Compaction Very Useful

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COMPACTION

In construction of highway embankments, earth dams and many other engineering structures, loose soils

must be compacted to improve their strength by increasing their unit weight. Compaction is the densification of soil by removing air voids using mechanical equipment. The dense state is achieved through the reduction of the air voids in the soil, with little or no reduction in the

water content. In general, soil densification includes compaction and consolidation.

Compactionis one kind of densification that is realized by rearrangement of soil particles without outflowof water. It is realized by application of mechanic energy. It does not involve fluid flow, but with moisture

changing altering.Compaction is the application of mechanical energy to a soil to rearrange the particlesand reduce the void ratio.

Consolidationis another kind of densification with fluid flow away. Consolidation is primarily for clayey

soils. Water is squeezed out from its pores under load.

Objectives for Compaction

Increasing the bearing capacity of foundations

!ecreasing the undesirable settlement of structures

Control undesirable volume changes "eduction in hydraulic conductivity

Increasing the stability of slopes.

General Compaction Methods

Priodeep Chowdhury;Lecturer;Dept. of CEE;Uttara University//| Soil Compaction

Compaction Effect


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Laboratory Compaction

Oriin

The fundamentals of compaction of fine #grained soils are relatively new. !"!" Proctorin the early $%&'(s was

building dams for the old )ureau of Waterworks and *upply in +os ngeles, and he developed the principles of

compaction in a series of articles in -ngineering ews#"ecord. In his honor, the standard laboratory compactiontest which he developed is commonly called the proctor test.

P#rpose

The purpose of a laboratory compaction test is to determine the proper amount of mi/ing water amount ofmi/ing water to use when compacting the soil in the field and the resulting degree of denseness which can be

e/pected from compaction at this optimum water.

Impact compaction

The proctor test is an impact compaction. hammer is dropped several times on a soil sample in a mold. The

mass of the hammer, height of drop, number of drops, number of layers of soil, and the volume of the mold are

specified.

Types and $etails of Laboratory Compaction Techni%#es

Two types of compaction tests are routinely performed0

&'( The *tandard 1roctor Test, and

&)( The 2odified 1roctor Test.

-ach of these tests can be performed in three different methods as outlined in the attached Table $.

In the *tandard Proctor Test, the soil is compacted by a 3.3 lb hammer falling a distance of one foot into a

soil filled mold. The mold is filled with three equal layers of soil, and each layer is sub4ected to 53 drops of

the hammer. The Modified Proctor Test is identical to the *tandard 1roctor Test e/cept it employs, a $' lb hammer

falling a distance of $6 inches, and uses five equal layers of soil instead of three. There are two types of compaction molds used for testing. The smaller type is 7 inches in diameter and has a

volume of about $8&' ft&9%77 cm&:, and the larger type is ; inches in diameter and has a volume of about

$8$&.&&& ft&95$5& cm&:. If the larger mold is used each soil layer must receive 3; blows instead of 53 9*ee

Table $:

Table ' Alternative Proctor Test Methods



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Standard Reference:

A*TM $ +,-# *tandard Test 2ethods for +aboratory Compaction Characteristics of *oil '' =#m8m&::

Laboratoy Compaction Test *#mmary

Compaction Enery

Compaction -ffort is calculated with the following parameters0

'"2old volume )"o of Compaction +ayer 0"Weight of ?ammer 1"@ree fall ?eight ."o of blows

Compaction Enery

E=no of blows per layerno of layersweight of hammerFree fallheight

Volume of mold*o, for *tandard Proctor Test0

E=25(no of blows per layer )3(no of layers)5.5(weight of hammer )1(Free fall height)

1 /30 (Volume of mold ) 2 ')0/. ft3

lb4lb0

*o, for Modified Proctor Test0

E=25 (no of blows per layer )5 ( no of layers )10 ( weight of hammer )1.5(Free fall height)

1/30 (Volume of mold ) 2 .+).5 ft3

lb4lb0

Comparison

*inificance of the Tests



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2echanical compaction is one of the most common and cost effective means of stabilizing soils. n

e/tremely important task of geotechnical engineers is the performance and analysis of field control tests to

assure that compacted fills are meeting the prescribed design specifications. !esign specifications usually state

the required density 9as a percentage of the Ama/imumB density measured in a standard laboratory test:, and the

water content. In general, most engineering properties, such as the strength, stiffness, resistance to shrinkage,

and imperviousness of the soil, will improve by increasing the soil density.

The optimum water content is the water content that results in the greatest density for a specified

compactive effort. Compacting at water contents higher than 9wet of: the optimum water content results in arelatively dispersed soil structure 9parallel particle orientations: that is weaker, more ductile, less pervious

softer, more susceptible to shrinking, and less susceptible to swelling than soil compacted dry of optimum to the

same density. The soil compacted lower than 9dry of: the optimum water content typically results in a

flocculated soil structure 9random particle orientations: that has the opposite characteristics of the soil

compacted wet of the optimum water content to the same density.

*tandard Proctor Test6 Proced#re 7 $etails

In the 1roctor test, the soil is compacted in a mold that has a volume of %7&.& cm &.The diameter of themold is $'$.; mm. !uring the laboratory test, the mold is attached to a base plate at the bottom and to an

e/tension at the top. The soil is mi/ed with varying amounts of water and then compacted in three equal layersby a hammer that delivers 53 blows to each layer. The hammer weighs 57.7 9mass 5.3 kg:, and has a drop of&'7.6 mm. @or each test, the moist #nit 8eihtof compaction can be calculated as

where

W weight of the compacted soil in the mold9m: volume of the mold 9%7&.& cm&:

@or each test, the moisturecontent of the compacted soil is determined in the laboratory.

With known moisture content, the dry unit weight !d can be calculated as

9where w 9D: percentage of moisture content.:


*tandard Proctor Test e%#ipment & Mold 7 9ammer(


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The values of !ddetermined from -q. above can be plotted against the corresponding moisture contentsto obtain the ma/imum dry unit weight and the optimum moisture content for the soil. @ollowing figure shows

such a compaction for a silty clay soil.

@or a given moisture content, the theoretical ma/imum dry unit weight is obtained when there is no air in the

void spacesEthat is, when the degree of saturation equals $''D. Thus, the ma/imum dry unit weight at a givenmoisture content with :ero air voidscan be given by

To obtain the variation of !zavwith moisture content, use the following procedure0'" !etermine the specific gravity of soil solids.

)" =now the unit weight of water 9!w:.0" ssume several values of w, such as 3D, $'D, $3D, and so on.


*tandard Proctor compaction test res#lts for a silty clay


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1"


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Effect of Compaction Effort

With the development of heavy rollers and their uses in field

compaction, the *tandard 1roctor Test was modified to better

represent field compaction.

s the compaction effort increases,

the ma/imum dry unit weight of compaction increase

The optimum moisture content decreases to some e/tend

The preceding statements are true for all soils. ote, however,that the degree of compaction is no tdirectly proportional to the

compaction effort.

Effect of *oil type and radation

fine grain soil needs more water to reach optimum and

Coarse grain soil needs less water to reach optimum.

Compaction curves for different soils with the same compact effort fine grain soil needs more water to

reach optimum and coarse grain soil needs less water to reach optimum.

Effect of Moist#re content

Below wopt (dry side of optimum)0

s the water content increases, the particles develop larger and largerwater films around them, which tend to AlubricateB the particles and make them

easier to be moved about and reoriented into a denser configuration.t wopt0

The density is at the ma/imum, and it does not increase any further.!ove wopt (wet side of optimum)0

Water starts to replace soil particles in the mold, and since F wHH F s the

dry density starts to decrease



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General Information

-ach data point on the curve represents a single compaction test, and usually four or five individual

compaction tests are required to completely determine the compaction curve.

t least two specimens wet and two specimens dry of optimum, and water contents varying by about

5D. Gptimum and water contents varying by about 5D.

Gptimum water content is typically slightly less than the plastic limit 9*T2 suggestion:. Typica

values of ma/imum Typical values of ma/imum dry density are around $.; to 5.' $ 2g8m&& with the

ma/imum range from about $.& to 5.7 2g8m&.

Typical optimum water contents are between $'D and 5'D, with an outside ma/imum range of about

3D to 7'D. With an outside ma/imum range of about 3D to

7'D.

Effect of Compaction on Clay *tr#ct#re

@or a given compactive effort and dry density, the soil tends

to be more flocculated 9random: for compaction on the dryside as compared on the wet side.

@or a given molding water content, increasing the compactiveeffort tends to disperse 9parallel, oriented: the soil, especially

on the dry side.

Effect of Compaction on Permeability

Increasing the water content results in a decrease in permeability on the dry side of the optimum moisture content

and a slight increase in permeability on the wet side of optimum.

Increasing the compactive effort reduces the permeability since it both increases the dry density, thereby reducing

the voids available for flow, and increases the orientation of particles.

Effect of Compaction on Compressibility

t low stresses the sample compacted on the wet side is more compressible than the one compacted on the dry

side.

t the high applied stresses the sample compacted on the dry side is more compressible than the sample

compacted on the wet side.



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Effect of Compaction on *8ellin

*welling of compacted clays is greater for those

compacted dry of optimum. They have a relatively compacted

dry of optimum. They have a relatively greater deficiency of

water and therefore have a greater tendency to adsorb water

and thus swell more.

Enineerin Properties *#mmary

Compaction


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"ro!lem#%: The laboratory test data for a standard 1roctor test are given in the table. @ind the ma/imum dryunit weight and the optimum moisture content.

Solution:

We can prepare the following table0

The plot of&d against wisshown in @igure. @rom the graph, we observe

2a/imum dry density )5)5 =4m0

Gptimum moisture content '0>



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Compaction Very Useful

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