2.2. 2COMPACTIONCOMPACTION FOR BEGINEERS TO UNDERSTAND 12.2. 2COMPACTIONCOMPACTION FOR BEGINEERS TO UNDERSTAND 12.2. 2COMPACTIONCOMPACTION FOR BEGINEERS TO UNDERSTAND 12.2. 2COMPACTIONCOMPACTION FOR BEGINEERS TO UNDERSTAND 1

1. COMPACTION

2. Factors affecting Compaction
 Water Content
 Amount of Compaction
 Method of Compaction
 Type of Soil
 Addition of Admixtures

4. Effect of Water Content
 With increase in water content, compacted
density increases up to a stage, beyond which
compacted density decreases.
 At lower water contents than OMC, soil particles
are held by electrical forces that prevents the
development of diffused double layer leading to low
inter-particle repulsion.

5.  Increase in water results in expansion of double
layer and reduction in net attractive force between
particles. Water replaces air in void space
 Particles slide over each other easily increasing
lubrication, helping in dense packing.
 After OMC is reached, air voids remain constant.
Further increase in water, increases the void
space, thereby decreasing dry density

6. Method of Compaction
 Weight of compacting equipment
 Type of compaction Impact, Kneading, Rolling, Static
Pressure
 Area of contact of compacting equipment with soil
 Time of exposure
 Each of these approaches will yield different
compactive effort Further, suitability of a particular
method depends on type of soil.

7. Influence of compaction on soil properties
Soil Structure
Density
Shear strength
Permeability
Bearing Capacity
Settlement
Stress Strain characteristics
Swelling & Shrinkage

9. Soil
Structure
B
C
Flocculated
E
Dispersed
A
Highly
Flocculated
Highly
Dispersed
D

10. At low water contents, attractive forces between
clay particles predominate, creating a more or less
random orientation of plate like particles. (results in
low density)
The addition of water increases repulsion between
more parallel
particles leading them to
assume orientation near OMC.
If compacted wet of optimum
parallel
orientation is
is described
as
further increased leading to
what dispersed structure.

11. Fig: Effect Of Soil Type

12.  Soil type, grain size, shape of the soil
grains, amount and type of clay minerals
present and the the γd
Gs of soil solids, have a great influence
on and OMC.
 In poorly graded sands γd initially decreases as the
moisture content increases, and then increases to a
maximum value with further increase in moisture.
 At lower moisture contents, the capillary tension
inhibits the tendency of the soil particles to move
around and be compacted.

13. Fig: Effect of Compactive
Effort

14.  Applying more energy to a soil will reduce the air
voids content further and increase the dry unit
weight.
 More compaction energy can be beneficial
especially for soils dry of OMC If a soil is
already moist, weaker and above OMC then
applying more energy is wasteful since air can
quickly be removed.

15. Fig: Effect of Permeability

16.  Permeability at
decreases
with
constant
compactive increasing
watercontent
effort
and
reaches a minimum at about the optimum.
 If compactive effort is increased, the permeability
decreases because the void ratio decreases.

17. Voi
d
ratio
,
e
0
Pressure, natural scale
Low pressure consolidation
Pressure, log scale
Voi
d
ratio
,
e
0
High-pressure consolidation
Dry compacted or
undisturbed sample
Low stress level: Clay compacted wet of optimum are
more compressible and in High stress level: The opposite
is true
Effect of Compressibility
or
Drycompacted
undisturbed sample
Rebound for both samples
Wet compacted or
Remolded sample
Wet compacted or
Remolded sample

18. Higher
Swelling
tial
Pote
n
Wet Side
Dry Side
Dry
Density
(g
d
)
Higher
Swelling
Potential
Higher
Shrinkage
Potential
SHRINKAGE
 During drying, soils compacted in the wet side tend
to show more shrinkage than those compacted in the
dry side. In the wet side, the more orderly orientation
of particles allows them to pack more efficiently.
 samples compacted wet of optimum have the highest
shrinkage

19. SWELLING
 Due to a higher water deficiency and partially
developed water films in the dry side, when given
access to water, the soil will soak in much more
water and then swell more.
 Swelling of compacted clays is greater for those
compacted dry of optimum.
 They have a relatively greater deficiency of water
and therefore have a greater tendency to adsorb
water and thus swell more.

20.  Field compaction control depends on Placement
water content, Type of equipment for compaction,
lift thickness and Number of passes based on soil
type & degree of compaction desired.
 Placement water content is the water content at
which the ground is
 compacted in the field. It is desirable to compact at
or close to optimum moisture content achieved in
laboratory so as to increase the efficiency of
compaction.
Field Compaction Control