Permeability is defined as the property of porous material which permits the passage or seepage of water through its interconnected voids. It is the ease with which water can flow through the soils. The coefficient of permeability is found using the following methods:

1. Laboratory methods:

   1. Variable head test
      The variable head test is used for fine grained soils like silts and silty clays.
      k = 2.203 * a * L * log₁₀ (h₁ / h₂) / (A * t)
      

      where, k = Coefficient of permeability at T^oC (cm/sec)
      a = Cross Sectional area of stand pipe (cm²)
      L = Length of soil specimen (cm)
      A = Cross-sectional area of soil sample inside the mould (cm²)
      t = (t1 – t2) = Time interval for the head to fall from h1 to h2
      h1 = Initial head of water at time t1 in the pipe, measured above the outlet
      h2 = Final head of water at time t2 in the pipe, measured above the outlet
      

   2. Constant head test
      The Constant head test is suitable for coarse grained soils like sands and sandy silts.
      k = Q / (t * i * A) = Q * L / (t * h * A)
      

      where, k = Coefficient of permeability at T^oC (cm/sec)
      L = Length of soil specimen (cm)
      A = Cross-sectional area of soil sample inside the mould (cm²)
      i = Hydraulic gradients
      Q = Quantity of water collected in measuring jar
      t = Total time required for collecting ‘Q’ quantity of water
      h = Difference in the water levels of the overhead and bottom tank
      

2. Field methods:

   1. Pumping out test
   2. Pumping in test

3. Indirect tests:

   1. Computation from grain size or specific surface
   2. Horizontal capillarity test
   3. Consolidation test data

The derivation of the coefficient of permeability is based on the assumption of the validity of Darcy’s law to the flow of water in soil. The law is based on the fact that the flow between two points is directly proportional to the pressure differences between the points, the distance, and the connectivity of flow within rocks between the points. The term coefficient of permeability implies the velocity of flow of water through the soil under unit hydraulic gradient, and consequently has the same units as that of velocity.