chapter 4

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SOIL COMPACTION

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Definition:

Soil compaction is defined as the method of mechanically increasing the density of soil by reducing volume of air.

Solids Water

Air

Solids Water

Air

Compressed soil Load

Soil Matrix

γ

soil (1)

= W

_T1

V

_T1

γ

soil (2)

= W

_T1

V

_T2

γ

soil (2)

> γ

soil (1)

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Why Soil Compaction:

1- Increase Soil Strength 2- Reduce Soil Settlement 3- Reduce Soil Permeability 4- Reduce Frost Damage

5- Reduce Erosion Damage

Soil Compaction in the Field:

1- Rammers

2- Vibratory Plates

3- Smooth Rollers

4- Rubber-Tire

5- Sheep foot Roller

6- Dynamic Compaction

Soil Compaction in the Lab:

1- Standard Proctor Test 2- Modified Proctor Test

Standard Proctor Test Modified Proctor Test

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The Proctor Test (after Ralph R. Proctor, 1933)

95 96 97 98 99 100 101 102 103 104 105

15 17 19 21 23 25 27 29

w %

D ry D e n s it y , p c f

Maximum dry

density = 102.5 pcf

optimum w % = 21.0

zero air

voids

curve

Example:

The laboratory test for a standard proctor is shown below. Determine the optimum water content and maximum dry density. If the G

_s

of the soil is 2.70, draw the ZAV curve.

Solution:

γ

_dry

=

γ

_wet

W

_c

100 1+ %

Volume of Proctor Mold

(ft³) 1/30 1/30 1/30 1/30 1/30 1/30

Weight of wet soil in the

mold (lb) 3.88 4.09 4.23 4.28 4.24 4.19

Water Content (%)

12 14 16 18 20 22

Volume of Mold

(ft³) 1/30 1/30 1/30 1/30 1/30 1/30

Weight of wet soil in the

mold (lb) 3.88 4.09 4.23 4.28 4.24 4.19

Water Content

(%) 12 14 16 18 20 22 Wet Unit

Weight (lb/ft³)

116.4 122.7 126.9 128.4 127.2 125.7

Dry Unit Weight

(lb/ft³) 103.9 107.6 109.4 108.8 106.0 103.0

100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115

10 11 12 13 14 15 16 17 18 19 20 21 22 23

γ

ZAV

=

G

_s

γ

_w

W

_c

G

_s

1+

S

_r

Water Content Dry Density

Effect of Energy on Soil Compaction

Higher Energy

Increasing compaction energy Lower OWC and higher dry density

In the field

increasing compaction energy

= increasing number of passes or reducing lift depth

In the lab

increasing compaction energy

= increasing number of blows

Soil Compaction in the Lab:

1- Standard Proctor Test

ASTM D-698 or AASHTO T-99

2- Modified Proctor Test

ASTM D-1557 or AASHTO T-180

Energy = 12,375 foot-pounds per cubic foot

Energy = 56,520 foot-pounds per cubic foot

Number of blows per layer x Number of layers x Weight of hammer x Height of drop hammer Volume of mold

Energy =

Moisture Content Dry Density

γ

^{d max}

Compaction Curve for Standard Proctor

(OMC)

γγγγ

^{d max}

(OMC)

Zero Air Void Curve S_r< 100%

Zero Air Void Curve S_r=100%

Zero Air Void Curve S_r= 60%

Compaction Curve for Modified Proctor

For standard Proctor test

) ft / lb ft 375 ,

12 ( m / kJ 7 . 592

m 10

944 .

0

) layer /

blows 25

)(

layers 3

)(

m 3048 .

0 )(

s / m 81 . 9 ( kg 495 .

E 2

3 3

3 3

2

⋅

−

=

= ×

Effect of Soil Type on Soil Compaction

Nuclear Density Sand Cone