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3. Soil Compaction

Soil Mechanics2010 - 2011

Dr. Manal A. Salem – Soil Mechanics

Soil Compaction Is the application of mechanical energy to densify the soil by reducing the volume of air, and thus reducing the total volume.

Applications (examples):Soil replacementEarth damsPavement structures

Benefits of compaction:Increase soil’s strengthReduce soil settlementReduce soil permeability

The dry density of the soil changes due to compaction, where:

T

sdry V

W=γ

2


Factors controlling compaction1. Water content2. Energy of compaction3. Soil type


Effect of water contentThe dry density – water content relationship (Compaction Curve) passes through 4 phases during compaction: γdry

water content (%)

1

2 3

4

1. Hydrationsoil particles absorb water to saturate particles + provide minor lubrication, γdry increases

2. Lubricationwater forms thin films around particles, acts as lubricant, γdry increases, densest packing

3


Effect of water content

γdry

water content (%)

1

2 3

4

3. Swellingadding more water causes solid particles to move away from each other, γdry decreases

4. Saturationadding more water fills most of the voids between particles, γdrydecreases, sample is close to saturation

The dry density – water content relationship (Compaction Curve) passes through 4 phases during compaction:


Effect of water content

γdry

water content (%)

γdmax

OMC

From compaction curve, get:γdry,max = maximum dry densityOMC = optimum moisture content

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Effect of compaction energy

γdry

water content (%)

E3

E2

E1

E1 < E2 < E3


Effect of compaction energyAs the compaction energy increases:1. γdry,max increases2. OMC decreases

Compaction curve is the relationship between dry density (γd) and water content (w) for a specific soil compacted at constant energy. Determined in the laboratory.

Compaction curves for different energies never intersect.

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Effect of soil typeDifferent soil types compacted at the same energy:

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

0 5 10 15 20 25 30 35 40

Water Content (%)

γdr

y (t/m

3 )Gravel

Sand

Silt

Clay


Effect of soil type

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

0 5 10 15 20 25 30 35 40

Water Content (%)

γdr

y (t/m

3 )

Gravel

Sand

Silt

Clay

OMCclay > OMCsilt > OMCsand > OMCgravel

(remember: OMC at end of lubrication stage. Lubrication is directly proportional to the surface area of solid particles)

Curvature of compaction curve for clay > silt > sand > gravel.

As the curvature increases, the effect of water content on γdry

increases.

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Saturation CurveSaturation curve is the relationship between dry density (γd) and water content (w) for a specific soil compacted at constant degree of saturation (S).

γdry

water content (%)

γdmax

OMC

Saturation curve


Saturation CurveCalculated, not measured in lab.

Find γd in terms of w, Gs, and S:

Assume Ws = 1

γdry

water content (%)

γdmax

OMC

Saturation curve

sww

T

sd

GSwV

W

γγ

γ 11

+==

ws

w WWWw ==

Air

Water

Solid

WeightsVolumes

1

w

1/Gsγw

w/γw

w/γwSww

ww

wWVγγ

==

wss G

Vγ1

=

w

wv S

wS

VVγ

==

7


Saturation Curve

To draw 100% saturation curve, substitute S = 1. To draw 50% saturation curve, use S = 0.5.

100% saturation curve never intersect compaction curve.

γdry

water content (%)

γdmax

OMC

100% saturation curve“Zero air voids curve”

50% saturation curve

sww

d

GSw

γγ

γ 11

+=


Laboratory compaction tests1. Standard Proctor Test2. Modified Proctor Test

Determine compaction curves

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Standard Proctor Test


Standard Proctor TestVolume of compaction mold (V) = 1/30 ft3

Weight of hammer (W) = 5.5 lb

Hammer drop (H) = 12 inches = 1ft

Four to five soil samples are mixed at different water contents.

Each sample is compacted in the mold in 3 equal layers, each layer is hit by N = 25 blows with the hammer, which is falling freely (drop = 1ft).

Compaction energy (E) = WHN x no. of layers/V

= 5.5 x 1 x 25 x 3 / (1/30)

~ 12000 lb ft/ft3

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Standard Proctor TestAfter compacting each specimen, get the weight of the compacted wet soil in the mold (W).

Calculate γwet = W/Vmold

Measure the water content (w) of the wet soil sample in the mold

Given: w, γwet. Calculate: γdry

Plot γdry versus w, and get γdry,max and OMC.

wwet

dry +=

1γγ

wet

T

sdry wV

W

γ

γ+

== 11

Air

Water

Solid

WeightsVolumes

1

w

(1+w)/γwet


Example

654321Test No.

605061206230620060455900Wt. of Mold + Soil (gm)

12.89.88.27.16.35.1Water Content (%)

6050-4263

=1787

6120-4263

=1857

6230-4263

=1967

6200-4263

=1937

6045-4263

=1782

5900-4263

=1637Wt. Soil (gm)

1.891.962.082.05

1.671.791.921.91

Mold data: The volume = 946 cm3

The weight = 4263 gm

)/( 3mtVW

moldwet =γ 73.1

9461637

= 88.1946

1782=

)m/t(

100w1

3wetdry

+

γ=γ 65.1

1001.51

73.1=

+77.1

1003.61

88.1=

+

10


Example

1.6

1.65

1.7

1.75

1.8

1.85

1.9

1.95

4 5 6 7 8 9 10 11 12 13 14

γdry

(t/m3)

water content (%)

OMC = 7.65 %

γdmax=1.93 t/m3


Exampleγdry

(t/m3)

water content (%)

Not AcceptableAdjust your scale

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Modified Proctor TestVolume of compaction mold (V) = 1/30 ft3

Weight of hammer (W) = 10 lb

Hammer drop (H) = 18 inches = 1.5 ft

Four to five soil samples are mixed at different water contents. Each sample is compacted in the mold in 5 equal layers, each layer is hit by N = 25 blows with the hammer, which is falling freely (drop = 1.5 ft).

Compaction energy (E) = WHN x no. of layers/V

= 10 x 1.5 x 25 x 5 / (1/30)

~ 56000 lb ft/ft3


Standard versus modified Proctor testsγdry

(t/m3)

water content (%)

Modified

Standard

Emodified > Estandard

γmodified > γstandard

OMCmodified < OMCstandard

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Field CompactionSoil is compacted in layers, 20 to 50 cm thick.

Based on laboratory defined compaction curves, target field water content is defined to obtain target field dry density.

Equipment for field compaction

Smooth-wheel rollers (Static compaction)

Rubber-tired roller

Sheepsfoot rollers (Kneeding compaction)

Vibratory rollers

Vibratory plates


Field Compaction Equipment

Smooth-wheel roller suitable for all types of soil

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Rubber-tired rollers suitable for all types of soil



Sheepsfoot rollers suitable for fine-grained soil

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Vibratory rollers suitable for coarse-grained soil (vibrators attached

to smooth-wheel or rubber-tired rollers)



Vibratory plate compactor suitable for all types of soil

Hand-heldMounted on machine

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Specifications for field compactionIn most specifications, it is required to achieve a dry density of 90 to 98% of maximum dry density obtained from standard or modified Proctor tests.

This is a specification for relative compaction:

Relative Compaction

Field dry density can be determine by conducting “Sand Cone Test”

100max

×=d

dfield

γγ


Sand Cone Test

Sand cone apparatus

Cone

Valve

JarStandard sandγstandard sand

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Sand Cone Test

Metal plate

Open valve

For excavated soil, determine:- Weight- Water content


Sand Cone Test

W2After test

W1Before test

W1 – W2 = weight of standard sand that filled (cone + hole)

Volume of (cone + hole) = (W1 – W2)/γstandardsand

Volume of hole = Volume of (cone + hole) – Volume of cone

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Sand Cone TestW1 – W2 = weight of standard sand that filled cone + hole

Volume of (cone + hole) = (W1 – W2)/γstandardsand

Volume of hole = Volume of (cone + hole) – Volume of cone

Volume of hole = Volume of excavated soil = VT

γwet = WT/VT

wwet

fieldd +=

1,γγ

100max

, ×=d

fielddRCγγ


Sand Cone TestExcavate of a hole in the ground.

The hole is filled with standard sand using the sand cone apparatus.

The volume of hole equals the volume of standard sand that filled the hole.

γwet is calculated using weight of excavated soil divided by the volume of hole.

The water content of the extracted soil is measured.

γdfield is calculated.

RC is calculated.

The field compaction is compared to RC specification

If RC < specified value, field compaction is rejected, recompacted until RC

meets the specification.

3. soil compaction-print - unimasr.com 3. soil compaction soil mechanics 2010 - 2011 dr. manal a....

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