Flowable fill as backfill of undercut footings
Flowable fill as backfill of undercut footings
(OP)
We are on a site where unsuitable soils are to be removed up to 3 or 4 feet below planned bottom of footing (BOF). Flowable fill (no air, 300-500 psi) can be used to re-establish original BOF. Can anyone tell me the recommeded length of curing of the flowable fill prior to normal-wt concrete placement?





RE: Flowable fill as backfill of undercut footings
RE: Flowable fill as backfill of undercut footings
Another vote for 24 hours however. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Flowable fill as backfill of undercut footings
The best way to test something is to squeeze it, slowly, until it breaks!
RE: Flowable fill as backfill of undercut footings
Would you find it acceptable to place 3 to 4 feet of crushed stone below a footing? We wouldn't recommend that (not 3 to 4 feet), mostly becuase of difficulty in being able to determine proper compaction. 18 inches is typically the maximum, although there was a job where we allowed 4 feet of coarse aggregate to be placed. I am new at this and have a lot to learn, and would appreciate your opinion.
As to the OP, I agree that 24 hours should be good.
RE: Flowable fill as backfill of undercut footings
Sure, I'd have no problem with a thick interval of open-graded aggregate (i.e., not bank run) beneath a footing. I'd compact it in 2-ft thick lifts using the backhoe bucket. I'd recognize to what extent there are shrink swell soils and reconsider accordingly; however. I can't tell you how many different jobs I've seen this done withoug a problem - typically using AASHTO #57 aggregate.
f-d
¡papá gordo ain't no madre flaca!
RE: Flowable fill as backfill of undercut footings
RE: Flowable fill as backfill of undercut footings
Here's the typical application of a thickened interval of open-graded aggregate beneath footings. Let's say you have a site with existing fill or a surface horizon with a lower than desired soil modulus. You look at the foundation settlements using Schmertman or some other integration of stress v. depth and soil modulus profile. You calculate 2 or 3 inches of settlement potential. The majority of this settlement coming from the bearing soils within 4 ft of the bearing grade.
Well, if you design an engineered subbase (i.e., with a higher soil modulus value - for example open-graded aggregate), you can limit the settlement potential of the bearing soils immediately below the footing. It may be you can lower the footing (i.e., buy more concrete or work in a trench), but it can also be that you dig an undercut and provide an engineered subbase.
There is no "correct" answer, other then to take each site, each structure, each owner (i.e., their tolerance to risk) and consider various options. An engineered subbase of 57 stone is one approach and I'm not alone as a geotechnical engineer in Central Virginia on this one. . .
Then again, I do like to work toward an agreeable solution with structural engineers too - ha.
f-d
¡papá gordo ain't no madre flaca!
RE: Flowable fill as backfill of undercut footings
That could be why we don't do coarse aggregate in this area. Most of our soils are fat clays with high LL. We do frequently recommend a thin (6") layer of coarse aggregate on the bottom of footing excavations to aid in dewatering and to prevent disturbing the bottom. Particularly during the rainy season.
The case where we allowed 4 feet had stiff sandy lean clay w/ gravel, and was used to backfill the hole left after removing an old cistern.
I would like to additional comments.
RE: Flowable fill as backfill of undercut footings
f-d
¡papá gordo ain't no madre flaca!
RE: Flowable fill as backfill of undercut footings
I do see what you are saying.
RE: Flowable fill as backfill of undercut footings