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Page 1: Soil Mechanics

MECHANICS OF SOILSMECHANICS OF SOILS

COURSE INTRODUCTIONCOURSE INTRODUCTION

According to Ralph Pech:

Soil Engineering is an Art

Soil Mechanics is an Engineering Science

Three Attributes of a Successful Soil Engineer:

Knowledge of Precedents (Experience)

F ili it ith S il M h iFamiliarity with Soil Mechanics

Working Knowledge of Geology

Purpose of this Course: To Familiarize the Student with the Fundamental Principles of Soil Mechanics

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The Solution of Soil The Solution of Soil Engineering Problemg g

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Complicating Characteristics of Soil Deposits

1) Soil does not possess a linear or unique stress-strain relationship.

2) S il b h i d d ti d i t2) Soil behavior depends on pressures, time, and environment.3) The soil at essentially every location is different.

I l ll th f il i l d i d d4) In nearly all cases the mass of soil involved is underground and cannot be seen in its entirety but must be evaluated on the basis of small samples obtained from isolated locationsbasis of small samples obtained from isolated locations.

5) Most soils are very sensitive to disturbance from sampling, and thus the behavior measured by a laboratory test may bethus the behavior measured by a laboratory test may be unlike that of the in situ soil.

Nearly all soil problems are statically indeterminate to high degree.

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Applications in Soil Engineering

1) Analysis and Design of Earth Structures such as Dams and Embankments

2) Stability of Artificial and Natural Slopes

3) Foundations Supports for Various Structures

4) Lateral Pressures against Various Structures

5) Prediction of Water Movement through the Soil

6) Improvement of Soil Properties by Chemical and Mechanical Methods

Geotechnical Materials

1) Soils are discrete particles derived from rock minerals and have extreme variability

2) Soils are cheap and readily available construction materials

3) Soils support all structures located above and below ground

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GENERAL PROCEDURE FOR MOST GEOTECHNICAL PROJECTS

D fi P j t C t purpose schedule location plansDefine Project Concept

Site Reconnaissance

purpose, schedule, location, plans

review of information, site inspection

Working Hypothesis Subsurface investigations, soil conditions, design parameters

Model for Analysis

g p

physical, analytical, numerical models

Alternative Schemes

S ifi R d ti

evaluate various solutions

cost, benefit, time, reliability, environmental Specific Recommendations

Plans and Specifications

cost, benefit, time, reliability, environmental impact, etc.

plans and specifications for recommended l tip

Supervision and Consultation

solutions

inspection of construction operations revisions of plans due to new information

Performance Feedback

f p f

observe long range performance

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Figure 1.1 Examples of Geotechnical Engineering Construction

Figure 1.2 Principles of Mechanics

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Fig. 1.3 Branches of Mechanics used in Geotechnical Engineeringused in Geotechnical Engineering

Fig. 1.4 Compression and Distortion

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The Particulate Nature The Particulate Nature of Soil

“The discrete particles that make up soil are not strongly bonded together in the way that the crystal of astrongly bonded together in the way that the crystal of a metal are, and hence the soil particles are relatively free to move with respect to one another.”p“The soil particles are solid and cannot move relative to each other as easily as the elements in a fluid.”y

(Lambe and Whitman, 1979)

It is this basic fact that distinguishes soil mechanics from solid mechanics and fluid mechanics.

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Consequences of the particulate t f ilnature of soil

1st consequence: Nature of soil deformationq“The deformation of a mass of soil is controlled by interactions

between individual particles, especially by sliding (and also adhesion) b t i di id l ti l B lidi i li dbetween individual particles. Because sliding is a nonlinear and irreversible deformation, we must expect that the stress-strain behavior of soil will be strongly nonlinear and irreversible.” {various constitutiveof soil will be strongly nonlinear and irreversible. {various constitutive soil models}

2nd consequence: Role of pore phase -- Chemical interaction “Soil is inherently multiphase, and the constituents of the pore

h ill i fl th t f th i l f d h ff tphase will influence the nature of the mineral surfaces and hence affect the processes of force transmission at the particle contacts. This interaction between the phases is called chemical interaction.” {double p {layer water; plasticity of soils; swelling potential, compression, strength, fluid conductivity}

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Consequences of the particulate t f il ( t’d)nature of soil (cont’d)

3rd consequence: Role of pore phase -- Physical interactionq p p y“Water can flow through soil and thus interact with the

mineral skeleton, altering the magnitude of the forces at the contacts between particles and influencing the compression andcontacts between particles and influencing the compression and shear resistance of the soil.” {effective stress concept and consolidation theory}

4th consequence: Role of pore phase – Sharing the load “When the load applied to a soil is suddenly changed thisWhen the load applied to a soil is suddenly changed, this

change is carried jointly by the pore fluid and by the mineral skeleton. The change in pore pressure will cause water to move through the soil hence the properties of the soil will change withthrough the soil, hence the properties of the soil will change with time (hydrodynamic time lag).” {basis of consolidation theory of Terzaghi, the “father of soil mechanics”. This marked the b i i f d il i i }beginning of modern soil engineering}

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Soil Forming Process

Definition of Soil

Soil - All materials, organic or inorganic, overlying bedrock

Soil are natural aggregates of mineral grains that can be separated by such gentle h i l it ti i t hil k t l t fmechanical means as agitation in water, while rocks are natural aggregates of

minerals connected by strong and permanent cohesive forces.

Based on Origin

Inorganic Soil - derived from chemical and mechanical weathering

Organic Soil - significant parts are derived from growth and decay of plant and animal lifea d a a e

Inorganic Soils

Residual Soil - located at a place where it was formed

Transported Soil - the soil has been moved to another location by gravity, water or wind

Alluvium – river and stream deposits (very heterogeneous mixture of gravels, sands, and p ( y g g , ,silts/clays)

Lacustrine – lake deposits; Marine – salt water deposits (beach, swamps); Deltas – deposits at mouth of streams and rivers

Wind blown – loess – uniform mixture of silts, fine sands and clays

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Nature of Soil

VOIDS

SOLIDS

Soil is composed of particles

Coarse-Grained Soils (large particles)

Volume >> Surface Area(large particles)

Gravitational Force governs BehaviorFRICTION

Surface Area >> Volume

(Sand/Gravel)

Surface Area >> VolumeSurface Force (electrical) governs the behavior

Fine-Grained Soils(small particles)

DOUBLE LAYER

clay

COHESION LAYER

clayDouble layer expands – repulsionDouble layer contracts - attraction(Clay/Silt)

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Dispersive Soil

Replace N + C ++ M ++

+ +

Replace Na - Ca , Mg

Clay

+++

+

Cl

++

++

++

+ Na+

Clay

++

Nearer

+

Farther repulsion

Nearer attraction

++

++

clay

+ clay++

+

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Types of Soil

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Types of SoilTypes of Soil

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Origin of Clay MineralsOrigin of Clay Minerals1) Inheritance. The clay mineral was formed by reactions

that occurred in another area, was transported to its present site, and is stable enough to remain inert in its present environmentpresent environment.

2) Neoformation The clay has precipitated from solution2) Neoformation. The clay has precipitated from solution or has formed from reaction of amorphous material.

3) Transformation. An inherited clay has undergone chemical reaction. Two reactions are possible,

l i h d l t f ti Inamely, ion exchange and layer transformation. In layer transformation, the arrangements of octahedral, tetrahedral or fixed interlayer cations are modifiedtetrahedral, or fixed interlayer cations are modified.

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Importance of Soil Mineralogy in Geotechnical EngineeringGeotechnical Engineering

It is a controlling factor determining the sizes, shapes, and g g , p ,surface characteristics of particles in the soil. It determines interactions with fluid phases. Together, these factors determine:determine:

PlasticitySwellingSwellingCompressionStrengthgFluid conductivity behavior

It is essential when dealing problems involving environmental g gproblems, such as: safe disposal and containment of hazardous and nuclear wastes; clean up of contaminated sites; and protection of ground water Compositional characteristics ofprotection of ground water. Compositional characteristics of soils and their relation to the long-term physical and chemical properties are of most concerned.

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STRUCTURE OF CLAY MINERALS

Two fundamental building laws can be noticed with clay minerals

1) Silica – tetrahedron unit 2) Octahedral Unit (with Al+3 or Mg+2)

(one tetrahedron shares 3 oxygens with other tetrahedrons)

Al (OH)6-3

with other tetrahedrons)

(each Si has one O-aton and shares 3 other oxygens – SiO4 – unit has neg. charge of 1

(each OH is shared by 2 Al-ions

charge of 1

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Basic Structural Unit of Clay MineralBasic Silicate Unit: (1) Silicon Tetrahedron

Block Symbol =

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Basic Silicate Unit: (2) Aluminum or Magnesium Aluminum or Magnesium

Octahedron

Block Symbol =Block Symbol

Gibbsite sheet: if cations are mainly AluminumGibbsite sheet: if cations are mainly AluminumBrucite sheet: if cations are mainly Magnesium

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KAOLINITE consists basically of repeating layers of one tetrahedral (silica) sheet and one octahedral (alumina or gibsite) sheet.

Successive layers are held together by hydrogen bonds between the hydroxyls of the octahedral sheet and the oxygen of the tetrahedral Since the hydrogen bond is very strong it preventssheet and the oxygen of the tetrahedral. Since the hydrogen bond is very strong, it prevents hydration and allows the layers to stack up to make up 70 to 100 layers thick.

Halloysite is related to kaoline. It somehow became hydrated between layers causing distortions and random stacking of the crystal lattice so that it is tubular in shape The water can be easilyand random stacking of the crystal lattice so that it is tubular in shape. The water can be easily driven out from between layers by heating or air drying and the process is irreversible.

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MONTMORILLONITE sometimes called smectite – composed of 2 silica and one alumnica (gibsite) sheet

Because the bonding by van deer Waal’s forces (common attraction between matter) between the g y ( )tops of the silica sheets is weak and there is a net negative charge deficiency in the octahedral sheet, water and exchangeable ions can enter and separate the layers – very susceptible to swelling

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ILLITE – it has similar structure as montmorillonite but the interlayers are bonded together with a potassium ions.

Illites have strong bonds of potassium atom that fills the hexagonal holethe hexagonal hole.

Other minerals: chlorite, vermiculite, etc.

Identification of Clay Minerals

1) X-ray diffraction – crystal structure will diffract x-rays – compare

2) DTA – heated in electrical furnace and certain changes in temperature occur because of particular structure of the clay minerals

3) El i i i3) Electrom microscopy – quatitative

4) Use of Plasticity Charts

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Table 8 Summarized Properties of Clay-Mineral Groups

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Summary of Occurrence of Clay Minerals in Soils

Clay Mineral Group OccurrenceClay Mineral Group OccurrenceKaolinite Highly weathered soils with good drainage. Generally

in older soils. Common in tropical and subtropical areas

Chl it A f t hi t k C i iChlorite Areas of metamorphic parent rock. Common in marine sediments and sedimentary rocks. Not normally present in dominant proportion

Cl Mi I il d i d f th i f di t kClay Mica In soils derived from weathering of sedimentary rocks. Dominant mineral in slate and shale

Montmorillonite Results from weathering of volcanic rocks or ash under poor drainage Common in sediments of arid areasdrainage. Common in sediments of arid areas

fIn flush sites, where cations are being added, and perhaps silica too, e.g. tropical swamps, then there is a build-up to montomorillonite or illite. As a simplification in tropical areas, there are kaolinites in the hillslopes, and montmorillonites in the valley. In temperate regions, there is less extreme variation and illite is more common and vermiculite are common topsoils for someis less extreme variation and illite is more common and vermiculite are common topsoils for some reasons.

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Most Common Clay Minerals

a) Kaolinite - two-layer unit of Gibbsite and Silica Sheet (7.2oA)

- strong linkage by hydrogen bonding and secondary valence forces

b) Illite - three-layer unit (9.5oA) formed by silica sheet sandwiched by 2 Gibbsite sheets

t bl b di b d l f d t i i- very stable bonding by secondary valence forces and potassium ions.

c) Montmorillonite - three-layer unit (9.5oA) formed by silica sheet sandwiched by 2 Gibbsite sheets

- very weak bonding by secondary valence forces and exchangeable ion linkagevery weak bonding by secondary valence forces and exchangeable ion linkage

- isomorphous substitution of magnesium or iron for aluminum, changes the character of montmorillonites

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Casagrande Plasticity ChartPlasticity Chart

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Atterberg Limits for Atterberg Limits for Common Clay Mineralsy

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4.6 Clay-water system behavior

The clay particles interact with soil water (pore water) surrounding them. All clay particles are y p (p ) g y pcharged and they can therefore attract ions on the surface

The origin of the charge is the result of:

-Isomorphous substitution

-Imperfections in crystal lattice or broken bonds at edges

4.6.1 Isomorphous Substitution

4.6.2 Broken bonds: The clay crystal is continuous in two directions, however at the edges there must be broken bonds between oxygen and silicon and between oxygen and aluminum. The amount of this charge per unit weight of clay increases with decreasing particle size, because the proportion of edge area to total area is increased. These b k b d h d (H+) h d l (OH ) i f h Th broken bonds attract hydrogen (H+) or hydroxyl (OH-) ions from the pore water. The ease with which the hydrogen ion can be exchanged increases as the pH of the pore water increases.

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BROKEN BONDS

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ATTRACTIVE AND REPULSIVE FORCES

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Flocculated and Dispersive Structures

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Colloidal micella

Interaction between negatively charged mineral particles and the surrounding aqueous l ti (Fi 1)solution (Fig. 1)

-Surface of clay particles are negatively charged.

-Attract the positively charged cations in the porewater.

Adjacent water particles undergo alteration and become structured-Adjacent water particles undergo alteration and become structured.

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Double-Layer Theory

Basic units have sheeted structure – flatly – shaped part. Surface dimensions many times greater than part. ThicknessSurface dimensions many times greater than part. Thickness affected by cations in porewater and nature of individual particlesColloidal micella – interaction between negatively charged g y gmineral particles and the surrounding aqueous solution.

Surface of clay particles are negatively charged

Attract the positively charged cations in the porewaterAttract the positively charged cations in the porewater.

Adjacent water particles undergo alteration and become structured

Stern electric double layer – cations formed a positively charged one together ith the negati el charged clacharged zone together with the negatively-charged clay particle surface.

Cations distribute themselves around the negatively charged particles Greatest density near the surface decreasing

Cations formed a positively charged zone together with the negatively-charged clay particle surface

particles. Greatest density near the surface – decreasing with distance

g y p

Cations distribute themselves around the negatively charged particles. Greatest density near the surface – decreasing

ith di t

Fig. 2 Stern Double Layer

with distance

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Three possible mechanisms that water molecules can be electrically attracted y

toward the surface of clay particlesa) Attraction between the negatively charged faces of clay and the ) g y g y

positive ends of dipolesb) Attraction between cations in the double layer and the negatively

charged ends of dipoles. The cations are in turn attracted by the g p ynegatively charged faces of clay particles.

c) Sharing of the hydrogen atoms in the water molecules by hydrogen bonding between the oxygen atoms in the clay particles and thebonding between the oxygen atoms in the clay particles and the oxygen atoms in the water molecules.

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Double-Layer Water(After T. W. Lambe, Compacted Clay:

Structure, Trans. ASCE, vol. 125, 1960)

(a) Typical kaolinite particle, 10000 by 1000 A.

(b) Typical montmorillonite particle, 1000 by 10 A.

Since the innermost layer of double-layer water is very strongly held by a clay particle, it is referred to as adsorbed water.

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Moisture in Soils

4

Solid5 1 2 3

123drainage

air drying

4

5 air drying

oven drying

Categories of water surrounding clay particles

1) Absorbed water – held by powerful electrical forces virtually in solid state and very thin1) Absorbed water held by powerful electrical forces virtually in solid state and very thin (0.005 mm)

- cannot be removed by oven drying at 110oC

2) C b d b d i t b i d i2) Can be removed by oven drying not by air drying.

3) Capillary water held by surface tension, removed by air drying

4) Gravitational water – removable by drainage4) Gravitational water removable by drainage

5) Chemically combined water – water hydration

within crystal structure – not removable by over drying

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EFFECT OF WATER CONTENT

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A) Consistency of Cohesive Soils) y

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I) Plasticity of SoilsI) Plasticity of Soils

The term “plasticity” is normally encountered and applied in fine-grained soils, such as clay.

The plasticity property of clayey soils is due to the f d bl l t th t i tt t dpresence of double layer water that is attracted on

the surface of clay minerals.

The plasticity property of soils can be reflected from the Atterberg Limits:the Atterberg Limits:

Liquid limit Plastic limitPlastic limitShrinkage limit

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A) Atterberg Limits) g(For soil fraction passing #40 sieve)1) Liquid Limit (LL)

It is the lowest water content that a soil can behave like a viscous liquidviscous liquid. It is the water content of the soil at which the soil, when place in a standard liquid limit device in a specified manner, will cause 25 number of blows to close a specific width groove for a specified length.

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A) Atterberg Limits A) Atterberg Limits (cont’d)( )

2) Plastic Limit (PL)It is the lowest water content that a given soil can stillIt is the lowest water content that a given soil can still behave plastically, i.e., above PL the soil can be deformed without volume change or cracking and will retain its deformed shapedeformed shape. It is the water content of the soil at which the soil, when rolled into 3mm diameter threads, will start to crumble.

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A) Atterberg Limits A) Atterberg Limits (cont’d)( )

3) Shrinkage LimitIt is the highest water content at which the soil mass stops to shrink or decrease its pvolume upon further decrease of water content.It is determined as the water content after just enough water is added to fill all thejust enough water is added to fill all the voids of a dry pat of soil.

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B) Physical Significance B) Physical Significance of Atterberg Limitsg

The greater the amount of water a soil contains theThe greater the amount of water a soil contains, the less interaction there will be between adjacent particles and the more the soil should behave like aparticles and the more the soil should behave like a liquid.

Generally, the water that is attracted to the surface of soil should not behave as a liquid. Thus, a soil, which has a greater tendency to attract water to the particle surface, should have a larger liquid limit.

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B) Physical Significance … (cont’d)

Plasticity Index (PI) = LL – PLPlasticity Index (PI) LL PLThis is an index of soil’s strength. PI decreases, strength increases.g

Flow Index (If) = Slope of flow curve. ( f) pA flow curve is a plot of the water content vs. the number of blows (in log scale).PI

fIPI

Toughness Index (It) = fI

PI

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B) Physical Significance … (cont’d)

Liquidity Index (LI) = PLN −ωq y ( )

Expected Soil Behavior:

PI

Expected Soil Behavior:LI < 0 Brittle behavior0<LI<1 Plastic beha ior0<LI<1 Plastic behaviorLI>1 Viscous liquid behavior

ωN<PL

ωN >LL

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C) Relationship of Atterberg Limits to Composition of Soilto Composition of Soil

Atterberg limits for a soil are related to the amount of water that is attracted to the surfaces of the soilwater that is attracted to the surfaces of the soil particles. Since the surface area increases significantly with decreasing size of soil particles, it g y g p ,may be expected that the amount of attracted water will be largely influenced by the amount of clay that is present in the soil. On the basis of this reasoning, Skempton (1953) defined a quantity called “Activity.”

2thfii htb%PI)A(Activity =

m2thanfinerweightby% µ

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Approximate values for the specific pp psurface of some common soil grains

(after Mitchell, 1976)( , )

Soil grain Specific surface(m2/g)(m /g)

Clay minerals:M t ill it U t 840Montmorillonite Up to 840Illite 65-200Kaolinite 10-20

Clean sand 2 x 10-4Clean sand 2 x 10 4

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Activity of Clay (cont’d)Activity of Clay (cont d)

Activity of clay may provide information

l ti t llirelative to swelling potential (relative swell at 1psi psurcharge) and stability.

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Atterberg limits of clay Atterberg limits of clay mineral

Mineral Liquid Plastic Plasticity limit limit Index

Montmorillon 140-710 54 to 98 67 to 656Montmorillonite

140 710 54 to 98 67 to 656

Illite 79 to 110 45 to 60 33 to 67Illite 79 to 110 45 to 60 33 to 67

Kaolinite 38 to 59 27 to 35 11 to 23Kaolinite 38 to 59 27 to 35 11 to 23

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SIEVE ANALYSIS

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GRAIN SIZE CURVE

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GRAIN SIZE CURVES

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GRAIN SIZE CURVES

Well-graded

Gap-graded

Poorly-graded

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Fig. 4.2 Sequence of Mineral Transformation Observed from Soil Thin Sections

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Fig. 1.25 Profile residual soil arearesidual soil area,

indicating stages of transition from rock to

soilsoil

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Fig. 4.6 a – Grain Size Distribution of Samples from Line I, Site No. 1

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Fig. 4.7 – Cumulative Percentage of Grain Size Distribution from Site 1 and Site 2 Profiles

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HYDROMETER ANALYSIS

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Fig. 7.1 Schematic representation of sedimentaion of different particle sizes, and sampling for particle size analysis (Modified after Chu et al. [140])

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Fig. 4.27 Representation of

di t tisedimentation process

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Fig. 1.11 Soil-separate size limits of M.I.T., FAA, AASHTO, Corps of

E i d USBREngineers and USBR

Sieve AnalysisAnalysis

Hydrometer Analysisy

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UNIFIED SOIL CLASSIFICATION

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Example 1Classify the following soil using USSC. The soil data available for classification are as follows:

D = 0 2mmD10 = 0.2mmD30 = 0.73mmD60 = 1 27mmD60 1.27mm% passing No. 4 sieve = 77.5 (This means 22.5% gravel)% passing No. 200 = 4.0% pass g o 00 0

Solution:1) Passing No. 200 sieve <50% (coarse-grained soil)2) Passing No. 4 sieve >50% (Sand)3) For <5% passing #200 SW? or SP?4) Use gradation curve, i.e., calculate Cu and Cc

Cu = D60/D10 = 6.35 >6 Cc = (D30)2/(D10D60) = 2.1 (1<2.1<3)5) The soil is SW. Soil description is “well-graded sand with gravel.”

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AASHTO CLASSIFICATION

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AASHTO Classification System

Major Division/Group:Major Division/Group:

a) >35% passing #200: A-4, A-5, A-6, A-7(Silt Clay Materials)(Silt-Clay Materials)

b) <35% passing #200: A-1, A-2, A-3(G l M t i l )

Required Soil Data:(Granular Materials)

From sieve analysis: Passing #10, #40 and #200 sievesAtterberg limits: Liquid Limit (LL) and Plasticity Index (PI)

Group Index (GI) = 0.2a + 0.005ac + 0.02bdwhere: a = %passing #200 – 35 (1 ≤ a ≤ 40)

b = %passing #200 15 (1 ≤ b ≤ 40)b = %passing #200 – 15 (1 ≤ b ≤ 40)c = LL - 40 (1 ≤ c ≤ 20)d = PI – 10 (1 ≤ d ≤ 20)

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Example 2Example 2Classify the following soil using AASHTO The soilClassify the following soil using AASHTO. The soil data available for classification are as follows:

% passing #200 =5 7% passing #200 =5.7% passing #40 = 38.6% passing #10 = 67.4

Are the data of LL and PI necessary? Why?y y

Ans The soil is: A 1b (0)Ans. The soil is: A-1b (0)

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Embankment MaterialsEmbankment Materials

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Fig. 31 Location Plan of the SecondPlan of the Second Bangkok International Airport Field TestAirport Field Test Site

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Fig. 26 Index Properties of Foundation Soils (Section A/1)

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Fig. 27 Strength and Compressibility Characteristics of Foundation Soils (Section 1A/1)

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Mineralogy and Chemistry… Bangkok Clay and Ariake Clay (Soils and Foundation, Vol. 40, No. 1, pp. 11-21, 2000)

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Ariake ClayAriake Clay

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Bangkok Clay

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Soil PhasesSoil Phases

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II) Soil Fabric

If the net interparticle forces between two clay particle in suspension is repulsion, each clay particle will settle individually and will form a dispersed structure of clayand will form a dispersed structure of clay.

If it is attraction, flocs will be formed and these flocs will settle, forming a flocculated clay.

High salt concentration will depress the double layer of clay andHigh salt concentration will depress the double layer of clay and, hence, will decrease the force of repulsion. Attractive force (largely attributed by Van der Waal’s force) will dominate and effect an orientation approaching parallelism (face-to-face type)

salt type flocculation– salt-type flocculation.

In a soil-water suspension with low salt concentration, the electrostatic force of attraction (positive at the edge and negative at face of clay particle) may produce a flocculation with an orientation approaching perpendicular array– non-salt flocculation.

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