The document discusses permeability in soil, which refers to the ability of soil to allow water to flow through it. Permeability depends on factors like soil particle size, shape, void ratio, saturation, and temperature. Darcy's law states that the rate of water flow through soil is proportional to the hydraulic gradient driving the flow. It is valid for laminar flow conditions typically found in groundwater flow. Permeability is an important consideration in applications like construction dewatering, slope stability, and earth dam design.

1. Permeability

2. Permeability
• The ability of soil to allow flow of water through it is called as
permeability of soil.
• It is very important factor for the structures which are in contact with
water.
• Flow of water in soil takes place through void spaces, which are
interconnected.
• Water does not flow in a straight line, but in a winding path.
• However in soil mechanics flow is considered to be in a straight line at
an effective velocity. The velocity of flow depends on size of pores.

3. Permeability

4. Importance of Permeability of Soil
• Permeability influences the rate of settlement of a saturated soil
under load.
• The stability of slopes and retaining structures can be greatly affected
by the permeability involved.
• The design of earth dams is very much based upon the permeability
of soil used.
• Filters made of soils are designed based upon their permeability.

5. Properties of Permeability of Soil
• Solving problems involving pumping seepage water from construction
excavation.
• Estimating the quantity of underground seepage.
• Stability analysis of earth structures and earth retaining walls
subjected to seepage forces.

6. Factors Affecting Permeability of Soil
• Size of soil particle
• Specific Surface Area of Soil Particle
• Shape of soil particle
• Void ratio
• Soil structure
• Degree of saturation
• Water properties
• Temperature
• Adsorbed water
• Organic Matter

7. Factors Affecting Permeability of Soil
• Grain size or Particle size
The above equation is given by Alan Hazen. Permeability depends on
shape and soil of soil particles. Permeability varies with square of
particle size diameter.
Permeability varies according to size of soil particle. If the soil is coarse
grained, permeability is more and if it is fine grained, permeability is
low. The relation between coefficient of permeability (k) and particle
size (D) can be shown from equation (1) as follows.

8. Factors Affecting Permeability of Soil
• Specific Surface Area of Particles
Specific surface area of soil particles also effects the permeability.
Higher the specific surface area lower will be the permeability.

9. Factors Affecting Permeability of Soil
• Shape of Soil Particle
Rounded Particles will have more permeability than angular shaped. It
is due to specific surface area of angular particles is more compared to
rounded particles.

10. Factors Affecting Permeability of Soil
• Void Ratio
In general, Permeability increases with void ratio. But it is not
applicable to all types of soils. For example, Clay has high void ratio
than any other types of soil but permeability for clays is very low. This is
due to, the flow path through voids in case of clays is extremely small
such that water cannot permit through this path easily.

11. Factors Affecting Permeability of Soil
• Soil Structure
Structure of any two similar soil masses at same void ratio need not be
same. It varies according to the level of compaction applied. If a soil
contains flocculated structure, the particles are in random orientation
and permeability is more in this case.

12. Factors Affecting Permeability of Soil
• Soil Structure
If the soil contains dispersed structure, the particles are in face to face
orientation hence, permeability is very low. The permeability of
stratified soil deposits also varies according to the flow direction. If the
flow is parallel, permeability is more. If it is perpendicular, permeability
is less.

13. Factors Affecting Permeability of Soil
• Degree of Saturation
Partially saturated soil contain air voids which are formed due to
entrapped air or gas released from the percolating fluid or water. This
air will block the flow path thereby reduces the permeability. Fully
saturated soil is more permeable than partially saturated soil.

14. Factors Affecting Permeability of Soil
• Water Properties
Various properties of water or fluid such as unit weight and viscosity
also effects the permeability. However, unit weight of water will not
affect much since it does not change much with temperature.
But when temperature is increased viscosity decreases rapidly. From
equation (1), permeability increase when viscosity decreases.

15. Factors Affecting Permeability of Soil
• Temperature
• Temperature also affects the permeability in soils. From equation (1),
permeability is inversely proportional to the viscosity of the fluid. It is
known that viscosity varies inversely to the temperature. Hence,
Permeability is directly related to temperature.
• Greater the temperature, higher will be the permeability. That is the
reason, seepage is more in summer seasons than in winter.

16. Factors Affecting Permeability of Soil
• Adsorbed Water
Adsorbed water is the water layer formed around the soil particle
especially in the case of fine-grained soils. This reduces the size of the
void space by about 10%. Hence, permeability reduces.
• Organic matter
Presence of organic matter decreases the permeability. This is due to
blockage of voids by the organic matter.

17. Factors Affecting Permeability of Soil
• Structural Arrangement
Remolding of natural soil reduces permeability. If soil contains more
rounded particles, the permeability is more.
• Stratification
When flow of water is parallel to strata, permeability will be more
when compared with flow perpendicular to strata.

18. Factors Affecting Permeability of Soil
• Presence of foreign particles and entrapped air
This affects the permeability as it reduces void space and it blocks the
inter-connectivity between the pores.
• Degree of saturation
If the soil is dry or partly saturated the permeability of soil is always
less.

19. Permeability Range of Different Soil Mass

20. Darcy’s Law
• In 1856, french hydraulic engineer Henry Darcy published a report on
the water supply of the city of Dijon in France.
• In that report, Darcy described the result of an experiment designed
to study the flow of water through a porous medium.
• Darcy’s experiment resulted in the formulation of mathematical law
that describes fluid motion in porous media.
• Darcy’s law states that the rate of fluid flow through porous medium
is proportional to the potential energy gradient within that fluid.

21. Darcy’s Law
• The constant of proportionality is the Darcy’s permeability of soil.
• Darcy’s permeability is a property of both porous medium and the
fluid moving through the porous medium.
• In fact, Darcy’s law is the empirical equivalent of the Navier-Strokes
equation.
• Darcy’s flow velocity for laminar flow is defined as the quantity of
fluid flow along the hydraulic gradient per unit cross sectional area.
• Velocity of flow through a porous media is directly proportional to the
hydraulic gradient responsible for flow.

22. Darcy’s Law

23. Assumptions of Darcy’s Law
• Soil is fully saturated.
• Temperature during testing is 27°C.
• Flow through soil is laminar
• Entire cross sectional area is available for flow
• Flow is continuous and steady.

24. Validity of Darcy’s Law
• Darcy’s law is valid only for slow and viscous flow, fortunately most
groundwater flow cases fall in this category.
• Typically any flow with a Reynolds number less than 1 is clearly
laminar and it would be valid to apply Darcy’s law.
• Experimental tests have shown that flow regimes with values of
Reynolds number up to 10 may still be Darcine.