What is a common tool used for measuring the plastic limit of soil?

The plastic limit of soil is a crucial parameter in geotechnical engineering that indicates the moisture content at which soil changes from a plastic state to a semi-solid state. A common tool used for determining the plastic limit is a glass plate.

During the testing process, a small amount of soil is rolled into a thread on the glass plate. The test involves rolling the soil until it breaks at a specific diameter, which helps assess the moisture content at that critical transition from plastic to brittle behavior. The use of a glass plate provides a smooth, non-absorbent surface that allows for easy observation of the soil's behavior during the test, facilitating accurate measurements.

Other tools mentioned, such as a cylinder mold, cone penetrometer, and salinity meter, are used for different purposes in soil and materials engineering. The cylinder mold is generally used for making soil specimens for compaction tests, the cone penetrometer measures soil resistance or consistency, and a salinity meter is used for measuring the salinity of a solution, not for soil plasticity.