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

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

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

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

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

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.

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

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

BA

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

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.

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

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

is this a bridge, dam or garden shed?

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

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

Quote (BARetired)

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.

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

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

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