These slides describes the permeability of soil in a very lucid manner. This has been posted specially for the students of Diploma and Degree Engineering courses.

1. GEOTECHNICAL
ENGINEERING
CHAPTER-III
PERMEABILITY AND SHEAR
STRENGTH OF SOIL
PART-I
PERMEABILITY OF SOIL
PREPARED BY
GOURHARI BISWAS

2. PERMEABILITY OF SOIL
• A material is porous if it contains
interstices(voids).
• The porous material is permeable if the
voids are interconnected or continuous.
• A liquid can flow through a permeable
material.
• Electron photomicrographs of even very
stiff clay soil indicate that the voids are
interconnected.

3. Permeability-Definition
• Permeability- Permeability is defined as
the property of a porous material which
permits the passage or seepage of water(
or any liquid) through its interconnecting
voids.
• Permeability in fluid mechanics and the
earth sciences (commonly symbolized as
κ, or k) is a measure of the ability of a
porous material (often, a rock or an
unconsolidated material) to allow fluids to
pass through it.

4. Facts about Permeability
• It is a very important Engineering property
of soil.
• Gravels are highly permeable and stiff clay
is least permeable.
• A material having contentious voids is
known as permeable. A soil is called highly
permeable when water can flow through it
very easily.
• The permeability of a medium is related to
the porosity, but also to the shapes of the
pores in the medium and their level of
connected ness.

5. LAMINAR AND TURBULANT FLOW
• Flow of water through soils may either be
a laminar flow or a turbulent flow.
• In laminar flow condition each fluid particle
travel along a definite path which never
crosses the path of any other particle.
• While in turbulent flow travel paths of
particles are irregular and twisting,
crossing at random.

6. Fields of Civil Engineering where Knowledge
of Permeability is Required
• Calculation of uplift pressure under hydraulic
structure and their safety against piping
• Ground water flow towards wells and drainage
of soil
• Calculation of seepage through the body of
earth dams and stability of slopes
• Determination of rate of settlement of a
saturated compressible soil layerors .
• Construction of foundation of structures below
ground water table.

7. DARCY’S LAW
• Darcy's law is an equation that describes the
flow of a fluid through a porous medium. The law
was formulated by Henry Darcy based on the
results of experiments on the flow of water
through beds of sand, forming the basis of
hydrogeology, a branch of earth sciences.
• Darcy's law states that there is a linear
relationship between flow velocity (v) and
hydraulic gradient (i) for any given saturated,
isotropic and homogeneous soil under steady
laminar flow conditions, i.e, flow velocity through
a porous media is proportional to hydraulic
gradient.

8. Mathametical Expression of Darcy’s Law
If the rate of flow is q
(volume/time) through cross-
sectional area (A) of the soil
mass, Darcy's Law can be
expressed as
v ∞ i
Or, v = q/A = k.i
where k = permeability of the
soil
i = ∆h/L = Hydraulic Gradient
∆h = difference in total heads
L = length of the soil mass

9. Coefficient of permeability(k)
Darcy’s Law v = q/A = k.i
When hydraulic gradient is unity, i.e. i = 1, q
= kA Or v = k, since v=q/A
From this relation we can define Coefficient
of permeability(k) as the average velocity of
flow that will occur through the total cross
sectional area of soil under unit hydraulic
gradient.
Unit of k is cm/sec. M/day etc.

10. Coefficient of Permeability
Permeability (k) is an
engineering property of
soils and is a function of
the soil type. Its value
depends on the average
size of the pores and is
related to the distribution of
particle sizes, particle
shape and soil structure.
For different soil types as
per grain size, the orders of
magnitude for permeability
are as follows:
Soil k (cm/sec)
Gravel 100
Coarse sand 100to 10-1
Medium sand 10-1to 10-2
Fine sand 10-2to 10-3
Silty sand 10-3to 10-4
Silt 1 x 10-5
Clay 10-7to 10-9

11. Factors affecting permeability of soils
• Particle Size : It was studied by Allen Hazen that the
coefficient of permeability (k) of a soil is directly
proportional to the square of the particle size (D). Thus
|permeability of coarse grained soil is very large as
compared to that of fine grained soil. The permeability of
coarse sand may be more than one million times as
much that of clay.
• Impurities in water : Any foreign matter in water has a
tendency to plug the flow passage and reduce the
effective voids and hence the permeability of soil.
• Void ratio (e) : For a given soil, the greater the void ratio,
the higher the value of the coefficient of permeability.
Here 'e' is the void ratio.

12. contd...
• Degree of Saturation : If the soil is not fully saturated, it
contains air pockets. The permeability is reduced due to the
presence of air which causes a blockage to the passage of
water. Consequently, the permeability of a partially saturated
soil is considerably smaller than that of fully saturated soil.
In fact, Darcy's Law is not strictly applicable to such soils.
• Adsorbed water : Fine grained soils have a layer of
adsorbed water strongly attached to their surface. This
adsorbed layer is not free to move under gravity. It causes
an obstruction to the flow of water in the pores and hence
reduces the permeability of soils.
• Entrapped air and organic matter : Air entrapped in the soil
and organic matter block the passage of water through soil,
hence permeability considerably decreases. In permeability
tests, the sample of soil used should be fully saturated to
avoid errors.

13. CONSTANT HEAD PERMEABILITY TEST
In the constant head
permeameter, the head
causing flow through the
specimen remains constant
throughout the test. The
coefficient of permeability (k)
is obtained from the relation
Where q= discharge,
Q=total volume of water,
t=time period, h=head
causing flow, L= length of
specimen, A= cross-
sectional area.

14. VARIABLE HEAD PERMEABILITY TEST
The variable head
permeameter is used to
measure the permeability of
relatively less pervious soils.
The coefficient of
permeability is given by
Where h1 = initial head, h2
= final head, t= time interval,
a= cross-sectional area of the
liquid stand pipe, A=cross-
sectional area of the
specimen, L= length of
specimen.

15. NEED AND SCOPE OF PERMEABILITY
TESTS OF SOIL
The test results of the permeability experiments
are used:
1.To estimate ground water flow.
2.To calculate seepage through dams.
3.To find out the rate of consolidation and settlement of
structures.
4.To plan the method of lowering the ground water table.
5.To calculate the uplift pressure and piping.
6.To design the grouting.
7.And also for soil freezing tests.
8.To design pits for recharging.