Lecture 5 soil compaction

INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
‫وا‬ ‫م‬ ‫ا‬ ‫و‬ ‫ا‬ ‫ا‬
CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303322 - Soil Mechanics
Soil Compaction
Dr. Abdulmannan Orabi
Lecture
2
Lecture
5

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

Compaction of Soil
Higher resistance
to deformation
Higher resistance
to frost damage
Increased stability
Decreased permeability
Increased bearing
capacity
Increased durability
Poor
compaction
Good
compaction
Poor
compaction
Good
compaction
Poor
compaction
Good
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
Air
Water
Solids
Air
Water
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
=

1 +

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
16
18
14
12
20
8 14 16 18 20 221210
Dryunitweight()γ
Water Content (Wc)

Compaction Curve
16
18
14
12
8 14 16 18 20 221210
OMC
Dryunitweight()γ
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 maximum
density generally increases and the OMC decreases.
Water Content (Wc)
16
18
14
12
8 14 16 18 20 221210
OMC1
Dryunitweight()γ
OMC2
15

Compaction Curve
16
18
14
12
8 14 16 18 20 221210
OMC
Dryunitweight()γ
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
16
18
14
12
8 14 16 18 20 221210
Dryunitweight()γ
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 particles
and 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.
16
18
14
12
20
8 14 16 18 20 221210
Dryunitweight()γ
Water Content (Wc)
Water content below OMC
OMC

Water content at OMC
The density is at the maximum, and it
does not increase any further.
16
18
14
12
20
8 14 16 18 20 221210
Dryunitweight()γ
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
16
18
14
12
20
8 14 16 18 20 221210
Dryunitweight()γ
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.

DryUnitWeight
Water Content
High Compactive
Effort
Low Compactive Effort
Flocculated Structure, or
Honeycomb Structure, or
Random
Intermediate
structure
Dispersed Structure
or
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)
16
18
14
12
8 14 16 18 20 221210
OMC1
Dryunitweight()γ
OMC2
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 that
1. 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
LaboratoryField
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 controlling
construction 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 compactive
effort.
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 (compactive
effort) 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 amounts
of 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
4
3
1 2
5
7
6
8
4
etc.
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
305mm
• 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
305mm
• 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 of
each compacted sample are measured
=
1 +

16
18
14
12
8 14 16 18 20 221210
OMC
Dryunitweight()γ
Water Content (Wc)
"Zero Air Voids"
S = 100%
Plot the dry unit weight versus water content for
each 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 blows
per 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
3 3 3
591.3
kN.m/m^3
591.3
kN.m/m^3
591.3
kN.m/m^3
562525
Specification of standard Proctor test ( Based on ASTM Test
Designation 698)
Item

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.
Item

•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

3
7
2
8
1
6
4
5
9
Rammer Pattern for compaction in
152,4 mm Mold
Uniformly distribution of the
blows over the surface
457.2mm
# 1
# 3
# 2
# 5
# 4
44.5 N(10 lb)

Diameter of mold
Volume of mold
Weight of hammer
Height of hammer drop
Number of hammer blows
per 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
5 5 5
2696
kN.m/m^3
2696
kN.m/m^3
2696
kN.m/m^3
562525
Designation 698)
Item

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.
Item
Designation 698) ( con.)

Dryunitweight(γd)
Standard Procter
Test
Modified Procter
Test
Water Content (wc)
( * .)
(,- . .)
OMC
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 rollers
3. Sheepsfoot rollers
4. 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

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 relative
compaction, 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 method
2. Rubber balloon method
3. 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 Examples
Example 2
Given
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.

Lecture 5 soil compaction

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