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Expansive soil design Q

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MrStohler

Structural
May 1, 2001
81
US
I am preparing a proposal to design foundations for large industrial upgrades (column loads from 10k to 300k, 150k uplift). Large industrial fans (3000 hp) and a tall chimney are also part of the project.

The geotechnical report provided by the owner indicates that the soil consists three strata:
Fill for 1 to 2 ft
Lean Clay with silt
"Claystone Bedrock" at 5 to 8 ft below grade
The "Claystone" (that's what the report calls it) extends down at least 50ft where the borings end. It has a SPT blow count that varies from 25 to 50.

The report advises that large loads should be supported on drilled shafts into the bedrock with a skin friction of 3ksf and an end-bearing of 30ksf.

The report further indicates that the "claystone bedrock" possesses a moderate swell potential..."Swelling pressures were generally in the range of 8 to 10 ksf"

The current plan would be to use mats supported by the drilled shafts for the foundation systems.

Q: As part of designing the embedment for the shafts, would one need to apply the 10ksf swell pressure against the minimum DL? Could anyone cite a reference for this type of condition?

It appears to be unusual in that the layer subject to swell is also the layer used to anchor the foundations. (PS geotech does not reply to inquiries from bidders...owner assures access to successful design team)
 
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