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?
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Flowable fill as backfill of undercut footings
- Thread starter jharm
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civilperson
Structural
If the total weight of addition concrete is less than 10 psi, (10 feet deep), then 24 hours after initial set should be plenty.
lovethecold
Civil/Environmental
fattdad,
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.
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.
lovethecold,
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!
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!
Fair enough hokie66 (p.s., I got my M.Eng. at VT).
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!
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!
lovethecold
Civil/Environmental
fattdad
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.
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.
Following the removal of some buried ruin, I'd use open-graded aggregate in a setting with stiff sandy lean clay. One (additional) item to note: If you use open-graded aggregate for this fill and of the original excavation is wider than the footing needs to be, you are stuck using forms for the footings. This can add expense. If you use earthfill or flowable fill, you can dig for the footing and use earth form. Sometimes, I've seen open-graded aggregate to the bearing grade with compacted earth above that to allow earth forms. Just seems like too much finesse however. . .
f-d
¡papá gordo ain’t no madre flaca!
f-d
¡papá gordo ain’t no madre flaca!
lovethecold
Civil/Environmental
That's interesting that most contractors in your area use the earth to form their footings. I have only seen earth forms used once for footings. It can be assumed that all footings around here are formed with old fashioned wood. They do this, but I can't see the reason for it in most cases.
I do see what you are saying.
I do see what you are saying.
I need to eat some of my words. I may have misread the original post, and thought it was just talking about soft spots. If it about all the footings, I would agree with fattdad, and have on a number of occasions used engineered filled as subgrade improvement under footings.
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