About permeability of soil and tests

2. Soil water:
Water present in the void spaces of a soil mass is called soil water
Soil water may be In the form of free water , gravitational water and held w

3. Free surcace water
Gravitational water – water that drains freely through the
soil by force of gravity

4. HELD WATER: 1. adsorbed water is thin film of around the mineral

5. 2. Structural water: water that is chemically combined as a part of the crystallin
the mineral of the soil grains.

6. Capillary tightly in small capillary pores
by H bonding
Height of capillary rise inversely
related to radius of the tube
Smaller pore diameter, greater
the movementwater – held

7. • Capillary water & Saturation
Percentage
– Capillary water – held tightly in small
capillary pores by H bonding
• Water in minute tubes that will rise through
soil matrix to needed areas
• Height of capillary rise inversely related to
radius of the tube
– Smaller pore diameter, greater the movement

8. What is permeability?
• Property of a soil which permits the flow of water
• Permeability is defined as the property of a porous
material which permits the passage or seepage of water
through its interconnecting voids.
• It is a very important Engineering property
gravels highly permeable
stiff clay least permeable

10. • 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 layer
Permeability through soil is important for the
following engineering problems:

11. Flow of water through soils may either be a
laminar flow or a turbulent flow
Each fluid particle travel
along a definite path
which never crosses
the path of any other
particle
Paths are irregular and
twisting, crossing at
random

12. Permeability
• Three types of permeability
• Absolute permeability - the permeability of a porous
medium with only one fluid present (single-phase
flow).
• When two or more fluids are present permeability of
the rock to a flowing fluid is called effective
permeability (ko, kg, kw).
• Relative permeability is the ratio of absolute and
effective permeabilities kro=ko/k, krg=kg/k, krw=kw/k.

13. Coefficient Of
Permeability
Depends not only on the
properties of soil but also
on the properties of water
Absolute permeability
Independent of the
properties of water
It depends only on the
characteristics of soil
The absolute permeability
only depends on the
geometry of the pore-
channel system.
Relative permeability is the ratio of
effective permeability of a particular fluid to its
absolute permeability.

14. DARCY’S LAW
The law of flow of water through soil was first studied by Darcy in 1856.
The Darcy’s law is,
“For laminar flow through saturated soil mass, the discharge per unit time is
proportional to the hydraulic gradient”.
q = k.i.A
= k.i = v
v = k.i ……. Darcy’s Law
Where, q = Discharge per unit time (rate of flow)
A = Total c/s area of soil mass
i = Hydraulic gradient = h/L
k = Darcy’s coefficient of Permeability
v = Velocity of flow (discharge velocity)

16. FACTORS AFFECTING PERMEABILITY OF SOILS
The following factors affect the permeability of soils:-
1. Particle size
2. Properties of pore fluid.
3. Void ratio of soil.
4. Shape of particles.
5. Structure of soil mass.
6. Degree of saturation.
7. Absorbed water.
8. Impurities in water.

17. Variable-head
Permeability
Test
Determination Of Coefficient Of
Permeability
Laboratory
Methods
Constant-head
Permeability
Test

18. Typical Soil Permeability
The constant head test method is used for permeable soils (k>10-4
cm/s), and the falling head test is mainly used for less permeable soils
(k<10-4 cm/s).

19. Test Apparatus
• Permeameters
• Ruler
• Tamper
• Balance
• Watch (or Stopwatch)
• Thermometer
• Filter
Constant Head Permeability Test

21. Test Procedure:-
1. Measure internal dimensions of the mould. Apply a little grease on the inside to the
mould.
2. Take about 2.5kg of the soil, from a thoroughly mixed wet soil, in the mould.
Compact the soil at the required dry density using a suitable compacting device.
3. Remove the collar and base plate. Trim the excess soil level with the top of the
mould.
4. Clean the outside of the mould. Find the mass of the soil in the mould. Take a
small specimen of the soil in container for the water constant determination.
5. Saturate the porous stones.
6. Place the porous stone (disc) on the drainage base and keep a filter paper on the
porous stone.
7. Place the mould with soil on the drainage base.
8. Place a filter paper and a porous stone on the top of specimen.
9. Connect the constant head tank to the drainage cap inlet.
10. Open the stop cock, and allow the water downward so that all the air is removed,
then close the stop cock.
11. Now, again open the stop cock and at the same time start the stopwatch. Collect
the water flowing out of the base in a measuring flask for some convenient time
interval.
12. Measure the difference of head (h) in levels between the constant
head tank and the outlet in the base.

22. •c/s area of specimen = A = (π/4) x D2 (cm2)
•Volume of mould = V = (π/4) x D2 x L (cm3)
•Mass of wet soil in the mould = M = M2 – M1
Where, M1 = mass of empty mould.
M2 = mass of mould + wet soil
•Bulk density of soil, = _____ gm/cm3
• Dry density of soil, = _____ gm/cm3

23. • Quantity of water that flows under a given hydraulic gradient
through a soil sample of known length & cross sectional area in a
given time
• Water is allowed to flow through the cylindrical sample of soil
under a constant head
• For testing of pervious, coarse grained soils
K = Coefficient of permeability
Q = total quantity of water
t = time
L = Length of the coarse soil
Aht
QL
k 

24. FALLING HEAD PERMEABILITY TEST :-
Object:-
To determine the coefficient of permeability of a soil specimen by falling head
method.
Equipments:-
All the equipments required for the constant head permeability test.

25. Test Procedure:-
1. Prepare the remoulded soil specimen in the permeameter and saturate it.
2. Keep the permeameter mould in the bottom tank and fill the bottom tank with
water up to its outlet.
3. Connect the water inlet nozzle of the mould to the stand pipe filled with water.
Permit water to flow for some time till steady state of flow is reached.
4. Now open the valve of stand pipe and record the time (t) to fall the head from h1
to h2. Repeat this step at least twice.
Normally,
 Constant head permeability test is used for more permeable soils like
sand.
 Falling head permeability test is used for less permeable soils like
clay.

26. • Relatively for less permeable soils
• Water flows through the sample from a standpipe attached
to the top of the cylinder.
• The head of water (h) changes with time as flow occurs
through the soil. At different times the head of water is
recorded.
t = time
L = Length of the fine soil
A = cross section area of soil
a= cross section area of tube
K = Coefficient of permeability
2
1
10log
30.2
h
h
At
aL
k 

27. Permeability in Stratified Soils
H
HkHkHk
k
iHkHkHkiHkq
kiAq
nn
x
nnx






2211
2211 )(
Flow parallel to the plans of stratification

28. Flow normal to the plans of stratification