Permeability Of Soil🔥 | Darcy's Law | Seepage velocity | Soil Mechanics

12.3 PERMEABILITYOFSOIL

* Permeability of soil is the property that permits the seepage of water through its interconnected

voids. The gravels are highly permeable while clay is least permeable.

* Darcy’s Law: Darcy (1856) experimentally demonstrated that for laminar flow conditions the rate of

flow (i.e., discharge) per unit time, through a saturated soil, is proportional to the hydraulic

gradient. Thus,

q = k i A

where A = Total cross-sectional area of soil mass normal to the direction of flow.

i = Hydraulic gradient

k = Darcy’s coefficient of permeability.

The above equation may be written as

where v is the velocity of flow or discharge velocity. From this relation the coefficient of

permeability may be defined as the velocity of flow which occurs through the total cross-sectional

area of soil under a unit hydraulic gradient.

Seepage velocity Flow can occur through voids only, i.e., it cannot occur through solids. Hence, the

velocity inside the soil pores is different form velocity of flow. The actual velocity through the

pores is known as seepage velocity. If Av

is cross-sectional area of the voids, then

q = A × v = Av

× Vs

.

or Vs =se

 = where n is porosity.

or 

Thus, Vs 

i

where Kp

is coefficient of percolation.

Thus, kp =

The coefficient of permeability k is expressed in terms of cm/sec.

* Table 12.1 gives the coefficient of permeability of some soils.

Table 12.1 Coefficient of permeability of some soils

1. Constant water level method

2. Falling water level method

3. Rising water level method

* Factors affecting permeability

1. Size and shape of particles The angular sands of greater specific surface will be less permeable

at a given porosity than rounded sands.

2. Properties of water: Permeability varies directly with the density and inversely with the visocity

of water. It may be noted that viscocity of water varies considerably with temperature.

3. Voids ratio: For coarse-grained soils, the following relations are almost correct.

even for fine grained soil it has been found that a semi-logarithmic plot of void ratio versus

permeability is approximately a straight line.

4. Structure of soil: Stratified soil masses have different permeabilities in the direction parallel

and perpendicular to stratification. The permeability is greatly affected by shrinkage cracks,

fissures, joints, and warping of layers.

5. Degree of saturation: Tiny air bubbles entrapped and foreign matter present in water may plug

the soil pores and reduce the permeability considerably.

6. Adsorbed water: The adsorbed water surrounding the fine soil particles reduces the flow of

water.