Chapter 1
Soil Deposits and Grain-Size Analysis
Introduction
Geotechnical Engineering is the study of the behavior of soils under the influence of loading forces and
soil-water interactions.
The term "soil" can have different meanings, depending upon the field in which it is considered.
To a geologist, it is the material in the relative thin zone of the Earth's surface within which roots occur, and
which are formed as the products of past surface processes. The rest of the crust is grouped under the term
"rock".
To an engineer, it is a material that can be:
built on: foundations of buildings, bridges
built in: basements, culverts, tunnels
built with: embankments, roads, dams
supported: retaining walls
Soil Mechanics is a discipline of Civil Engineering involving the study of soil, its behavior and application
as an engineering material.
Physical weathering reduces the size of the parent rock material, without any change in the original
composition of the parent rock.
Origin and Characteristics of Soil Deposits
Rock - Harder, consolidated material (parent material for all soil)
Soil - Unconsolidated deposits of particulate material.
Types of Rock
1. Igneous Rock - from molten rock that has hardened during cooling
a. Intrusive - formed slowly under high pressure, large crystals
b. Extrusive - formed rapidly or under low pressure small crystals
2. Sedimentary Rock - Deposits of soil particles, precipitate or organisms that are cemented together
a. Sandstone - quartz or rock fragments
b. Shale - composed of very fine grained material
c. Limestone - crystalline calcium carbonate
d. Dolestone - harder types of limestone
2
3. Metamorphic Rock - Igneous or sedimentary rock that is changed chemically by both high pressure and
heat.
a. Limestone to marble
b. Sandstone to quartzite
c. Shale to slate or schist
d. Coal to diamond
Decomposition of Rock to Soil
1. Igneous rocks
a. Intrusive rocks (acidic) - decompose to coarse grained soils, sand and gravel.
b. Extrusive rocks (basic) - decompose to fine grained soils, clay and silt.
2. Sedimentary rocks
a. sandstone to sand and gravel
b. shale to silts and clay
c. limestone to silt and clay
3. Metamorphic rocks - decompose to the soil that the parent rock would decompose to.
Soil Types According to Geologic Origin
1. Residual Soils - soil that originates from weathered rock and remains at its original site.
2. Transported Soils
a. Gravity - material moves down sides of hills
b. Wind Blown Deposits
1) Sand - blown relatively short distances into dunes or ridges
2) Silts (Loess) - blown large distances (hundreds of miles) to form large accumulations
Example - Mississippi river valley consisting of near vertical bluffs
c. Glacial Deposits - soil transported either directly or indirectly by glaciers.
d. River Deposits (alluvial deposits) - soils carried by flowing water.
e. Lake Deposits - Sand and gravel deposited along edges forming beaches due to wave action. Fine
size particles are deposited in the center. The fines are generally soft, compressible deposits referred
to as lacustrine deposits.
f. Marine Sediments - silt and clay combined with organisms to form weak deposits of clay. These
occur along many coastlines.
g. Beach Deposits - coarse materials deposited because of currents wave action.
h. Swamp and Marsh Deposits - contain large amounts of organics and silts and clay. Usually black
and has an odor. Will usually be soft and compressible and is not suited for engineering purposes.
3
Soil - Particle Size
The sizes of particles that make up soil vary over a wide range. To describe soils by their particle sizes,
several organizations have developed particle-size classifications.
Table 1.1
Particle-Size Classifications
Name of organization
Gravel
Massachusetts Institute of Technology (MIT) >2
U.S. Department of Agriculture (USDA)
>2
American Association of State Highway and
Transportation Officials (AASHTO)
76.2 to 2
Unified Soil Classification System
76.2 to 4.75
(U.S. Army Corps of Engineers, U.S.(i.e., silts and clays)
Bureau of Reclamation, and American
Society for Testing and Materials)
Grain size (mm)
Sand
Silt
2 to 0.06
0.06 to 0.002
2 to 0.05
0.05 to 0.002
2 to 0.075
4.75 to 0.075
Clay
<0.002
<0.002
0.075 to 0.002 <0.002
Fines
<0.075
4 Major Categories of Soils
1. Gravels are pieces of rocks with occasional particles of quartz, feldspar, and other minerals.
2. Sand particles are made of mostly quartz and feldspar. Other mineral grains also may be present at times.
3. Silts are the microscopic soil fractions that consist of very fine quartz grains and some flake-shaped
particles that are fragments of micaceous minerals.
4. Clays are mostly flake-shaped microscopic and submicroscopic particles of mica, clay minerals, and other
minerals.
Specific Gravity, GS
Specific gravity is defined as the ratio of the unit weight of a given material to the unit weight of water. The
specific gravity of soil solids is often needed for various calculations in soil mechanics.
Table 1.2
Specific Gravity of Common Minerals
Mineral
Specific gravity, Gs
Quartz
2.65
Kaolinite
2.6
Illite
2.8
Montmorillonite
2.65–2.80
Halloysite
Potassium feldspar
Sodium and calcium feldspar
Chlorite
Biotite
2.0–2.55
2.57
2.62–2.76
2.6–2.9
2.8–3.2
Muscovite
Hornblende
Limonite
Olivine
2.76–3.1
3.0–3.47
3.6–4.0
3.27–3.7
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Mechanical Analysis of Soil
Mechanical analysis is the determination of the size range of particles present in a soil, expressed as a
percentage of the total dry weight.
Two methods generally used to find the particle-size distribution of soil:
(1) Sieve analysis—for particle sizes larger than 0.075mm in diameter, and
(2) Hydrometer analysis—for particle sizes smaller than 0.075 mm in diameter.
Sieve Analysis
Sieve analysis consists of shaking the soil sample through a set of sieves that have progressively smaller
openings. U.S. standard sieve numbers and the sizes of openings are given in Table 2.3.
Table 1.3 U.S. Standard Sieve Sizes
Sieve no.
Opening (mm)
4
4.75
5
4.00
6
3.35
7
2.80
8
2.36
10
2.00
12
1.70
14
1.40
16
1.18
18
1.00
20
0.850
25
0.710
30
0.600
Sieve no.
35
40
50
60
70
80
100
120
140
170
200
270
Opening (mm)
0.500
0.425
0.355
0.250
0.212
0.180
0.150
0.125
0.106
0.090
0.075
0.053
To conduct a sieve analysis, one must first oven-dry the soil and then break all lumps into small particles.
The soil then is shaken through a stack of sieves with openings of decreasing size from top to bottom (a pan
is placed below the stack).
Steps in Calculating the Percentage Passing for each sieve:
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 the soil: M1+M2+. . .+Mi+. . . +Mn+Mp=M
3. Determine the cumulative mass of soil retained above each sieve. For the ith sieve, itis 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
5
F
 M   M1  M 2  ...  M i 
x 100
M
Once the percent finer for each sieve is calculated (step 5), the calculations are plotted on semi-logarithmic
graph paper (Figure 2.2) with percent finer as the ordinate (arithmetic scale) and sieve opening size as the
abscissa (logarithmic scale). This plot is referred to as the particle-size distribution curve.
Fig. 1.1 Particle-size distribution curve
Particle-Size Distribution Curve
A particle-size distribution curve can be used to determine the following four parameters for a given soil
(Figure 1.2):
1. Effective size (D10) is the diameter in the particle-size distribution curve corresponding to 10% finer. The
effective size of a granular soil is a good measure to estimate the hydraulic conductivity and drainage
through soil.
2. Uniformity coefficient (Cu) is defined as
Cu 
D60
D10
whereD60= diameter corresponding to 60% finer.
3. Coefficient of gradation (Cc) is defined as
Cc 
D230
D60 x D10
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4. Sorting coefficient (S0) is another measure of uniformity and is generally encountered in geologic works
and expressed as
So 
D75
D25
Fig. 1.2 Definition of D75, D60, D30, D25, and D10
The percentages of gravel, sand, silt, and clay-size particles present in a soil can be obtained from the
particle-size distribution curve. As an example, we will use the particle-size distribution curve shown in
Figure 2.3 to determine the gravel, sand, silt, and clay size particles as follows (according to the Unified Soil
Classification System—see Table 2.1):
Fig. 1.3 Particle-size distribution curve - sieve analysis and hydrometer analysis
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The particle-size distribution curve shows not only the range of particle sizes present in a soil, but also the
type of distribution of various-size particles. Such types of distributions are demonstrated in Figure 2.4.
Curve I represents a type of soil in which most of the soil grains are the same size. This is called poorly
graded soil.
Curve II represents a soil in which the particle sizes are distributed over a wide range, termed well graded
soil. A well-graded soil has a uniformity coefficient greater than about 4 for gravels and 6 for sands, and a
coefficient of gradation between 1 and 3 (for gravels and sands).
Curve III represents a combination of two or more uniformly graded fractions. This type of soil is termed
gap graded soil.
Fig. 2.4 Different types of particle-size distribution curves
Example 1.1
Following are the results of a sieve analysis. Make the necessary calculations and draw a particle-size
distribution curve and determine the following:
a. D10, D30, and D60
b. Uniformity coefficient, Cu
c. Coefficient of gradation, Cc
d. the percentages of gravel, sand, silt, and clay-size particles present. Use the Unified Soil
Classification System.
U. S. sieve no.
4
10
20
40
60
80
100
200
Pan
Mass of soil retained
on each sieve (g)
0
40
60
89
140
122
210
56
12
Cumulative Mass (g)
Mass Passing
% Finer
0
40
100
189
329
451
661
717
729
729
689
629
540
400
278
68
12
0
100
94.513
86.283
74.074
54.87
38.134
9.328
1.646
0
8
Solution:
(a) By linear interpolation
0.425  D60
14.074

0.175
19.204
D60  0.297 mm
0.18  D30
8.134

0.03
28.806
D30  0.172 mm
0.18  D10
28.134

0.03
28.806
D10  0.151 mm
(b) Cu 
(c ) Cc 
D60
0.297 mm

 0. 146
D10
0.151 mm
D302
( 0.172 mm)2

 0.66
 0.297   0.151
D60 x D10
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Hydrometer Analysis
The hydrometer method was originally proposed in 1926 by Prof. Bouyoucos of Michigan Agriculture
College, and later modified by Casagrande (1931).The density of the suspension is measured with a
hydrometer at determined time intervals. The coarsest diameter of particles in suspension at a given time and
the percentage of particles finer than that coarsest (suspended) diameter are computed based on Stokes'
formula.
Stokes' Law
Stokes (1856), an English physicist, proposed an equation for determining the terminal velocity of a falling
sphere in a liquid. If a single sphere is allowed to fall through a liquid of indefinite extent, the terminal
velocity, v can be expressed as,
v
 s  w 2
D
18 
where,
v =terminal velocity of fall of a sphere through a liquid =
distance L

time
t
s = unit weight of solid sphere
w = unit weight of liquid
 = absolute viscosity of liquid
D = diameter of sphere.
After substituting for v, we have
D
18
 Gs 1  w
L
t
If L is in cm, t is in min, w in g/cm3,  in (g – sec)/cm2 and D in mm, the above equation may be written as
D  mm 
10
D

18
 Gs 1  w
30 
 Gs 1  w
L
t x 60
L
t
or
DK
L
t
10
where, K =
30 
 Gs 1
Table 2.4 gives the values of K for the various values of Gs at different temperatures T
Table 2.4 Values of K for several specific gravity of solids and temperature combinations
Table 2.5 Properties of distilled water
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ASTM 152H Hydrometer
Definition of L in hydrometer test
Fig. 2.5
The value of L (cm) for the ASTM 152H hydrometer can be given by the expression
V 
1
L  L1   L2  B 
2
A
where
L1= distance along the stem of the hydrometer from the top of the bulb to the mark for a hydrometer
reading (cm)
L2= length of the hydrometer bulb = 14 cm
VB = volume of the hydrometer bulb = 67 cm3
A= cross-sectional area of the sedimentation cylinder = 27.8 cm2
The value of L1 is 10.5 cm for a reading of R= 0 and 2.3 cm for a reading of R= 50. Hence, for any reading R,
L1 10.5 
10.5  2.3 R 10.5  0.164 R (cm)
50
Thus,
1
67 
L  10.5  0.164 R  14 
  16.29  0.164 R
2
27.8 
where R = hydrometer reading corrected for the meniscus
The variations of L with the hydrometer readings R are given in Table 2.6.
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Table 2.6Variation of L with Hydrometer Reading — ASTM 152H Hydrometer
Temperature Correction
The readings of the hydrometer are affected by the rise in temperature during the test. The temperature
correction is a constant. If the temperature during the test is quite high, the density of water will be equally
less and hydrometer will sink too deep.
Zero Correction
The use of a dispersing agent also affects the hydrometer reading. Correction for this can be obtained by
using a sedimentation cylinder of water from the same source and with the same quantity of dispersing agent
as that used in the soil-water suspension to obtain a zero correction.
The actual hydrometer reading Ra has to be corrected as follows:
1. Correction for meniscus Cm.
2. Zero correction Co and temperature correction CT for obtaining percent finer.
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R = Ra + Cm
(reading for obtaining D)
Rc = Ra – Co + CT
(reading for obtaining percent finer)
Table 2.7
Temperature correction factors, CT
Determination of percent finer
The 152 H hydrometer is calibrated for a suspension with a specific gravity of solids Gs = 2.65. If the
specific gravity of solids used in the suspension is different from 2.65, the percent finer has to be corrected
by the factor Csg expressed as
Csg 
1.65 Gs
2.65  Gs 1
Typical values of Csg are given in Table 2.8.
The percent finer with correction factor Csg may be expressed as
% finer, 
where
Csg Rc
Ms
x 100
Rc = grams of soil in suspension at some elapsed time t
Ms = mass of soil used in suspension in grams (not more than 60 g for 152 H hydrometer)
Table 2.8
Temperature correction factors, Csg for specific gravity of solids
14
Example 1.2
In a hydrometer test, the results are as follows: Gs= 2.60, temperature of water = 24 °C, and R = 43 at 60 min
after the start of sedimentation. What is the diameter, D, of the smallest-size particles that have settled
beyond the zone of measurement at that time (that is, t = 60 min)?
.
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