5. Permeability Test
Permeability is defined as the ability of water to flow through a soil volume. It controls the
strength and deformation behavior of soil.
In the design of engineering projects, one of the most important soil properties of the soils
engineer is permeability. To some degree, permeability will play a role in the design of almost
any structure. For example, in designs that make use of earthen materials (soils and rock) the
permeability of these materials will usually be of great importance.
Soils are permeable (water may flow through them) because they consist not
only of solid particles, but a network of interconnected pores. The degree to which soils are per
meable depends upon a number of factors, such as soil type, grain size distribution and soil
history. This degree of permeability is characterized by the coefficient of permeability.
In the laboratory, twocommon tests are generally used to determine this soil property. These two
tests are the falling head permeability test and the constant head permeability test. Which test is
used depends upon the type of soil to be tested. For soils of high permeability (sands and
gravels) a constant head test is used. For soils of intermediate to low permeability (silts and
clays) a falling head test is used.
NEED AND SCOPE
The knowledge of this property is much useful in solving problems involving yield of water
bearing strata, seepage through earthen dams, stability of earthen dams, and embankments of
canal bank affected by seepage, settlement etc.
Part One: Constant Head Permeability Test in Sand
OBJECTIVE:
This test method covers determination of the coefficient of permeability by a constant-head
method for the laminar flow of water through granular soils.
The rate of flow of water through a soil specimen of gross cross-sectional area, A, can be
expressed as
𝑣𝑜𝑙𝑢𝑚𝑒
q=
𝑡𝑖𝑚𝑒
𝑣𝑜𝑙𝑢𝑚𝑒
V* A=
𝑡𝑖𝑚𝑒
𝐻
From Darcy law, V= K* i where i =
𝐿
, 𝑄.𝐿
So, KT=
𝐻.𝐴.𝑡
Where:
K = coefficient of permeability at temperature T, cm/sec.
L = length of specimen in centimeters.
t = time for discharge in seconds.
Q = volume of discharge in cm3.
A = cross-sectional area of specimen in cm2.
D = inside diameter of the specimen in centimeters.
H = hydraulic head difference in cm.
EQUIPMENT:
1. Constant head permeability cell.
2. Graduated cylinder (250 ml or 500 ml)
3. Thermometer
4. Rubber tubing
5. Stop watch
A schematic diagram of constant head permeability is shown in the Figure below. This can be
assembled in the laboratory at very little cost. It essentially consists of a plastic soil specimen
cylinder, two porous stones and constant head chamber, the plastic cylinder may have an inside
diameter of 2.5 in. (63.5 mm). The length of the specimen tube may be about 12 in. (304.8 mm).
Permeability is defined as the ability of water to flow through a soil volume. It controls the
strength and deformation behavior of soil.
In the design of engineering projects, one of the most important soil properties of the soils
engineer is permeability. To some degree, permeability will play a role in the design of almost
any structure. For example, in designs that make use of earthen materials (soils and rock) the
permeability of these materials will usually be of great importance.
Soils are permeable (water may flow through them) because they consist not
only of solid particles, but a network of interconnected pores. The degree to which soils are per
meable depends upon a number of factors, such as soil type, grain size distribution and soil
history. This degree of permeability is characterized by the coefficient of permeability.
In the laboratory, twocommon tests are generally used to determine this soil property. These two
tests are the falling head permeability test and the constant head permeability test. Which test is
used depends upon the type of soil to be tested. For soils of high permeability (sands and
gravels) a constant head test is used. For soils of intermediate to low permeability (silts and
clays) a falling head test is used.
NEED AND SCOPE
The knowledge of this property is much useful in solving problems involving yield of water
bearing strata, seepage through earthen dams, stability of earthen dams, and embankments of
canal bank affected by seepage, settlement etc.
Part One: Constant Head Permeability Test in Sand
OBJECTIVE:
This test method covers determination of the coefficient of permeability by a constant-head
method for the laminar flow of water through granular soils.
The rate of flow of water through a soil specimen of gross cross-sectional area, A, can be
expressed as
𝑣𝑜𝑙𝑢𝑚𝑒
q=
𝑡𝑖𝑚𝑒
𝑣𝑜𝑙𝑢𝑚𝑒
V* A=
𝑡𝑖𝑚𝑒
𝐻
From Darcy law, V= K* i where i =
𝐿
, 𝑄.𝐿
So, KT=
𝐻.𝐴.𝑡
Where:
K = coefficient of permeability at temperature T, cm/sec.
L = length of specimen in centimeters.
t = time for discharge in seconds.
Q = volume of discharge in cm3.
A = cross-sectional area of specimen in cm2.
D = inside diameter of the specimen in centimeters.
H = hydraulic head difference in cm.
EQUIPMENT:
1. Constant head permeability cell.
2. Graduated cylinder (250 ml or 500 ml)
3. Thermometer
4. Rubber tubing
5. Stop watch
A schematic diagram of constant head permeability is shown in the Figure below. This can be
assembled in the laboratory at very little cost. It essentially consists of a plastic soil specimen
cylinder, two porous stones and constant head chamber, the plastic cylinder may have an inside
diameter of 2.5 in. (63.5 mm). The length of the specimen tube may be about 12 in. (304.8 mm).
