What is the laboratory test used to indicate the grain size of fine-grained soil?

The hydrometer analysis is the laboratory test used to determine the grain size distribution of fine-grained soils. This method is particularly effective for soils with very small particle sizes, such as silts and clays, where traditional sieve analysis is not applicable due to the small dimensions of the particles.

In hydrometer analysis, a soil sample is dispersed in water and a hydrometer is used to measure the specific gravity of the suspension at timed intervals. Because fine-grained soils have very small particles that settle slowly, the hydrometer readings can be correlated to particle size. The test provides a way to plot a grain size distribution curve that shows how much of the soil sample falls within specified size ranges.

In contrast, sieve analysis is mainly used for coarse-grained soils as it requires particles to be larger than the standard sieve openings. The Atterberg limits test determines the plasticity characteristics of fine-grained soils, which is important for understanding their behavior but does not provide direct information about grain size. The California bearing ratio (CBR) test is a measure of the strength of subgrade soil and is not specific to grain size distribution.

Thus, hydrometer analysis is the most suitable method for obtaining grain size information for fine-grained soils.