Concrete vs Soil fill

[JT-1995]

In foundation construction (my cases are typically industrial and within existing facilities) I have formed the opinion that it is better to excavate a foundation, place reinforcement, and fill the entire excavation with specified concrete as opposed to a thinner structural section at the bottom of the excavation, cure, then form and pour a pier(s) to base plate elevation, cure, and then place compacted structural soil fill. The placed concrete is maybe 3-4 times more expensive per yard than soil (depending on location in the facility) but assuming the volume is not extraordinary I have convinced my self the time savings and quality control advantages make the concrete fill a better option. For consideration, say the bottom of a mat footing is 4ft below grade and the total dimension of the excavation is 20ft square. Hypothetically, we could design a 24" footing with say (4) 2ft pier(s) and backfill with soil (= 31yds concrete and 29yds compacted fill probably placed in 3 lifts) or using concrete fill we would end up with 60yds of concrete.

I would appreciate opinions on the above.

From a design perspective, As(min) becomes large in the thick footing and must be accounted for in the thick foundation design...correct?

Thanks

 

[phamENG]

The downsides are certainly the amount of reinforcing and mass concrete requirements. Is there a reason you can't excavate the bad material, backfill with engineered fill, and then pour your foundation? Or are you dealing with onerous frost depths? Your profile indicates you're in Virginia. So am I. In my area, frost depth is a whopping 8". So I'd be backfilling, compacting, and then pouring the footing at a manageable thickness.

 

[JT-1995]

Thanks for the thought. The footing is amongst a forest of existing footings that bear at (-)4ft, so I am putting the new footing at the same depth to avoid extending my added load over soil that is already occupied by existing loads.

 

[phamENG]

Got it. Then it would probably depend on the schedule. If you have a weekend shutdown in which to get it done, dig a hole, put in a load of rebar, and pour a bunch of concrete (make sure the added weight doesn't negatively impact surrounding footings...that's a lot of concrete and will weigh more than the soil it's displacing). If you have more time, I like the footing and pedestals.

 

[JT-1995]

Thanks.

As for Va, I am based out of Va but we work in many states. I have not looked at frost in Va for a long time, most of my career has been in SC. I am shocked the frost depth you noted in Va is only 8", that does seem shallow. Even in SC our standard minimum bearing depth was -12" and usually we ended up at -24" because of exterior details and such.

Thanks for your thoughts today.

 

[phamENG]

Yeah, it's shallow here. I don't go less than 12", and usually not less than 24" just so the footing isn't sticking out of the ground.

 

[AskTooMuch]

Yes, construction labor is more expensive than concrete material specially in industrial.
Having to compact soil fill on top of foundation, we usually just dump flowable fill instead.
I would do that instead of 4' thick footing which could be harder to put rebars in.

 

[AskTooMuch]

Although 20' area thats just 4'×5', I wouldnt worry about it, I'd use soil backfill.