In the plasticity test, what does a long ribbon indicate?

In the plasticity test, what does a long ribbon indicate?

• A. Long ribbon indicates high sand content
• B. Long ribbon indicates low moisture
• C. Long ribbon indicates high clay content
• D. Long ribbon indicates high silt content

Answer: (C)

A long ribbon in the plasticity test points to high clay content because clay minerals make a paste very cohesive and plastic when moist, allowing it to be stretched into a continuous ribbon. The finer, plate-like clay particles bond together more strongly than sand or silt, so soils with more clay form longer ribbons before tearing. In contrast, sandy soils don’t bind well and crumble, low moisture reduces plasticity and shortens or prevents ribbon formation, and silt can form only shorter ribbons than clay-rich soils. So observing a long ribbon directly indicates high clay content.

A long ribbon in the plasticity test points to high clay content because clay minerals make a paste very cohesive and plastic when moist, allowing it to be stretched into a continuous ribbon. The finer, plate-like clay particles bond together more strongly than sand or silt, so soils with more clay form longer ribbons before tearing. In contrast, sandy soils don’t bind well and crumble, low moisture reduces plasticity and shortens or prevents ribbon formation, and silt can form only shorter ribbons than clay-rich soils. So observing a long ribbon directly indicates high clay content.