Co-efficient of permeability is evaluated using Darcy's law which states that the rate of flow of a liquid through a porous medium is directly proportional to hydraulic gradient across it, subject to the conditions that the flow is steady, laminar and the soil is saturated. IS : 2720 (part XVII) recommends two methods for laboratory determination of permeability viz. (i) Constant Head Method (ii) Variable Head Method. The first method is for sandy soils and the second for silts and clays.

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