5.PERMEABILITY:

Permeability of soil defined as property of soil which permits the passage of water or other fluids through its interconnecting voids. And this continuous voids are called permeable.

*Gravels are Highly permeable.

*Stiff clay is least permeable.

            So the permeability is mainly depends on the type of soil.

Darcy’s Law:

It states that for laminar flow in a saturated conditions the discharge per unit time is directly proportional to its hydraulic gradient.

                                                       q = k.i.A

                    Hydraulic gradient (i)  = h_f/L

                    Here h_f is head and L is length of soil specimen   

              There are two laboratory methods available to determine permeability of soil.

1.      Constant head Permeability test:

This test used only when coarse grained soil has to be tested for permeability and where the reasonable discharge can be collected during the test.

                   

                                           

    

2.    Falling Head Permeability Test:

This test actually used for all less permeable soils and where the discharge is small.

                   

                t- Time taken for the water level to fall from h1 to h2 in stand pipe.
                L- length of sample

                A - Area of sample

                a -  Area of stand pipe

Whenever the flow occurs on deposited layered soils the average co-efficient of permeability depends on the direction of bedding planes. They are..

i.            FLOW PARALLEL TO BEDDING:

              When the  flow occurs horizontal to the bed as shown below the average permeability is Calculated as below.

   

         

                   Actual discharge of soil

      Q = k.i.A

      Total discharge for the layered soil is equal to sum of discharge all the layers

  Q= k₁. i.A₁ + k₂. i.A₂ + k₃. i.A₃+ k₄. i.A₄ +....

                                        Here A = Z x1

So           

ii.           FLOW PERPENDICULAR TO BEDDING:

     Whenever the flow occurs over vertical bedding then the average co-efficient of permeability is calculated as below.

                     

               Note: K_h always greater than K_v

               Seepage velocity of soil having porosity “n” and velocity “v” = v/n

Example problems

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