50
Origin of Soil and Grain Size Chapter (2) Instructor : Dr. Jehad Hamad 2017-2016
Origin of Soil and Grain
Size
Chapter (2)
Instructor : Dr. Jehad Hamad
2017-2016
Soil
Soil is more or less taken for granted by the average person. It
makes up the ground on which we live, it is for growing crops, and
it makes us dirty. Beyond these observations, most people are not
overly concerned with soil. There are, however, some people who
are deeply concerned. These include certain engineers as well as
geologists, contractors, hydrologists, farmers, agronomists, soil
chemists, and others .
Most structures of all types rest either directly or indirectly upon
soil, and proper analysis of the soil and design of the structure’s
foundation are necessary to ensure a safe structure free of undue
settling and/or collapse. A comprehensive knowledge of the soil in
a specific location is also important in many other contexts.
Thus, study of soils should be an important component in the
education of civil engineers.
ROCKS—THE SOURCES OF SOILS
Soil is composed of particles, large and small, and it may be necessary to
include as “soil” not only solid matter but also air and water. Normally, the
particles are the result of weathering (disintegration and decomposition) of
rocks and decay of vegetation.
Some soil particles may, over a period of time, become consolidated under the
weight of overlying material and become rock. In fact, cycles of rock
disintegrating to form soil, soil becoming consolidated under great pressure and
heat to form rock, rock disintegrating to form soil, and so on have occurred
repeatedly throughout geologic time. The differentiation between soil and rock
is not sharp; but from an engineering perspective, if material can be removed
without blasting, it is usually considered to be “soil,” whereas if blasting is
required, it might be regarded as “rock.”
Rocks can be classified into three basic groups that reflect their origin and/or
method of formation: igneous, sedimentary, and metamorphic
Rock cycle and the origin of soil
The final products due to weathering
are soils
Note
igneous sedimentary
formed by cooling of
molten magma (lava)
formed by gradual
deposition, and in layer formed by alteration
of igneous &
sedimentary rocks by
pressure/temperature
e.g., limestone, shale
e.g., marble
e.g., granite
metamorphic
different processes threeRocks formed by one of these
~ in situ weathering (by
physical & chemical
agents) of parent rock
~ weathered and
transported far away
by wind, water and ice.
Soil-Separate-Size Limits by Various Systems
Clay Minerals
Clay minerals are made of two distinct structural units.
0.26 nm
0.29 nm
Silicon tetrahedron Aluminium Octahedron
Several tetrahedrons joined together form a tetrahedral sheet.
tetrahedron
: sheet by tetrahedralFor simplicity, let’s represent silica
Si
: sheet by octahedralalumina and
Al
Different combinations of tetrahedral and octahedral sheets form different clay minerals:
Different combinations of tetrahedral and octahedral sheets form different clay minerals:
2. Clay Mineral (e.g., montmorillonite, illite)
Si
Al
Si
Al
Si
Al
Si
Al
joined by strong H-bond
no easy separation
0.72 nm
Typically 70-100 layers
joined by oxygen
sharing
Si
Al
Si
Si
Al
Si
Si
Al
Si
0.96 nm
weakjoined by
van der Waal’s bond
easily separated
by water
Si
Al
Si
Si
Al
Si
Si
Al
Si
0.96 nm
joined by K+ ions
fit into the hexagonal
holes in Si-sheet
+ + +
+
+
+ +
+
+
+
+
+
+
+
+ + +
+
+
+ +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+
+ + +
+
+ +
+
+
+
+
+
+
+
+ + +
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+
- -
- -
- -
- -
- -
- -
- -
Cation concentration drops with distance from clay particle
19
- -
- -
- -
- -
- -
- -
- -
It is the ratio of the unit weight of a given material to the
unit weight of water
WaterofVolumeEqualanofWeight
ceSubsaofWeightGravitySpecific
tan
Unit weight of Water, w
w = 1.0 g/cm3 (strictly accurate at 4° C)
w = 62.4 pcf
w = 9.81 kN/m3
Note
The size distribution is often of critical importance
to the way the material performs in use.
Sieve analysis consists of shaking the soil sample
through a set of sieves that have progressively smaller
openings.
1
• Determine the mass of soil retained on each sieve (i.e,M1,M2…..Mn)and in the pan (i.e.,Mp)
2
• Determine the total mass of soil : M1+M2+..+Mi+..+Mn+Mp=∑M.
3
• Determine the cumulative mass pf soil retained above each sieve . For the ith sieve, it is M1+M2+…Mi
4
• The mass of soil passing the ith sieve is ∑M-(M1+M2+…+Mi)
5
• The percent of soil passing the ith sieve (or percent finer ) is
100*
)..( 21
M
MMMMF
i
• This parameter good measure to estimate the hydraulic conductivity and drainage through soil.
Effective Size (D10)
Uniformity coefficient
(Cu)
Coefficient of graduation
(Cz)
• This parameter is another measure of uniformity Sorting coefficient
(S0)
10
60
D
DCu
1060
2
30
*DD
DCz
25
750
D
DS
What is the Cu for a soil
with only one grain size?
D
Fin
er
Grain size distribution
110
60 D
DCu
The results of a particles size analysis are shown in the
)g469: The total mass Giventable below (
1-Plot the particle size distribution curve
2-Determine the coefficient of uniformity
3-Determine the coefficient of graduation
The calculations are summarized in the
table below
Particle size distribution curve
• This parameter good measure to estimate the hydraulic conductivity and drainage through soil.
Effective Size (D10)
Uniformity coefficient
(Cu)
Coefficient of graduation
(Cz)
• This parameter is another measure of uniformity
Sorting coefficient
(S0)
10
60
D
DCu
1060
2
30
*DD
DCz
25
750
D
DS
The coefficient of uniformity = 3.1336.0
8.4
10
60 D
DCU
The coefficient of graduation = 2.336.0*8.4
35.2 2
1060
2
30 DD
DCz
What is the Cu for a soil
with only one grain size?
D
Fin
er
Grain size distribution
110
60 D
DCu
Hydrometer analysis is based on the principle of
sedimentation of soil grains in water. When a
soil specimen is dispersed in water, the particles
settle at different velocities, depending on their
shape, size and weight, and the viscosity of the
water.
Stock’s Law
V1 V2 <
Bouncy Force
F = V ρ g
Based on the principle of sedimentation of soil grains
in water and bouncy principle.
2
18Dv ws
v = velocity
= density of soil particles
= density of water
= viscosity of water
D = diameter of soil particles
where
s
w
t
L
GD
G
t
LvD
ws
wss
wsws
)1(
18
1818
If the units of are (g.sec)/cm2, is in g/cm3, t is in min,
and D is in mm , then
w
60*(min)
)(
/1
/sec.18
10
)(3
2
t
cmL
cmgG
cmgmmD
ws
Assume to be approximate equal to 1g/cm3, so that w
130
(min)
)()(
sGK
where
t
cmLKmmD
R
L
L1
L2
=16.29+0.164R
The shape particles present in a soil mass is equally as
important as the particle size distribution because it has
significant influence on the physical properties of a given soil.
Bulky
Needle Flaky
Rounded
Angular
sphereinscribedimumtheofRadius
edgesandcornersofradiusAverage
A
max
p
e
L
DA
36
VDe
De = equivalent diameter of particle
V = volume of particle
Lp = length of particle
