List the factors that affect the permeability of a given soil. Determine the precautions that should be taken so that satisfactory permeability data may be obtained.

Explaining the test details of a falling head permeameter, derive the formula used in the computation. Also assess the types of soil material for which falling head and variable head permeameters are used in the laboratory. Compare the relative merits and demerits of laboratory and field methods of explaining the coefficient of permeability.