soil mechanics arranged)

8/6/2019 Soil Mechanics Arranged)

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oil mechanics

omeBased on part of the GeotechniCAL reference pac

by Prof. John Atkinson, City University, Lon

oil mechanics

q Basic mechanics of soils

q Description and classificationq Effective stress

q Volume change

q Shear strength

oil mechanics describes the mechanical behaviour of a granular material as it is compressed or shea

d as water flows though it.

o design of structures in the ground we need to describe

q soil strength

q soil compressibility and stiffness

q seepage

asic theories

q friction

q logarithmic compression

q plasticity

q Darcy seepage

Volume change Back to Soil mecha

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oil mechanics

r Compression and swelling

r Consolidation

r Compaction

Saturated soil contains only mineral grains and water. Both are relatively incompressible so t

volume can only change if water can drain out. In unsaturated soil volume changes can occu

air compresses or bleeds out. In both cases loading will bring the grains closer together and th

specific volume will reduce.

1. Reduction in volume leads to

2. increase in strength

3. increase in stiffness

4. settlement of foundations

If soil is unloaded it will swell as the grains move apart. Swelling leads to reduction in strengt

and stiffness and heave of excavations.

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asic mechanics of soils

ack to Soil MechanicsBased on part of the GeotechniCAL reference pac

by Prof. John Atkinson, City University, Lo

Basic mechanics of soils

q Analysis of stress and strain

q Strength

q Stiffness

q Material behaviour

oads from foundations and walls apply stresses in the ground. Settlements are caused by strains in tound. To analyse the conditions within a material under loading, we must consider the stress-straihaviour. The relationship between a strain and stress is termed stiffness. The maximum value of st

at may be sustained is termed strength.

Analysis of stress and strain Back to Basic mechanics of

q Special stress and strain states

q Mohr circle construction

q Parameters for stress and strain

resses and strains occur in all directions and to do settlement and stability analyses it is often neces

relate the stresses in a particular direction to those in other directions.

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ormalress= Fn / A

hear stress= Fs / A

normal strain = z / zo

shear strain= h / zo

ote that compressive stresses and strains are positive, counter-clockwise shear stress and strain areositive, and that these are total stresses (see effective stress).

Special stress and strain states Analysis of stress and s

n general, the stresses and strains in the threeimensions will all be different.

here are three special cases which are importantn ground engineering:

General case princpal stresses

xially symmetric or triaxial statestresses and strains in two dorections are equal.'x = 'y and x = y

elevant to conditions near relatively small

oundations, piles, anchors and other concentratedoads.

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lane strain:train in one direction = 0

y = 0

elevant to conditions near long foundations,mbankments, retaining walls and other longructures.

ne-dimensional compression:train in two directions = 0

x = y = 0

elevant to conditions below wide foundations orelatively thin compressible soil layers.

niaxial compression'x = 'y = 0

his is an artifical case which is only possible foroil is there are negative pore water pressures.

Mohr circle construction Back to Analysis of stress and strain Forward to Param

alues of normal stress and shear stress must relate toparticular plane within an element of soil. In general,

he stresses on another plane will be different.

o visualise the stresses on all the possible planes, araph called the Mohr circle is drawn by plotting anormal stress, shear stress) point for a plane at everyossible angle.

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There are special planes on which the shear streis zero (i.e. the circle crosses the normal stressaxis), and the state of stress (i.e. the circle) candescribed by the normal stresses acting on thes

planes; these are called the principal stressesand '3 .

Parameters for stress and strain Analysis of stress and s

common soil tests, cylindrical samples are used in which the axial and radial stresses and strains aincipal stresses and strains. For analysis of test data, and to develop soil mechanics theories, it is uscombine these into mean (or normal) components which influence volume changes, and deviator (earing) components which influence shape changes.

stress strain

meanp' = ('a + 2'r) / 3

s' = 'a + 'r) / 2

ev = V/V = (a + 2r)

n = (a + r)

eviatorq' = ('a - 'r)

t' = ('a - 'r) / 2es = 2 (a - r) / 3

= (a - r)

the Mohr circle construction t' is the radius of the circle and s'fines its centre.

ote: Total and effective stresses are related to pore pressure u:p' = p - u

s' = s - u

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q' = q

t' = t

Strength Back to Basic mechanics of

q Types of failure

q Strength criteria

q Typical values of shear strength

he shear strength of a material is most simply described as the maximum shearress it can sustain: When the shear stress is increased, the shear strain increases;ere will be a limiting condition at which the shear strain becomes very large ande material fails; the shear stress fis then the shear strength of the material. The

mple type of failure shown here is associated with ductile or plastic materials. Ife material is brittle (like a piece of chalk), the failure may be sudden andtastrophic with loss of strength after failure.

Types of failure Back to Stre

aterials can fail under different loading conditions. In each case, however, failure is associated we limiting radius of the Mohr circle, i.e. the maximum shear stress. The following common example shown in terms of total stresses:



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hearinghear strength = f

nf= normal stress at failure

niaxial extensionensile strength tf= 2f

niaxial compressionompressive strength cf= 2f

ote:ater has no strength f= 0.



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ence vertical and horizontal stresses are equal and the Mohr circle becomes a point.

Strength criteria

Back to Stre

strength criterion is a formula which relates the strength of a material to some other parameters: the material parameters and may include other stresses.

or soils there are three important strength criteria: the correct criterion depends on the nature of the d on whether the loading is drained or undrained.

General, course grained soils will "drain" very quickly (in engineering terms) following loading.hefore development of excess pore pressure will not occur; volume change associated with incremeeffective stress will control the behaviour and the Mohr-Coulomb criteria will be valid.

ne grained saturated soils will respond to loading initially by generating excess pore water pressurd remaining at constant volume. At this stage the Tresca criteria, which uses total stress to represen

ndrained behaviour, should be used. This is the short term or immediate loading response. Once theore pressure has dissapated, after a certain time, the effective stresses have incresed and the Mohr-oulomb criterion will describe the strength mobilised. This is the long term loading response.

q Tresca criterion

q Mohr-Coulomb (c=0) criterion

q Mohr-Coulomb (c>0) criterion



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Tresca criterion Back to Strength criteria Forward to Mohr-Coulomb (c

he strength is independent of the normal stressnce the response to loading simple increases theore water pressure and not the effective stress.

he shear strength fis a material parameter which is

nown as the undrained shear strength su.

= (a - r) = constant

Mohr-Coulomb (c'=0) criterion Back to Strength criteria Forward to Mohr-Coulo(c

he strength increases linearly with increasingormal stress and is zero when the normal stress isro.= '

ntan'

is the angle of friction

the Mohr-Coulomb criterion the materialrameter is the angle of friction and materialshich meet this criterion are known as frictional. Inils, the Mohr-Coulomb criterion applies when the

ormal stress is an effective normal stress.

Mohr-Coulomb (c'>0) criterion Back to Strength cri

he strength increases linearly with increasingormal stress and is positive when the normal stresszero.



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= c' + 'n tan'

is the angle of frictionis the 'cohesion' intercept

soils, the Mohr-Coulomb criterion applies whene normal stress is an effective normal stress. Inils, the cohesion in the effective stress Mohr-Coulomb criterion is not the same as the cohesion (or

ndrained strength su) in the Tresca criterion.

Typical values of shear strength Back to Stre

Undrained shear strength su (kPa)

Hard soilsu > 150 kPa

tiff soil su = 75 ~ 150 kPa

irm soil su = 40 ~ 75 kPa

oft soil su = 20 ~ 40kPa

Very soft soil su < 20 kPa

Drained shear strength c (kPa) (deg)

Compact sands 0 35 - 45

Loose sands 0 30 - 35

Unweathered overconsolidated clay

ritical state 018 ~25

eak state 10 ~ 25 kPa20 ~28

esidual 0 ~ 5 kPa 8 ~ 15

ften the value ofc' deduced from laboratory test results (in the shear testing apperatus) may appeardicate some shar strength at ' = 0. i.e. the particles 'cohereing' together or are 'cemented' in some wften this is due to fitting a c', ' line to the experimental data and an 'apparent' cohesion may beduced due to suction or dilatancy.

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Produced by Dr. Leslie Davison, University of the West of England, Bristol, May 2000

in association with Prof. Sarah Springman, Swiss Federal Technical Institute, Zurich

mailto://[email protected]/http://fbe.uwe.ac.uk/mailto://[email protected]/http://www.igt.ethz.ch/http://www.igt.ethz.ch/mailto://[email protected]/http://fbe.uwe.ac.uk/mailto://[email protected]/


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oil classification

escription & classification

ic characteristics of soils

Soil as an engineering material

Size range of grains

r Identification

Shape of grains

r SAND grains

r CLAY grainsr Specific surface

Composition of grains

Structure or fabric

gins, formation and mineralogy

Origins of soils from rocks

Weathering of rocks

Clay minerals

Transportation and deposition

Loading and drainage history

ding and composition

Coarse soils

r Particle size tests

r Simulation

r Typical grading curves

r Grading characteristics

r Sieve analysis example

Fine soils

r Consistency

r Simulation

r Plasticity index

r Plasticity chart

r Activity

Specific gravity

ume-weight properties

Volume: the soil model

Simulation

r Degree of saturation

r Air-voids content

Masses of solid and water

Densities and unit weights

Laboratory measurements

r Water content

r Unit weight

Field measurements

rent state of soil

Deposition and erosion

Ageing

Density index

Liquidity index

Predicting stiffness and strength

system for description and classification

BS description system

Back to home pageBased on part of the GeotechniCAL reference p

by Prof. John Atkinson, City University, L

Soil description and classification

q Basic characteristics of soils

q Origins, formation and mineralogy

q Grading and composition

q Volume-weight properties

q Current state of soil

q British Standard system

It is necessary to adopt a formal system of soil description and classification in orddescribe the various materials found in ground investigation. Such a system must b

comprehensive (covering all but the rarest of deposits), meaningful in an engineercontext (so that engineers will be able to understand and interpret) and yet relativconcise. It is important to distinguish between description and classification:

Description of soil is a statement describing the physical nature and state of the socan be a description of a sample, or a soil in situ. It is arrived at using visualexamination, simple tests, observation of site conditions, geological history, etc.

Soil classification is the separation of soil into classes or groups each having simicharacteristics and potentially similar behaviour. A classification for engineeringpurposes should be based mainly on mechanical properties, e.g. permeability, stiffstrength. The class to which a soil belongs can be used in its description.

Description and classification

Basic characteristics of soils

q Soil as an engineering material

q Size range of grainsq Shape of grains

q Composition of grains

q Structure or fabric

Soils consist of grains (mineral grains,rock fragments, etc.) with water and air inthe voids between grains. The water andair contents are readily changed bychanges in conditions and location: soilscan be perfectly dry (have no water

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oil classification

Definitions of terms

British Soil Classification Systemcontent) or be fully saturated (have no aircontent) or be partly saturated (with bothair and water present). Although the sizeand shape of the solid (granular) contentrarely changes at a given point, they canvary considerably from point to point.

First of all, consider soil as a engineering

material - it is not a coherent solidmaterial like steel and concrete, but is aparticulate material. It is important tounderstand the significance of particle size, shape and composition, and of a soil'sinternal structure or fabric.


Soil as an engineering material

The term "soil" means different things to different people: To a geologist it represethe products of past surface processes. To a pedologist it represents currently occuphysical and chemical processes. To an engineer it is a material that can be:

built on: foundations to buildings, bridges.built in: tunnels, culverts, basements.built with: roads, runways, embankments, dams.supported: retaining walls, quays.

Soils may be described in different ways by different people for their differentpurposes. Engineers' descriptions give engineering terms that will convey some sea soil's current state and probable susceptibility to future changes (e.g. in loading,drainage, structure, surface level).

Engineers are primarily interested in a soil's mechanical properties: strength, stiff

permeability. These depend primarily on the nature of the soil grains, the current s

the water content and unit weight.



q Aids to size identification

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oil classification

The range of particle sizes encountered in soil is very large: from boulders with a

controlling dimension of over 200mm down to clay particles less than 0.002mm (2

Some clays contain particles less than 1 in size which behave as colloids, i.e. d

settle in water due solely to gravity.

In theBritish Soil Classification System, soils are classified into named Basic Soil

groups according to size, and the groups further divided into coarse, medium and fsub-groups:

Very coarse

soils

BOULDERS > 200 mm

COBBLES 60 - 200 mm

Coarse

soils

GGRAVEL

coarse 20 - 60 mm

medium 6 - 20 mm

fine 2 - 6 mm

SSAND

coarse 0.6 - 2.0 mm

medium 0.2 - 0.6 mm

fine 0.06 - 0.2 mm

Fine

soils

MSILT

coarse 0.02 - 0.06 mm

medium 0.006 - 0.02 mm

fine 0.002 - 0.006 mm

C CLAY < 0.002 mm


Aids to size identification

Soils possess a number of physical characteristics which can be used as aids to sizidentification in the field. A handful of soil rubbed through the fingers can yield thfollowing:

SAND (and coarser) particles are visible to the naked eye.SILT particles become dusty when dry and are easily brushed off hands and bootsCLAY particles are greasy and sticky when wet and hard when dry, and have to bscraped or washed off hands and boots.

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oil classification


Shape of grains

q Shape characteristics of SAND grains

q Shape characteristics of CLAY grains

q Specific surface

The majority of soils may be regarded as either SANDS or CLAYS:

SANDS include gravelly sands and gravel-sands. Sand grains are generally brokenparticles that have been formed by physical weathering, or they are the resistantcomponents of rocks broken down by chemical weathering. Sand grains generallya rotund shape.

CLAYS include silty clays and clay-silts; there are few pure silts (e.g. areas formewindblown Less). Clay grains are usually the product of chemical weathering or and soils. Clay particles have a flaky shape.

There are major differences in engineering behaviour between SANDS and CLAYg. in permeability, compressibility, shrinking/swelling potential). The shape and sithe soil grains has an important bearing on these differences.

Shape of grains

Shape characteristics of SAND grains

SAND and larger-sized grains are rotund. Coarse soil grains (silt-sized, sand-sizelarger) have different shape characteristics and surface roughness depending on thamount of wear during transportation (by water, wind or ice), or after crushing inmanufactured aggregates. They have a relatively low specific surface (surface area

Click on a link below to see the shapeRounded: Water- or air-worn; transported sediments

Irregular: Irregular shape with round edges; glacial

sediments (sometimes sub-divided into 'sub-rounded' and

'sub-angular')Angular: Flat faces and sharp edges; residual soils, grits

Flaky: Thickness small compared to length/breadth; clays

Elongated: Length larger than breadth/thickness; scree,

broken flagstoneFlaky & Elongated: Length>Breadth>Thickness; broken schists and slates

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oil classification

Shape of grains

Shape characteristics of CLAY grains

CLAY particles are flaky. Their thickness is very small relative to their length &breadth, in some cases as thin as 1/100th of the length. They therefore have high tohigh specific surface values. These surfaces carry a small negative electrical chargthat will attract the positive end of water molecules. This charge depends on the so

mineral and may be affected by an electrolite in the pore water. This causes someadditional forces between the soil grains which are proportional to the specific surThus a lot of water may be held asadsorbed water within a clay mass.

Shape of grains

Specific surface

q Examples

Specific surface is the ratio of surface area per unit wight.Surface forces are proportional to surface area (i.e. to d).Self-weight forces are proportional to volume (i.e. to d).

ThereforeSurface force

1

self weight forces d

Also, specific surface =area

1

* volume d

Hence, specific surface is a measure of the relative contributions of surface forces self-weight forces.

The specific surface of a 1mm cube of quartz ( = 2.65gm/cm) is 0.00023 m/N



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oil classification

SAND grains (size 2.0 - 0.06mm) are close to cubes or spheres in shape, and havespecific surfaces near the minimum value.

CLAY particles are flaky and have much greater specific surface values.

Examples of specific surface

The more elongated or flaky a particle is the greater will beits specific surface.

Click on the following examples:cubes, rods, sheets

Examples of mineral grain specific surfaces:

Mineral/SoilGrain width

d (m)Thickness

Specific Surface

m/N

Quartz grain 100 d 0.0023

Quartz sand 2.0 - 0.06 d 0.0001 - 0.004

Kaolinite 2.0 - 0.3 0.2d 2

Illite 2.0 - 0.2 0.1d 8

Montmorillonite 1.0 - 0.01 0.01d 80

See also clay minerals


Structure or fabric

Natural soils are rarely the same from one point in the ground to another. The contand nature of grains varies, but more importantly, so does the arrangement of these

The arrangement and organisation of particles and other features within a soil mastermed its structure or fabric. This includes bedding orientation, stratification, laythickness, the occurrence of joints and fissures, the occurrence of voids, artefacts,

roots and nodules, the presence of cementing or bonding agents between grains.

Structural features can have a major influence on in situ properties.

q Vertical and horizontal permeabilities will be different in alternating layersfine and coarse soils.

q The presence of fissures affects some aspects of strength.q The presence of layers or lenses of different stiffness can affect stability.q The presence of cementing or bonding influences strength and stiffness.

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oil classification


Origins, formation and mineralogy

q Origins of soils from rocks

q Weathering of rocks

q Clay minerals

q Transportation and deposition

q Loading and drainage history

Soils are the results of geological events (except for the very small amount producman). The nature and structure of a given soil depends on the geological processesformed it:breakdown of parent rock: weathering, decomposition, erosion.transportation to site of final deposition: gravity, flowing water, ice, wind.environment of final deposition: flood plain, river terrace, glacial moraine, lacustor marine.subsequent conditions of loading and drainage - little or no surcharge, heavy surcdue to ice or overlying deposits, change from saline to freshwater, leaching,

contamination.



All soils originate, directly or indirectly, from solid rocks in the Earth's crust:

igneous rockscrystalline bodies of cooled magmae.g. granite, basalt, dolerite, gabbro, syenite, porphyry

sedimentary rockslayers of consolidated and cemented sediments, mostly formed in bodies of water lakes, etc.)e.g. limestone, sandstones, mudstone, shale, conglomerate

metamorphic rocksformed by the alteration of existing rocks due to heat from igneous intrusions (e.gmarble, quartzite, hornfels) or pressure due to crustal movement (e.g. slate, schist,gneiss).


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oil classification

Physical weatheringPhysical or mechanical processes taking place on the Earth's surface, including theactions of water, frost, temperature changes, wind and ice; cause disintegration anwearing. The products are mainly coarse soils (silts, sands and gravels). Physicalweathering produces Very Coarse soils and Gravels consisting of broken rock partbut Sands and Silts will be mainly consists of mineral grains.

Chemical weathering

Chemical weathering occurs in wet and warm conditions and consists of degradatidecomposition and/or alteration. The results of chemical weathering are generally soils with separate mineral grains, such as Clays and Clay-Silts. The type of claymineral depends on the parent rock and on local drainage. Some minerals, such asquartz, are resistant to the chemical weathering and remain unchanged.

quartzA resistant and enduring mineral found in many rocks (e.g. granite, sandstone). It principal constituent of sands and silts, and the most abundant soil mineral. It occuequidimensional hard grains.haematite

A red iron (ferric) oxide: resistant to change, results from extreme weathering. It isresponsible for the widespread red or pink colouration in rocks and soils. It can forcement in rocks, or a duricrust in soils in arid climates.micasFlaky minerals present in many igneous rocks. Some are resistant, e.g. muscovite;are broken down, e.g. biotite.clay mineralsThese result mainly from the breakdown of feldspar minerals. They are very flakytherefore have very large surface areas. They are major constituents of clay soils,although clay soil also contains silt sized particles.


Clay minerals

Clay minerals are produced mainly from the chemical weathering and decompositfeldspars, such as orthoclase and plagioclase, and some micas. They are small in s



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oil classification

and very flaky in shape.

The key to some of theproperties of clay soils, e.g.plasticity, compressibility,swelling/shrinkage potential,lies in the structure of clayminerals.

There are three main groups ofclay minerals:kaolinites(include kaolinite, dickite andnacrite) formed by thedecomposition of orthoclasefeldspar (e.g. in granite); kaolinis the principal constituent inchina clay and ball clay.illites(include illite and glauconite) are the commonest clay minerals; formed by the

decomposition of some micas and feldspars; predominant in marine clays and shalg. London clay, Oxford clay).montmorillonites (also called smectites or fullers' earth minerals) (include calcium and sodiummomtmorillonites, bentonite and vermiculite) formed by the alteration of basic ignrocks containing silicates rich in Ca and Mg; weak linkage by cations (e.g. Na+, Cresults in high swelling/shrinking potential

For more information on mineralogy see http://mineral.gly.bris.ac.uk/mineralogy/


Transportation and deposition

The effects of weathering and transportation largely determine the basic nature ofsoil (i.e. the size, shape, composition and distribution of the grains). The environminto which deposition takes place, and subsequent geological events that take placthere, largely determine the state of the soil, (i.e. density, moisture content) and th

structure or fabric of the soil (i.e. bedding, stratification, occurrence of joints orfissures, tree roots, voids, etc.)

TransportationDue to combinations of gravity, flowing water or air, and moving ice. In water or grains become sub-rounded or rounded, grain sizes are sorted, producing poorly-gdeposits. In moving ice: grinding and crushing occur, size distribution becomes wideposits are well-graded, ranging from rock flour to boulders.

DepositionIn flowing water, larger particles are deposited as velocity drops, e.g. gravels in riv

http://mineral.gly.bris.ac.uk/mineralogy/http://mineral.gly.bris.ac.uk/mineralogy/


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terraces, sands in floodplains and estuaries, silts and clays in lakes and seas. In stilwater: horizontal layers of successive sediments are formed, which may change wtime, even seasonally or daily.

q Deltaic & shelf deposits: often vary both horizontally and vertically.q From glaciers, deposition varies from well-graded basal tills and boulder cl

poorly-graded deposits in moraines and outwash fans.q In arid conditions: scree material is usually poorly-graded and lies on slopeq Wind-blown Less is generally uniformly-graded and false-bedded.


Loading and drainage history

The current state (i.e. density and consistency) of a soil will have been profoundlyinfluenced by the history of loading and unloading since it was deposited. Changedrainage conditions may also have occurred which may have brought about changwater content.

Loading /unloading history

Initial loadingDuring deposition the load applied to a layer of soil increases as more layers aredeposited over it; thus, it is compressed and water is squeezed out; as depositioncontinues, the soil becomes stiffer and stronger.

UnloadingThe principal natural mechanism of unloading is erosion of overlying layers. Unlocan also occur as overlying ice-sheets and glaciers retreat, or due to large excavatimade by man. Soil expands when it is unloaded, but not as much as it was initiallycompressed; thus it stays compressed - and is said to be overconsolidated. The degof overconsolidation depends on the history of loading and unloading.

Drainage history

Chemical changesSome soils initially deposited loosely in saline water and then inundated with freshwater develop weak collapsing structure. In arid climates with intermittent rainy

periods, cycles of wetting and drying can bring minerals to the surface to form acemented soil.

Climate changesSome clays (e.g. montmorillonite clays) are prone to large volume changes due towetting and drying; thus, seasonal changes in surface level occur, often causingfoundation damage, especially after exceptionally dry summers. Trees extract watefrom soil in the process of evapotranspiration; The soil near to trees can therefore shrink as trees grow larger, or expand following the removal of large trees.



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Grading and composition

q Coarse soils

q Fine soils

q Specific gravity

The recommended standard for soil classification is the British Soil ClassificationSystem, and this is detailed in BS 5930 Site Investigation.


Coarse soils

q Particle size tests

q Typical grading curves

q Grading characteristics

q Sieve analysis example

Coarse soils are classified principally on the basis of particle size and grading.

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

SSAND

coarse 0.6 - 2.0 mm

medium 0.2 - 0.6 mm

fine 0.06 - 0.2 mm

Coarse soils

Particle size tests

The aim is to measure the distribution of particle sizes inthe sample. When a wide range of sizes is present, thesample will be sub-divided, and separate tests carried outon each sub-sample. Full details of tests are given in BS1377: "Methods of test for soil for civil engineeringpurposes".



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Particle-size tests

Wet sieving to separate fine grains from coarse grains iscarried out by washing the soil specimen on a 60m sievemesh.Dry sieving analyses can only be carried out on particles >60 m. Samples (with fines removed) are dried and shakenthrough a nest of sieves of descending size.

Sedimentation is used only for fine soils. Soil particlesare allowed to settle from a suspension. The decreasingdensity of the suspension is measured at time intervals.Sizes are determined from the settling velocity and times recorded. Percentagesbetween sizes are determined from density differences.

Particle-size analysis

The cumulative percentage quantities finer than certain sizes (e.g. passing a given sieve mesh) are determined by weighing. Points are then plotted of% finer (passiagainst log size. A smooth S-shaped curve drawn through these points is called a

grading curve. The position and shape of the grading curve determines the soil clGeometrical grading characteristics can be determined also from the grading cur

Coarse soils

Typical grading curves

Both the position and the shape of the grading curve for a soil can aid its identity adescription.Some typical grading curves are shown in the figure:A - a poorly-graded medium SAND (probably estuarine or flood-plain alluvium)



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B - a well-graded GRAVEL-SAND (i.e. equal amounts of gravel and sand)C - a gap-graded COBBLES-SANDD - a sandy SILT (perhaps a deltaic or estuarine silt)E - a typical silty CLAY (e.g. London clay, Oxford clay)

Coarse soils

Grading characteristics

A grading curve is a useful aid to soil description. Grading curves are often includground investigation reports. Results of grading tests can be tabulated using geomproperties of the grading curve. These properties are called grading characteristi

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 sized10

Uniformity coefficientCu = d60 / d10

Coefficient of gradationCk= d30 / d60 d10

Both Cu and Ck will be 1 for a single-sized soilCu > 5 indicates a well-graded soil

Cu < 3 indicates a uniform soil

Ck between 0.5 and 2.0 indicates a well-graded soil

Ck < 0.1 indicates a possible gap-graded soil



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Coarse soils

Sieve analysis example

The results of a dry-sieving test are given below, together with the grading analysigrading curve. Note carefully how the tabulated results are set out and calculated. grading curve has been plotted on special semi-logarithmic paper; you can also doanalysis using a spreadsheet.

Sieve meshsize (mm)

Massretained (g)

Percentageretained

Percentagefiner (passing)

14.0 0 0 100.0

10.0 3.5 1.2 98.8

6.3 7.6 2.6 86.2

5.0 7.0 2.4 93.8

3.35 14.3 4.9 88.9

2.0 21.1 7.2 81.7

1.18 56.7 19.4 62.3

0.600 73.4 25.1 37.2

0.425 22.2 7.6 29.6

0.300 26.9 9.2 20.4

0.212 18.4 6.3 14.1

0.150 15.2 5.2 8.9

0.063 17.5 6.0 2.9

Pan 8.5 2.9

TOTAL 292.3 100.0

The soil comprises: 18% gravel, 45% coarse sand, 24% medium sand, 10% fine sa3% silt, and is classified therefore as:a well-graded gravelly SAND



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Fine soils

q Consistency limits and plasticity

q Plasticity index

q The plasticity chart and classification

q Activity

In the case of fine soils (e.g. CLAYS and SILTS), it is the shape of the particles rathan their size that has the greater influence on engineering properties. Clay soils hflaky particles to which water adheres, thus imparting the property ofplasticity.

Fine soils

Consistency limits and plasticity

Consistency varies with the water content of the soil. The consistency of a soil canrange from (dry) solidto semi-solidtoplastic to liquid(wet). The water contents awhich the consistency changes from one state to the next are called consistency li(or Atterberg limits).

Two of these are utilised in the classification of fine soils:Liquid limit (wL) - change of consistency from plastic to liquid

Plastic limit (wP) - change of consistency from brittle/crumbly to plastic

Measures of liquid and plastic limit values can be obtained from laboratory tests.

Fine soils

Plasticity index



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The consistency of most soils in the ground will be plastic or semi-solid. Soil strenand stiffness behaviour are related to the range of plastic consistency. The range owater content over which a soil has a plastic consistency is termed the Plasticity I(IP or PI).

IP= liquid limit - plasticlimit

= wL - wP

Fine soils

The plasticity chart and classification

In the BSCS fine soils are divided into ten classes based on their measured plastici

index and liquid limit values: CLAYS are distinguished from SILTS, and five diviof plasticity are defined:

Low plasticity wL = < 35%

Intermediate plasticity wL = 35 - 50%

High plasticity wL = 50 - 70%

Very high plasticity wL = 70 - 90%

Extremely high plasticity wL = > 90%

A plasticity chart is provided to aid

classification.

http://glossary%28%27gloss_b.htm/#BSCS')http://glossary%28%27gloss_b.htm/#BSCS')


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Fine soils

Activity

So-called 'clay' soils are not 100% clay. The proportion of clay mineral flakes (< 2size) in a fine soil affects its current state, particularly its tendency to swell and shrwith changes in water content. The degree of plasticity related to the clay content icalled the activity of the soil.

Activity= P / (% clay particles)

Some typical values are:

Mineral Activity Soil Activity

Muscovite 0.25 Kaolin clay 0.4-0.5

Kaolinite 0.40 Glacial clay and loess 0.5-0.75

Illite 0.90 Most British clays 0.75-1.25

Montmorillonite > 1.25 Organic estuarine clay > 1.25


Specific gravity

Specific gravity (Gs) is a property of the mineral or rock material forming soil grai

It is defined as

Method of measurementFor fine soils a 50 ml density bottle may be used; for coarse soils a 500 ml or 1000jar. The jar is weighed empty (M1). A quantity of dry soil is placed in the jar and t

weighed (M2). The jar is filled with water, air removed by stirring, and weighed ag

(M3). The jar is emptied, cleaned and refilled with water - and weighed again (M4



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[The range of Gs for common soils is 2.64 to 2.72]


Volume-weight properties

q Volumes of solid, water and air: the soil model

q Masses of solid and water: water content

q Densities and unit weights

q Laboratory measurements

q Field measurements

The volume-weight properties of a soil define its state. Measures of the amount ofspace, amount of water and the weight of a unit volume of soil are required inengineering analysis and design.

Soil comprises three constituent phases:Solid: rock fragments, mineral grains or flakes, organic matter.Liquid: water, with some dissolved compounds (e.g. salts).Gas: air or water vapour.

In natural soils the three phases are intermixed. To aid analysis it is convenient to

consider a soil model in which the three phases are seen as separate, but still in thecorrect proportions.


Volumes of solid, water and air: the soil model



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q Degree of saturation

q Air-voids content

The soil model is given dimensional values for the solid, water and air componentTotal volume,V = Vs + Vw + Va

Since the amounts of both water and air are variable, the volume of solids present taken as the reference quantity. Thus, the following relational volumetric quantitie

be defined:

Note also that:n = e / (1 + e)e = n / (1 - n)v = 1 / (1 - n)

Typical void ratios might be 0.3 (e.g. for a dense, well graded granular soil) or 1.5for a soft clay).


Degree of saturation



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The volume of water in a soil can only vary between zero (i.e. a dry soil) and thevolume of voids; this can be expressed as a ratio:

For a perfectly dry soil:Sr = 0

For a saturatedsoil:Sr = 1

Note: In clay soils as the amount water increases the volume and therefore the volof voids will also increase, and so the degree of saturation may remain at Sr = 1 w

the actual volume of water is increasing.


Air-voids content

The air-voids volume, Va , is that part of the void space not occupied by water.

Va = Vv - Vw

= e - e.Sr

= e.(1 - Sr)



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Air-voids content, Av

Av = (air-voids volume) / (total volume)

= Va / V= e.(1 - Sr) / (1+e)

= n.(1 - Sr)

For a perfectly dry soil:

Av = nFor a saturatedsoil:Av = 0


Masses of solid and water: water content

The mass of air may be ignored. The mass of solid particles is usually expressed interms of their particle density or grain specific gravity.

Grain specific gravity

Hence the mass of solid particles in a soil

Ms = Vs .Gs .w(w = density of water = 1.00Mg/m)

[Range of Gs for common soils: 2.64-2.72]

Particle densitys = mass per unit volume of particles

= Gs .w

The ratio of the mass of water present to the mass of solid particles is called the wcontent, or sometimes the moisture content.



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From the soil model it can be seen thatw = (Sr .e .w) / (Gs .w)

Giving the useful relationship:w .Gs = Sr .e


Densities and unit weights

Density is a measure of the quantity ofmass in a unit volume of material.Unit weight is a measure of the weightof a unit volume of material.

There are two basic measures of density or unit weight applied to soils: Dry densitmeasure of the amount ofsolidparticles per unit volume. Bulk density is a measurthe amount ofsolid+ waterper unit volume.

The preferred units of density are:Mg/m, kg/m or g/ml.

The corresponding unit weights are:



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Water content, w= (mass of water) / (mass of drysoil)

= (37.82 - 34.68) / (34.68 - 16.16)

= 0.169

Percentage water content = 16.9 %

Laboratory measurements

Unit weight

Clay soils: Specimens are usually prepared in the form of regular geometric shapeg. prisms, cylinders) of which the volume is easily computed.Sands and gravels: Specimens have to be placed in a container to determine volug. a cylindrical can).

Example

A soil specimen had a volume of 89.13 ml, a mass before drying of 174.45 g and adrying of 158.73 g; the water content was 9.9 %. Determine the bulk and dry densand unit weights.

Bulk density

= (mass of specimen) / (volume ofspecimen)

= 174.45 / 89.13 g/ml

= 1.957 Mg/m

[1 g/ml = 1 Mg/m]

Unit weight

= 9.81m/s x Mg/m

= 19.20 kN/m

Dry density

d= (mass after drying) / (volume)

= 158.73 / 89.13

= 1.781 Mg/m

d= / (1 + w)

= 1.957 / (1+0.099)

= 1.781 Mg/m

Dry unit weight

d = / (1 + w)

= 19.20 / (1+0.099)

= 17.47 kN/m



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Field measurements

Measurements taken in the field are mostly to determine density/unit weight. The common application is the determination of the density of rolled and compacted fig. in road bases, embankments, etc.

Note: These methods are covered in detail by BS1377. You should understand the

general principle that density is calculated from the mass and volume of a sample.

a sample of known volume is obtained depends on the nature of the soil. You are n

expected to remember the details of each method.

The core cutter method

This method is suitable for soft fine grained soils.

A steel cylinder is driven into the ground, dug out and thesoil shaved off level. The mass of soil is found byweighing and deducting the mass of the cylinder. Smallsamples are taken from both ends and the water contentdetermined.

The sand-pouring cylinder method

This method is suitable for stony soils

Using a special tray with a hole in the centre, a hole isformed in the soil and the mass of soil removed is weighed.

The volume of the hole is calculated from the mass ofclean dry running sand required to fill the hole.

The sand-pouring cylinder is used to fill the hole in acontrolled manner. The mass of sand required to fill thehole is equal to the difference in the weight of the cylinder

before and after filling the hole, less an allowance for thesand left in the cone above the hole.

Bulk density = (mass of soil) / (volume of core cutter or hole)




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Current state of soil

q Soil history: deposition and erosion

q Soil history: ageing

q Density index (relative density)

q Liquidity index

q Predicting stiffness and strength from index properties

The state of soil is essentially the closeness of packing of the grains in the range:Closely packed Loosely packed

Dense Loose

Low water content High water content

Strong and stiff Weak and soft

The important indicators of the current state of a soil are:current stresses: vertical and horizontal effective stressescurrent water content: effecting strength and stiffness in fine soilsliquidity index: indicates state in fine soils

density index: indicates state of compaction in coarse soilshistory of loading and unloading: degree of overconsolidation

Engineering operations (e.g. excavation, loading, unloading, compaction, etc.) on bring about changes in its state. Its initial state is the result of processes of erosiondeposition. It is possible for the engineer to predict changes that could result from proposed engineering operation: changes from the soil's current state to a new futustate.


Soil history: deposition and erosion

Original depositionMost soils are formed in layers or lenses by deposition from moving water, ice or

One-dimensional compression occurs as overlying layers are added. Vertical and

horizontal stresses increase with deposition.

ErosionErosion causes unloading; stresses decrease; some vertical expansion occurs.

Plastic strain has occurred; the soil remains compressed, i.e. overconsolidated.

Subsequent changesSubsequent changes may occur in the depositional environment: further loading/unloading due to glaciation, land movement, engineering; and ageing processes.

http://glossary%28%27gloss_o.htm/#ONEDCOMPRESSION')http://glossary%28%27gloss_p.htm/#PLASTICSTRAIN')http://glossary%28%27gloss_p.htm/#PLASTICSTRAIN')http://glossary%28%27gloss_o.htm/#ONEDCOMPRESSION')


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Soil history: ageing

The term ageing includes processes that occur with time, except loading and unloa

Ageing processes are independent of changes in loading.

Vibration and compactionCoarse soils can be made more dense by vibration or compaction at essentially coneffective stress

CreepFine soils creep and continue to compress and distort at constant effective stress afprimary consolidation is complete.

Cementing and bondingIntergranular cementing and bonding occurs due to deposition of minerals fromgroundwater, e.g. calcium carbonate; disturbance due to excavation fractures thebonding and reduces strength.

WeatheringPhysical and chemical changes take place in soils near the ground surface due to thinfluence of changes in rainfall and temperature.

Changes in salinityChanges in the salinity of groundwater are due to changes in relative sea and landlevels, thus soil originally deposited in sea water may later have fresh water in its such soils may be prone to sudden collapse.


Density index (relative density)

The void ratio of coarse soils (sands and gravels) varies with the state of packing

between the loosest practical state in which it can exist and the densest. Someengineering properties are affected by this, e.g.shear strength, compressibility,

permeability.

It is therefore useful to measure the in situ state and this can be done by comparingin situ void ratio (e) with the minimum and maximum practical values (emin and e

to give a density indexD

http://glossary%28%27gloss_s.htm/#SHEARSTRENGTH')http://glossary%28%27gloss_c.htm/#COEFFCOMPRESS')http://glossary%28%27gloss_p.htm/#PERMEABILITY')http://glossary%28%27gloss_p.htm/#PERMEABILITY')http://glossary%28%27gloss_c.htm/#COEFFCOMPRESS')http://glossary%28%27gloss_s.htm/#SHEARSTRENGTH')


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oil classification

emin is determined with soil compacted densely in a metal mould

emax is determined with soil poured loosely into a metal mould

Density index is also known as relative densityRelative states of compaction are defined:

Density index State of compaction

0-15% Very loose

15-35 Loose

35-65 Medium

65-85 Dense

85-100% Very dense


Liquidity index

In fine soils, especially clays, the current state is dependent on the water content wrespect to the consistency limits (or Atterberg limits). The liquidity index (L or L

provides a quantitative measure of the current state:

wherewP = plastic limit and

wL = liquid limit

Significant values of IL indicating the consistency of the soil are:

http://glossary%28%27gloss_c.htm/#CONSISTENCYLIMITS')http://glossary%28%27gloss_c.htm/#CONSISTENCYLIMITS')


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oil classification

IL < 0 semi-plastic solid or solid

0 < IL < 1 plastic

1 < IL liquid


Predicting stiffness and strength from indexproperties

Preliminary estimates of strength and stiffness can provide a useful basis for earlydesign and feasibility studies, and also the planning of more detailed testingprogrammes. The following suggestions have been made; they are simple, but notnecessarily reliable, and should be not be used in final design calculations.

Undrained shear strength

su = 170 exp(-4.6 L) kN/m

[Schofield and Wroth (1968)]su = (0.11 + 0.37 P) 'vo kN/m

where 'vo = vertical effective stress in situ

[Skempton and Bjerrum (1957)]

StiffnessThe slope of the critical state line may be estimated from:

= P .Gs / 461

[After Skempton and Northey (1953)]

The compressibility index may be estimated from:

Cc = ln10 = P Gs / 200

(where P is in percentage units)

Description and classificationttp://fbe.uwe.ac.uk/public/geocal/SoilMech/classification/default.htm (29 of 33)23/03/2005 2:08:15
http://glossary%28%27%27%29c.htm/#CSLhttp://glossary%28%27gloss_c.htm/#COMPRINDEX')http://glossary%28%27gloss_c.htm/#COMPRINDEX')http://glossary%28%27%27%29c.htm/#CSL


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BS system for description and classification

q BS description system

q Definitions of terms used in description

q British Soil Classification System (BSCS)

BS 5930 Site Investigation recommends the terminology and a system for describi

and classifying soils for engineering purposes. Without the use of a satisfactory syof description and classification, the description of materials found on a site wouldmeaningless or even misleading, and it would be difficult to apply experience to fuprojects.


BS description system

A recommended protocol for describing a soil deposit uses ninecharacteristics; theshould be written in the following order:

compactnesse.g. loose, dense, slightly cementedbedding structuree.g. homogeneous or stratified; dip, orientationdiscontinuitiesspacing of beds, joints, fissuresweathered statedegree of weatheringcolourmain body colour, mottlinggrading or consistencye.g. well-graded, poorly-graded; soft, firm, hardSOIL NAMEe.g. GRAVEL, SAND, SILT, CLAY; (upper case letters) plus silty-, gravelly-, witfines, etc. as appropriatesoil class(BSCS) designation (for roads & airfields) e.g. SW = well-graded sand

geological stratigraphic name(when known) e.g. London clay

Not all characteristics are necessarily applicable in every case.

Example:(i) Loose homogeneous reddish-yellow poorly-graded medium SAND (SP), Floodalluvium(ii) Dense fissured unweathered greyish-blue firm CLAY. Oxford clay.



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Definitions of terms used in description

A table is given in BS 5930 Site Investigation setting out a recommended fieldindentification and description system. The following are some of the terms listed

use in soil descriptions:

Particle shapeangular, sub-angular, sub-rounded, rounded, flat, elongateCompactnessloose, medium dense, dense (use a pick or driven peg, or density index )

Bedding structurehomogeneous, stratified, inter-stratifiedBedding spacingmassive(>2m), thickly bedded (2000-600 mm), medium bedded (600-200 mm), thbedded (200-60 mm), very thinly bedded (60-20 mm), laminated (20-6 mm), thinl

laminated (2m), widely spaced (2000-6mm), medium spaced (600-200 mm), closely spaced (200-60 mm), very closely sp(60-20 mm), extremely closely spaced (


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oil classification

GRAVEL

G GW 0 - 5 Well-graded GRAVEL

GPu/GPg 0 - 5Uniform/poorly-gradedGRAVEL

G-F GWM/GWC 5 - 15Well-graded silty/clayeyGRAVEL

GPM/GPC 5 - 15Poorly graded silty/clayeyGRAVEL

GF GML, GMI... 15 - 35 Very silty GRAVEL [plastisub-group...]

GCL, GCI... 15 - 35Very clayey GRAVEL [..symbols as below]

SAND

S SW 0 - 5 Well-graded SAND

SPu/SPg 0 - 5 Uniform/poorly-graded SA

S-F SWM/SWC 5 - 15 Well-graded silty/clayey SA

GPM/GPC 5 - 15Poorly graded silty/clayeySAND

SF SML, SMI... 15 - 35Very silty SAND [plasticitygroup...]

SCL, SCI... 15 - 35Very clayey SAND [..symbas below]

Fine soils >35% fines Liquid limit%

SILT M

MG Gravelly SILT

MS Sandy SILT

ML, MI... [Plasticity subdivisions as fCLAY]

CLAY C

CG Gravelly CLAY

CS Sandy CLAY

CL 90 CLAY of extremely highplasticity

Organic soils O [Add letter 'O' to group sym

Peat Pt[Soil predominantly fibrousorganic]



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oil classification

Produced by Dr. Leslie Davison, University of the West of England, Bristol, May 20

in association with Prof. Sarah Springman, Swiss Federal Technical Institute, Zuric

mailto://[email protected]/http://fbe.uwe.ac.uk/mailto://[email protected]/http://www.igt.ethz.ch/http://www.igt.ethz.ch/mailto://[email protected]/http://fbe.uwe.ac.uk/mailto://[email protected]/


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oil mechanics

ack to home pageBased on part of the GeotechniCAL reference pac

by Prof. John Atkinson, City University, Lo

oil description and classification

q Basic characteristics of soils

q Origins, formation and mineralogy

q Grading and composition

q Volume-weight properties

q Current state of soil

q British Standard system

is necessary to adopt a formal system of soil description and classification in order to describe therious materials found in ground investigation. Such a system must be comprehensive (covering all e rarest of deposits), meaningful in an engineering context (so that engineers will be able to

nderstand and interpret) and yet relatively concise. It is important to distinguish between descriptiod classification:

escription of soil is a statement describing the physical nature and state of the soil. It can be a

scription of a sample, or a soil in situ. It is arrived at using visual examination, simple tests,bservation of site conditions, geological history, etc.

oil classification is the separation of soil into classes or groups each having similar characteristics aotentially similar behaviour. A classification for engineering purposes should be based mainly onechanical properties, e.g. permeability, stiffness, strength. The class to which a soil belongs can beed in its description.

escription and classification

asic characteristics of soils

q Soil as an engineering material

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oil mechanics

q Size range of grains

q Shape of grains

q Composition of grains

q Structure or fabric

oils consist of grains (mineral grains, rock fragments,

c.) with water and air in the voids between grains. Theater and air contents are readily changed by changes innditions and location: soils can be perfectly dry (have noater content) or be fully saturated (have no air content) orpartly saturated (with both air and water present).

lthough the size and shape of the solid (granular) contentrely changes at a given point, they can vary considerablyom point to point.

rst of all, consider soil as a engineering material - it isot a coherent solid material like steel and concrete, but isparticulate material. It is important to understand thegnificance of particle size, shape and composition, and ofsoil's internal structure or fabric.


oil as an engineering material

he term "soil" means different things to different people: To a geologist it represents the products ost surface processes. To a pedologist it represents currently occurring physical and chemicalocesses. To an engineer it is a material that can be:

uilt on: foundations to buildings, bridges.uilt in: tunnels, culverts, basements.uilt with: roads, runways, embankments, dams.pported: retaining walls, quays.

oils may be described in different ways by different people for their different purposes. Engineers'scriptions give engineering terms that will convey some sense of a soil's current state and probable

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sceptibility to future changes (e.g. in loading, drainage, structure, surface level).

ngineers are primarily interested in a soil's mechanical properties: strength, stiffness, permeability.

hese depend primarily on the nature of the soil grains, the current stress, the water content and uniteight.


ize range of grains

q Aids to size identification

he range of particle sizes encountered in soil is very large: from boulders with a controlling dimens

over 200mm down to clay particles less than 0.002mm (2m). Some clays contain particles less th

in size which behave as colloids, i.e. do not settle in water due solely to gravity.

theBritish Soil Classification System, soils are classified into named Basic Soil Type groups

cording to size, and the groups further divided into coarse, medium and fine sub-groups:

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

SSAND

coarse 0.6 - 2.0 mm

medium 0.2 - 0.6 mm

fine 0.06 - 0.2 mm

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Fine

soils

MSILT

coarse 0.02 - 0.06 mm

medium 0.006 - 0.02 mm

fine 0.002 - 0.006 mm

C CLAY < 0.002 mm

ze range of grains

Aids to size identification

oils possess a number of physical characteristics which can be used as aids to size identification in t

eld. A handful of soil rubbed through the fingers can yield the following:

AND (and coarser) particles are visible to the naked eye.LT particles become dusty when dry and are easily brushed off hands and boots.LAY particles are greasy and sticky when wet and hard when dry, and have to be scraped or washef hands and boots.


hape of grains

q Shape characteristics of SAND grains

q Shape characteristics of CLAY grains

q Specific surface

he majority of soils may be regarded as either SANDS or CLAYS:

ANDS include gravelly sands and gravel-sands. Sand grains are generally broken rock particles thave been formed by physical weathering, or they are the resistant components of rocks broken downemical weathering. Sand grains generally have a rotund shape.

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LAYS include silty clays and clay-silts; there are few pure silts (e.g. areas formed by windblowness). Clay grains are usually the product of chemical weathering or rocks and soils. Clay particlesve a flaky shape.

here are major differences in engineering behaviour between SANDS and CLAYS (e.g. inrmeability, compressibility, shrinking/swelling potential). The shape and size of the soil grains has

mportant bearing on these differences.

hape of grains

hape characteristics of SAND grains

AND and larger-sized grains are rotund. Coarse soil grains (silt-sized, sand-sized and larger) havefferent shape characteristics and surface roughness depending on the amount of wear duringansportation (by water, wind or ice), or after crushing in manufactured aggregates. They have alatively low specific surface (surface area).

ick on a link below to see the shapeounded: Water- or air-worn; transported sediments

regular: Irregular shape with round edges; glacial sediments (sometimes

b-divided into 'sub-rounded' and 'sub-angular')ngular: Flat faces and sharp edges; residual soils, grits

aky: Thickness small compared to length/breadth; clays

ongated: Length larger than breadth/thickness; scree, broken flagstone

aky & Elongated: Length>Breadth>Thickness; broken schists and slates

hape of grains

hape characteristics of CLAY grains

LAY particles are flaky. Their thickness is very small relative to their length & breadth, in some cathin as 1/100th of the length. They therefore have high to very high specific surface values. These

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rfaces carry a small negative electrical charge, that will attract the positive end of water moleculeshis charge depends on the soil mineral and may be affected by an electrolite in the pore water. Thisuses some additional forces between the soil grains which are proportional to the specific surface.

hus a lot of water may be held asadsorbed water within a clay mass.

hape of grains

pecific surface

q Examples

pecific surface is the ratio of surface area per unit wight.Surface forces are proportional to surface area (i.e. to d).

Self-weight forces are proportional to volume (i.e. to d).

hereforeSurface force

1

self weight forces d

lso, specific surface =area

1

* volume d

ence, specific surface is a measure of the relative contributions of surface forces and self-weight

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rces.

he specific surface of a 1mm cube of quartz ( = 2.65gm/cm) is 0.00023 m/N

AND grains (size 2.0 - 0.06mm) are close to cubes or spheres in shape, and have specific surfaces ne minimum value.

LAY particles are flaky and have much greater specific surface values.

xamples of specific surface

he more elongated or flaky a particle is the greater will be its specificrface.

ick on the following examples:

bes, rods, sheets

xamples of mineral grain specific surfaces:

Mineral/SoilGrain width

d (m)Thickness

Specific Surface

m/N

Quartz grain 100 d 0.0023

Quartz sand 2.0 - 0.06 d 0.0001 - 0.004

Kaolinite 2.0 - 0.3 0.2d 2

lite 2.0 - 0.2 0.1d 8

Montmorillonite 1.0 - 0.01 0.01d 80

ee also clay minerals


tructure or fabric

atural soils are rarely the same from one point in the ground to another. The content and nature ofains varies, but more importantly, so does the arrangement of these.

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he arrangement and organisation of particles and other features within a soil mass is termed itsructure or fabric. This includes bedding orientation, stratification, layer thickness, the occurrence ints and fissures, the occurrence of voids, artefacts, tree roots and nodules, the presence of cementibonding agents between grains.

ructural features can have a major influence on in situ properties.

q Vertical and horizontal permeabilities will be different in alternating layers of fine and coarsesoils.

q The presence of fissures affects some aspects of strength.q The presence of layers or lenses of different stiffness can affect stability.q The presence of cementing or bonding influences strength and stiffness.



q Origins of soils from rocks

q Weathering of rocks

q Clay minerals

q Transportation and deposition

q Loading and drainage history

oils are the results of geological events (except for the very small amount produced by man). Theture and structure of a given soil depends on the geological processes that formed it:

reakdown of parent rock: weathering, decomposition, erosion.ansportation to site of final deposition: gravity, flowing water, ice, wind.

nvironment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine.bsequent conditions of loading and drainage - little or no surcharge, heavy surcharge due to ice or

verlying deposits, change from saline to freshwater, leaching, contamination.

rigins, formation and mineralogy

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ll soils originate, directly or indirectly, from solid rocks in the Earth's crust:

neous rocksystalline bodies of cooled magmag. granite, basalt, dolerite, gabbro, syenite, porphyry

dimentary rocksyers of consolidated and cemented sediments, mostly formed in bodies of water (seas, lakes, etc.)g. limestone, sandstones, mudstone, shale, conglomerate

etamorphic rocksrmed by the alteration of existing rocks due to heat from igneous intrusions (e.g. marble, quartzite,

ornfels) or pressure due to crustal movement (e.g. slate, schist, gneiss).


Weathering of rocks

hysical weatheringhysical or mechanical processes taking place on the Earth's surface, including the actions of water,ost, temperature changes, wind and ice; cause disintegration and wearing. The products are mainlyarse soils (silts, sands and gravels). Physical weathering produces Very Coarse soils and Gravelsnsisting of broken rock particles, but Sands and Silts will be mainly consists of mineral grains.

hemical weatheringhemical weathering occurs in wet and warm conditions and consists of degradation by decompositi

d/or alteration. The results of chemical weathering are generally fine soils with separate mineralains, such as Clays and Clay-Silts. The type of clay mineral depends on the parent rock and on locaainage. Some minerals, such as quartz, are resistant to the chemical weathering and remain unchan

uartzresistant and enduring mineral found in many rocks (e.g. granite, sandstone). It is the principalnstituent of sands and silts, and the most abundant soil mineral. It occurs as equidimensional hardains.

aematite

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red iron (ferric) oxide: resistant to change, results from extreme weathering. It is responsible for thidespread red or pink colouration in rocks and soils. It can form a cement in rocks, or a duricrust inils in arid climates.icasaky minerals present in many igneous rocks. Some are resistant, e.g. muscovite; some are broken

own, e.g. biotite.ay minerals

hese result mainly from the breakdown of feldspar minerals. They are very flaky and therefore havery large surface areas. They are major constituents of clay soils, although clay soil also contains sil

zed particles.


lay minerals

ay minerals are produced mainly from the chemical weathering and decomposition of feldspars, suorthoclase and plagioclase, and some micas. They are small in size and very flaky in shape.

he key to some of the properties of clay soils, e.plasticity, compressibility, swelling/shrinkage

otential, lies in the structure of clay minerals.

here are three main groups of clay minerals:aolinitesnclude kaolinite, dickite and nacrite) formed bye decomposition of orthoclase feldspar (e.g. inanite); kaolin is the principal constituent inina clay and ball clay.ites

nclude illite and glauconite) are the commonestay minerals; formed by the decomposition ofme micas and feldspars; predominant in marineays and shales (e.g. London clay, Oxford clay).ontmorilloniteslso called smectites or fullers' earth minerals) (include calcium and sodium momtmorillonites,ntonite and vermiculite) formed by the alteration of basic igneous rocks containing silicates rich ind Mg; weak linkage by cations (e.g. Na+, Ca++) results in high swelling/shrinking potential

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or more information on mineralogy see http://mineral.gly.bris.ac.uk/mineralogy/


ransportation and deposition

he effects of weathering and transportation largely determine the basic nature of the soil (i.e. the siape, composition and distribution of the grains). The environment into which deposition takes placd subsequent geological events that take place there, largely determine the state of the soil, (i.e.nsity, moisture content) and the structure or fabric of the soil (i.e. bedding, stratification, occurrenjoints or fissures, tree roots, voids, etc.)

ransportationue to combinations of gravity, flowing water or air, and moving ice. In water or air: grains become unded or rounded, grain sizes are sorted, producing poorly-graded deposits. In moving ice: grindind crushing occur, size distribution becomes wider, deposits are well-graded, ranging from rock floboulders.

epositionflowing water, larger particles are deposited as velocity drops, e.g. gravels in river terraces, sands

oodplains and estuaries, silts and clays in lakes and seas. In still water: horizontal layers of successidiments are formed, which may change with time, even seasonally or daily.

q Deltaic & shelf deposits: often vary both horizontally and vertically.q From glaciers, deposition varies from well-graded basal tills and boulder clays to poorly-grad

deposits in moraines and outwash fans.q In arid conditions: scree material is usually poorly-graded and lies on slopes.q Wind-blown Less is generally uniformly-graded and false-bedded.


oading and drainage history

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he current state (i.e. density and consistency) of a soil will have been profoundly influenced by thestory of loading and unloading since it was deposited. Changes in drainage conditions may also havcurred which may have brought about changes in water content.

oading /unloading history

itial loading

uring deposition the load applied to a layer of soil increases as more layers are deposited over it; this compressed and water is squeezed out; as deposition continues, the soil becomes stiffer andronger.

nloadinghe principal natural mechanism of unloading is erosion of overlying layers. Unloading can also occoverlying ice-sheets and glaciers retreat, or due to large excavations made by man. Soil expands w

is unloaded, but not as much as it was initially compressed; thus it stays compressed - and is said toverconsolidated. The degree of overconsolidation depends on the history of loading and unloading.

rainage history

hemical changesome soils initially deposited loosely in saline water and then inundated with fresh water develop wellapsing structure. In arid climates with intermittent rainy periods, cycles of wetting and drying caning minerals to the surface to form a cemented soil.

limate changesome clays (e.g. montmorillonite clays) are prone to large volume changes due to wetting and dryingus, seasonal changes in surface level occur, often causing foundation damage, especially afterceptionally dry summers. Trees extract water from soil in the process of evapotranspiration; The soar to trees can therefore either shrink as trees grow larger, or expand following the removal of larg

ees.



q Coarse soils

q Fine soils

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q Specific gravity

he recommended standard for soil classification is the British Soil Classification System, and this tailed in BS 5930 Site Investigation.

rading and composition

oarse soils

q Particle size tests

q Typical grading curves

q Grading characteristicsq Sieve analysis example

oarse soils are classified principally on the basis of particle size and grading.

Very coarse

soils

BOULDERS > 200 mm

COBBLES 60 - 200 mm

Coarse

soils

G

GRAVEL

coarse 20 - 60 mm

medium 6 - 20 mmfine 2 - 6 mm

SSAND

coarse 0.6 - 2.0 mm

medium 0.2 - 0.6 mm

fine 0.06 - 0.2 mm

oarse soils

article size tests

he aim is to measure the distribution of particle sizes in the sample. Whenwide range of sizes is present, the sample will be sub-divided, andparate tests carried out on each sub-sample. Full details of tests are given

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BS 1377: "Methods of test for soil for civil engineering purposes".

article-size tests

Wet sieving to separate fine grains f

soil mechanics arranged)

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SOIL MECHANICS - · PDF filereview of soil mechanics, soil behavior, & geotechnical site investigations section 2 contents symbols used in this section. soil mechanics