undistubed soil vs 4" flowable fill below foundation

[delagina]

Can someone give me reference when it's ok to just use undisturbed soil vs requesting 4" flowable fill?
I'm not doing big, tall buildings. Just regular isolated spread, occasionally, combined footings.

 

[BAretired]

I'd say it is always ok to place footings on undisturbed soil.

BA

 

[delagina]

I'm currently checking foundation done by third party consulting engineers. They have all foundations on 4" flowable fill. Looking for a reference when to use this and other fill you put below foundation as opposed to just put on undisturbed soil

 

[oldestguy]

I've been around many years and have seen many a rule or guide for what or how you place foundations on. That method sounds like one person's idea of making up for some support question that may come up, but I don't know what it might be. It may or may not be any better than the natural soil depending on its make-up.. Is there any binding or cementing agent in that fill? If none, it may be less suitable than soil alone. Why the inspection? Some question?

 

[BUGGAR]

Sometimes contractors like to work on a "mud mat" and will pour lean mix for this purpose. In my experience, this is for the convenience of the contractor and his operations only. We did it when excavations had to remain open for a period of time.

 

[hokie66]

Buggar is correct. It helps a lot in adverse weather conditions.

 

[Ron]

Undisturbed soil is not always good. If you are doing lightly loaded shallow foundations, at least have a few manual auger borings done to characterize the near surface soils and locate shallow groundwater conditions. Then decide whether you need a "mud mat", blinding layer, flowable fill or just good compaction. When bearing conditions are wet, contractors often place a lean concrete layer as buggar described. This helps to prevent dilution/adulteration of the foundation concrete by water and prevent minor settlement of the footing as water is squeezed from mushy soil.

In my area, we sometimes encounter organic silts at or near the surface. Overexcavation and backfill is required for such conditions.

 

[hokie66]

Building footings in sand also often benefits from a blinding layer. If the sand above the bottom doesn't stand until the placement, at least when cleanout is done, you know where the bottom is.

 

[GeoPaveTraffic]

To add to what BUGGAR said. I have specified that a mud mat be placed when the foundation soils are easily disturbed. Mostly when soils are low plasticity silts and the groundwater table is high. Without a mud mat, just the iron workers placing footing steel and cause the soils to become soft and rutted. When confined, the soils are fine for support of the building, just have to protect them.

Mike Lambert

 

[BAretired]

I have never placed footings on silt, certainly not on organic silt and never near the surface (because of frost depth in my locale). If iron workers placing steel cause the soil to become soft and rutted, the soil can hardly be considered "undisturbed". Such conditions would require special consideration which would presumably be recommended by the geotechnical engineer.

BA

 

[oldestguy]

A question for the OP. Are we discussing flowable fill or is it weak concrete at your site? The mud mat thing is very common, but generally the term "flowable fill", at least to me, is a fill slightly cemented, sufficient that it does not require compaction for then supporting structures or doesn't settle. Which use?

 

[delagina]

Flowable fill 100 psi.