Lecture 5 soil compaction

INTERNATIONAL UNIVERSITY FOR SCIENCE & TECHNOLOGY

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CIVIL ENGINEERING AND

ENVIRONMENTAL DEPARTMENT

303322 - Soil Mechanics

Soil Compaction

Dr. Abdulmannan Orabi

Lecture

Dr. Abdulmannan Orabi IUST 2

Das, B., M. (2014), “ Principles of geotechnical Engineering ” Eighth Edition, CENGAGE Learning, ISBN-13: 978-0-495-41130-7.

Knappett, J. A. and Craig R. F. (2012), “ Craig’s Soil Mechanics” Eighth Edition, Spon Press, ISBN: 978-0-415-56125-9.

References

In the construction of highway embankments, earth dams, and many other engineering structures, loose soils must be compacted to increase their unit weights. To compact a soil, that is, to place it in a dense state. The dense state is achieved through the reduction of the air voids in the soil, with little or no reduction in the water content. This process must not be confused with consolidation, in which water is squeezed out under the action of a continuous static load.

Introduction

Compaction increases the strength characteristics of soils, which increase the bearing capacity of foundations constructed over them.

Compaction also decreases the amount of undesirable settlement of structures and increases the stability of slopes of embankments.

Compaction of Soil

Higher resistance

to deformation

Higher resistance

to frost damage

Increased stability

Decreased permeability

Increased bearing

capacity

Increased durability

compaction

1) Increased Shear Strength This means that larger loads can be applied to compacted soils since they are typically stronger. Increased Shear Strength =>increased bearing capacity, slope stability, and pavement system strength

2) Reduced Permeability

This inhibits soils’ ability to absorb water, and therefore reduces the tendency to expand/shrink and potentially liquefy

Purposes of compacting soil

3) Reduced Compressibility

This also means that larger loads can be applied to compacted soils since they will produce smaller settlements.

5) Reduce Liquefaction Potential

4) Control Swelling & Shrinking

Purposes of compacting soil

Compaction, in general, is the densification of soil by removal of air, which requires mechanical energy. Simplistically, compaction may be defined as the process in which soil particles are forced closer together with the resultant reduction in air voids.

Compaction of Soil

Definition:

compacted Soil before compacted

compacted

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

or soil grains·

Soil before compacted

Principles of Compaction

Solids

Loose soil Compacted soil

Compaction Effect

The 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.

The more compacted a soil is:

• the smaller its void ratio (e) will be.

• the higher its dry unit weight ( ) will be

�� =��

��Dr. Abdulmannan Orabi IUST 11

Compaction Curve

The compaction curve is relationship between a soil water content and dry unit weight.

Soil sample was computed at different water contents in a cylinder of volume 1000 cc and dry unit weight were obtained.

�� =�

1 + ��

Compaction curve is plotted between the water content as abscissa and the dry density as ordinate.

It is observed that the dry density increases with an increase in water content till the max. density is attained. With Further increase in water content, the dry density decreases.

Compaction Curve

8 14 16 18 20 221210

Water Content (Wc)

Compaction Curve

8 14 16 18 20 221210

��

Water Content (Wc)

Optimum moisture content (OMC) :The water content corresponding to maximum dry unit weight is called optimum moisture content.

Note that the maximum dry unit weight is only a maximum for a specific compactive effort and method of compaction.

Compaction Curve

Optimum moisture content (OMC) :

Each compactive effort for a given soil has its own OMC.As the compactive effort is increased, the maximumdensity generally increases and the OMC decreases.

Water Content (Wc)

8 14 16 18 20 221210

��

Compaction Curve

8 14 16 18 20 221210

��

Water Content (Wc)

Zero air voids curve or saturation line

Theoretical unit weight is given as �� =��

1 + �� ∗ ��

"Zero Air Voids"

S = 100%

The curve represent the fully saturated condition ( S= 100%). ( It can not be reached by compaction )

Compaction Curve

8 14 16 18 20 221210

Water Content (Wc)

Line of Optimums

"Zero Air Voids"

S = 100%

A line drawn through the peak points of several compaction curves at different compactive efforts for the same soil will be almost parallel to a zero air voids curve , it is called the line of optimums

Line of Optimums

Factors affecting Compaction

• Water content of the soil • Amount of compaction• Type of soil being compacted • The amount of compactive energy used • Method of compaction• Thickness of layer• Saturation line• Admixtures• Stone content

Water content of the soil

As water is added to a soil ( at low moisture content) it acts as a softening agent on the soil particlesand becomes easier for the particles to move past one another during the application of the compacting forces. As the soil compacts the voids are reduced and this causes the dry unit weight ( or dry density) to increase.

As the water content increases, the particles develop larger and larger water films around them, which tend to “lubricate” the particles and make them easier to be moved about and reoriented into a denser configuration.

8 14 16 18 20 221210

Water Content (Wc)

Water content below OMC

Water content at OMC

The density is at the maximum, and it does not increase any further.

8 14 16 18 20 221210

OMC Water Content (Wc)

Water starts to replace soil particles in the mold and the dry unit weight starts to decrease.

Water content above OMC

8 14 16 18 20 221210

OMC Water Content (Wc)

Soil type

Soil type, grain size, shape of the soil grains, amount and type of clay minerals present and the specific gravity of the soil solids, have a great influence on the dry unit weight and optimum moisture content

Uniformly graded sand or poorly graded in nature is difficult to compact them.

Soil type

In poorly graded sands the dry unit weight initially decreases as the moisture content increases and then increases to a maximum value with further increase in moisture content.

At lower moisture content, the capillary tension inhibits the tendency of the soil particles to move around and be compacted.

Soil type

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

Water Content

High CompactiveEffort

Low Compactive Effort

Flocculated Structure, orHoneycomb Structure, orRandom

Intermediatestructure

Dispersed Structureor

parallel

Structure of Compacted Clay

Effect of Compaction effort

The compaction energy per unit volume used for the standard Proctor test can be given as

��. ��

��×

��. ��

��×�!"ℎ$��

ℎ�%%�×

ℎ!"ℎ$��

&��

'��(%��%��&

Increased compactive effort enables greater dry unit weight. It can be seen from this figure that the compaction curve is not a unique soil characteristic. It depends on the compaction energy.

Effects of increasing compactive effort

Water Content (Wc)

8 14 16 18 20 221210

��

High compactive effort curve

Low compactive effort curve

Effects of increasing compactive effort

For this reason it is important when giving values of

(γdry)max and OMC to also specify the compaction procedure (for example, standard or modified).

From the preceding observation we can see that1. As the compaction effort is increased, the maximum dry unit weight of compaction is also increased.2. As the compaction effort is increased, the optimum moisture content is decreased to some extent.

Coarse-grained soils Fine-grained soils

� Rubber-tired rollers

Vibration

Vibrating hammer

Kneading

Static loading and press

General Compaction Methods

� Hand-operated vibration plates

� Motorized vibratory rollers

� Rubber-tired equipment

� Free – falling weight

dynamic compaction

� Hand-operated tampers

� Sheep-foot rollers

Falling weight and hammers

Kneading compactors

Laboratory Compaction Tests

Laboratory compaction tests provide the basis for determining the percent compaction and molding water content needed to achieve the required engineering properties, and for controllingconstruction to assure that the required compaction and water contents are achieved.

The aim of the test is to establish the maximum dry unit weight that may be attained for a given soil with a standard amount of compactiveeffort.

When a series of samples of a soil are compacted at different water content the plot usually shows a distinct peak.

The fundamentals of compaction of fine-grained soils are relatively new. R.R. Proctor in the early 1930’s developed the principles of compaction.

The proctor test is an impact compaction. A 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.

There are several types of test which can be

used to study the compactive properties of soils.

1. Standard Procter Test is not sufficient for airway and highways,

2. Modified Procter Test was later adopted by AASHTO and ASTM

Soil is compacted into a mould in 3-5 equal layers, each layer receiving 25 blows of a hammer of standard weight. The energy (compactiveeffort) supplied in this test is 595 kJ/m3. The important dimensions are

Volume of mould Hammer mass Drop of hammer

1000 cm^3 2.5 kg 300 mm

Standard Procter Test

Standard Proctor test equipment

Proctor established that compaction is a function of four variables:

• Dry density (ρd) or dry unit weight γd.

• Water content wc

• Compactive effort (energy E)

• Soil type (gradation, presence of clay minerals, etc.)

Several samples of the same soil , but at different water contents, are compacted according to the compaction test specification

The soil is mixed with varying amountsof water to achieve different water contents

• Apply 25 blows from the rammer dropped from a height of 305 mm above the soil.

• Distribute the blows uniformly over the surface and ensure that the rammer always falls freely and is not obstructed.

Rammer Pattern for compaction in 101.6 mm Mold

The first four blows The successive blows

The soil is in mold will be divided into three lifts

Soil sample 3 layers

2.5 kg (5.5lb) 25 blows per layer

• Place a second quantity of moist soil in the mould such that when compacted it occupies a little over two-thirds of the height of the mould body.

Each Lift is compacted 25 times

Soil sample 3 layers

2.5 kg (5.5lb) 25 blows per layer

• Repeat procedure once more so that the amount of soil used is sufficient to fill the mould body, with the surface not more than 6mm proud of the upper edge of the mould body.

Derive the dry unit weight from the known unit weight and water content

The unit weight and the actual water content ofeach compacted sample are measured

�� =�

1 + ��

8 14 16 18 20 221210

��

Water Content (Wc)

"Zero Air Voids"

S = 100%

Plot the dry unit weight versus water content foreach compacted sample.

Determine the maximum dry weight and OMC

Diameter of mold

Volume of mold

Weight of hammer

Height of hammer drop

Number of hammer blowsper layer of soil

Number of layers of compaction

Energy of compaction

Method A Method B Method C

101.6 mm 101.6 mm 152.4 mm

943.3 cm^3 943.3 cm^3 2124 cm^3

24.4 N 24.4 N 24.4 N

304.8 mm 304.8 mm 304.8 mm

591.3 kN.m/m^3

562525

Specification of standard Proctor test ( Based on ASTM Test Designation 698)

Specification of standard Proctor test ( Based on ASTM Test Designation 698) ( con.)

Soil to be used Portion passing No.4 ( 457mm)sieve .May be used if 20% or less by weight of material is retained on

No.4 sieve.

Portion passing 9.5 mm sieve .May be used if retained on No.4 sieve is more than 20% and 20% or less by weight of material is retained on 9.5

mm sieve.

Portion passing 19- mm sieve .May be used if more than 20% by weight of material is retained on 9.5 mm sieve and less than 30% by weight of material is retained on 19-

mm sieve.

•Was developed during World War II

•By the U.S. Army Corps of Engineering

• For a better representation of the compaction required for airfield to support heavy aircraft.

Modified Procter Test

Same as the Standard Proctor Test with the following exceptions:

� The soil is compacted in five layers

� Hammer weight is 10 Lbs or 4.54 Kg

� Drop height h is 18 inches or 45.72cm

� Then the amount of Energy is calculated

Rammer Pattern for compaction in

152,4 mm Mold

Uniformly distribution of the blows over the surface

44.5 N(10 lb)

Diameter of mold

Volume of mold

Weight of hammer

Height of hammer drop

Number of hammer blowsper layer of soil

Number of layers of compaction

Energy of compaction

101.6 mm 101.6 mm 152.4 mm

943.3 cm^3 943.3 cm^3 2124 cm^3

44.5 N 44.5 N 44.5 N

457.2 mm 457.2 mm 457.2 mm

2696 kN.m/m^3

562525

Specification of standard Proctor test ( Based on ASTM Test Designation 698)

Soil to be used Portion passing No.4 (457mm)sieve May be used if 25% or less by weight of material is retained on No.4 sieve. If this gradation requirement cannot be met, then Methods B or C may be used.

Portion passing 9.5 mm sieve .May be used if soil retained on No.4 sieve is more than 25% and 25% or less by weight of material is retained on 9.5

mm sieve.

Portion passing 19- mm sieve .May be used if more than 20% by weight of material is retained on 9.5 mm sieve and less than 30% by weight of material is retained on 19-

mm sieve.

Specification of standard Proctor test ( Based on ASTM Test Designation 698) ( con.)

Water Content (wc)

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��(,-�.�.)

Comparison-Curves

Standard Proctor Test

� Mold size: 943.3cm^3

� 304.8 mm height of drop

� 24.4 N hammer

� 3 layers

� 25 blows/layer

� Energy 591.3 kN.m/m^3

Modified Proctor Test

� Mold size: 943.3cm^3

� 457.2 mm height of drop

� 44.5 N hammer

� 5 layers

� 25 blows/layer

� Energy 2696 kN.m/m^3

Comparison-Summary

Filed Compaction

Compaction Equipment

Most of the compaction in the field is done with rollers. The four most common types of rollers are:

1. Smooth-wheel rollers (or smooth-drum rollers)2. Pneumatic rubber-tired rollers3. Sheepsfoot rollers4. Vibratory rollers

Smooth-wheel rollers are suitable for proof rolling subgrades and for finishing operation of fills with sandy and clayey soils. These rollers provide 100% coverage under the wheels, with ground contact pressures as high as 310 to 380 kN/m^2. They are not suitable for producing high unit weights of compaction when used on thicker layers.

Filed Compaction

Smooth-wheel rollers

oone steel drum and rubber tired drive wheels

o two steel drums one of which is the driver

o effective for gravel, sand, silt soils

Pneumatic rubber-tired rollers are better in many respects than the smooth-wheel rollers. The former are heavily loaded with several rows of tires. These tires are closely spaced—four to six in a row.

Pneumatic rubber-tired rollers

Pneumatic rollers can be used for sandy and clayey soil compaction.

Compaction is achieved by a combination of pressure and kneading action.

Pneumatic rubber-tired rollers

Sheepsfoot rollers are drums with a large number of projections. The area of each projection may range from 25 to 85 cm2. These rollers are most effective in compacting clayey soils. The contact pressure under the projections can range from 1400 to 7000 kN/m2.

Sheepsfoot rollers

During compaction in the field, the initial passes compact the lower portion of a lift.Compaction at the top and middle of a lift is

done at a later stage.

Sheepsfoot rollers

Vibratory rollers are extremely efficient in compacting granular soils. Vibrators can be attached to smooth-wheel, pneumatic rubber-tired, or sheepsfoot rollers to provide vibratory effects to the soil. The vibration is produced by rotating off-center weights.

Vibratory rollers

For field compaction, soil is spread in layers and a predetermined amount of water is sprayed on each layer (lift) of soil, after which compaction is initiated by a desired roller.In addition to soil type and moisture content, other factors must be considered to achieve the desired unit weight of compaction in the field.

Factors Affecting Field Compaction

These factors include the thickness of lift, the intensity of pressure applied by the compacting equipment, and the area over which the pressure is applied. These factors are important because the pressure applied at the surface decreases with depth, which results in a decrease in the degree of soil compaction.

During compaction, the dry unit weight of soil also is affected by the number of roller passes.

The dry unit weight of a soil at a given moisture content increases to a certain point with the number of roller passes.

Specifications for Field Compaction

In most specifications for earthwork, the contractor is instructed to achieve a compacted field dry unit weight of 90 to 95% of the maximum dry unit weight determined in the laboratory by either the standard or modified Proctor test.

This is a specification for relativecompaction, which can be expressed as

where R = relative compaction

/ =�&�!�&

�& 012 ��

× 100%

For the compaction of granular soils, specifications sometimes are written in terms of the required relative density Dr or the required relative compaction.

Relative density should not be confused with relative compaction.

/ =/�

1 − 6� 1 − /�where :

/� =�&(%!7)

�& 012

Correlation between relative compaction (R) and the relative density Dr

Determination of Field Unit Weight of Compaction

When the compaction work is progressing in the field, knowing whether the specified unit weight has been achieved is useful. The standard procedures for determining the field unit weight of compaction include

1. Sand cone method2. Rubber balloon method3. Nuclear method

Sand Cone Method

Sand Cone Method (ASTM Designation D-1556)

The sand cone device consists of a glass or plastic jar with a metal cone attached at its top

Determination of Field Unit Weight of Compaction

Nuclear Method Rubber Balloon method

The results of a standard Proctor test are given in the following table.

Determine the maximum dry unit weight of compaction and the optimum moisture content Also, determine the moisture content required to achieve 95% of (γdry)max .

Worked Examples

Volume of Proctor

Mold (cm^3 )

944 944 944 944 944 944 944 944

Mass of wet soil in the

mold ( kg)

1.68 1.71 1.77 1.83 1.86 1.88 1.87 1.85

Water content ( % ) 9.9 10.6 12.1 13.8 15.1 17.4 19.4 21.2

Example 1

Worked ExamplesExample 2

1) The in situ void ratio of a borrow pit’s soil is 0.72.

2) The borrow pit soil is to be excavated and transported to fill a construction site where it will be compacted to a void ratio of 0.42.

3) The construction project required 10000 m^3 of compacted soil fill

Required

Volume of soil that must be excavated from the borrow pit to provide the required volume of fill.

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