Practice exam question soil

Can you explain this further? I thought clay is the type of soil that is most susceptible to liquefaction? Liquefaction causes the building to sink if it occurs.

A building is to be located in an area with high seismic risk. A site must be chosen. The site should be one that minimizes liquefaction of the soil.

Which soil type is the least susceptible to liquefaction during a seismic event?

Clay
Correct. Liquefaction is when a soil behaves like a liquid, such as when seismic “S” waves cause vibrations during earthquakes. Loose and uncompacted granular soils such as sand and sandy (non-plastic) silts are the most susceptible to liquefaction. During liquefaction, soils lose their shear strength. A clay content of 15% or more is considered adequate protection from liquefaction.

Silt
Incorrect. The more granular and loose the soil, the greater the susceptibility to liquefaction.

Sand
Incorrect. The more granular and loose the soil, the greater the susceptibility to liquefaction.

Gravel
Incorrect. The more granular and loose the soil, the greater the susceptibility to liquefaction.

Reference: Site Planning and Design Handbook

Hello @sergioclaure93 -

Perhaps @coachchrishopstock could further elaborate this question.

Elise

Hi @sergioclaure93
Liquefaction is a condition that has been associated with fine to medium grained sands and silts - i.e. soils with low cohesion and low density. Clay has high cohesion and density and is therefore not susceptible to liquefaction. In fact, a clay content of 15% or greater in soils has been found to be adequate protection from liquefaction.
To try to conceptualize this, think of the textures of modeling clay vs. sand from the beach. The clay is heavier and sticky, which means it will be less likely to liquefy during a seismic event. It’s easy to imagine how the sand could liquefy under the same conditions.

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Ok I see, so because clay’s characteristics is heavy and sticky makes it harder to liquefy compared to sand. Makes sense, great that helps.