How much of a sieved portion of sample is needed if only the plastic limit of a soil is required?

To determine the plastic limit of a soil, a specific amount of the sieved portion of the sample is necessary to ensure accurate testing. In geotechnical engineering, the plastic limit is defined as the moisture content at which a soil changes from a plastic state to a semi-solid state.

For the determination of the plastic limit, a sample size of 20 grams is required. This quantity is generally sufficient to conduct the test accurately while allowing for the necessary manipulation of the soil to assess its plasticity. During the test, the soil sample is rolled out into threads, and observing its ability to retain shape and consistency at different moisture contents is crucial for determining the plastic limit.

Using a smaller or larger sample could lead to inaccuracies in the results, as there may not be enough material to adequately represent the properties of the soil being tested, or it may be too cumbersome to manipulate effectively. Hence, the selected amount of 20 grams is optimal for reliable measurements and assessments of the soil's plasticity.