It is not clear, but this paper seems to point at <35% fines is sand-like regardless of PI.

http://ussdams.com/proceedings/2014Proc/29-38.pdf

moe333, thanks for sharing that document. It is a good reference and comes at good time since we are doing some liquefaction analysis for clayey sands. I was looking at the Bray and Sancio (2006) paper and they observed that even soils with more than 50% fines (more than 50% passing #200) can liquefy. It is indicated on page 8 of the document provided by moe333's. So, my interpretation is that the soil classification does not matter.

Their conclusion is interesting: "...As a result of the different purposes and
terminologies used, it is important to recognize that soils that are susceptible to
liquefaction are not equivalent to sand-like soils, and soils that are not susceptible to
liquefaction are also not equivalent to clay-like soils..."

"...Before using these liquefaction susceptibility methods (the ones presented in the document) it is important to understand
whether the soil is coarse-controlled or fines-controlled. When the coarse-grained
particles control the behavior (coarse-controlled), the liquefaction susceptibility methods
are not applicable and traditional liquefaction evaluation procedures should be used. On
the other hand, when the fines content controls the behavior (fines-controlled), the
liquefaction susceptibility criteria are applicable..."

It is also important distinction in the referenced paper and the studies in general as to whether you are considering strength loss (flow liquefaction), or cyclic mobility...they are different.

Great paper and summary of the 3 studies I am somewhat familiar with, mostly use the Bray and Sancio. It would be nice to have a 'formula' to follow which it seems is the attempt, however I think each 'borderline' soil must be looked at individually with engineering judgment.

Failure modes: There is also as mentioned the traditional pore water pressure increase until liquefaction occurs, and then soil mobility and other strength losses, to make it more complex. The PI is a good indicator if all soil passes the #40. Question, how would you classify a clayey fine to medium sand with a PI of 15, can it be a medium plasticity clayey fine to medium sand?

Hi ocgeo, I would classify it following ASTM D2487. I guess that your question is related to the soil description rather than the soil classification. So, to answer your question, considering that PI of 15 is the threshold for slightly and medium plastic, but since you are indicating that it is a "clayey" sand, so I concur with your description above: it can be medium plasticity clayey fine to medium sand.