chapter 7 soil formation

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CHAPTER 7CHAPTER 7

SOIL FORMATIONSOIL FORMATION



Definition and Terminology in SoilsDefinition and Terminology in Soils

Application of Soils in Civil EngineeringApplication of Soils in Civil Engineering

Origin and Composition of SoilsOrigin and Composition of Soils

Formation of Soil and Major Soil TypesFormation of Soil and Major Soil Types Basic Properties of SoilBasic Properties of Soil

Determination of Specific Gravity of Soil ParticlesDetermination of Specific Gravity of Soil Particles

Determination of Porosity and Void RatioDetermination of Porosity and Void Ratio

CONTENTS OF THIS CHAPTER CONTENTS OF THIS CHAPTER



SoilSoil

‘ ‘Soil is a natural body comprised of solidsSoil is a natural body comprised of solids(minerals and organic matter), liquid, and gases(minerals and organic matter), liquid, and gasesthat occurs on the land surface, occupies space,that occurs on the land surface, occupies space,

and is characterised by one or both of theand is characterised by one or both of thefollowing: horizons, or layers, that arefollowing: horizons, or layers, that aredistinguishable from the initial material as adistinguishable from the initial material as aresult of additions, losses, transfers, andresult of additions, losses, transfers, andtransformations of energy and matter or thetransformations of energy and matter or the

ability to support rooted plants in a naturalability to support rooted plants in a naturalenvironment.’ environment.’

Agriculturalists’ point of view.Agriculturalists’ point of view.

(USDA, 2004)(USDA, 2004)

DEFINITIONS AND TERMSDEFINITIONS AND TERMS



‘ ‘Soil is considered as a naturally (mostly) occurringSoil is considered as a naturally (mostly) occurringparticulate material of variable composition havingparticulate material of variable composition having

properties of compressibility, permeability andproperties of compressibility, permeability andstrength.’ strength.’

Engineers’ point of view.Engineers’ point of view.

(Whitlow, 2001)(Whitlow, 2001)

‘ ‘Soil comprises of layers of loose unconsolidatedSoil comprises of layers of loose unconsolidatedmaterial extending from the surface to solid rock,material extending from the surface to solid rock,which have been formed by the weathering andwhich have been formed by the weathering anddisintegration of the rocks.’ disintegration of the rocks.’

Geologists’ point of viewGeologists’ point of view

(Whitlow, 2001)(Whitlow, 2001)



Soil MechanicsSoil Mechanics

Soil mechanics is the branch of science that dealsSoil mechanics is the branch of science that dealswith the study of the physical properties of soil andwith the study of the physical properties of soil andthe behaviour of soil masses subjected to variousthe behaviour of soil masses subjected to varioustypes of forces (Das, 1997).types of forces (Das, 1997).

Soil EngineeringSoil Engineering

Soil engineering is the application of the principles of Soil engineering is the application of the principles of soil mechanics to practical problems (Das, 1997).soil mechanics to practical problems (Das, 1997).The publication of ‘erdbaumechanik’ by Karl TerzaghiThe publication of ‘erdbaumechanik’ by Karl Terzaghi

(1925) ‘father of soil mechanics’ gave birth to(1925) ‘father of soil mechanics’ gave birth tomodern soil mechanics which include fundamentalmodern soil mechanics which include fundamentalprinciples of soil mechanics on which advance studiesprinciples of soil mechanics on which advance studiesare made.are made.



GEOTECHNICAL ENGINEERINGGEOTECHNICAL ENGINEERING

Geotechnical engineering is defined as theGeotechnical engineering is defined as the

subdiscipline of civil engineering that involvessubdiscipline of civil engineering that involvesnatural materials found close to the surface of thenatural materials found close to the surface of the

earth with the inclusion of principles of soilearth with the inclusion of principles of soil

mechanics and rock mechanics applications formechanics and rock mechanics applications for

the design of foundations, retaining structuresthe design of foundations, retaining structures

and earth structures (Das, 1997).and earth structures (Das, 1997).



Application of Soil MechanicsApplication of Soil Mechanics

Geotechnical engineering is a branch of civil engineeringGeotechnical engineering is a branch of civil engineeringand is closely related to engineering geology which is aand is closely related to engineering geology which is abranch of geology. The discipline that relatesbranch of geology. The discipline that relatesgeotechnical engineering principles with engineeringgeotechnical engineering principles with engineeringgeology principles is geotechnics. Some of thegeology principles is geotechnics. Some of theapplications of geotechnics include:applications of geotechnics include:

i)i) Shallow foundation of structures such as bridges,Shallow foundation of structures such as bridges,buildings, highways and road embankments.buildings, highways and road embankments.

ii)ii) Deep foundations of structures such as piled high riseDeep foundations of structures such as piled high risebuildings, structures on difficult ground conditions,buildings, structures on difficult ground conditions,tunnelling and excavation.tunnelling and excavation.

iii)iii)Ground improvement of difficult soil conditions such asGround improvement of difficult soil conditions such asgeotextiles, stabilisation using chemicals, dewatering,geotextiles, stabilisation using chemicals, dewatering,vibrocompaction etc.vibrocompaction etc.

iv)iv)Retaining structures and slopes such as reinforced earthRetaining structures and slopes such as reinforced earthwall, retaining walls, cofferdams etc.wall, retaining walls, cofferdams etc.



CIVILENGINEERING

GEOLOGY

GeotechnicalEngineering

EngineeringGeology

Geomorphology

Paleontology

Petrology

EconomicGeology

Geophysics andSeismology

Structural

Engineering

EnvironmentalEngineering

Water ResourceEngineering

ConstructionEngineering

Surveying andMapping

TransportationEngineering

Geotechnics is a Discipline Related to GeotechnicalEngineering and Engineering Geology



Some Application of Soil MechanicsSome Application of Soil Mechanics

Bridge FoundationCofferdam

Geotextiles

RoadEmbankment

Dynamic CompactionTunnelling

ExcavationGrout

Curtain

Shallow Foundation

Embankment DamReinforced Earth Wall



ORIGIN AND COMPOSITION OF SOILORIGIN AND COMPOSITION OF SOIL

Soils are formed by the disintegration (technicallySoils are formed by the disintegration (technicallyknown as weathering) of rocks.known as weathering) of rocks.

The disintegrated or weathered materials areThe disintegrated or weathered materials areeither found deposited at its original location oreither found deposited at its original location ortransported by weathering agents such as water,transported by weathering agents such as water,

wind, ice, etc. before deposition.wind, ice, etc. before deposition. In the first case, the resultant soil is known asIn the first case, the resultant soil is known as

residual soilresidual soil whereas the second case is knownwhereas the second case is knownasas transported soiltransported soil..

Three stages involved in the formation transportedThree stages involved in the formation transported

soil is described as:soil is described as:i)i) weatheringweathering

ii)ii) transportationtransportation

iii)iii) deposition of weathered materialsdeposition of weathered materials



WeatheringWeathering

Weathering is the process of breaking down rocksWeathering is the process of breaking down rocksby mechanical and chemical processes into smallby mechanical and chemical processes into smallpieces. Weathering is divided into two distinctpieces. Weathering is divided into two distinctprocesses:processes:

i)i) Mechanical WeatheringMechanical Weathering

Mechanical weathering may be caused by theMechanical weathering may be caused by theexpansion and contraction of rocks from continuousexpansion and contraction of rocks from continuousgain and loss of heat, which results in ultimategain and loss of heat, which results in ultimate

disintegration. Frequently, water seeps through thedisintegration. Frequently, water seeps through thepores and existing cracks in rocks and as thepores and existing cracks in rocks and as thetemperature drops, the water freezes and expandstemperature drops, the water freezes and expandscausing the expansion of volume strong enough tocausing the expansion of volume strong enough tobreak down larger rocks.break down larger rocks.



ii)ii) Chemical WeatheringChemical Weathering

The original rock minerals are transformed intoThe original rock minerals are transformed intonew minerals by chemical reaction is known asnew minerals by chemical reaction is known aschemical weathering. Water and carbon dioxideschemical weathering. Water and carbon dioxides

from the atmosphere form carbonic acid, whichfrom the atmosphere form carbonic acid, whichreacts with the existing rock minerals to formreacts with the existing rock minerals to formnew minerals and soluble salt. Soluble saltsnew minerals and soluble salt. Soluble saltspresent in the groundwater and organic acidspresent in the groundwater and organic acidsformed from decayed organic matter also causeformed from decayed organic matter also causechemical weathering.chemical weathering.



The product of soils formed from the weatheringThe product of soils formed from the weatheringprocess at its origin is known to be residual soil.process at its origin is known to be residual soil.

On the contrary, the transported soil can beOn the contrary, the transported soil can beclassified into several groups, depending on theclassified into several groups, depending on the

mode of transportation and deposition:mode of transportation and deposition:i)i) Glacial soilsGlacial soils - formed by transportation- formed by transportation

and deposition of glaciers.and deposition of glaciers.

ii)ii) Alluvial soilsAlluvial soils - transported by running- transported by runningwater and depositedwater and deposited

along streams.along streams.iii)iii) Lacustrine soilsLacustrine soils - formed by deposition in- formed by deposition in

quiet lakes.quiet lakes.

iv)iv) Marine soilsMarine soils - formed by deposition in the- formed by deposition in theseas.seas.

v)v) Aeolian soilsAeolian soils - transposted and deposited- transposted and depositedby wind.by wind.

vi)vi) Colluvial soilsColluvial soils - formed by movement of - formed by movement of soil from its original placesoil from its original placeby gravity such as duringby gravity such as during

landslides.landslides.



A soil profile consists of severalA soil profile consists of several soilsoilhorizonshorizons and they are numberedand they are numberedroughly alphabetically, beginning at theroughly alphabetically, beginning at theground surface, going downwards:ground surface, going downwards:

i)i) O horizonO horizon – This is the layer of humus on theThis is the layer of humus on the

ground surface.ground surface.

ii)ii) A horizonA horizon – Top soil.Top soil.–

Rich in organic matter and typicallyRich in organic matter and typicallydark in color.dark in color.

– Also called zone of Also called zone of leachingleaching..

iii)iii) B horizonB horizon – Subsoil.Subsoil.– Also called zone of accumulation.Also called zone of accumulation.

– May contain soluble minerals suchMay contain soluble minerals suchas calcite in arid climates (caliche).as calcite in arid climates (caliche).

iv)iv) C horizonC horizon– Weathered bedrock or saproliteWeathered bedrock or saprolite

(rotten rock).(rotten rock). – Bedrock lies below the soil profile.Bedrock lies below the soil profile.



i) Pedalfer– These soils are rich in Al and Fe.

– They form in humid climates, such as the southeastern U.S.

ii) Pedocal

– These soils are rich in Calcium Carbonate.

– They form in arid climates, such as the southwestern U.S.

– These soils commonly contain caliche (or hardpan), a calciumcarbonate deposit which accumulates in the soil.

iii) Laterite

– These soils have been depleted of nearly all elements except iron andaluminum oxides.

– Laterites are derived from the weathering of basalt (mafic parent

rock).– They form in tropical climates (hot and wet) with very high rainfall.– The high rainfall has caused leaching of most of the elements and

nutrients from the soil.

– This is the soil typical of a tropical rainforest. When used foragriculture, the small amount of nutrients is quickly depleted, andthe soil dries to become as hard as a brick.

MAJOR SOIL TYPESMAJOR SOIL TYPES



Mineral CompositionMineral Composition

As a summary, soil can be classifiedAs a summary, soil can be classified

into two major groups given as:into two major groups given as:

I)I) Coarse SoilsCoarse Soils

Coarse soils are classified as havingCoarse soils are classified as havingparticle sizes > 0.06 mm such asparticle sizes > 0.06 mm such as

SANDS and GRAVELS. The grainsSANDS and GRAVELS. The grainswill either bewill either be rounded rounded oror angular angular and usually consist of fragments of and usually consist of fragments of rock or jasper with iron oxide,rock or jasper with iron oxide,calcite and mica present. Thecalcite and mica present. Therelatively equidimensional shape is arelatively equidimensional shape is a

function of the crystalline structurefunction of the crystalline structureof the minerals including the degreeof the minerals including the degreeof rounding depends upon theof rounding depends upon theamount of wear that have takenamount of wear that have takenplace.place.



ii)ii) Fine SoilsFine Soils

Fine soils are finer than 0.06 mm andFine soils are finer than 0.06 mm andtypically flaky in shape such as SILTS andtypically flaky in shape such as SILTS andCLAYS. Very fine oxides, sulphides andCLAYS. Very fine oxides, sulphides andsometimes organic matter may be present.sometimes organic matter may be present.The most important engineering context of The most important engineering context of fine soils is the flakiness of the clay mineralsfine soils is the flakiness of the clay mineralswhich give rise to very large surface areas.which give rise to very large surface areas.

Organic matter Organic matter originates from plant ororiginates from plant oranimal remains and the end product, knownanimal remains and the end product, knownasas humushumus which is a complex mixture of which is a complex mixture of organic compound.organic compound.

Featured in topsoil occurring in the upperFeatured in topsoil occurring in the upperlayer of usually not more than 0.5 mlayer of usually not more than 0.5 mthickness.thickness.

Peat Peat are predominantly fibrous organicare predominantly fibrous organicmaterial.material.

Organic matter has undesirable propertiesOrganic matter has undesirable propertieswhich are highly compressible and absorbwhich are highly compressible and absorblarge amount of water that will change inlarge amount of water that will change inload or moisture content producingload or moisture content producing

considerable changes in volume.considerable changes in volume.



Large majority of soilsLarge majority of soilsconsist of mixtures of consist of mixtures of inorganic mineral particlesinorganic mineral particles

together with some watertogether with some waterand air and it is expedientand air and it is expedientto consider a soil as threeto consider a soil as threephase model composed of phase model composed of solid, liquid.solid, liquid.

Gas

Liquid

Solid

Air, water vapour

Water, dissolved

salt

Rock fragments,Mineral grains,

Organic matter

Water Water is a fundamental part of natural soil and in factis a fundamental part of natural soil and in facthas a great effect on engineering properties such ashas a great effect on engineering properties such ascompressibility, seepage and permeability.compressibility, seepage and permeability.

Water has no shear strength but relativelyWater has no shear strength but relativelyincompressible, hence it transmits direct pressureincompressible, hence it transmits direct pressuretherefore drainage conditions in a soil mass are of greattherefore drainage conditions in a soil mass are of great

significance when considering shear strength.significance when considering shear strength. Air Air is compressible and water vapour can freeze. Soilis compressible and water vapour can freeze. Soilmay be considered to be perfectly dry or fully saturatedmay be considered to be perfectly dry or fully saturatedor in the condition somewhere between these twoor in the condition somewhere between these twoextremes. In dry soil, water vapour is present while in aextremes. In dry soil, water vapour is present while in afully saturated soil, 2 % of air voids may be present.fully saturated soil, 2 % of air voids may be present.

Three-Phase Diagram



BASIC PROPERTIES OF SOILBASIC PROPERTIES OF SOIL

The soil mass is quantified into three constituentThe soil mass is quantified into three constituent

phase materials which are solid, liquid and gas.phase materials which are solid, liquid and gas. In a partially saturated state, the soil may consistIn a partially saturated state, the soil may consist

of these phase as shown below, however, in aof these phase as shown below, however, in afully saturated state or fully dry state, the soil willfully saturated state or fully dry state, the soil willbehave as a two phase system.behave as a two phase system.

In a fully saturated state, water will fill in the voidIn a fully saturated state, water will fill in the voidspaces in the soil mass whereas in fully dry state,spaces in the soil mass whereas in fully dry state,the void spaces will be filled with air.the void spaces will be filled with air.

Air

Water

Solid

Ma =0

Mw

Ms

Va

Vw

Vs

VM

Vv

mass volume



Definition of Mass-VolumeDefinition of Mass-Volume

Water Content (W) and Degree of Saturation (SWater Content (W) and Degree of Saturation (Srr))

Moisture content is expressed as the ratio of mass of Moisture content is expressed as the ratio of mass of

water to the mass of solid and is given in percentage of water to the mass of solid and is given in percentage of

water content (%). The water content is given by:water content (%). The water content is given by:

Degree of saturation is expressed as the quantity of Degree of saturation is expressed as the quantity of

water in the soil based on the fraction of the voidswater in the soil based on the fraction of the voids

volume and is given in percentage of saturation (%).volume and is given in percentage of saturation (%).

For a perfectly dry soil, Sr = 0 and for a fully saturatedFor a perfectly dry soil, Sr = 0 and for a fully saturatedsoil, Sr = 1. The degree of saturation is given by:soil, Sr = 1. The degree of saturation is given by:

w = =Mass of water Mw

Mass of solid Ms

Sr = =Volume of water Vw

Volume of voids Vv



Air Voids VolumeAir Voids Volume

The volume of air voids is the part of voids volume notThe volume of air voids is the part of voids volume notoccupied by water. The volume of air voids is aoccupied by water. The volume of air voids is a

percentage from the total volume of soil and is givenpercentage from the total volume of soil and is given

as:as:

Void Ratio and PorosityVoid Ratio and Porosity

Void ratio is the volume which is not occupied byVoid ratio is the volume which is not occupied bysolids and it may be occupied by water or air or by asolids and it may be occupied by water or air or by a

mixture of both. The void ratio is given by:mixture of both. The void ratio is given by:

Va

= Volume of Voids – Volume of Water = Vv

- Vw

e = = =Volume of voids Vv Vw + Vv

Volume of solids Vs Vs



Density of SoilDensity of Soil

Density is referred to as the mass of solids per unitDensity is referred to as the mass of solids per unitvolume of the soil which is the quantity of materialvolume of the soil which is the quantity of material

related to the amount of space it occupies given asrelated to the amount of space it occupies given as

kg/m3. Several density relationship can be establish:kg/m3. Several density relationship can be establish:

i)i) Dry DensityDry Density

ii)ii) Bulk DensityBulk Density

ρ dry = =Mass of solid Ms

Total volume V

ρ b = =Total Mass M

Total volume V



iii)iii) Saturated DensitySaturated Density

Saturated density is the bulk density of the soil whenSaturated density is the bulk density of the soil whenit is fully saturated, Sit is fully saturated, S

rr= 1.= 1.

iv)iv) Submerged DensitySubmerged Density

Submerged density or effective density of a soil is theSubmerged density or effective density of a soil is thenotional effective mass per unit total volume , whennotional effective mass per unit total volume , whenthe soil is submerged. When a unit total volume of soilthe soil is submerged. When a unit total volume of soilis is submerged in water, the net mass of a unitis is submerged in water, the net mass of a unitvolume of soil is given by:volume of soil is given by:

ρ ’ = ρ sat. - ρ w



Solid Particle Density and Specific GravitySolid Particle Density and Specific Gravity

Solid particle density is defined as the ratio of theSolid particle density is defined as the ratio of themass of solids to the volume of solids given as kg/m3.mass of solids to the volume of solids given as kg/m3.

The expression is given by:The expression is given by:

Specific gravity is the ratio of the mass of solids to theSpecific gravity is the ratio of the mass of solids to the

density of water expressed as:density of water expressed as:

ρ s = =Mass of solid Ms

Volume of solid Vs

Gs = Ms

Vsρ w



Relationships of Mass-VolumeRelationships of Mass-VolumeMassMass VolumeVolume

Water content and degree of saturation are derived fromWater content and degree of saturation are derived from

previous equation given as:previous equation given as:

AirWater

Solid

Ma =0

Mw = wGsρ w

Ms = Gsρ w

Va = e(1 - sr)

Vw = Sre = wGs

Vs = 1

V = 1 + e

e

SreGs

w =

wGs

eSr =



The porosity is derived based on the total volume andThe porosity is derived based on the total volume and

the volume of void in the soil expressed as:the volume of void in the soil expressed as:

Solid particle density derived from the aboveSolid particle density derived from the above

relationship is given by:relationship is given by:

The air voids volume is expressed as:The air voids volume is expressed as:

The air voids content is the ratio of the volume of airThe air voids content is the ratio of the volume of airvoids to the total volume given as:voids to the total volume given as:

e

1 + en =

Gsρ w

ρ s =

Va = e(1 – Sr)

e – wGs

1 + eAv =



The dry density and bulk density are derived from theThe dry density and bulk density are derived from theprevious expression given as:previous expression given as:

The saturated density is derived from the bulk densityThe saturated density is derived from the bulk density

when the soil is fully saturated given by:when the soil is fully saturated given by:

Gsρ w ρ s

1 + e 1 + eρ d = =

Gsρ w + Sreρ w ρ s +Sreρ w

1 + e 1 + e

ρ b = =

Gsρ w + eρ w Gs + e1 + e 1 + e

ρ sat

=

= ρ w



Unit Weights of SoilUnit Weights of SoilDry unit weight,Dry unit weight, γ γ drydry == ρρ drydry gg

Bulk unit weight,Bulk unit weight, γ γ bb == ρρ bbgg

Saturated unit weight,Saturated unit weight, γ γ satsat == ρρ satsat ggUnit weight of water,Unit weight of water, γ γ ww == ρρ wwgg

Submerged unit weight,Submerged unit weight, γ γ '' == γ γ satsat –– γ γ ww

The gravitational acceleration, g is taken as 9.81 m/sThe gravitational acceleration, g is taken as 9.81 m/s22..The unit for the given unit weights is kN/mThe unit for the given unit weights is kN/m33..

The actual void ratio of a soil lies somewhere betweenThe actual void ratio of a soil lies somewhere between

the possible minimum and maximum values dependingthe possible minimum and maximum values dependingon the state of compaction.on the state of compaction.

In the case of sands and gravels, a good deal of In the case of sands and gravels, a good deal of variation is attainable between two extremes and thevariation is attainable between two extremes and therelationship between the void ratio values is termed asrelationship between the void ratio values is termed asdensity index or relative density given by:density index or relative density given by:

Relative DensityRelative Density



From the definition of dry unit weight, the relationshipFrom the definition of dry unit weight, the relationshipcan be expressed in terms of maximum and minimumcan be expressed in terms of maximum and minimum

possible dry unit weight given as:possible dry unit weight given as:

emax. – e

emax. – emin.

RD =

γ d - γ d(min)

γ d(max)

γ d(max) – γ d(min) γ d

RD =

Relative Density (%) Description of Compactness

0 – 15 Very Loose

15 – 50 Loose50 – 70 Medium

70 – 85 Dense

85 – 100 Very Dense



DETERMINATION OF SPECIFICDETERMINATION OF SPECIFIC

GRAVITY OF SOIL PARTICLESGRAVITY OF SOIL PARTICLES

Specific gravity of fine soils can be conducted using a 50 mlSpecific gravity of fine soils can be conducted using a 50 mldensity bottle whereas for coarse soils, a 500 ml or 1000 mldensity bottle whereas for coarse soils, a 500 ml or 1000 mlcontainer is used either an ordinary gas jar shown incontainer is used either an ordinary gas jar shown in FigFig4.14.1 or a special gas jar fitted with a conical screw topor a special gas jar fitted with a conical screw topknown as a pyknometer shown inknown as a pyknometer shown in Fig 4.2.Fig 4.2.

The soil is first dried and placed into the jar and weighed.The soil is first dried and placed into the jar and weighed.

The jar is then filled with de-aired water and agitated toThe jar is then filled with de-aired water and agitated toremove any air bubbles.remove any air bubbles. Once it is carefully topping up with water, the jar is weighedOnce it is carefully topping up with water, the jar is weighed

again.again. Finally, the jar is emptied and filled with de-aired water andFinally, the jar is emptied and filled with de-aired water and

again weighed. The masses are indicated below toagain weighed. The masses are indicated below to

determine specific gravity of the soil given as:determine specific gravity of the soil given as:M1 = Mass of empty jarM1 = Mass of empty jar

M2 = Mass of jar including dry soilM2 = Mass of jar including dry soil

M3 = Mass of jar, soil including waterM3 = Mass of jar, soil including water

M4 = Mass of jar including water onlyM4 = Mass of jar including water only



Alternatively, another procedure can beAlternatively, another procedure can be

adopted where the empty jar and theadopted where the empty jar and the jar filled with water is weighed. jar filled with water is weighed.

A pre-weighed soil quantity is thenA pre-weighed soil quantity is thenpoured into the jar and stirred.poured into the jar and stirred.

The water is carefully filled into the jarThe water is carefully filled into the jar

and weighed.and weighed. The masses are indicated below toThe masses are indicated below to

determine the specific gravity of thedetermine the specific gravity of thesoil given as:soil given as:

M1 = Mass of empty jarM1 = Mass of empty jar

Ms = Mass of soilMs = Mass of soil M3 = Mass of jar, soil including waterM3 = Mass of jar, soil including water M4 = Mass of jar including water onlyM4 = Mass of jar including water only

Mass of Soil M2 – M1

Mass of Water Displaced by Soil (M4 – M1) – (M3 – M2)Gs = =

Ms

M4 – M3 + Ms

Gs =

Fig 4.1Fig 4.1 Gas Jar

Fig 4.2Fig 4.2 Pyknometer



DETERMINATION OF POROSITYDETERMINATION OF POROSITY

AND VOID RATIOAND VOID RATIO

Porosity and void ratio of a granular soil can bePorosity and void ratio of a granular soil can bedetermined by filling a suitable mould or container withdetermined by filling a suitable mould or container withwater and adding the soil to fill the mould.water and adding the soil to fill the mould.

The volume occupied by the soil particles may beThe volume occupied by the soil particles may bedetermined by comparing the masses of water in thedetermined by comparing the masses of water in themould with the mould filled with soil and water.mould with the mould filled with soil and water.

The minimum void ratio can be determined using aThe minimum void ratio can be determined using astandard compaction mould placed under water indicatedstandard compaction mould placed under water indicatedas Mas M

11.. The soil is then compacted into three layers of equalThe soil is then compacted into three layers of equal

thickness with each thoroughly compacted using athickness with each thoroughly compacted using a

vibrating hammer.vibrating hammer. The collar of the mould is then removed and the mass of The collar of the mould is then removed and the mass of mould including soil from its surface struck-off level andmould including soil from its surface struck-off level andwater is determined indicated as Mwater is determined indicated as M

22.. V indicates the volume of mould used in the laboratoryV indicates the volume of mould used in the laboratory

determination.determination.



Saturated density,Saturated density,

Assuming the soil is fully saturated, sAssuming the soil is fully saturated, srr= 1,= 1,

The porosity is given by:The porosity is given by:

ρ sat = M2 – M1

V

ρ sat(max) = Gs + emin

1 + emin

emin = Gsρ w -

ρ sat(max)

ρ sat(max) - ρ w

nmin = emin

1 + emin



The maximum void ratio is determined by pouring theThe maximum void ratio is determined by pouring the

soil quickly into the mould placed under water.soil quickly into the mould placed under water.

The collar is then removed and the weighed of theThe collar is then removed and the weighed of the

mould including soil at its struck off level with water ismould including soil at its struck off level with water is

weighed. The expression is given by:weighed. The expression is given by:

emax = Gsρ w - ρ sat(min)

ρ sat(min) - ρ w

nmax = emax

1 + emax

chapter 7 soil formation

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