311 GEOTECHNICAL ENGINEERING

1. FCE 311 GEOTECHNICAL ENGINEERING
9. SOIL COMPACTION
Department of Civil & Construction Engineering
University of Nairobi

2. 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).

3. 9.1 Introduction
• A good measure of compaction is
needed in the construction of
– Road embankments,
– improvement of road subgrade, subbase
and base layers.

4. 9.2 Theory of Compaction
9.2.1 General
• The degree of compaction is measured
quantitatively by the dry density (γd).
w
b
d
+
=
1
γ
γ

5. 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.

6. 9.2 Theory of Compaction
9.2.1 General
• Typical Compaction Curve

7. 9.2 Theory of Compaction
9.2.1 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.

8. 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. 9.2 Theory of Compaction
9.2.1 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).

10. 9.2 Theory of Compaction
9.2.1 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.

11. 9.2 Theory of Compaction
9.2.1 General
wd
e
G
γγ
+
=
1
e
Gwe
A s
+
−
=
1
*
s
ws
d
wG
AG
+
−
=
1
)1(*γ
γ

12. 9.2 Theory of Compaction
9.2.2 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.

13. 9.2 Theory of Compaction
9.2.2 Variation in Compaction Curve
A)Variation in compaction effort

14. 9.2 Theory of Compaction
9.2.2 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.

15. 9.2 Theory of Compaction
9.2.2 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).

16. 9.2 Theory of Compaction
9.2.2 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.

17. 9.2 Theory of Compaction
9.2.2 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.

18. 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.

19. 9.3 Laboratory Compaction
Tests
14
16
18
20
22
0 5 10 15 20 25 30
Dryunitweight(kN/m2)
Moisturecontent(%)
2.5 kg rammer
4.5 kg rammer
Vibratory rammer
Power(0%air voids)
Power(10%air voids)

20. 9.4 Field Compaction
9.4.1 Compaction
• The smooth wheeled rollers

21. 9.4 Field Compaction
• The pneumatic–tired rollers

22. 9.4 Field Compaction
• The sheep’s foot rollers

23. 9.4 Field Compaction
• Rammers

24. 9.4 Field Compaction
• Vibrators

25. 9.4 Field Compaction
Construction Equipment
• Lorries

26. 9.4 Field Compaction
Construction Equipment
• Scrapers

27. 9.4 Field Compaction
Construction Equipment
• Excavators

28. 9.4 Field Compaction
Construction Equipment
• Graders

29. 9.4 Field Compaction
9.4.2 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.

30. 9.4 Field Compaction
9.4.3 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.

31. 9.4 Field Compaction
9.4.3 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.