If we can tell that a finegrained soil is a clay or silt fro

If we can tell that a fine-grained soil is a clay or silt from a plasticity chart, why would we have to perform a hydrometer test?

Solution

We perform hydrometer analysis to get specific gravity of particles which is otherwise required for calculating unit weights of soil mass. This cannot be interpreted from plasticity chart. Other rreason is to get size of particles from hydrometer analysis. Size of particles gives us idea about the permeability.