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Basic Soil Mechanics
ATGB2513
Learning Outcome
Upon completion of this course, students should be able:-
To differentiate the properties and behaviour of various types
of soil
To interpret a soil report
To explain various methods of dewatering
To discuss the various methods of soil testing
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Basic Soil Mechanics
Core reading list
Craig, R.F., Craigs Soil Mechanics, 7th Edition, Spon Press
(2004)
Whitlow, R., Basic Soil Mechanics, 4th Edition, Prentice Hall
(2001)
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Module 6
Testing, Measurement and Evaluation
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Reference Text
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Library ref: 624.151 36 LIU
Chapters 4, 6, 7, 8, 10 12, 14, 18 & 23
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Module 1
Soil Materials
Synonym
AASHTO American Association of State Highway and Transportation
Officials
USCS Unified Soil Classification System
ASTM American Society for Testing and Materials
BS British Standards
USDA United States Department of Agriculture
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Soil Materials
Soil and Soil Engineering
To a Pedologist Soil is the substance existing on earths
surface, which grows and develop plant life
To a Geologist Soil is the material in the relative thin surface
zone within which roots occur and all the rest of the crust is
grouped under the term ROCK irrespective of hardness
To an Engineer Soil is the un-aggregated deposits of mineral
and/or organic particles or fragment covering large portion of the
earths crust
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Soil Materials
Soil and Soil Engineering
Soil Mechanics is one of the youngest disciplines of Civil
Engineering involving study of soil, its behavior and application
as engineering material
Geotechnical Engineering Is a broader term for Soil
Mechanics
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SOIL
Geologic definition: Loose surface of the earth as distinguished
from solid bedrock; support of plant life not required.
Traditional definition: Material which nourishes and supports
growing plants; includes rocks, water, snow, air.
Component definition: Mixture of mineral matter, organic matter,
water, and air.
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Soil Formation/Nature of Soil Deposits
Soil material is a product of rock
May be defined as an accumulation of solid particles produced by
mechanical and chemical disintegration of rock
May contain organic material
Soils are derived from the weathering of rocks and are commonly
described by external textural terms such as gravels, sands, silts
and clays.
What is rock?
Naturally occurring material
Composed of mineral particles firmly bonded
Difficult to separate; blasting, crushing, chemical 12
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Engineering Soils Soil Types Coarse-grained soils Fine-grained
soils
Description Boulders Gravel Sand Silt Clay
Feel Hard, Gritty and Bulky Slight Grittiness Smooth
Size (mm) > 75 75 to 4.75 4.75 to 0.075 0.075 to 0.002 <
0.002
Characterization Particle size Particle size and
mineralogy
Basic Geology Knowledge of geology is important for practice of
geotechnical
engineering;
The earths surface (lithosphere) is fractured into about 20
mobile plates. Interaction of these plates causes volcanic
activities and earthquakes;
The three groups of rocks are igneous, sedimentary and
metamorphic. Igneous rocks are formed from magma (molten rock
materials) emitted from volcanoes that have cooled and solidified.
Sedimentary rocks are formed from sediments, animals and plant
materials that are deposited in water or on land on the earths
surface and then subjected to pressures and heat. Metamorphic rocks
are formed deep within the earths crust from the transformation of
igneous and sedimentary rocks into denser rocks;
Sedimentary rocks are of particular importance to geotechnical
engineers because they cover about 75% of the earths crust surface
area; and
Rock masses are inhomogeneous and discontinuous.
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Categories of rock
Igneous rock solidification of molten material; by intrusion or
extrusion to earth surface
Sedimentary rock deposition, under water, disaggregation,
preexisting
Metamorphic rock igneous or sedimentary rock, change or
metamorphose under heat and pressure
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Geological process that produces soil
General controlling factors
Nature and composition of parent rock
Climatic conditions; temperature, humidity
Topographic and general terrain; degree of shelter or exposure,
density, type of vegetation
Length of time under particular prevailing conditions
Interferences by other agencies; storms, earthquakes, action of
human
Mode and conditions of transport
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Geological process that produces soil Weathering physical and
chemical
Physicalnatural mechanical and abrasion
Coarse soil
Aggregate/gravel
Sand
Retained the same composition of parent rock
Physical weathering causes reduction in the size of the parent
rock without change in its composition.
Chemicalwater- acidic, alkaline, oxygen, carbon dioxide
Small particles, crystalline form, two dimensional, flaky
Clayey soil characteristic depends on parent rock, environment,
duration of alteration
Chemical weathering causes reduction in size and chemical
composition different from the parent rock.
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Geological process that produces soil
Weathering effects;
Frosts within pore space; splitting, sharp and angular
Wind and water; attrition- rounded
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Type of soil produced by different weathering
Boulders; size
Sand (physical)
Silt (cohesive)
Clay (chemical)
Moisture
Dry, saturated (fully and partially)
Shapes and textures different
Particle size is used to distinguish various soil textures.
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Transportation process sorting out process
Not been transported residual soil, usually by chemical
means,
flat terrain
Water alluvial soil (river deposit)
Estuarine mixture of marine and alluvial soil; favourable
foundation
Lacustrine fresh water; good foundation
Coastal, marine (blackish water); velocity, suspension,
deposition
Ice glacial soil (large to smaller)
Wind Aeolin soil
Sand dunes, loess; long distance, arid or coastal areas
Gravity... Colluvial soil (unsorted)
below slide areas, cliff base; future difficulties for
foundation 20
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Soil Materials
Types of soils
a) Organic b) Residual c) Alluvial d) Colluvial
a) Organic soil is a mixture of mineral and organic material.
Usually dark in colour and with an odour.
b) Residual soil is weathered remains of rock after going
through the transportation process.
c) Alluvial soil is material (sand and gravel) deposited by
streams and rivers.
d) Colluvial soil is material transported and deposited by
gravity like landslides.
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Loading and Drainage History
The current state (i.e. density and consistency) of a soil;
influenced by the history of loading and unloading since it was
deposited.
Changes in drainage conditions; may have brought about changes
in water content.
Initial loading
During deposition the load applied to a layer of soil increases
as more layers are deposited over it; thus, it is compressed and
water is squeezed out; as deposition
continues, the soil becomes stiffer and stronger.
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Loading and Drainage history
Unloading The principal natural mechanism of unloading is
erosion of
overlying layers. Unloading can also occur as overlying
ice-sheets and glaciers retreat, or due to large excavations made
by man.
Soil expands when it is unloaded, but not as much as it was
initially compressed; thus it stays compressed - and is said to be
overconsolidated. The degree of overconsolidation depends on the
history of loading and unloading.
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Loading and drainage history
Drainage history
Chemical changes
Some soils initially deposited loosely in saline water and then
inundated with fresh water develop weak collapsing structure.
In arid climates with intermittent rainy periods, cycles of
wetting and drying can bring minerals to the surface to form a
cemented soil.
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Loading and drainage history
Drainage history
Climate changes
Some clays (e.g. montmorillonite clays) are prone to large
volume changes due to wetting and drying; seasonal changes in
surface level occur, often causing foundation damage, especially
after exceptionally dry summers.
Trees extract water from soil in the process of
evapotranspiration; The soil near to trees can therefore either
shrink as trees grow larger, or expand following the removal of
large trees.
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Overburden pressure
Pressure/stress caused by weight of all material; more dense for
soil located deeper
Normal consolidation
Usually clay, subjected to pressure imposed by the overburden
since its formation; a soil which current state corresponds to the
maximum consolidation pressure
Over consolidation
Usually clay, subjected to access and extreme pressure besides
from the overburden since formation; a soil which have present day
overburden pressure less than the highest historic consolidation
pressure
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Definition of clay based on stress history
Normally consolidated, whose present effective overburden
pressure is the maximum pressure that the soil was subjected to in
the past.
Overconsolidated, whose present effective pressure is less that
that which the soil experienced in the past. The maximum effective
past pressure is called the preconsolidated pressure.
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Clays are composed of three main types of mineral kaolinite,
illite and montmorillonite.
The clay minerals consist of silica and alumina sheets that are
combined to form layers. The bonds between layers play a very
important role in the mechanical behavior of clays. The bond
between the layers of montmorillonite is very weak compared with
kaolinite and illite. Water can easily enter between the layers in
the montmorillonite, causing swelling.
A thin layer of water, called absorbed water, is bonded to the
mineral surfaces of soils. This layer significantly influences the
physical and mechanical characteristics of fine-grained soils.
Fine-grained soils have much larger surface areas than
course-grained soils and are responsible for the major physical and
mechanical differences between course-grained and fine-grained
soils.
The engineering properties of fine-grained soils depend mainly
on mineralogical factors.
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Main components of soil description
Nature of soil
Shape, size and particles distribution
State of soil
Density, relative density, water content
Fabric of soil
Homogeneity or layer sequences
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Soil Color
Indicator of different soil types
Indicator of certain physical and chemical characteristics
Due to humus content and chemical nature of the iron compounds
present in the soil
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Major Forms of Iron and Effect on Soil Color
Form Chemical Formula Color
Ferrous oxide FeO Gray
Ferric oxide
(Hematite) Fe2O3 Red
Hydrated ferric oxide
(Limonite) 2Fe2O3 3H2O Yellow
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Field Classification; site exploration; preliminary; with some
in-situ testing procedures
Identify particle size; visual and feel; proportion of size
range (shaking with water)
Course soils (British Standards)
Seen with unaided eye; Gravel > 2mm, Sand (0.06 mm <
2.0mm); gritty feeling; over 65% (coarse)
Clean sand or gravel; W or P gradation
Dirty sand and gravel; M or C gradation (fines are silty or
clayey
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Field Classification
Fine soil (British Classification)
More than 35% < 0.06mm; visually
Need magnification to see grain
Silt (0.002mm < 0.06mm); slightly abrasive
Clay (< 0.002mm); greasy feel
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Field Classification
Compactness (field strength)
Hand spade, wooden peg Loose, dense and slightly cemented
Structure; trial pit, cutting
Homogeneous; one type of soil
Inter-stratified; alternating or bands of different layers
Intact; non fissured fine soil
Fissured; direction, size and spacing
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Field Classification
Cohesion, plasticity and consistency
Remove particles > 2.0mm; squeeze handful; descript feeling;
soft, firm, hard and crumbly
Dilatancy; fine sand and inorganic silt
Dry strength; high (clay), low (inorganic silt, powdery)
Weathering; unweathered, slightly, moderately, highly and
fully
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Soil Description and Classification
System to group soil;
Physical characteristic of soil particles
Performance of soil particles when subjected to certain tests or
condition of service
Textural classification
Assign descriptive name; e.g. clayey sand
Assign particle size limits to soil fraction and % compositions
corresponding to the descriptive names
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Soil Description and Classification
Textural classification
Use more for construction; coarse grain soil (sand and gravel);
performance base on relative amount of sizes of particles
Fine soil (silt and clay); produce little information for
engineering use; behavior depend more than size eg plasticity
characteristic
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Soil Index Properties
As a guide; descriptive nature of soil constituents
Relates to engineering behavior of soil
Behavior of sands and gravels may inferred shape, size and
density
Behavior of silts and clays; interaction of particles with
water
Properties of soil that indicate the type and conditions of
soil
Provide structural properties; strength, compressibility,
permeability
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Soil Index Properties
Particle size distribution curve for coarse grain soil
Mechanical; sieve analysis
Plasticity characteristics for fine grain soil and relationship
to natural water content
Hydrometer
Phase relationships (air, water, sand and solid) for soil
mass
Consistency limits; L.L, P.L and P.I
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Sieve Analysis
A sieve analysis is used to determine the grain size
distribution of coarse-grained soils
The particle size distribution plot is used to delineate the
different soil textures (percentage of gravel, sand, silt, and
clay) in a soil.
For fine-grained soils, a hydrometer analysis is used to find
the particle size distribution
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Soil Description
The identification and defining basic soil types, British
Classification System;
e.g. boulders, cobbles, gravel, sand, silt and clay
Including organic clay and silt or sand and peat
In terms of particle size range; Figure 1.6
Different percentage of particle sizes defines grouping
Dependent on sizes; two major groups
Coarse grain soil; > 65% sand and gravel sizes
Fine grain soil; > 35% silt and clay sizes
Sand & gravel further subdivided into coarse, medium and
fine fraction; Figure 1.6
Names in capital letter in a soil description; e.g. S for
Sand
Mixture of basic soil types are referred to as composite
type
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British Standard Range of Particle Sizes
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Soil Description Sand & gravel can further described as well
graded, poorly graded, uniform or gap-
graded
Well graded; if there is no excess of particles in any size
range and if no intermediate sizes are lacking; smooth concave
distribution curve
Poorly graded; if high proportion of particles have sizes within
narrow limits; uniform graded
Poorly graded; if particles of both large and small sizes are
present but relatively low proportion of particles of intermediate
particle; gap graded
In the case of gravels, particle shape (angular, sub angular,
sub rounded, rounded, flat, elongated) & texture (rough,
smooth, polished) can be described
Particle composition can also be described; sandstone in gravel
and quartz in sand; Table 1.1
Firmless or strength of in-situ soil description and can also be
assessed by means of tests ; Table 1.2
Description of soil structure can also be established; Table
1.3
Example of soil description;
Dense, reddish-brown, sub angular, well graded, gravelly SAND
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Table 1.1 Description of Composite Soil Types
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Table 1.2 Firmless or Strength of In-situ Soil Description
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Table 1.3 Descriptions for Structure of Soil Deposit
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Soil Classification System BSCS
British Soil Classification System is shown in Table 1.4.
Soil group in the classification denoted by group symbols
composed of main and qualifying descriptive letters having meanings
given in Table 1.5
Reference should also to be made to the Plasticity Chart for
fine material; Figure 1.7;
Plasticity Index and Liquid Limit parameters; determined from
laboratory
Determine the plasticity characteristic of fine soil represented
by a point on chart
Classification according to zone in the chart within which the
point lies.
If point is above A-line, soil is Clay; and
If point is below A-line, soil is Silt
Any boulders or cobles (particles retained on a 63 mm BS sieve)
are to be removed before classification tests are carried out but
their percentages in the total sample should be determined or
estimated
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48 Table 1.4 British Soil Classification Systems for Engineering
Purposes
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Table 1.5 Classifications Qualifying Descriptive Letters
(BSCS)
Primary Letters Secondary Letters
Coarse-grained Soil
G = GRAVEL
S = SAND
W = well graded
P = poorly graded
Pu = uniform
Pg = gap graded
Fined grained Soil F = FINES
M = SILT
C = CLAY
L = low plasticity (wL < 35)
I = intermediate plasticity(wL : 35-50)
H = high plasticity(wL :50-70)
V = very high plasticity(wL : 70-90)
E = extremely high plasticity(wL >90)
Organic soils Pt = PEAT O = organic
Sub-group symbols in the British Soil Classification system
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Figure 1.7 Plasticity Chart: British System (BS 5930: 1981)
Soil Classification System USCS
Classification Systems vary from country to country, but most
are based on the US system (The Unified Soil Classification System,
USCS), or the British Standard Soil Classification System. The
Australian Standard Soil Classification System is similar to the
British Standard, but the USCS is widely used in Australia and the
SE Asia region.
USCS with primary and secondary descriptive letter and meaning
is shown in Table 1.6.
USCS with primary and secondary descriptive letter and
laboratory classification criteria is shown in Table 1.7
The associated Plasticity Chart should be used as shown in
Figure 1.8
In the USCS system, the divisions are slightly different, but
given the wide range of particle sizes, these differences are not
important (for example, in the BS system, fines includes silt and
clay, and is defined as being < 60 m, but in the USCS, fines is
< 75m). Australia and the SE Asia region.
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Table 1.6 Group Symbols with Primary and Secondary Descriptive
Letters
(Unified Soil Classification System)
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Table 1.7 Unified Soil Classification System
Unified Soil Classification System
CU 4 1 CC 3 GW < 15% sand Well-graded gravel
15% sand Well-graded gravel with sandCC < 1 or CC > 3 GP
< 15% sand Poorly-graged gravel
15% sand Poorly-graded gravel with sandCU < 4
ML or MH GW-GM < 15% sand Well-graded gavel with siltCU 4 1
CC 3 15% sand Well-graded gravel with silt and sand
CL, CH, or CL-ML GW-GC < 15% sand Well-graded gravel with
clay (or silty clay)
15% sand Well-graded gravel with clay & sand (or silty clay
& sand)
Gravel CC < 1 or CC > 3 ML or MH GP-GM < 15% sand
Poorly-graded gravel with silt
% sand < % gravel 15% sand Poorly-graded gravel with silt and
sand
CL, CH, or CL-ML GP-GC < 15% sand Poorly-graded gravel with
clay (or silty clay)CU < 4 15% sand Poorly-graded gravel with
clay & sand (or silty clay & sand)
ML or MH GM < 15% sand Silty gravel
15% sand Silty gravel with sand
CL or CH GC < 15% sand Clayey gravel
15% sand Clayey grvel with sand
CL-ML GC-GM < 15% sand Silty, clayey gravel
15% sand Silty, clayey gravel with sand
CU 6 1 CC 3 SW < 15% gravel Well-graded sand
15% gravel Well-graded sand with gravelCC < 1 or CC > 3 SP
< 15% gravel Poorly-graded sand
15% gravel Poorly- graded sand with gravelCU < 6
ML or MH SW-SM < 15% gravel Well-graded gsand with siltCU 6 1
CC 3 15% gravel Well-graded sand with silt and gravel
CL, CH, or CL-ML SW-SC < 15% gravel Well-graded sand with
clay (or silty clay)
15% gravel Well-graded sand with clay & gravel (or silty
clay & sand)
Sand CC < 1 or CC > 3 ML or MH SP-SM < 15% gravel
Poorly-graded sand with silt
% sand % gravel 15% gravel Poorly-graded sand with silt and
gravel
CL, CH, or CL-ML SP-SC < 15% gravel Poorly-graded sand with
clay (or silty clay)
CU < 6 15% gravel Poorly-graded sand with clay & gravel
(or silty clay & sand)
ML or MH SM < 15% gravel Silty sand
15% gravel Silty sand with gravel
CL or CH SC < 15% gravel Clayey sand
15% gravel Clayey sand with gravel
CL-ML SC-SM < 15% gravel Silty, clayey sand
15% gravel Silty, clayey sand with gravel
Flow chart for classification of coarse-grained soils
Coduto, D.P., Yeung, R.M. and Kitch, W.A. 2011. Geotechnical
Engineering: Principles and Practices . 2nd edn. USA:
Pearson(Adapted from ASTM D2487).
50% or more of
coarse fraction
passes the 4.75
mm (#4) sieve
50% or more of
coarse fraction
retained on the
4.75 mm (#4)
sieve
> 12% pass #200
> 12% pass #200
More than
50%
retained on
the 0.075
mm (#200)
sieve
5 - 12% pass #200
< 5% pass #200
Course-
Grained
Soils
5 - 12% pass #200
< 5% pass #200
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Table 1.7 Unified Soil Classification System
Unified Soil Classification System
85% pass #200 Lean clay
70% pass #200 70 - 84% pass #200 % sand % gravel Lean clay with
sand
% sand < % gravel Lean clay with gravel
% sand % gravel < 15% gravel Sandy lean clay
50 - 69% pass #200 15% gravel Sandy lean clay with gravel
% sand < % gravel < 15% sand Gravelly lean clay
15% sand Gravelly lean clay with sand
85% pass #200 Silty clay
70% pass #200 70 -84% pass #200 % sand % gravel Silty clay with
sand
Silts and Clays % sand < % gravel Silty clay with gravel
Liquid Limit < 50% % sand % gravel < 15% gravel Sandy
silty clay
50 - 69% pass #200 15% gravel Sandy silty clay with gravel
% sand < % gravel < 15% sand Gravell silty clay
15% sand Gravelly silty clay with sand
85% pass #200 Silt
70% pass #200 70 -84% pass #200 % sand % gravel Silt with
sand
% sand < % gravel Silt with gravel
% sand % gravel < 15% gravel Sandy silt
50 - 69% pass #200 15% gravel Sandy silt with gravel
% sand < % gravel < 15% sand Gravelly silt
15% sand Gravelly silt with sand
85% pass #200 Fat clay
70% pass #200 70 -84% pass #200 % sand % gravel Fat clay with
sand
% sand < % gravel Fat clay with gravel
% sand % gravel < 15% gravel Sandy fat clay
50 - 69% pass #200 15% gravel Sandy fat clay with gravel
Silts and Clays % sand < % gravel < 15% sand Gravelly fat
clay
Liquid Limit 50% 15% sand Gravelly fat clay with sand
85% pass #200 Elastic silt
70% pass #200 70 -84% pass #200 % sand % gravel Elastic silt
with sand
% sand < % gravel Elastic silt with gravel
% sand % gravel < 15% gravel Sandy elastic silt
50 - 69% pass #200 15% gravel Sandy elastic silt with gravel
% sand < % gravel < 15% sand Gravelly elastic silt
15% sand Gravelly elastic sitl with sand
Flow chart for classification of inorganic fine-grained
soils
Coduto, D.P., Yeung, R.M. and Kitch, W.A. 2011. Geotechnical
Engineering: Principles and Practices . 2nd edn. USA:
Pearson(Adapted from ASTM D2487).
Fine-Grained
Soils
ML
CH
MH
More than
50% passes
the 0.075 mm
(#200) sieve
CL
CL -ML
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Figure 1.8 Plasticity Chart (Unified Soil Classification
System)
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Basic characteristic of soil constituents (British Soil
Classification System);
Majority consist of mixtures of inorganic mineral particles,
with water and air(void); solid, water and gas phases
Rock fragments
Fairly large (> 2mm) eg sand to gravel(stone)
Soundness depends on extent of mineral decomposition
Mineral grain
Separate particles of mineral eg sand (quartz)
Size from 2mm to clay (1m)
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Basic characteristic of soil constituents(British Soil
Classification System)
Dependent on sizes; two major groups
Coarse grain soil; > 65% sand and gravel sizes
Fine grain soil; > 35% silt and clay sizes
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Coarse grain
Individual grain; wet or dry condition
Particle size > 0.06mm; sands and gravels
Rounded and angular (depend on degree of wear)
Fragments of rock, quartz or jespar, iron oxide, calcite,
mica
Equidimentional shape; crystalline structure of mineral
Cohesionless
Fine grain
Particle size < 0.06mm; silts and clays
Flaky shape; large surface area
Very fine sulphides and oxides
Organic (infrequent)
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60
British Standard Range of Particle Sizes
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Very coarse
soils
BOULDERS > 200 mm
COBBLES 60 - 200 mm
Coarse
soils
G
GRAVEL
coarse 20 - 60 mm
medium 6 - 20 mm
fine 2 - 6 mm
S
SAND
coarse 0.6 - 2.0 mm
medium 0.2 - 0.6 mm
fine 0.06 - 0.2 mm
Fine
soils
M
SILT
coarse 0.02 - 0.06 mm
medium 0.006 - 0.02 mm
fine 0.002 - 0.006 mm
C CLAY < 0.002 mm
Soil Classification- Basic Soil Type Group
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Gravel fraction(BSCS)
The fraction of a soil composed of particles between the sizes
of 60 mm and 2 mm. The gravel fraction is subdivided as
follows:
Coarse gravel 60 mm to 20 mm
Medium gravel 20 mm to 6 mm
Fine gravel 6 mm to 2 mm
Sand fraction(BSCS)
The fraction of a soil composed of particles between the sizes
of 2.0 mm and 0.06 mm. The sand fraction is subdivided as
follows:
Coarse sand 2.0 mm to 0.6 mm
Medium sand 0.6 mm to 0.2 mm
Fine sand 0.2 mm to 0.06 mm
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Silt fraction(BSCS)
The fraction of a soil composed of particles between the sizes
of 0.06 mm (63 m) and 0.002 mm. The silt fraction is subdivided as
follows:
Coarse silt 0.06 mm to 0.02 mm
Medium silt 0.02 mm to 0.006 mm
Fine silt 0.006 mm to 0.002 mm
Clay fraction(BSCS)
The fraction of a soil composed of particles smaller
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65
Very coarse
soils
BOULDERS > 200 mm
COBBLES 60 - 200 mm
Coarse
soils
G
GRAVEL
coarse 20 - 60 mm
medium 6 - 20 mm
fine 2 - 6 mm
S
SAND
coarse 0.6 - 2.0 mm
medium 0.2 - 0.6 mm
fine 0.06 - 0.2 mm
Fine
soils
M
SILT
coarse 0.02 - 0.06 mm
medium 0.006 - 0.02 mm
fine 0.002 - 0.006 mm
C CLAY < 0.002 mm
British Soil Standard Range of Particle Sizes and
Classification
Unified Soil Classification System
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ASTM Particle Size Classification (ASTM D2487)
Sieve Size Particle Size
Passes Retained on (inch) mm
12 in. >12 >300 Boulder
Rock Fragment 12 in (300 mm) 3 in. 3 12 75 300 Cobble
3 in. (75 mm) in 0.75 3 19.0 75 Coarse gravel
Soil
in. (19 mm) #4 0.19 0.75 4.75 19.0 Fine gravel
#4 (4..75 mm) #10 0.079 0.19 2.00 4.75 Coarse sand
#10 (2.00 mm) #40 0.017 0.079 0.425 2.00 Medium sand
#40 (0.425 mm) #200 0.003 0.017 0.075 0.425 Fine sand
#200 (0.075 mm) < 0.003 < 0.075 Fines (silt + clay)
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Plasticity of fine grain soils
Ability to undergo unrecovered deformation without cracking or
crumbling
Due to presence of high clay content
Ratio of mass of water to mass of soil
Water reduction; Liquid>plastic>semi-solid
Cohesion; negative pressure>capillary action>suction
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69
Consistency Relationship of Cohesive Soil
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Liquid limit Two main types of test are specified.
The first is the cone penetrometer method, which is
fundamentally more satisfactory than the alternative because it is
essentially a static test depending on soil shear strength. It is
also easier to perform and gives more reproducible results.
The second is the much earlier Casagrande type of test which has
been used for many years as a basis for soil classification and
correlation of engineering properties. This test introduces dynamic
effects and is more susceptible to discrepancies between
operators.
For both types of test an alternative rapid one-point procedure
is given, which may give less accurate results.
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Liquid limit Cone penetration (Penetrometer) Method;
This method covers the determination of the liquid limit of a
sample of soil in its natural state, or of a sample of soil from
which material retained on a 425 m test sieve has been removed.
Proceed from drier to wetter state; range approx. from 15mm to
25mm; water content against penetration graph (20mm defines the
liquid limit)
The amount of water added shall be such that a range of
penetration values of approximately 15 mm to 25 mm is covered by
the four or more test runs and is evenly distributed.
74
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38
75
Penetration Water Content Graph
Liquid limit Casagrande Method
This is an alternative method for the determination of the
liquid limit of a sample of natural soil, or of a sample of soil
from which material retained on a 425 m test sieve has been
removed.
Flat metal cup; grove; dropping of cup
Turn the crank handle at the rate of 2 revolution/sec so that
the cup is lifted and dropped, counting the number of bumps.
Continue until the two parts of the soil come into contact at the
bottom of the groove along a distance of 13 mm, measured with the
end of the grooving tool or with a ruler. Record the number of
bumps at which this occurs.
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39
77
Flow Curve - Interpolate Liquid Limit water content at 25
blows
Log N
Plastic limit This method covers the determination of the
plastic limit of a soil
sample, i.e. the lowest moisture content at which the soil is
plastic. The sample shall be of soil in its natural state, or of
soil from which material retained on a 425 m test sieve has been
removed.
flat, glass plate, smooth and free from scratches, on which
threads are rolled. A convenient size of plate is about 10 mm thick
and 300 mm square.
A length of rod, 3 mm in diameter and about 100 mm long.
78
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40
Plastic limit Roll the thread between the fingers, from
finger-tip to the second
joint, of one hand and the surface of the glass rolling plate.
Use enough pressure to reduce the diameter of the thread to about 3
mm in five to 10 complete, forward and back, movements of the hand.
Some heavy clays will require 10 to 15 movements when the soil is
near the plastic limit because the soil hardens at this stage. It
is important to maintain a uniform rolling pressure; do not reduce
the pressure as the thread diameter approaches 3 mm.
Pick up the soil, mould it between the fingers to dry it
further, form it into a thread and roll it out again
Repeat until the thread shears both longitudinally and
transversely when it has been rolled to about 3 mm diameter, as
gauged by the rod. Do not gather the pieces of soil together after
they have crumbled, in order to reform a thread and to continue
rolling; the first crumbling point is the plastic limit.
Determine water content
79
Organic matter
Originates from plant and animal; decomposed
Topsoil < 0.5m from surface
Peat fibrous organic material
Undesirable properties (engineering)
High absorbance and compressibility (why), low bearing capacity,
settlement, shear failure
High cost for stabilization
80
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41
Water
Ever present, fundamental,
Substantial influence on soil properties
Seepage and permeability (pros and cons)
Compressibility, containment, drainage
Pressure on retaining system
Shear strength
Chemical reaction (sulphate ions; Portland cement concrete),
harmful to structures
81
Nature and structure of clay minerals
Weathering of felspars and micas
Layer-lattice minerals; small, flaky; ion of silicon, magnesium,
aluminium
Four main groups; kaolinite, illite, montmorillonite,
vermiculite
Kaolinite weathering of felspar; kaolin and china clay
Illite degradation of micas under marine conditions; shale and
marine clay
Montmorillonite further degardation of illite; high swelling and
shrinkage
Vermiculite weathering from biotite and chloride; swelling and
shrinkage
82
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42
Some important properties of clay minerals
Surface area; large with lesser weight
Surface charge and absorption; ability to absorb water
Base exchange capacity; ability to absorb water
Flocculation and dispersion; thin layer structure and high
liquid limit
Swelling and shrinkage; absorption and dispersion of water
83
Soil quality
Detrimental effects on embedded structures; testing samples of
ground water
Soluble sulphates
Reacts with certain constituents of Portland cement; inhibits
hardening and disruption to aggregate binding process
Organic acids; in peat soil
Reacts with lime in cement to form calcium salts; deteriorate
concrete with high water/cement content
pH value; presence of industrial waste or other pollutants
Corrosion of buried iron, steel and some concrete 84
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43
Physical Properties of Soil
Soil texture
Soil structure
Soil color
Bulk density
85
Particle size distribution; sieve analysis; description of type
of soil eg poorly or well graded, sandy, silty, gravel
Engineering properties
Uniformity Coefficient CU ; measure particle size range CU = d60
/ d10 ; Hazen Coefficient; permeability; d60 maximum
size of the smallest 60 % of the sample
Coefficient of Curvature or Gradation CC or CZ or CG = ( d30
)
2 / ( d60 x d10 ) ; measure the shape of the particle size
curve
Coefficient of Permeability k = CK ( d10 )
2 m/sec ; CK is coefficient of permeability range from 0.01 to
0.015
86
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44
87
A grading curve is a useful aid to soil description. Grading
curves are often included in ground investigation reports. Results
of grading tests can be tabulated using geometric properties of the
grading curve. These properties are called grading
characteristics
First of all, three points are located on the grading curve: d10
= the maximum size of the smallest 10% of the sample d30 = the
maximum size of the smallest 30% of the sample d60 = the maximum
size of the smallest 60% of the sample
From these the grading characteristics are calculated: Effective
size d10 , d30 and d60 Uniformity coefficient CU = d60 / d10
Coefficient of gradation CC or CG or CZ = ( d30 )
2 / ( d60 x d10 )
88
Grading characteristics
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45
Both CU and CC or CG or CZ will be 1 for a single-sized soil CU
> 5 indicates a well-graded soil CU < 3 indicates a uniform
(one type) soil
CC between 0.5 and 2.0 indicates a well-graded soil CC < 0.1
indicates a possible gap-graded soil
Two coefficients the uniformity coefficient, CU , and the
coefficient of curvature CC or CG or CZ , are used to characterize
the particle size distribution.
Poorly graded soils have uniformity coefficient < 4 and steep
gradation curves (USCS).
Well-graded soils have uniformity coefficients > 4,
coefficients of curvature between 1 and 3, and flat gradation
curves(USCS).
Gap-graded soils have coefficients of curvature < 1 or >
3, and one or more humps on the gradation curves(USCS).
89
A - a poorly-graded medium SAND B - a well-graded GRAVEL-SAND
(i.e. equal amounts of gravel and sand) C - a gap-graded
COBBLES-SAND D - a sandy SILT E - a typical silty CLAY
Typical grading curves
90
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46
Very coarse
soils
BOULDERS > 200 mm
COBBLES 60 - 200 mm
Coarse
soils
G
GRAVEL
coarse 20 - 60 mm
medium 6 - 20 mm
fine 2 - 6 mm
S
SAND
coarse 0.6 - 2.0 mm
medium 0.2 - 0.6 mm
fine 0.06 - 0.2 mm
Fine
soils
M
SILT
coarse 0.02 - 0.06 mm
medium 0.006 - 0.02 mm
fine 0.002 - 0.006 mm
C CLAY < 0.002 mm
Soil Classification- British Basic Soil Type Group
91
92
Figure 1.7 Plasticity Chart: British System (BS 5930: 1981)
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47
93
94
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48
Soil Classification Example Using British and Unified Soil
Classification System
Example Using British and Unified Soil Classification System
95
96
For classification of Fine grained soils and Fine-grained
fraction of Coarse-Grained
Soils Equation of A Line: Horizontal at PI = 4 to LL = 25.5,
then PI = 0.73 (LL-20) Equation of "U" Line: Vertical at LL = 16 to
PI = 7, then PI = 0.9 (LL-8)
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49
97
Fine- grained soils can exist one of four states: solid,
semisolid, plastic and liquid.
Water is the agent that is responsible for changing the states
of soils.
A soil gets weaker if its water content increases.
Three limits are defined based on the water content that causes
a change of state.
These are the liquid limit the water content that caused the
soil to change from liquid to a plastic state;
The plastic limit the water content that cause the soil to
change from plastic to semisolid; and
The shrinkage limit the water content that caused the soil to
change from a semisolid to a solid state.
All these limiting water contents are found from laboratory
tests
The plasticity index defines the range the water content for
which the soil behaves like a plastic material.
The liquidity index gives a measure of strength.
The soil strength is the lowest at the liquid state and the
highest at the solid states.
98
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50
99
Liquid Limits
Ranges from 35% to 55% moisture content
Normally consolidated soils, (consolidated or densified under
their own weight). If deposited through gentle agitation from slow
moving rivers, deltaic fans, ice melt or quiet marine shore
conditions, the LL value can be much lower. The soil can be classed
as sensitive.
Ranges from 60% to 100% moisture content
Over consolidated soils, (subjected to high surcharge loads,
such as a thick layer of ice, rock formations that have been
denuded or eroded away, or subject to many wetting and drying
cycles).
100
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51
Plastic Limits
Low values below 10% tell us that the soil is normally
consolidated, but sensitive.
Values between 10% and 25% tell us that the soil is normally
consolidated and medium to low sensitivity.
Values above 30% moisture content tell us that the soil is over
consolidated and insensitive.
101
Low Plasticity WL= < 35%
Intermediate High Plasticity WL= < 35% - 50%
High Plasticity WL= < 50% - 70%
Very High Plasticity WL= < 70% - 90%
Extremely High Plasticity WL= > 90%
Plasticity Chart and Classification
102
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52
Soil Description and Classification
Textural classification
Textural soil classification chart (USDA)
Index line for clay is horizontal
Index line for silt is down and towards the left
Index line for sand is upwards towards the left
When all index lines meet, the intersection point descript the
soil
103
Sieve No Opening (mm) Sieve No Opening (mm)
4 4.75 35 0.500
5 4.00 40 0.425
6 3.35 50 0.355
7 2.80 60 0.250
8 2.36 70 0.212
10 2.00 80 0.180
12 1.70 100 0.150
14 1.40 120 0.125
16 1.88 140 0.106
18 1.00 170 0.090
20 0.850 200 0.075
25 0.710 270 0.053
30 0.600 0.6 mm
104
U.S. Standard Sieve Sizes
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53
US Standard Sieve Size British Standard Sieve Size
3 in. (75mm) 75 mm
2 in. (50.0mm) 63 mm
1.5 in. (38.1mm) 50 mm
3/4 in. (19.0mm) 37.5 mm
in. (12.5mm) 20.0 mm
3/8 in. (9.5mm) 14.0 mm
No. 4 (4.75mm) 10.0 mm
No. 10 (2.0mm) 6.3 mm
No. 20 (0.85mm) 5.0 mm
No. 40 (425mm, ) 2.0 mm
No. 60 (0.25mm) 1.18 mm
No. 100 (0.15mm) 0.6 mm
No. 200 (75m, 0.075mm) 0.425 mm
0.3 mm
0.212 mm
0.15 mm
0.063 mm
105
106
BSS, ASTM and ISS Aperture
APERTURE SIZE
BSS Mesh
No.
(410/1969)
ASTM Mesh
No.
(11-70)
ISS
(469/1972)
Microns
4 5 4.00mm 4000
5 6 3.35mm 3353
6 7 2.80mm 2812
7 8 2.36mm 2411
8 10 2.00mm 2057
10 12 1.70mm 1680
12 14 1.40mm 1405
14 16 1.18mm 1204
16 18 1.00mm 1003
18 20 .850mm 850
22 25 .710mm 710
25 30 .600mm 600
30 35 .500mm 500
36 40 .425mm 420
44 45 .355mm 355
52 50 .300mm 300
60 60 .250mm 250
72 70 .212mm 210
85 80 .180mm 180
100 100 .150mm 150
120 120 .125mm 120
150 140 .106mm 105
170 170 .090mm 90
200 200 .075mm 75
240 230 .063mm 63
300 270 .053mm 53
350 325 .045mm 45
400 400 .037mm 37
500 .025mm 35
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54
Phase Relationship/Basic Properties
Serve as indices for better description of soils; physical
state
Soil can be in 2 or 3 phases composition
Solid, liquid and gas; phase diagram
107
108
(a) Soil element in natural state; (b) three phases of soil
element
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55
109
Three separate phases of soil element with volume of solids
equal to one
Saturated soil element with volume of soil solids equal to
one
S : Solid Soil particle
W: Liquid Water (electrolytes)
A: Air Air
110
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56
Phase Diagram
111
112
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57
113
114
-
58
115
Phase Diagram
116
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59
Compaction
Compaction is the densification of a soil by the expulsion of
air and rearrangement of soil particles.
The Proctor test is used to determine the maximum dry unit
weight and the optimum water content and serves as the reference
for field identifications of compaction.
Higher compaction effort increases the maximum dry unit weight
and reduces the optimum water content.
Compaction increases strength, lowers compressibility and
reduces the permeability of soils.
A variety of field equipment is used to check the dry unit
weights achieved in the field. Popular field equipment includes the
sand cone apparatus, the balloon apparatus and the nuclear density
meter.
117
Compaction Rolling and tampering
Reduction of air-void volume
No change in solid volume and water content; increase in
density
Effect permeability
Increase shear strength; bearing capacity
Reduce settlement and damage to structures
118
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60
Compaction
Effectiveness
Nature and type of soil
Water content during compaction
Maximum possible state of compaction; attainable and field
conditions
Type of construction plant
Sheep foot roller
119
Compaction
Degree of compaction
Depend on maximum dry/water content
Increase in water allows soil particles to be pack more closely;
increase in density; beyond certain water limit/content density
reduces
Maximum dry density; at optimum moisture content
Maximum possible state of compaction; attainable and field
conditions
Construction specification; 90% to 95% of the optimum moisture
content; locations and usage of fill ground
120
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61
Degree of compaction
Proper compaction is very important. The degree of compaction
depends on the soil type, compaction method, compactive effort and
the as-compacted moisture content.
121
Compaction When clay is compacted than optimum moisture content,
clay tends to
have a flocculated fabric consisting of platy particles oriented
randomly. When is compacted wetter than optimum moisture content,
clay tends to have a more oriented or dispersed fabric, in which
platy particles are aligned parallel to one another.
Difference in soil fabric leads to differences in various soil
properties,
eg. Drier than optimum moisture content will give higher
hydraulic conductivity than clay with wetter than optimum moisture
content
Eg. Drier than optimum moisture content will have a greater
shear strength than wetter than clay with wetter than optimum
moisture content
122
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62
123
Compaction
Effect of increased compaction effort
Maximum dry density increases
Optimum moisture content decreases
Air-void content remain the same
124
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63
Soil type and its effect on the optimum moisture and density
Well graded; high density
Plasticity and % of fine increase, lower density
125
Field Density Measurement
Core cutter method
Sand material
Steel rammer and dolly; 100mm dia. and 130mm long
Sand replacement method
Dig a hole ; 150mm
Place in sand to measure volume of hole
126
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64
Field Density Measurement
127
Core cutter method Sand material Steel rammer and dolly; 100mm
dia. and 130mm long
Sand replacement method Dig a hole ; 150mm Place in sand to
measure volume of hole
End of Module 1