This document defines permeability as the property of soil that allows water to flow through due to interconnected voids. It describes two laboratory methods to measure permeability - constant head and falling head tests. Darcy's law is explained, relating flow rate to permeability and hydraulic gradient. Typical permeability values are given for different soil types from gravel to clay. The constant head test procedure and calculations are outlined, along with data sheets to record measurements.

1. Erbil Polytechnic University Soil Mechanics 3rd
Stage /Civil Eng. Dept.
Prepared by: Mr. Diyar I. Karash Technical Engineering College Page 1
PERMEABILITY OF SOILS
DEFINITION
Permeability is the property of soil which permits flow of water through it from
points of high energy to points of low energy, due to existence of interconnected
voids.
INTRODUCTION
 The degree of permeability is characterized by the permeability coefficient k. Two
general laboratory methods are available for determining the coefficient of
permeability of a soil directly. These are the constant head method for soils of high
permeability (e.g. gravel & sand) and the falling head method for soils of
intermediate and low permeable (e.g. silts and clays).
 A soil is highly pervious when water can flow through it easily. In an impervious
soil, the permeability is very low and water cannot easily flows through it.
DARCY’S LAW
In 1856 Darcy defined a coefficient of permeability, from a study of water passing
through a saturated porous medium. He demonstrated experimentally that for
laminar flow in a homogeneous soil, the velocity of flow (v) is given by:
v = k i
If the quantity of water (discharge) flowing through the soil in unit times is q,
then:
q = v A = k i A
Where:
q = quantity of fluid flowing through the soil in unit time (volume/ time).
k = coefficient of permeability (length/ time).
i = hydraulic gradient. i =
L
h

A = cross sectional area of soil mass includes both the solids and the voids.
 Coefficient of permeability is generally expressed in cm/sec or m/sec in SI units
and in ft/min or ft/ day in English units.

2. Erbil Polytechnic University Soil Mechanics 3rd
Stage /Civil Eng. Dept.
Prepared by: Mr. Diyar I. Karash Technical Engineering College Page 2
 The value of (k) varies widely for different soils. Some typical values for saturated
soils are given in table (1). The coefficient of permeability for unsaturated soils is
lower and increases rapidly with the degree of saturation.
Table (1): gives the typical values of the coefficient of permeability of different soils.
Soil type k (cm/s) Degree of permeability
Gravel Over 10-1 High
Sandy gravel, clean sand, fine sand 10-1 to 10-3 Medium
Sand, dirty sand, silty sand 10-3 to 10-5 Low
Silt, silty clay 10-5 to 10-7 Very low
Clay Less than 10-7 Practically Impermeable
Fluid viscosity: The coefficient of permeability is standardized at 20°C, and the coefficient
of permeability at any temperature T is related to k20:
k20 = kT
20

T
The variation of
20

T
with the test temperature is given in table (2).
Table (2): Variation of
20

T
.
Temperature, T
(°C)
20

T Temperature, T
(°C)
20

T
10 1.301 21 0.976
11 1.265 22 0.953
12 1.230 23 0.931
13 1.197 24 0.910
14 1.165 25 0.889
15 1.135 26 0.869
16 1.106 27 0.850
17 1.077 28 0.832
18 1.051 29 0.814
19 1.025 30 0.797
20 1.000

3. Erbil Polytechnic University Soil Mechanics 3rd
Stage /Civil Eng. Dept.
Prepared by: Mr. Diyar I. Karash Technical Engineering College Page 3
PERMEABILITY TEST
PURPOSES
Permeability is a very important engineering property of soils. Knowledge of
permeability is essential in a number of soil engineering problems:-
- To determine the amount of seepage through and beneath earth structures.
- To control seepage velocities such that fine particles of soil mass not eroded.
- Rate of settlement (consolidation) studies.
- The permeability of soils is also required in design of filter used to prevent
piping in hydraulic structures.
STANDARD LABORATORY TESTS
Two standard laboratory tests are used to determine the coefficient of permeability
of soil:
I- CONSTANT HEAD TEST
In the constant head test, water is made to flow through a column of soil under
application of a pressure difference which remains constant, i.e. under a constant
head. See Figure (1).
APPARATUS
1- Permeameter cell as shown in Figure (1), with, loading piston, perforated plates,
flow tube connections, piezometer connections and air bleed valve.
2- Glass piezometer tubes mounted on a stand with a graduated scale.
3- Graduated cylinder.
4- Inlet water tank with overflow to maintain a constant head.
5- Outlet water tank with overflow to maintain a constant head.
6- Supply of clean water.
7- Small tools, funnel, tamping rod…etc. (sample preparation)
8- Timer.
9- Thermometer.

4. Erbil Polytechnic University Soil Mechanics 3rd
Stage /Civil Eng. Dept.
Prepared by: Mr. Diyar I. Karash Technical Engineering College Page 4
10-Ruler (scale).
11-Vacuum device.
PROCEDURE
1- Remove a loading piston of permeameter cell, then measure the inside diameter,
height of it and the distance between centers of each piezometer connection
points.
2- Place graded gravel filter materials in the bottom of the cell to a depth about
40mm and level the surface, then put a wire gauze on it.
3- Place the sample in the permeameter cell by one of three following methods,
a) Compacting by rod. b) Dry pouring. c) Pouring through water.
4- After placing the sample in the cell measure the length of it at two or three
positions and calculate the average length (Ls).
5- Put a wire gauze on the top of the soil sample, then place graded gravel filter
materials in the top of the soil sample to a depth about 40mm and level the surface,
then assembly the top part of the permeameter cell.
6- Saturated the specimen by immersing in water for water (15 min) or by other
methods.
7- Connect the top of the cell to the inlet water tank and the base to the outlet
water tank; also connect the piezometer to the cell.
8- Turn on the water supply to the constant head device which should be at low
level.
9- Allow water to flow through the sample until the conditions appears to be steady
and water level in the piezometer tubes remains constant.
10- To start a test run, empty the measuring cylinder and start time at the instant
the cylinder is placed under the outlet overflow.
11- Record the clock time at which the first run is started.
12- Read the level of water in the piezometer tubes (h1, h2 and h3), and measure
the water temperature (T) in the outlet tank, when the level in the cylinder reaches
a predetermined level.
13- Empty the cylinder and make two to three repeat runs at about (5 min.)
intervals.

5. Erbil Polytechnic University Soil Mechanics 3rd
Stage /Civil Eng. Dept.
Prepared by: Mr. Diyar I. Karash Technical Engineering College Page 5
CALCULATION
1- Calculate
i = Δh/ L
k20 = kT
20

T
Where:
Q = volume of water collected.
A = area of cross section of the specimen.
Δh = the difference between elevation of water level inside piezometer tubes.
L = distance between piezometer connections.
t = duration of water collection.
i = hydraulic gradient.
2- Calculate unit weight of the soil.
Q
L
h
A
kT =
ht
A
QL

Δ
a
b
c
Air valve Loading piston
Perforated
plate
Wire
gauze

6. Erbil Polytechnic University Soil Mechanics 3rd
Stage /Civil Eng. Dept.
Prepared by: Mr. Diyar I. Karash Technical Engineering College Page 6
Fig (1): Principle of constant head test.
DISCUSSION
1- What is the effect of the temperature of the permeability? Explain in detail.
2- What is the effect of the shape of the soil particle on the permeability? Explain.
3- What is the effect of the void ratio on the permeability? Explain.
4- Find degree of permeability of the soil.

7. Erbil Polytechnic University Soil Mechanics 3rd
Stage /Civil Eng. Dept.
Prepared by: Mr. Diyar I. Karash Technical Engineering College Page 7
CONSTANT HEAD TEST DATA SHEET
Name: Class: Group No.
Weight of the sample = 1500 gm
Specimen height H = 18 cm
Specimen diameter D = 7.5 cm
Height above Datum:
Piezometer a ……………cm a ……………cm a ……………cm a ……………cm
b …………… cm b …………… cm b …………… cm b …………… cm
c …………… cm c …………… cm c …………… cm c …………… cm
Distance between a & b =
b & c =
a & c =
Hydraulic gradient between a and b =
a and c =
No.
Time for each
Measured flow
(min.) t
Measured flow
(cm)3 Q
Temperature
(Co )
1
2
3
4
5
kT =
ht
A
QL

Signature: Date: 25 /01/ 2017