when can typical footings be on undisturbed subgrade as opposed to compacted subgrade?

[delagina]

without geotech, i'm seeing a go by drawing with both undisturbed subgrade and 6" compacted subgrade.. when is it ok to use undistrubed?

 

[KootK]

Don't we found most of our shallow footings on undisturbed soil?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

Undisturbed soil does not guarantee a satisfactory bearing surface. A silty soil with a high water table could be problematic, for example.

BA

 

[jayrod12]

For me it usually depends on how they plan on excavating it. If I know it's a small thing where they may not have a good excavator with a smooth bucket, or it's tight quarters, I'll spec a compacted base.

 

[cvg]

kind of depends on what type of footing and what type of structure.

is this a bridge, dam or garden shed?

[ul]
[li]over-excavate to competent foundation material / depth[/li]
[li]inspect by engineer[/li]
[li]scarify 8 inches deep[/li]
[li]re-compact to 95%[/li]
[li]then QC and QA testing to confirm[/li]
[/ul]

 

[KootK]

Undisturbed soil does not guarantee a satisfactory bearing surface.

In the jurisdictions where I've practiced, it's usually this for shallow foundations:

1) Found on undisturbed bearing strata.
2) Geotechnical to confirm bearing capacity on site.
3) Mud sill at GC's option but, if you omit it and something goes wrong, you're screwed.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[GeoPaveTraffic]

There is no one answer to your question.

As others have pointed out, it depends on the type of structure, loads, allowable settlement, and the soil.

Mike Lambert