FCE 311 GEOTECHNICAL ENGINEERING

9. SOIL COMPACTION

Department of Civil & Construction Engineering

University of Nairobi

9.1 Introduction

• Is the process of increasing the density of soil by packing the particles together with reduction in volume of air.

• The process does not involve removal of water.

• The process primarily results in the increase of soil unit weight (density).

9.1 Introduction

• A good measure of compaction is needed in the construction of

– Road embankments,

– improvement of road subgrade, subbase and base layers.

9.2 Theory of Compaction

9.2.1 General

• The degree of compaction is measured quantitatively by the dry density (γd).

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d

1

9.2 Theory of Compaction

9.2.1 General

• The increase in dry density of soil is a function of the moisture content of the soil and the compaction effort.

• The variation of dry density and moisture content (M) are presented in a Compaction Curve.

• Curve is usually plotted by obtaining the dry density from bulk density and water content measurements.

9.2 Theory of Compaction9.2.1 General

• Typical Compaction Curve

9.2 Theory of Compaction9.2.1 General

• Low water content - soil is stiff and difficult to compact.

• Increase in water content - soil is workable facilitating compaction and reduction in air.

• Dry density increases.

9.2 Theory of Compaction• As air is reduced and replaced by water, at

certain water content the voids are occupied by water and prevent any appreciable decrease in the air voids.

9.2 Theory of Compaction9.2.1 General

• The result is increase in the void ratio and consequent decrease in the dry density.

• The curve has a peak which shows an optimum moisture content (OMC) at which the soil would need to be compacted to achieve the maximum dry density (MDD).

9.2 Theory of Compaction9.2.1 General

• If it was possible to remove all the air at various water contents, then the dry density achieved at the zero air voids would be the maximum achievable for the various water contents.

• This curve can be drawn by the relationship of dry density and Gs γw, and Sr and assuming the degree of saturation Sr is equal to one.

9.2 Theory of Compaction9.2.1 General

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9.2 Theory of Compaction9.2.2 Variation in Compaction Curve

A) Variation in compaction effort

• Increasing the compaction effort results in the increase in the maximum dry density and a decrease in the optimum moisture content as shown.Thus if light compaction is used more water would be needed to overcome the resistance of the soil grains to packing. With more compaction effort the soil grains need less water to occupy the available pore spaces.

9.2 Theory of Compaction9.2.2 Variation in Compaction Curve

A)Variation in compaction effort

9.2 Theory of Compaction9.2.2 Variation in Compaction Curve

A)Variation in compaction effort

• Light compaction – use more water would to overcome the resistance of the soil grains to packing.

• With more compaction effort the soil grains need less water to occupy the available pore spaces.

9.2 Theory of Compaction9.2.2 Variation in Compaction Curve

B) Variation in soil grading type

Well graded soil

• There is a good representation of all the particle sizes

Poorly graded soil

• There is an excess or a deficiency of certain particle sizes within the limits of the minimum and maximum sizes, or if most of the particles are of about the same sizes (uniformly graded).

9.2 Theory of Compaction9.2.2 Variation in Compaction Curve

B) Variation in soil grading type

• The particle size distribution influences the arrangement of particles in the compaction of soils.

• A well-graded soil will compact to lesser voids than a poorly graded soil.

9.2 Theory of Compaction9.2.2 Variation in Compaction Curve

B) Variation in soil grading type

• The effect is that the well-graded soils will have an increased density and improved properties such as bearing capacity on compaction.

9.3 Laboratory Compaction Tests

• Light manual compaction test with 2.5kg hammer.

• Heavy compaction test in which much greater compaction is achieved by use of 4.5kg hammer.

• Vibrating hammer and is intended mainly for granular soils passing 37.5 mm test sieve with no more than 30% retained on a 20mm test sieve.

9.3 Laboratory Compaction Tests

14

16

18

20

22

0 5 10 15 20 25 30

Dry

uni

t wei

ght (

kN/m

2 )

Moisture content (%)

2.5 kg rammer4.5 kg rammerVibratory rammerPower (0% air voids)Power (10% air voids)

9.4 Field Compaction9.4.1 Compaction

• The smooth wheeled rollers

9.4 Field Compaction• The pneumatic–tired rollers

9.4 Field Compaction

• The sheep’s foot rollers

9.4 Field Compaction

• Rammers

9.4 Field Compaction

• Vibrators

9.4 Field Compaction

Construction Equipment

• Lorries

9.4 Field Compaction

Construction Equipment

• Scrapers

9.4 Field Compaction

Construction Equipment

• Excavators

9.4 Field Compaction

Construction Equipment

• Graders

9.4 Field Compaction9.4.2 Field Control of Compaction

• Tests for determination of bulk density and water content during earthwork construction must be carried out at every layer of the compacted earthworks. In the least at least four density tests per 8 hour shift with a minimum one test for every 400m3 of earthworks are needed.

9.4 Field Compaction9.4.3 Specification of the Field

Compacted Density

• The compacted density is usually specified as a % of the MDD as determined in the laboratory.

• In some instances the moisture content is required to be within a small band above or below the OMC.

9.4 Field Compaction9.4.3 Specification of the Field

Compacted Density

• The specification usually ranges from 95-100% MDD when a 2.5 kg hammer is used the field control.

• A minimum number of roller passes is usually required to reach the desired compaction. The usual range is between 3 and 12 passes.