Its for filling space, not for structural applications.

Einstein gave the same test to students every year. Why would he have done that? "Because the answers had changed."

 

[EireChch]

Run some Uniaxial compressive strengths tests (with elastic modulus measurements) of the flowable fill.

Check your bearing capacity and settlement.

If you backfilled it with gravel you would be doing the same thing, bearing capacity and settlement checks.

Although if its as extremely high loading as you said then then cast a portion of reinforced structural concrete (i.e. a foundation) and bear it onto rock.

 

[cvg]

Definition: self-consolidating cementitious mixture that is intended to result in a compressive strength of 1200 psi (8.3 MPa) or less.

 

[EireChch]

8.3MPa could be more than the rock hes founding on....

 

[1503-44]

Maybe 1200 psF?

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[cvg]

that definition is straight from the ACI website. mix design can run from half sack to 2 sack or more of cement resulting in a wide range of strengths.
lower strength is better for trench backfill where it needs to be "diggable" in the future. higher strengths are better for structural applications.

https://www.concrete.org/topicsInConcrete/topicdetail/CLSM?search=CLSM

 

[1503-44]

That would be a pretty stiff mix to come in at half of normal (lower) strength concrete.
For a structural load application, I'd go the full monty and sleep peacefully at night.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[SwinnyGG]

While placing flow fill under structural elements isn't the 'normal' way to do it... 1000 psi is 144,000 psf. That's a potential level of settlement or shear failure resistance which is higher than compacted hard clay by more than a factor of 10.

If the right product is correctly placed, flow fill is a valid solution here. If the trench is truly cut into rock, or soil with a reasonably high bearing capacity, the flow fill is confined.

It's more complicated to get a true strength assessment than if you poured a big tall footing to the bottom of the trench, but it's do-able.