PI of Soil Structural Note

[KevinChez]

Hi, I came across a note on a set of drawings I was reviewing in Austin TX:

"For PI of less than 15 or on solid rock, notify engineer of record"

The plans note the design PI as 32.

This is for a residential addition.

Thank you.

 

[PEinc]

So what is your question?

http://www.PeirceEngineering.com

 

[KevinChez]

Why would the contractor notify the engineer of record? To redesign the foundation?

 

[dik]

What effect does a low Plasticity Index have on a foundation?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KevinChez]

I would think that a low plasticity index or bedrock would provide a higher bearing capacity so the footings could potentially be smaller or not as deep?

 

[dik]

Wouldn't the bearing and depth be determined by other means?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[emmgjld]

In Austin Texas, probably a limit between the low to non-expansive soils and the medium to high expansive soils

 

[hokie66]

Kevin,

You are probably correct. If the footings are designed for a presumptive PI of 32, and the actual site conditions are much better, there may be savings achievable by redesign.

 

[dirt-nerd]

I'll second the last few responses.

It sounds like it's likely a PT slab due to expansive soil and the PT slab was designed for a PI of 32. If PI is less than 15, it's not considered expansive per the IBC. It sounds like the designer is telling the construction side to reach out to them if the PI is less than 15 or on solid rock as it's then not considered expansive and the PT slab is overdesigned; therefore, the slab can be redesigned and will likely be thinner, less reinforcement, cheaper, etc. Good on the designer.