Sedimentation analysis is used when the soil fraction is finer than ____. 

Explanation:

(i) Mechanical analysis is defined as the determination of the mass of particles in the fraction containing particles of only a certain size.

• Sieve Analysis: Suitable for coarse soil (greater than 75-micron particles) e.g. sand, gravel
• Sedimentation Analysis: Suitable for fine soil (fines - particle size less than 75 microns) e.g. clay, silt

(ii) ​Sedimentation analysis is used only for the soil fraction finer than 75 microns.

(iii) The sedimentation analysis procedure is as follows:

• Soil particles are allowed to settle from a suspension.
• The decreasing density of the suspension is measured at various time intervals.
• The procedure is based on the principle that in a suspension, the terminal velocity of a spherical particle is governed by the diameter of the particle and the properties of the suspension