08 · 2017. 8. 4. · prof. b v s viswanadham, department of civil engineering, iit bombay....

Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay

08


Soil Compaction -1


(After Bell, 1993)

Range of the degree ofexpansiveness of a claybased on the activity

Swell-Shrinkage response of clay = f (Period, magnitude of precipitation and evapotranspiration)

Activity

Kaolinite – Smallest swelling capacity

Illite – May swell up to 15% and intermixed illite and montmorillonite may swell up to 60-100%

Swelling of Ca Montmorillonite swell up to 50-100% and Na Montmorillonite swell up to 2000%


For a soil specimen, given:Passing 2 mm sieve = 100 %; Passing 0.425 mm sieve = 85 %;

Passing No. 200 sieve = 38 %

LL = 20 % and PI = 12 %

Classify the soil by the Unified soil classification system

Example 1


Solution for Example 1

- Soil is a coarse grained soil (Percent passing No. 200 sieve < 50).

- Sands (percent of coarse fraction passing No. 4 sieve > 50)

- Since more than 12 % passes No. 200 sieve, it must be SM or SC

- PI = 20 – 12 = 8 > 7 [above A-line]

Hence the soil classification is SC


Example 2

For a soil specimen, given:Passing No. 4 sieve = 92 %; Passing No. 40 sieve = 78 %;

Passing No. 10 sieve = 81 %; Passing No. 200 sieve = 65 %

LL = 48 % and PI = 32 %

Classify the soil by the unified Soil classification system

No. 4 sieve = 4.75 mm

No. 10 sieve = 2 mm;

No. 40 sieve = 0.425 mm sieve



Since more than 50 % is passing through a No. 200 sieve, it is a fine-grained soil, i.e., it could be ML, CL, OL, MH, OH, CH or OH.

Now, if we plot LL =48 and PI = 32 on the plasticity chart, it falls in the zone CL.

So the soil is classified as CL


Example 3

Limit tests performed on a clay indicate a liquid limit of 67 and a plastic limit of 32. From a hydrometer analysis to determine particle sizes, it is found that 80 % of the sample consists of particles smaller than 0.002 mm. From this information, indicate the activity classification and the probable type of clay mineral.



PI = LL – PL

= 67 – 32

= 35 CPIAc =

Ac = 35/80 = 0.44

The clay mineral is Kaolinite as Ac: 0.3 – 0.5


Example 4:

Use the grain-size distribution curve shown below toclassify soils A and B using the USCS. Soil B’s Atterberglimits are LL = 49% and PL = 45%.


For soil A, G = 2%, S = 98%;

M = 0% & C = 0%.

CU = 1.4/0.5 = 2.8; Cc = 0.952/(1.4)*(0.5) = 1.29

Soil A is a poorly graded sand (SP)

For soil B, G = 0%, S = 61%;

M = 35% & C = 4%.

CU = 0.45/0.005 = 90

Soil A is a very well graded silty sand (SM)


Compaction

In many situations soil itself used as a construction material.

- Such as: Highway embankmentsRailway embankmentsEarth damsHighway/Airfield pavementsBackfilled trenchesLandfills

The purpose of compaction is to produce a soil having physical properties appropriate for a particular project.

⇒When soil is used as a foundation material, it is desirable that the in-place material possess certain properties.


Compaction

Compaction is defined as the process ofincreasing the unit weight of soil by forcing the soilsolids into a dense state and reducing the air voids(No significant change in volume of water in thesoil)

This is achieved by applying static or dynamicload to the soil.

Compaction is measured quantitatively in terms ofthe dry unit weight γd of the soil.


Compaction generally leads to the following desirable effects on soils:


Purpose of Compaction

1. Maximum shear strength occurs approximatelyat minimum void ratio.

2. Large air voids may lead to compaction underworking loads, causing settlement of thestructure during service.

3. Larger voids if left may get filled with waterwhich reduces the shear strength .

4. Increase in water content is also accompaniedby swelling and loss of shear strength in someclays.


Advantages of Compaction

1. Settlements can be reduced or prevented.2. Soil strength increases and stability can be

improved.3. Load carrying capacity of pavement sub-grades

can be improved.4. Undesirable volume changes (by frost action,

swelling, shrinkage) may be controlled.


When loose are soils are applied to a construction site, compressive mechanical energy is applied to the soil using special equipment to densify the soil (or reduce the void ratio).

Densification ⇒ Reduction in Volume of Air Voids

It is almost an instantaneous phenomena and soil is always partially saturated.

Typically applies to soils that are applied or re-applied to a construction site.

Compaction

Compaction is a old technique adopted in Ancient China/India (e.g. Great Wall of China/Tajmahal)


Compaction of cohesion-less soilsWhen speaking of cohesion-less soils, gravelly soils, there are many possibilities:

Loose, Angular soil Dense, Angular soil

Honeycombed soil, Very Loose Loose Dense


Compaction of cohesion-less soilsSoils in loose or honeycombed state are avoided, or compacted before being built upon, since they are prone to densification when subjected to vibratory or shock loading (as from earthquakes or vibrating machinery)


Compaction of cohesion-less soils

The relative looseness of a soil in its natural, in-situ state is determined by measuring/computing its relative density, DrThe smaller Dr is for a given soil deposit, the more prone that soil deposit will be to densification and settlement.

For uniformly (poorly graded) spherical grained soils, the theoretical range of void ratios is 0.35 < e



For non-uniform, well-graded soils, the possible range of void ratios is much smaller.

Well-graded, sub-angular sand: 0.35 < e < 0.75

Well-graded, silty sand: 0.25 < e < 0.65

The range of void ratios for well-graded soils is less than that for uniformly graded soils.

That is why it is generally preferred to use well-graded soils in geotechnical applications as opposed to uniform soils.



Cohesion-less soils are compacted by vibration.

Static load produces very little compaction of loose sand.

Medium and fine sands do not get compacted easily when moist because of the shear strength developed by capillary forces.

Dry or submerged sands can be compacted by Vibration.


Compaction of Clayey soils

Clays cannot be compacted by vibration.

Shaking or vibration does not change the volume.

A very small amount of static pressure produces a large volume decrease of the platelet particles (like mica flakes).

In compacting the clay, position of the particles must be changed by forcing the contact points along adjacent surfaces to positions nearly more parallel with reduced voids.



Loose structure of clay before compaction

Dense structure of clay after compaction

Platelet particles

Thickness of adsorbed water + Free water

= f(water content)



When the clay has a higher water content less than saturation, a thick layer of free water surrounds the particles (low viscosity). Under this condition only a small amount of pressure is required to force the particles to new positions.

But a high degree of compaction cannot be produced with this high water content because the thick layer of free water prevents the particles from being forced close together.


Proctor’s Theory - After R.R. Proctor (1930)

Proctor showed that:1. There exists a definite relationship between the

soil moisture content and the degree of drydensity to which a soil may be compacted.

2. That for a specific amount of compactionenergy applied on the soil there is onemoisture content termed Optimum MoistureContent (OMC) at which a particular soilattains maximum dry density.


Proctor’s Theory

Proctor proposed tests to determine relationshipbetween w, γd or e of a compacted soil in astandard manner and to determine the OMC(optimum moisture content) for the soil.

Compaction = f [ γd , compactive effort, and soiltype (gradation, presence of clay minerals, etc.) ]

Compactive effort is a measure of mechanical energy applied to a soil mass.


Measuring compaction of soils in the laboratory

1. Standard Proctor compaction test

2. Modified Proctor compaction test


Standard Proctor Test

ScopeThis method covers the determination of therelationship between the moisture content anddensity of soils compacted in a mould of a givensize with a 2.5 kg rammer dropped from a height of305 mm.


Compactive Energy E applied to soil per unit volume

3363 594mmkg57187.5

10100.3052.5 3 25Effort Compactive

mkJ

==×

×××= −

VNnWhE =

N = No. of blows per layer

n = No. of layers

W = Hammer weight

h = Height of drop

V = Volume of mould


Standard Proctor Test – Procedure

Dry unit weight Calculation

Volume of Proctor mould = V

Bulk unit weight of soil,γb = W/V

Dry unit weight of soil, γd = γb / (1+w)


Typical moisture content-dry unit weight relationship

γd, maxOMC


Principle of compaction and moisture-density relations

Compaction of soils is achieved by reducing the volume of voids. It is assumed that the compaction process does not decrease the volume of solids or soil grains.


Principles of compaction and moisture-density relationsThe degree of compaction of a soil is measured by the dry unit weight of the skeleton. The dry unit weight correlates with the degree of packing of the soil grains.

eG ws

d +=

1γγ

Recall that:

The more compacted a soil is:

-The smaller its void ratio will be and thus

-The higher its dry unit weight γd will be


Principle of compaction and moisture-density relations

Water plays a critical role in the soil compactionprocess:

-It lubricates the soil grains so that they slide more easily over each other and can thus achieve a more densely packed arrangement.

-While a little bit of water facilitates compaction. Too much water inhibits compaction.


Principle of compaction and moisture-density relations• At low values of watercontent most soils tend to be stiffand are difficult to compact.• As the water content isincreased the soil becomesmore workable, facilitatingcompaction and resulting inhigher dry densities.• At high water contents,however, the dry densitydecreases with increasing watercontent, an increasingproportion of the soil volumebeing occupied by water.


AIR

WATER

SOLIDS

AIR

WATER

SOLIDS

WATER

SOLIDS

Un-compacted soil Compacted soil Theoretically maximum degree of compaction

Compaction In practice this dry unit weight is never achieved but it represents theoretical upper bound.


Principles of compaction and moisture-density relations

w

sr

sd

GSw

G γγ

+

=1

( ) cra

ws

asd

naSnnwG

nG

=−=

+−

=

11

)1( γγ

Air-Void lines:

Saturation lines:

ac = Va/VV na = Va/V


Saturation Line or Zero Voids Line

1. Saturation line is a hypothetical line.2. Points on the line denote density for

completely saturated condition at respectivewater contents.

3. It is the maximum possible dry density for anysoil.

4. Practically it is not possibleto achieve this density.

5. Dry density for saturation lineis given by

+

=w

Gs

wd 11γγ


AASHTO or Modified Proctor Test

1. Standard Proctor test is not sufficient for airwaysand highways.

2. US Army Corps of Engineers developed ModifiedProctor Test which used greater levels ofcompaction and produced higher dry densities.

3. Modified Proctor Test was later adopted byAASHTO & ASTM.


Modified Proctor Test Specifications

No. of blows : 25 per layer

No. of layers : 5 layers

Wt. of hammer : 4.5 kg

Falling height : 0.45m

36-3 mm kg253125

10102550.454.5EffortCompactive =

×

×××=


Dry Density-Moisture content curve

+

=w

Gs

wd 11γγ


Importance of Proctor Test1. It gives the density that must be achieved in the

field.2. Provides the moisture range that allows for

minimum compactive effort to achieve density.3. Provides data on the behaviour of the material in

relation to various moisture contents.4. It is not possible to determine whether a density

test passes or fails without it.


Nature of Effort

Moisture Content

Amount of Effort

Soil Type Effect of Compactive Effort

Factors Influencing Compaction

Load Duration Area of Contact

Factors influencing Compaction


Soil Type1. Soil type, grain size, shape of the soil grains,

amount and type of clay minerals present andthe Gs of soil solids, have a great influence onthe γd and OMC.

2. In poorly graded sands γd initially decreasesas the moisture content increases, and thenincreases to a maximum value with furtherincrease in moisture.

3. At lower moisture contents, the capillarytension inhibits the tendency of the soilparticles to move around and be compacted.


Compaction curve – effect of soil typegravel-sand-clay

Compactibility or ease with which soils can be compacted will depend on the soil type


Soil Type

At a given moisture content a clay with low plasticity will be stronger than a heavy or high plastic clay, as it will be easier to compact.

The reason is attributed to:

For a given compactive effort the air voids can be removed more easily for a low plasticity clay and because it will have a lower moisture content anyway, a higher dry unit weight can be obtained.


Dry density-water content curves for a range of soil types


Effect of Compactive Effort

Amount of compactive effort

1. Maximum dry unit weight increases withincrease in compactive effort.

2. Increase in compactive effort decreasesoptimum moisture content to some extent.



Increasing compaction energy

Applying more energy to a soil will reduce the air voids content further and increase the dry unit weight.

More compaction energy can be beneficial especially for soils dry of OMC


Effect of Compactive EffortIf a soil is already moist, weaker and above OMC then applying more energy is wasteful since air can quickly be removed.

Applying large amounts of energy to a very moist soil may be damaging since no more air can be expelled but high pore water pressures can build up which could cause:

-Slope Instability during construction

- Consolidation settlements as they dissipate after construction.



Nature of Effort - Load Duration and Contact Area

1. Longer time duration leads to reduced shear

stiffness response and greater compaction.

2. Greater contact area leads to greater depth of

influence



Degree of compaction generally increases with increasing compactive effort.

However, beyond a certain point, increased compactive effort produces only very small increase in dry unit weight. i.e. It takes a great deal of additional compactive effort E to see significant increase in dry unit weight.


Moisture Density Relationships of Cohesion-less Soils

Surface tension induces apparent cohesive strength, resisting compaction initially decreasing in dry density.

Thin water filmgrain

Bulking phenomena


Effect of compaction on soil structure

Low strength, low k, more shrinkage, less swelling

A

B

C

D

E

Highly Flocculated

Flocculated

Highly Dispersed

DispersedHigh strength, more k, less shrinkage, more swelling

Direction of increasing dispersion


Effect of compaction on soil structure

1. At low water contents, attractive forcesbetween clay particles predominate, creatinga more or less random orientation of plate likeparticles. (results in low density)

2. The addition of water increases repulsionbetween particles leading them to assumemore parallel orientation near OMC.

3. If compacted wet of optimum parallelorientation is further increased leading to whatis described as dispersed structure.


Compaction equipment

In the field, fill soils are typically imported to a siteand applied to the existing grade level in layerswhich are called lifts.

When a lift of soil is placed, it will be very loose.Special compaction equipment is then used tocompact this lift of the soil.

Rollers, Rammers and Vibrators


Types of Rollers

Smooth-wheel rollers

Vibratory rollers

Pneumatic-tire rollers

Sheepsfoot rollers


Rammers

Dropping weight (including piling equipment)

Internal combustion type

Pneumatic type


Smooth Wheeled Rollers

1. Conventional three wheel type - 18 tonsTandem rollers - 1 to 14 tonsThree axle tandem rollers - 12 to 18 tonsWeight increased by ballasting the rolls withwater or by a heavy sliding weight.

2. Performance is affected by the load unit widthunder the compaction rolls, and the width anddiameter of the rolls.

100 % coverage area under wheel with ground contact pressures upto 380 kPa.


Smooth Wheeled Rollers

3. Load per unit width and diameter control thepressure in the surface layer of soil; dimensionof the rolls affect rate with which this pressuredecreases with depth.

4. Suitable for gravels, sands, hardcore, crushedrock and any material where crushing action isneeded.


Pneumatic-tyred Roller80 % coverage area (i.e. 80 % of area is covered by tires)With tire pressures upto 700 kPa

• Suitable for fine grained soils (closely gradedsands). Best performance on cohesive soilsobtained when moisture content is 2-4% belowPL.

• Depth of compaction:Light rollers (200kN) – 150 mmMed. rollers (500kN) – 300 mmHeavy rollers (1800kN) – 450 mm


Sheepsfoot Rollers

Area of protrusions range from 30 to 80 cm2.8 – 12 % coverage, very high contact pressuresare possible, ranging from 1400 to 7000 kPa

• Compaction is by tampingand kneading

• Sheepsfoot rollers are mostsuitable for fine soils, bothplastic and non-plastic,especially at water contents dryof optimum.


Vibrators

Out of balance type

Pulsating hydraulic type


Out of balance type vibrator


Vibrators

1. Vibrators consist of a vibrating unit of eitherthe out-of-balance weight type or apulsating hydraulic type mounted on a plateor roller.

2. Vibrators give maximum dry density much inexcess of the corresponding compactiontest value at OMC.

3. Frequencies range 1500-2500 cycles/min.(Frequency range within natural frequencyof most soils)


Compaction equipmentEquipment type Soil typeSmooth wheel rollers Sands and Gravels

Pneumatic rubber tired rollers Silts and clays

Sheepsfoot rollers Silts and clays

Vibratory rollers Sands and Gravels

Vibratory tampers Sands and Gravels

To increase the compaction energy applied to the soil in the field:

a) Increase the mass/weight of the compaction equipment;

b) Decrease the thickness of lift thickness; and

c) Increase number of machinery passes.


Field compaction and specifications

Two categories of earthwork specifications:

1. End product specifications

2. Method specifications

With End product specifications, a certainrelative compaction or percent compaction, isspecified.

Slide Number 1Slide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12CompactionCompaction�Compaction generally leads to the following desirable effects on soils:Purpose of CompactionAdvantages of CompactionSlide Number 18Slide Number 19Slide Number 20Slide Number 21Slide Number 22Slide Number 23Slide Number 24Slide Number 25Slide Number 26Proctor’s Theory - After R.R. Proctor (1930)Proctor’s Theory Measuring compaction of soils in the laboratoryStandard Proctor TestCompactive Energy E applied to soil per unit volumeStandard Proctor Test – ProcedureSlide Number 33Slide Number 34Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39Saturation Line or Zero Voids LineAASHTO or Modified Proctor Test�Modified Proctor Test SpecificationsDry Density-Moisture content curveImportance of Proctor TestSlide Number 45Soil TypeSlide Number 47Slide Number 48Slide Number 49Effect of Compactive EffortSlide Number 51Slide Number 52Effect of Compactive EffortSlide Number 54Effect of Compactive EffortMoisture Density Relationships of �Cohesion-less SoilsEffect of compaction on soil structureSlide Number 58Slide Number 59Types of RollersRammersSmooth Wheeled RollersSmooth Wheeled RollersPneumatic-tyred RollerSheepsfoot RollersVibratorsSlide Number 67VibratorsSlide Number 69Slide Number 70

08 · 2017. 8. 4. · prof. b v s viswanadham, department of civil engineering, iit bombay....

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