What could cause a CPT and SPT soil behavior to not match up?

[JHeisenberg]

Have several adjacent CPTs and SPTs at a site that are showing clay like behavior in the CPT and then classifying as low PI sands in the SPT. Any ideas what could be the cause of this?

 

[EireChch]

Post you CPT plot, showing tip resistance, ic etc. and your SPT plot and also you fines content, atterbergs etc.

I have this happened occasionally and sometimes there is just no explanation. If you have dissipation plots then they can be a useful indicator of soil behavior type also.

 

[geotechguy1]

Agree can you share more information.

Also, which CPT-based soil classification system are you using and have you looked at the others?

 

[Jimpat]

It is my understanding that many "low" PI clayey sands behave much like low PI lean silt or lean clay.
The material behavior is mostly cohesive (at decent moisture contents) despite being more than 50% retained on the #200.
It is true that the flow curve will be fairly steep and in a saturated state the material likely won't behave very well.

the electrical resistivity, permeability and void ratio might be a bit high for ML/CL but you won't know unless you test it.
I'm guessing your CPT didn't have ER results, or to many fancy sensors.

It is my understanding that we as a field of science don't study the clayey sands and sandy clays enough. Tbh most studies i've read are based around clean sands or solid clays.

You may even have mixed (laminated sediments) sandy clay and clayey sand. This is fairly common in sandy alluvium deposits.


If it is "low PI" and not totally non-plastic I'd say that this is more an issue with the USCS interpretations than the CPT correlations. ¯\_(ツ)_/¯