1 introduction soil mechanics

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INTRODUCTION TO

SOIL MECHANICS


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1.0 Geotechnical Engineering and Soil Mechanics

What is Geotechnical

Engineering ?

• What is Soil? List anything that you know /heard.• What is Mechanics?

• How do they relates?

• What do you think that you will learn in this subject andHOW?

Instruction:On your own – answer all above Q’s (1-2 mins)Compare and discuss your answer with the person next to you (2 - 3 mins)


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The Father of Soil Mechanics Said

Terzaghi (1948):

Soil Mechanics is the

application of laws of

mechanics and hydraulics to

engineering problems dealing

with sediments and other

unconsolidated accumulations

of solid particles produced by

the mechanical and chemical

disintegration of rocks

regardless of whether or not

they contain an admixture of

organic constituent “Karl Terzaghi in 1951


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1.1 Term and Definition

Example of Soils

• Gravel, sand, silt, clay

Engineering Terms• Soft deposits in between top soil and base rock

• BS 892 – any soft or loose deposits that exit naturally, forming

part of earth crust and formed from weathering or disintegration ofrocks or degradation of vegetation.

• BS 1377 : part 1 : 1990 - An assemblage of discrete particles in

the form of deposit, usually of mineral composition but sometimes

of organic origin, which can be separated by gentle mechanicalmeans and which includes variable amounts of water and air (andsometimes of gases). A soil commonly consists of a naturallyoccurring deposit forming part of the earth crust but the term is

also applied to made ground consisting of replaced natural soil orman made materials exhibiting similar behaviour, e.g. crushed rock,crushed-blastfurnace slag, fly-ash.


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1.1 Term and Definition

Usage and Practicality

• Do geologist use the same definition and term – how do they differ

• How do you confirm• What is the problem if they are different – give example

Instruction:On your own – answer all above Q’s (1-2 mins)Compare and discuss your answer with the person next to you (2 - 3 mins)


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1.2 Soil Formation

GEODe II Interactive CD Presentation

-Recap on rock cycle

- Weathering


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1.2 Soil Formation

ROCK

WHEATHERING

PHYSICAL/MECHANICAL Agent – wind, temperature, water, frost,

glacier

Product – boulders, cobbles, gravel, sand

and rock flour Particle props.- similar chemical props. to

parent rock, bulky form; might be angular,

subangular or rounded, cover wide range

of sizes.

CHEMICAL/BOILOGICAL Agent –water, acid, alkali, carbon dioxide

Product – silt, clay minerals

Particle props.- chemical props. differ to

parent rock due to chemical reactionforming crystalline ptcl. of colloidal size (<

0.002 mm) with shape of either plate or

needle like having high specific surface.

Electrochemically active.

COARSE-GRAINED SOIL FINE-GRAINED SOIL


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Product of Physical Weathering

Particle shape

Boulders

(> 200 mm)

Cobbles

(60 - 200 mm)

Jigsaw – G1


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Coarse gravel(20-60 mm)

Fine gravel

(2 -6 mm)

Coarse sand(0.6 – 2 mm)

Medium sand(0.2 – 0.6 mm)

Fine sand(0.06 – 0.2 mm)

Jigsaw – G1


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Particles arrangement

single grain

Structuralarrangement depends

on minerals, how itsbeen transported anddeposited. Theengineering properties

depends oninteraction betweenneighbouring particles.

Strength (shear) – friction between soilparticles. Shear failure- mainly due to soilparticles roll and slide into new structuralarrangement without particles break intosmaller pieces.

Jigsaw – G1


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Product of Chemical Weathering

Particle –cannot be seen with naked eye- fine like flourParticle shape – under high power microscope

Long needle-shapePlate-like shape

Properties – cohesive and plastic, having high specific surface,electrochemically active, consistency depends on waterStrength – depends on cohesion

Jigsaw – G2


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1.5 Clay Minerals

BASIC UNITS

Alumina OctahedronSilica tetrahedron

a) One silica sheet consist of 6units of silica tetrahedron

b) silica sheet symbol

a) One alumina sheet is made of4 units of alumina octahedron

b) Alumina sheet symbol

Jigsaw – G2


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Structure of Clay Minerals

Kaolinit illit montmorillonit(15 m2 /g) (80 m2 /g) (800 m2 /g)

ActivityJigsaw – G2


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1.4 Properties of Clay Minerals

The surface of clay mineral particles – negatively charged

due to

1. Isomorphous subtitution of Al or Si by atoms of lower valency.2. Dissacociation of hydroxyl ion

3. ‘broken bonds’ at the edges of the particles

In dry condition

The negative charges are balanced by exchangeable such as Ca2+,Mg2+, Na+ and K + which are held by electrostatic attraction.

When water is added

Cations and a few anions float around the clay particles. Cations mayexchange.

Jigsaw – G2/G3


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Double-layer

When water is added / in wetcondition

• Cations float and attracted to the negative

surface of clay particle

• Cations tend to move away from each otherbecause of their thermal energy – resulting indiffuse layer of cations called double layer.

• The cation concentration decreasing withincreasing distance from the surface until theconcentration become equal to that of normalwater in the void space.

• The thickness of this layer depends onvalency and concentration of cations. Thehigher the valency and concentaration, the

lesser the thickness of this layer

Jigsaw – G3


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Adsorbed water

What is adsorbed water?

Water layer that formed aroundclay particles.

CLAY

Water layer

How does it formed? or

How does clay attract water (1) / water attracted to clayparticles (2).

(1) – look at clay properties – the role of negative charges and

double layer (explained)

(2) – need to investigate the properties of water molecule.

Jigsaw – G3


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water

Clayparticle

Formation of adsorbedwater

dipolar

Negative charges of clay surface,double layer

Formation of adsorbed water

Jigsaw – G3


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Identification of clay minerals

• X-ray diffarction

• Differential thermal analysis (DTA)

• Atterberg limit – Plasticity test to find Liquid Limit (LL) andPlastic Limit (PL).

• Plasticity Index (PI) = LL - PL

Locations of common clay minerals

in Casagrande Plasticity Chart

(Holtz and Kovacs, 1981)


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1.4 Clay Structure

Clay structures (a) dispersed, (b) flocculated, (c) bookhouse,

(d) turbostratic, (e) example of natural clay• Forces of attraction and repulsion act between adjacent clay mineral

particles

• Repulsion occurs between like charges of the double layer.

• Attraction is due to short range van der Waals forces (independent ofdouble layer characteristics) and decrease rapidly with increasing distancebetween the particles.

• It is the net interparticle forces influence the structural form

• Net repulsion – face-to-face orientation – dispersed structure

• Net attraction – edge-to-face/edge-to-edge – flocculated.


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1.5 Transportation and Deposition of

Weathered Materials

Transportation agent – water, wind, gravity, glacier etc..

• Residual soil

• Glacial Soils

• Alluvial Soils

• Lacustrine and Marine Soils

• Aeolian Soil• Colluvial Soils

• Organic Soils

Transported Soil

Di t ib ti f V i S il T i


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Distribution of Various Soil Types inSemenanjung Malaysia


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Residual Soil

• Top soil / laterite

• Product of chemical and biological attack not transported elsewhereand remains in its original place.

• Characteristics – the types of soil depend on the parent rock,boulders, bedding planes

• Granite – sandy residual soil,

• Igneous and metamorphic – silty and gravelly residual soil

• Sedimentary – clayey residual soil

(a and b) Residual soil profile Jigsaw – G4


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Residual Soil

Residual soil

classificationbased ondegree ofweathering

Jigsaw – G4


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Residual Soil

Jigsaw – G4


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Laterite in Malaysia

• Mainly from weatheredigneous rock due to rainand temperature

• Cemented with iron oxide(reddish brown) whichgives it a high drystrength.

• Strength ranging frompoor to good andgenerally improve withdepth

• Boulders still undergoingweathering process thatmight be unstableespecially in slopingsurface.

• Clay minerals – kaolinite, geothite,hematite and gibbsite

schematic diagram deposits of granitic material around

Bukit Antarabangsa (The Star 21.11.2002)

Jigsaw – G4


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Bukit Lanjan,6-12-2003


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Occurring of hard materials in thick weathering profile (mainly

residual soils) – ripping & blasting method may be expensive(lacking of sufficient information during SI)

Problems associated with residual soil


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Problems associated with residual soil

Problems during excavation


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Transported Soil

• Weathering product transpoted and deposited elsewhere.

• Tranporting agent – water, glacier, wind, etc

• Characteristics – material chemical composition differ from parent /surrounding rock, particle shape – rounded and less angular,uniform particle distribution, laminated soil mass, surface profilenormally not parallel with the profile of parent

• Types include - glacial, alluvial, lacustrine, marine, aeolian dancolluvial soils

(a) Transported soil profile (b) alluvial deposits at river mouthJigsaw – G5


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Transported Soil

Glacial soil• Soil being transported due to melting

glaciers (huge mass of ice).• Glacier’s movement resulting in

weathering product being groundedand transported to a long long

distance.• Characteristics – more rounded

particle shape, wide range of particlesize an hardness, complex and

heterogenous soil coming fromdifferent sources.

• 3 categories (Till, Glasiofluvial

and Glasiolacustrine)Jigsaw – G5

T t d S il


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Transported Soil

Alluvial / Fluvial Soil

• Transported by rivers and streams

• Materials deposited depends on velocity

of the streams• Rapid streams forces silts and clays in

suspension and finally depositeddownstream, sand, gravel and boulders

are deposited upstream.

• Slow water movement may causedeposition of fines before reaching

downstream.• The slowdown of the water velocity

resulting in alluvial fan

Jigsaw – G5

T t d S il


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Transported Soil

Marine and Lacustrine Soils

• Lacustrine – those depositedbeneath lakes – primarily silt andclay having poor to average bearingcapacity.

• Marine – those deposited

underwater except they formed inthe ocean eg. Deltas (river metocean) forming flat terrain.Primarily silt and clay and very soft.

• Both types are rather uniform andconsistently poor and hence morepredictable.

Jigsaw – G5

T t d S il


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Transported Soil

Aeolian Soils

• Those deposited by wind.

• Poorly graded soil (narrowrange of soil particles)because of the strongsorting power of wind,

generally very loose hencefair engineering properties.

• May form horizontal strataor hill (sand dunes) along

beaches or desert. Tendsto migrate downwind

• Prone to erosion and often

have deep gullies.

dust

coarse

sand

Jigsaw – G5


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Sand dunes

Jigsaw – G5

T o ted Soil


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Transported Soil

Colluvial Soils

• Transported downslope bygravity

• Downslope movement either –rapid or slow

Jigsaw – G5

Organic Soil


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Organic Soil

• Soil that contain > 20% organic materials

• Consist of degraded vegetation and other organic matter

• Peat soil > 75% organic material• Top soil ≤ 500 mm, contains high organic materials

• Not suitable for engineering work hence need to be removed beforeengineering work begins

• Engineering properties – low shear strength

and highly compressible

1 6 G l I f ti


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1.6 General Information

Relationship between agricultural soil and engineeringsoil

Tropical Residual Soil – Classification and


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pRecommended Use

Classification of Tropical Residual Soil by degree of weathering(Little, 1969)

Tropical Residual Soil


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Tropical Residual Soil

1 introduction soil mechanics

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