CHAPTER 6

SOIL COMPACTION

Omitted Sections
6.6, 6.7, 6.8
 SOIL COMPACTION

 In Geotechnical engineering practice, the soils at a given site

are often less than desirable for the intended purpose. They
may be:
 Weak (strength)
 Highly compressible
 Have a high permeability
Solution
Relocate the project
Articulate design for structure members
Stabilize or improve the properties of the soil
The third alternative may be in most cases the most economical
alternative. There are different techniques for improvement of soils
(This subject is covered in details in CE 486 “Improvement of
Geotechnical Materials”).

We will consider in this course only compaction.

 SOIL COMPACTION

 Compaction is also very important when soil is used as an

engineering material, that is the structure itself is made of soil.

Ex.
Earth dams
Highways
Airfields
etc.

Definition
Compaction is the densification of soils by removal of air
through the application of mechanical energy.

The degree of compaction is measured in terms of its dry unit

weight.
 SOIL COMPACTION

Increases unit weight

Increases shear strength

Increases bearing capacity

Increases stability of slopes of embankments

Decreases settlement of structures

 SOIL COMPACTION

You remember well-graded

reduced
Air
Air
Compaction
Water Water

Solid Solid
 SOIL COMPACTION

General Principle
The degree of compaction of soil is measured by its dry unit weight.
When water is added during compaction it acts as a softening agent
on the soil particles.
gd(max)
When the moisture content is
Soil Solid gradually increased, the weight
of the soil solids in a unit
water
volume gradually increases.

Soil Solid

 Optimum moisture content (OMC) is the water content at

g
which the maximum dry unit weight d (max) is attained.
SOIL COMPACTION

Soil Solid

water

Soil Solid
 SOIL COMPACTION

Types of Compaction Methods in the Laboratory

•Impact or dynamic (The most common type)
•Kneading
•Static
 The laboratory test generally used to obtain the maximum dry
unit weight of compaction and the optimum moisture content is
called the Proctor compaction test.
 It is named after R. R. Proctor (1933) (engineer in LA). He
established that compaction is a function of:
1.Moisture Content
2.Compactive Effort
3.Soil Type
 There are two methods or tests:
 Standard Proctor test (ASTM D-698 & AASHTO T-99)
 Modified Proctor test (ASTM D-1557 & AASHTO T-180)
 Standard Proctor Test

• Mold 1/30 ft3 in volume

• 3 layers
• 25 blows
• 5.5 lb hammer
• 12 inch drop

Mold Hammer

The procedure for the standard Proctor test is elaborated in ASTM

Test Designation D-698 (ASTM, 2007) and AASHTO Test
Designation T-99 (AASHTO, 1982).
Standard Proctor Test
 Standard Proctor Test

Process of Compaction
 Several samples are mixed at different water contents
 Compact according to the compaction test (standard or modified).
W
g moist 
Vmold
W = Weight of compacted soil in the mold
Vmold = Volume of the mold = (1/30 ft3)
 For each test find the moisture
content of the compacted soil.
 The dry unit weight is given by
g moist
gd 
1 w
 Plot gd vs. w
 From the plot, find OMC and g d (max)
 Standard Proctor Test

 In order to avoid a large number of compaction tests, it is

desirable to begin the first test at a moisture content that is about
4 to 5% below the approximate optimum moisture content.
 Standard Proctor Test

REMARKS
1. Each data point on the curve represent a single compaction test.

2. Four or five tests are required

3. The curve is unique for:

- A given soil type
- Method of compaction
- (constant) compactive effort
4. gd(max) is only a maximum for a specific compactive effort and method of
compaction. This does not necessarily reflect the maximum dry unit weight
that can be obtained in the field.
5. Typical OMC are between 10% and 20%. Outside maximum range 5% to
40%.
 Standard Proctor Test

6. Increasing the compactive effort tends to increase the maximum dry

density, as expected, but also decrease the OMC. (This is why the curve
never be to the right of zero curve).
7. In practice less amount of water is used but higher compactive effort or
vise versa.

8. For clay soils gd(max) tends to decrease as plasticity increases.

9. The approximation to field is not exact because the lab. test is a dynamic
impact type, whereas field compaction is essentially a kneading-type
compaction.

10. In the field, compactive effort is the number of passes or “coverage” of the
roller of a certain type and weight on a given volume of soil.
 Standard Proctor Test

Theoretical g d(max)
The maximumg d (max)is obtained when no air in the voids (i.e. s =100%)

Gs
gd  gw
1 e
but wGs  se
for S  100% e  wGs
Gs g
g z av  gw  1 w
1  wGs G
w
s

Where gzav = zero air void unit weight.

 The relationship between gzav and w can be obtained as shown in the figure across.
 Compaction curve is always to the left of the zero-air-void curve.
No matter how much water is added, the soil never
becomes completely saturated by compaction.
 Standard Proctor Test

To obtain the variation of gzav with moisture content, use

the following procedure:

Under no circumstances should any

part of the compaction curve lie to
the right of the zero-air-void curve.
 FACTORS AFFECTING COMPACTION

Besides moisture content, other important factors that affect

compaction are: 1) Soil type; 2) Compaction effort.

1. Effect of Soil Type

 Grain Size Distribution
 Shape of the soil grains
 Gs
 Amount of clay minerals
 Type of clay minerals
Fine grain soil needs more water
to reach optimum.
 FACTORS AFFECTING COMPACTION

Effect of Soil type and gradation

Fine grain soil

needs more water
to reach optimum.
 FACTORS AFFECTING COMPACTION

Effect of Soil type and gradation

Gs gw
g z av  gw  1 Typical Values
1  wGs G
w g d (max)(kN/m )
3 OMC (%)
s

Well graded sand SW 22 7

Sandy clay SC 19 12

Poorly graded sand SP 18 15

Low plasticity clay CL 18 15

Non plastic silt ML 17 17

High plasticity clay CH 15 25

 Gs is constant, therefore increasing maximum dry unit weight is associated

with decreasing optimum moisture contents.

 Do not use typical values for design as soil is highly variable.

 FACTORS AFFECTING COMPACTION

Compaction Curves Encountered in Soils

•The bell-shaped compaction
curve is typical for most clayey
soils.

Typical

•Some curves have more

than one peak others have no
peak.
 FACTORS AFFECTING COMPACTION

2. Effect of Compaction Effort

Standard Proctor

For the standard Proctor test

• The standard Proctor mold and

hammer were used to obtain these
compaction curves.
• For all cases the number of layers
was equal to 3.
g
Compaction effort d max
wopt.
 EXAMPLE 6.1

TEXT IN SI UNITS
 EXAMPLE 6.1

TEXT IN SI UNITS
 EXAMPLE 6.1

TEXT IN SI UNITS
EXAMPLE
EXAMPLE
 Example (2nd Midterm Exam Fall 40-41)

If you are checking the field compaction of a layer of soil and the compaction curve
for the soil is shown in Figure 1. From the specifications, the dry density of the
compacted soil should be at least 95% of the maximum value and within ± 1% of
the optimum water content. When you did the sand cone test, the volume of the soil
excavated was 1153 cm3. It weighed 2209 grams wet and 1875 grams dry.

a. What is the compacted dry density?

b. What is the field water content?
c. What is the relative compaction?
d. Does the test meet the specifications (explain)?
e. If it does not meet, what should be done to
improve the compaction so that it will meet the
specifications?
f. What is the degree of saturation of the field
sample?
g. If the sample were saturated at constant
density, what would be the water content?
 Modified Proctor Test

Modified Proctor Test (ASTM D-1557, AASHTO T-180)

 With the development of heavy rollers (also requirements of heavy

aircrafts and trucks) and their use in field compaction, the standard
Proctor test was modified for better representation of the field
conditions. This is sometimes referred to as modified Proctor test.

•Mold 1/30 ft3 in volume (same as for standard test)

•5 layers
•25 blows (same as for standard test)
•10 lb hammer
•18 inch drop
 Developed in WWII by U.S. Army Corps of Engineers to better
represent the compaction required for airfield to support heavy
aircraft.
 Modified Proctor Test

Modified Proctor
Standard Proctor Test
Test
Layer 5
Layer 4
Layer 3
Drop = 457.2 mm Layer 2
(18 in) Layer 1

Drop = 304.8 mm
(12 in)

hammer hammer
= 2.5 kg (5.5 lb) = 4.54 kg (10 lb)
 Modified Proctor Test

Standard Modified
Proctor Proctor
Test Test
Volume of mold 944 cm3 944 cm3

# of layers 3 5
Mass of hammer 2.5 kg 4.54 kg
Drop of hammer 30.5 cm 45.7 cm
# of hammer blows 25 25
 Compaction Energy for Unit Volume of Soil

 Standard Proctor Test

(25)(3)5.5(1)
E  12375 ft - lb/ft 3  592.5 kN  m / m3  600 kN  m / m3
(1 / 30)

 Modified Proctor Test

(25)(5)10(1.5)
E  56250 ft - lb/ft 3  2693.3 kN  m / m3  2700 kN  m / m3
(1 / 30)

 Because it increases compactive effort, the modified Proctor test results in

an increase of the maximum dry unit weight of the soil, and this is
accompanied by decrease in the optimum moisture content.

 Note: In the field, compactive effort is the number of passes of the roller of
a certain type and weight on a given volume of soil.
SOIL COMPACTION
FIELD COMPACTION
 FIELD COMPACTION

Most of the compaction in the field is done by means of ROLLERS.

The most common types are:

1. Smooth-wheel rollers (smooth-drum rollers)

2. Pneumatic rubber-tired rollers

3. Sheepsfoot rollers

4. Vibratory rollers
 FIELD COMPACTION

1. Smooth-wheel rollers (smooth-drum roller)

Proof rolling subgrades
Finishing operation of fills with sandy &clayey soils
Provide 100% coverage
Contact pressure 310 – 380 kN/m2
Not suitable for producing high g for thicker layers
 FIELD COMPACTION

2.Pneumatic rubber-tired rollers

Heavily loaded with several rows of tires

Tires are closely spaced 4 -6 in a row

Provide 70-80% coverage

Contact pressure 600 – 700 kN/m2

Combination of pressure and kneading

 FIELD COMPACTION

3.Sheepsfoot rollers
Drums with a large number of projections

Area of each projection 25 – 85 cm2

Most effective in compacting clayey soils

Contact pressure 1400 – 7000 kN/m2

 FIELD COMPACTION

4.Vibratory rollers
 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.

Figure 6.20 Principles of vibratory rollers

 FIELD COMPACTION

Handheld vibratory
Handheld vibratory plates can be used for effective compaction of
granular soils over a limited area.
 FACTORS AFFECTINGFIELD COMPACTION

There are several factors that must be considered to achieve the

desired unit weight of compaction in the field:

Soil type

Moisture content

 Thickness of lift

 Intensity of pressure

 Area over which the pressure is applied

No. of roller passes

FIELD COMPACTION

Compaction of Silty Clay

FIELD COMPACTION

Vibratory Compaction of Sand

 FIELD COMPACTION

In most cases, about 10 to 15 roller passes Lack of confining pressure

yield the maximum dry unit weight towards the surface
economically attainable.

Relationship between dry unit weight Relationship between dry unit weight,
and number of passes number of passes, and depth.
SPECIFICATIONS FOR FIELD COMPACTION

 Usually it is required for the contractor to achieve a compacted field dry unit weight
of say 90 to 95% of the maximum dry unit weight determined in the laboratory by
either the standard or modified Proctor test (Recall previous examples).

Relative compaction, R (a)

 For granular soils, specifications can be expressed in terms of relative

density.
Applicable if the soil
(b) contains less than 12%
fines (passing No. 200
sieve)
From (a) and (b)

where
 FIELD COMPACTION

Dividing by
R

1 R
 FIELD COMPACTION

Solve for R

 Approximate formula for granular soils

EXAMPLE 6.8
Determination of Field Unit Weight of Compaction

We know that both relative compaction or relative density are both

needed for determination of dry density in the field.

Common Methods:
1. Sand cone method

2. Rubber balloon method

3. Nuclear method
 Sand cone method

1. Sand cone method (ASTM Designation D-1556)

 Filling the jar with very uniform dry Ottawa sand

 W1 = weight of the jar, the cone, and the sand

filling the jar

 Excavating a small hole in the area where the

soil has been compacted

 W2 = weight of the moist soil excavated from

the hole.

 W3 = the dry weight of the soil =

 = moisture content
Recall
 = Ww/Ws
 Sand cone method
 The cone with the sand-filled jar attached
to it is inverted and placed over the hole.
 W4 = combined weight of the jar, the cone,
and the remaining sand filling the jar.
 W5 = weight of sand to fill both the hole and cone

 V = the volume of the excavated hole

Wc= weight of sand to fill the cone only

g d ( sand)  dry unit weigh t of Ottawa sand used

 The dry unit weight of compaction made in the field is

determined as W
g d ( field )  3
V
Field Density Test
Field Density Test
EXAMPLE 6.9

Proctor test

Sand Cone Test

EXAMPLE 6.9
EXAMPLE 6.9
EXAMPLE 6.10

Proctor test

Sand Cone Test

 Proctor test
EXAMPLE 6.10

Sand Cone Test

 RUBBER BALLOON METHOD

2. Rubber Balloon Method (ASTM Designation D-2167)

Similar to sand cone method except that the volume of the hole
is determined by introducing into it a rubber balloon filled with
water from a calibrated vessel.

Determine weight of dry soil

Determine volume of the hole
(can be read directly)
Determine dry unit weight
 Nuclear Method

3. Nuclear Method (ASTM D6938 - 15 )

 Nuclear density meter (Densometer)
o Dense soil absorbs more radiation
than loose soil.
o Measures the weight of wet soil per
unit volume and the weight of water
present in a unit volume of soil.
o The dry unit weight of compacted
soil can be determined by
subtracting the weight of water from
the moist unit weight of soil.

Operates either in drilled holes or on ground surface

Uses radioactive isotope source (Gamma rays)
Measure weight of wet soil per unit volume
It also measure weight of water per unit volume
Determine the dry unit weight of soil
Special Compaction Techniques
 Special Compaction Techniques

Vibroflotation

A technique for in situ

densification of thick layers of
loose granular soil deposits.
Special Compaction Techniques
Special Compaction Techniques
 GSD and compaction by vibrflotation

The most suitable

for compaction by
The rate of probe Vibroflotation.
penetration may be slow
and may prove
uneconomical in the long
run.
Difficult to compact

Lower limit of grain-size

distribution for which
compaction by vibroflotation is
effective.
 Special Compaction Techniques

RATING BACKFILL (Brown , 1977)

SN = Suitability No. for rating backfill

where D50, D20, and D10 are the diameters (in mm) through
which, respectively, 50, 20, and 10% of the material passes.

The smaller the value of SN, the

more desirable the backfill
material.
 Special Compaction Techniques

Typical patterns of Vibroflot probe spacings

for a column foundation

Compaction over
a large area
EXAMPLE 6.11
 Special Compaction Techniques

Dynamic Compaction
Densification of granular soil deposits
Dropping a heavy weight on the ground at regular intervals
Weight of hammer 80-360 kN
Hammer drop 7.5-30.5 m

Degree of compaction depends on:

Weight of hammer
Height of hammer drop
Spacing of locations at which
the hammer is dropped
 Special Compaction Techniques

Blasting
 Compaction (up to a relative
density of 80%) up to a depth of
about 18 m over a large area can
easily be achieved.
 Usually the explosive charges
are placed at a depth of about
two-thirds of the thickness of
the soil layer desired to be
compacted.
THE END