Chapter 1-Geotechnical Engineering
Chapter 1-Geotechnical Engineering
Chapter 1-Geotechnical Engineering
GEOTECHNICS
Chapter 1:
Composition and Soil
Classification
TABLE OF CONTENTS
1.0 Introduction
1.1 Phase relationship of soil
- Basic definitions, weight and volume
relationships, etc.
1.2 Composition of soil
1.3 Simple Physical Properties of Soils and
Classification tests
1
1.0 Introduction
The purpose of this chapter is to introduce you
to soils
You will learn some basic descriptions of soils
and some fundamental, physical, soil properties
that you should retain for future use in this text
and in geotechnical engineering practice.
Soils, derived from the weathering of rocks, are
very complex materials and vary widely.
There is no certainty that all soils will have the
same properties within a few centimeters of its
current locations.
2
1.0 Introduction (continued…)
Learning objectives:
You will learn how to:
Define phases by volume and mass or weight;
Derive terminology and terms related to soil parameters;
Identify the relationship of partially saturated, saturated
and dry; and
Solve problems related to the phase relationship.
Describe and classify soils
Determine particle size distribution in a soil mass
Determine the proportions of the main constituents in a
soil
Determine index properties of soils
Determine maximum dry unit weight and optimum water
content
3
1.1 Phase relationship (continued…)
Soils consist of solids and voids.
The solids are the particles and the voids
are the spaces between the particles,
which can be filled with gases (air) or
liquids (water) or both.
We are going to take a volume of soil, V,
and put all the solids together, all the liquid
components of the voids together and all
the gas components of the voids together.
4
1.1 Phase relationship (continued…)
5
1.1 Phase relationship (continued…)
(continued…)
1.1 Phase relationship (continued…
6
(continued…)
1.1 Phase relationship (continued…
Relationships
among Unit
Weight, Void
Ratio,
Moisture
Content, and
Specific
Gravity
7
Relationships
among Unit
Weight,
Porosity, and
Moisture
Content
(partially
saturated)
Relationships
among Unit
Weight,
Porosity, and
Moisture
Content
(fully
saturated)
8
Some derivations using Basic
Definitions
Derive: eSr = w. Gs
Link to file:
Example 1:
Using a phase diagram, find each of the following
relationships in terms of given quantities. Assume that the
unit weight of water, gw is always given.
(a) Given the porosity, n determine the void ratio, e?
(b) Given specific gravity of solids, Gs, and water content,
w, determine void ratio, e for a fully saturated soil?
(c) Given specific gravity of solids, Gs, water content, w
and degree of saturation, Sr, determine mass unit
weight,g.
weight,g
(d) Given specific gravity of solids, Gs, and water content,
w, determine the submerged unit weight, gb, of a fully
saturated soil?
9
UNITY
10
Origin & Formation of Soils and
Soil Categories
Rocks are the parent material from which the
soils are formed
When a rock surface gets exposed to the
atmosphere for an appreciable time,
considerable changes take place in its structure,
which is known as weathering of rocks
Weathering can be broadly divided into two
types which are physical and chemical
weathering
11
Origin & Formation of Soils and
Soil Categories (continued…)
Besides these complex factors that go to form
the soils, rocks are also of three major types –
Igneous,
Igneous, Sedimentary and Metamorphic
Nature of weathering and rate of weathering
depend upon the rock
Harder minerals will naturally be more resistant
to weathering action
Also more chemically stable minerals will retain
their identity when chemical changes tend to
occur
12
Origin & Formation of Soils and
Soil Categories (continued…)
The soils formed by weathering may
remain in position on the parent rock, in
which case they are termed as residual
soils
Or they may get transported by the various
transporting agencies such as wind, water,
ice or even gravity, etc. when they are
termed as transported soils.
soils.
13
Origin & Formation of Soils and
Soil Categories (continued…)
Table 1.1: Nomenclature for soils (Source: Sehgal, S.B., 1984)
Bil.
Bil. Transporting medium & deposit Name
location
1. Transported by running water, e.g. Alluvial
rivers and streams
2. Transported by wind Aeoline
3. Transported by Glaciers Glacial
4. Deposited in Lakes Lacustrine
5. Deposited in sea Marine
14
Soil Types (continued…)
Coarse-Grained Soils
These include sands, gravels and larger particles.
For these soils the grains are well defined and may be seen by the
naked eye.
The individual particles may vary from perfectly round to highly angular
reflecting their geological origins.
The engineering properties of coarse-grained soils depend mainly on the
grain size and the structural arrangement of the particles
Fine-Grained Soils
These include the silts and clays.
The particle sizes are generally smaller than 63μm.
The engineering properties of fine-grained soils depend mainly on
mineralogical factors
A thin layer of water is bonded to the mineral surfaces of soils and has
significant influences on the physical and mechanical characteristics of
fine-grained soils
Fine-grained soils have much larger surface areas than coarse-grained
soils and are responsible for the major physical and mechanical
differences between coarse-grained and fine-grained soils
15
Soil Types (continued…)
Silts:
These can be visually differentiated from clays because they exhibit
the property of dilatancy.
If a moist sample is placed and shaken on the palm of one’s hand,
water will appear on its surface.
If the sample is then squeezed between the fingers, the water will
disappear.
Their gritty feel can also identify silts.
Clays:
Clays exhibit plasticity, they may be readily remoulded when moist,
and if left to dry can attain high strengths.
Clays are composed of three main types of minerals – kaolinite, illite
and montmorillonite
The bonds between the layers play a very important role in the
mechanical behaviour of clays. The bond between the layers in
montmorillonite is very weak compared with kaolinite and illite.
Water can easily enter between the layers in Montmorillonite
causing swelling
Organic:
These may be of either clay or silt sized particles.
They contain significant amounts of vegetable matter.
The soils containing organic matter are usually dark grey
or black and have a noticeable odour from decaying
matter.
Generally, the organic matters are found on the ground
surface.
Peat layer may be found at few meters depth.
The organic soil can be classified as very poor soils for
most engineering purposes.
16
17
Activity of clay
Clay absorbs water to its surface
The water absorbed by a soil provides
some estimate of the amount of clay
present in that soil
Skempton proposed a relationship
between the plasticity index and the
percentage (by weight) of clay sizes finer
2mm , which is known as activity of
than 2m
clay, A
18
Activity of clay (continued…)
Ip
A=
% finer than 2 mm
Activity, A Classification
19
20
Sub Chapter 1.3:
Simple Physical
Properties of Soil and
Classification tests
21
Simple Physical Properties of Soil
& Classification test
To physically identify the soils in the field as also
in the laboratory and to determine their simple
physical properties such as water content,
specific gravity, void ratio, plasticity index and
particle size distribution, etc., some procedures
have been developed
These procedures are not based upon any
complicated theories but involve very simple
calculations
Many agencies such as the ASTM, BSI and MS
have laid down set procedures standardized to
ensure uniformity in practice
22
Simple Physical Properties of Soil
& Classification test (continued…)
Method of classifying the soils after laboratory
testing is called classification of the soils
Numerical results, which are obtained from such
tests are called the Index Properties of the soils
Index properties of a soil can be studied under
two distinct headings – properties concerning
the individual grains studied from disturbed soil
samples and the ones concerning the soil as an
aggregate, studied from the mode, history of
deposition and the structure of the soil
23
Particle Size and Grain Size
Distribution
The first analysis carried out in the
laboratory for soil classification is the
grain-size analysis
Grain size analysis of a soil is carried out
in two stages:
(a) Sieve analysis for the coarse grained
particles,
(b) Hydrometer analysis for the fine-
grained soils
24
25
Effective Size
The diameter D10 is called the effective size of
the soil
Established by Allen Hazen (1893) in connection
with his work on soil filters
Is the diameter of an artificial sphere that will
approximately produce the same effect of an
irregular shaped particle
The higher the D10 value, the coarser the soil
and the better the drainage characteristics
26
Coefficient of Curvature (CC)
Also known as coefficient of gradation and the
coefficient of concavity
The CC is between 1 and 3 for well-
well-graded soils
The absence of certain grain sizes, termed gap-
gap-
graded, is diagnosed by a CC outside the range
1 to 3 and a sudden change of slope in the
particle size distribution curve
27
In Short
Real soils consist of a mixture of particle sizes
The selection of a soil for a particular use may
depend on the assortment of particles it contains
Two coefficients have been defined to provide
guidance on distinguishing soils based on the
distribution of the particles
That are UC and CC
28
Fine Grained soils (continued…)
When the hydrometer is lowered into the
suspension, it will sink into the suspension until
the buoyancy force is sufficient to balance the
weight of the hydrometer.
The length of the hydrometer projecting above
the suspension is a function of the density,
density, so it
is possible to calibrate the hydrometer to read
the density of the suspension at different times.
The calibration of the hydrometer is affected by
temperature and the specific gravity of the
suspended solids.
Hence, must apply a correction factor to
hydrometer reading based on the test
temperatures.
18mz
D=
(Gs - 1)g w t D
29
Fine Grained Soils (continued…)
Typically, is conducted by taking
a small quantity of a dry fine-
fine-
grained soil (approximately 10
grams) and thoroughly mixing it
Hydrometer with distilled water to form a
paste
The paste is placed in a 1 liter
Increasing density
30
Table 1.2: Typical Atterberg Limits for Soils
31
Table 1.3: Plasticity Indices and Corresponding
States of Plasticity according to Burmister
(Source: Sehgal, S. B, 1984)
32
Casagrande Method (ASTM D4318-93
and BS1377: Part 2: 1990:4.5)(Continued…)
33
Casagrande
Method (ASTM
D4318-93
and BS1377:
Part 2:
1990:4.5)
(Continued…)
34
Cone Penetrometer Method
(BS 1377: Part 2: 1990: 4.3)
35
Cone Penetrometer Method
(continued…)
(BS 1377: Part 2: 1990: 4.3) (continued…
36
Plastic Limit
After removing particles larger than the 0.4 mm (No 40
sieve size), a specimen of soil about 10 cm3 in size is
moulded to the consistency of putty.
If too dry, water must be added and if sticky, the
specimen should be spread out in a thin layer and
allowed to lose some moisture by evaporation.
Then the specimen is rolled out by hand on a smooth
surface or between the palms into a thread about 3 mm
in diameter.
The thread is then folded and re-rolled repeatedly.
During this manipulation, the moisture content is
gradually reduced and the specimen stiffens, finally
loses its plasticity and crumbles when the plastic limit is
reached.
37
Example 2:
The results of limit tests on soil A are:
Liquid limit:
Cone 15.5 18.0 19.4 22.2 24.9
Penetration
Example 2 (continued…):
38
Example 2 (continued…):
Example 3:
Based on the results of limit test on soil A from
Example 2, given that:
Plastic limit:
Water 23.9 24.3
content (%)
39
Sub Chapter 1.3.3:
Compaction
N. Sivakugan
What is compaction?
A simple ground improvement technique,
where the soil is densified through external
compactive effort.
Compactive
effort
+ water =
40
Compaction Curve
Dry density (rd)
optimum
water content Water content
Compaction Curve
What happens to the relative quantities of the three phases
with addition of water? air
water
Dry density (rd)
soil
Water content
41
Zero Air Void Curve
Dry density (rd) - corresponds to 100% saturation
Water content
Increasing compactive
effort results in:
Lower optimum
water content
E2 (>E1) Higher maximum dry
density
E1
Water content
42
Compaction and Clay Fabric
Dry density (rd)
Water content
Line of Optimum
Dry density (rd)
Compaction curves
for different efforts
Line of optimum
Water content
43
Laboratory Compaction Test
(after BS1377:Part 4: 1990)
- to obtain the compaction curve and define the
optimum water content and maximum dry density for a
specific compactive effort.
compacted ground
44
Core-cutter method
core-cutter apparatus, which
Details of the core-
is suitable for cohesive soils, are given in
figure following
After the cutter has been first pressed into
the soil and then dug out, the soil is
trimmed to the size of the cutter and both
cutter and soil are weighed; given the
weight and dimensions of the cutter, the
bulk density of the soil can be obtained.
45
Sand Replacement method
For granular soils the apparatus shown in figure
following is used
A small round hole (about 100 mm diameter and
150 mm deep) is dug and the mass of the
excavated material is carefully determined
The volume of the hole thus formed is obtained
by pouring into it with sand of known density
from a special graduated container; given the
weight of sand in the container before and after
the test, the weight of sand in the hole and
hence the volume of the hole can be determined.
46
Figure: Sand
replacement
method
(Source: Smith, I.,
2006)
Link to file:
47
Sub Chapter 1.3.4:
Unified Soil
Classification System
(USCS) and AASHTO
48
Unified Soil
Classification
system
(USCS)
49
(continued…)
Unified Soil Classification System (continued…
(Continued…)
Unified Soil Classification System (Continued…
50
Unified Soil Classification System
(Continued…)
(Continued…
A special classification, (Pt), is reserved for the highly
organic soils, such as peat, which have many
undesirable engineering characteristics.
Soil have an organic content greater than 75% are
considered as peat (Pt) (Jarrett, P. M., 1995)
No laboratory criteria are established for these soils,
as they generally can be easily identified in the field
by their distinctive color and odor, spongy feel, and
frequently fibrous texture.
Particles of leaves, grass, branches, or other fibrous
vegetable matter are common components of these
soils.
Unified Soil
Classification
flowchart for
coarse-
coarse-grained
soils
51
Unified Soil
Classification Yes
flowchart for
fine-
fine-grained
soils
The Unified
Classification System
(Reprinted from
Cernica,
Cernica, 1995)
52
The Unified Classification
System (Reprinted from
Cernica,
Cernica, 1995)
Unified
Classification
System
(Based on
ASTM-
ASTM-2487)
53
Unified Soil Classification System (USCS)
Organic soils
and silts
U-line, defines the upper limit of the correlation between plasticity index and liquid limit
American
Association of State
Highway and
Transportation
Officials (AASHTO)
classification system
54
AASHTO Classification System
A-1, A-
A-2 & A-
A-3 are granular materials of which < 35%
of the particles pass through the No. 200 sieve
Soils of which > 35% pass through the No. 200 sieve
are classified under groups A-A-4, A-
A-5, A-
A-6, & A-
A-7,
these soils are mostly silt and clay-
clay-type materials
(continued….)
AASHTO Classification System (continued…
ii. Plasticity
The term silty is applied when the fine fractions of the soil have a
plasticity index of 10 or less. The term clayey is applied when the
fine fractions have a plasticity index of 11 or more
55
(continued….)
AASHTO Classification System (continued…
To classify a soil according to Table 1.3, one must apply the test
test
data from left to right
By process of elimination, the first group from the left into which
which
the test data fit is the correct classification
(continued….)
AASHTO Classification System (continued…
Table 1.3: Classification of Highway Subgrade Materials
56
(continued….)
AASHTO Classification System (continued…
Table 1.4: Description of Classification groups
Subgroup A-
A-1a Includes those materials consisting predominantly of stone fragments
fragments or
gravel
Subgroup A-
A-1b Includes those materials consisting predominantly of coarse sand,
sand, either with
or without a well-
well-graded soil binder
Subgroup A-
A-3 Fine beach sand or fine desert loess sand without silty or clay fines or with a
very small amount of nonplastic silt
Subgroup A-
A-2-4 and Include various granular materials containing 35% or less passing
passing the 0.075
A-2-5 mm sieve and with a minus 0.425 mm in having the characteristics of the A-
A-4
and A-
A-5 groups
Subgroup A-
A-2-6 and Include material similar to that described under subgrades A-2-4 and A- A-2-5,
A-2-7 except that a fine portion contains plastic clay having the characteristics
characteristics of the
A-6 or A-
A-7 group
Subgroup A-
A-4 The typical materials of this group are the nonplastic or moderately plastic silty
soils
Subgroup A-
A-5 Similar to that described under group 2-
2-4, except that it is usually of
diatomaceus or micaceous character
Subgroup A-
A-6 Usually a plastic clay having 75% or more passing the 0.075 mm sieve
sieve
Subgroup A-
A-7-5 Includes materials with moderate plasticity indexes in relation to liquid limit
Subgroup A-
A-7-6 Includes materials with high plasticity indexes in relation to liquid
liquid limit
(continued….)
AASHTO Classification System (continued…
57
(continued….)
AASHTO Classification System (continued…
Similarity:
based on the texture and plasticity of soil
divide the soils into coarse-
coarse-grained and fine grained,
as separated by the No. 200 sieve
58
Summary & Comparison between the
(Continued….)
AASHTO & USCS (Continued…
Differents:
Differents:
1. AASHTO, soil is considered fine- fine-grained > 35%
passes through the No. 200 sieve. However, for
USCS, soil is considered fine-
fine-grained > 50% passes
through the No. 200 sieve
* A coarse-
coarse-grained soil that has about 35% fine grains
will behave like a fine-
fine-grained material. This is because
enough fine grains exist to fill the voids between the
coarse grains and hold them apart. In this respect, the
AASHTO system appears to be more appropriate.
Differents:
Differents:
2. AASHTO, the No. 10 sieve separated gravel from
sand. However, for USCS, the No. 4 sieve is used
* From the viewpoint of soil-
soil-separate size limits, the No.
10 sieve is the more accepted upper limit for sand. The
limit is used in concrete and highway base-
base-course
technology
59
Summary & Comparison between the
(Continued….)
AASHTO & USCS (Continued…
Differents:
Differents:
3. USCS, the gravelly and sandy soils are clearly
separated; in the AASHTO system, they are not.
* The A-
A-2 group, in particular, contains a large variety
of soils. Symbols like GW, SM, CH, and others that
are used in the Unified systems are more
descriptive of the soil properties than the A symbols
used in the AASHTO system
Differents:
Differents:
4. USCS, classified organic soils as OL, OH, and Pt.
However, under the AASHTO system, there is no
place for organic soils.
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End of Chapter 1
61