Hi there! I am reviewing my answers for Programming + Analysis Practice exam #1 and am stumped on question 9, regarding liquefaction in soil types. The answer to this question states that clay is the least susceptible to liquefaction during a seismic event, which contradicts what i know to be true about clay (that it is highly plastic soil.) Can anyone expand on this?
Thank you!!
What were the other options? Clay typically will soften under seismic events, but it wont actually liquify like sand will.
Hi @SBPorzungolo,
Thanks for reaching out with your question! You are correct, in that clay is a highly plastic soil. This actually makes it least susceptible to liquefaction, which is the result of a soil behaving like a liquid. Loose and uncompacted granular soils are most susceptible. Therefore, of the other soil type options provided (gravel, silt, and sand), clay would be the least susceptible to liquefaction; as @devin.cressman has pointed out.
Also, it came to my attention that the referenced page number indicated for this answer explanation is incorrect. It should be Site Planning and Design Handbook, 2nd Edition, page 55. Sorry for the that confusion, but check it out and that should help clarify things more! We have fixed this reference in the practice exam summary.
We actually discussed these topics in our last PA Virtual Workshop: Soil Analysis [Exercise A], which is currently available for viewing on your dashboard. This replay will expire on Sunday at midnight CT, and the next live Soil Analysis workshop [Exercise B] will occur on Sunday, April 6th at 2:00pm CT.
Hope this helps! Happy studying 
Kiara Galicinao, AIA, NCARB
Black Spectacles
Hi Kiara and Devin,
thank you both for responding. I guess I have been confusing liquification and plasticity, but this helps a lot. I will definitely check out the PA Virtual workshop and will try to make the next one before my exam in April!
Thank you
Stella Blue Porzungolo
Hi Kiara,
I would like to better understand your explanation because clay is the most granular compared to gravel, sand, and silt (Site Planning and Design Handbook, 2nd Edition, p230). BS’s explanation provided for why “gravel” is incorrect doesn’t make sense either. Again, among comparing the 4 answers, gravel is the largest particle.
I understand adding 15% clay back to a soil type that has undergone liquefaction is to help stabilize the soil makeup’s cohesion, but that’s a different concept than what the exam question is asking in my opinion.
The exam question also does not elaborate whether the given soil types are compacted or not (ex. compacted sand has a lot higher shear strength and density than uncompacted sand, and thus less susceptible to liquefaction). Moreover, depending on its liquid limit, clays will liquify if saturated (ex. adding too much water to pottery clay causing the clay to become too liquidy to be workable.) SPD p55 also goes into explaining the concept of “clay liquefaction”.