Hydraulic conductivity

Principle

The rate of flow of a liquid through a porous medium depends on the size, distribution of the pores and temperature of the liquid, which is indexed as hydraulic conductivity. In saturated soil it is directly related to permeability of the porous medium. Using a constant head of water, water is passed through the soil column and the conductivity of water per unit time is calculated using the Darcy’s equation.

Materials required

         (i). Constant head water supply unit, (ii). Hydraulic rings, (iii). Measuring cylinder and (iv). Stop clock

Procedure

* Collect undisturbed soil core from the field by using core sampler.

* Shave off the excess soil from the cylinder that retains the sol core column.

*Cover the bottom portion of the sectional cylinder with a muslin cloth.

*  Keep the samples in a tray containing water for overnight aturation.

*  Place a filter paper on the surface of the soil to avoid the removal of soil particles.

* Connect the hydraulic rings over the saturated samples by means of wax coating or rubber bands.

*  Place the sample on a metal screen supported with an outflow unit.

* Fill up the constant head water supply unit and connect the discharge tube of constant head to the hydraulic rings.

* *Allow water to flow over the surface of the soil. Adjust the inflow and outflow rate of water.

*Start the experiment after 15 to 30 minutes.

*  Collect the outflow water that is passing through the soil column in a stipulated time. (For sandy soil 5 minutes interval is sufficient whereas for clay or heavy textured soil 15-30 minutes interval is necessary).

* Repeat this to get constant values. The hydraulic conductivity is calculated using the formula.

*   K=  x 60 cm/hr.

Where Q = Quantity of water that flows out at a time interval or ‘t’ in ml.

L=Length of the soil column (7 cm) in cm.

H=Length of the hydraulic head in cm (7+2.5 = 9.5 cm)

A=Cross sectional area of the soil column in cm2

(Where  r = radius of the soil column)

T=Time in hours

The permeability of soil is calculated from the hydraulic conductivity using the relationship.

K=dgk/n

Where

d=Density of the fluid

g=Gravitational acceleration

K=Hydraulic conductivity

n=Viscosity of the fluid

Calculation

The rate of flow of a liquid in a saturated soil column follows Darcy’s law;

Where,

         K = Rate of flow of water (cm / hr)

         Q = Quantity of water collected (ml)

         L = Length of soil column (cm)