Compaction of soil involves mechanically rearranging soil particles to reduce voids and increase dry density, which improves engineering properties like strength and reduces settlement. Standard compaction tests determine the optimum water content and maximum dry density for a given soil and compactive effort. Factors like water content, compactive effort, soil type, and method of compaction influence the engineering behavior of compacted soils.

1. CE6405 SOIL MECHANICS
V.Nageshwaran, M.E.,
Assistant Professor,
Department of Civil Engineering,
UCET

2. SOIL COMPACTION

3. COMPACTION OF SOIL
 Compaction is the application of mechanical energy to a soil so as
to rearrange its particles and reduce the void ratio, thus increase
its Dry Density.
 It is applied to improve the properties of an existing soil or in the
process of placing fill such as in the construction of embankments,
road bases, runways, earth dams, and reinforced earth walls.
 Compaction is also used to prepare a level surface during
construction of buildings. There is usually no change in the water
content and in the size of the individual soil particles.
1. To increase soil shear strength and therefore its bearing
capacity.
2. To reduce subsequent settlement under working loads.
3. To reduce soil permeability making it more difficult for water to
flow through.

4. CONT…
 Proctor – Definite relationship between the Soil Water
Content and Degree of Dry Density to which a Soil might be
Compacted and Specific Amount of Compaction Energy
applied on the Soil – Optimum Water Content – Maximum
Dry Density.
 Methods of Compaction – Dynamic or Impact, Kneading,
Static and Vibration.
 Laboratory Water-Density Relationships – Compaction Tests
– Standard & Modified Proctor Compaction, Harvard
Miniature Compaction, Abbot Compaction and Jodhpur-Mini
Compaction test.

5. CONT…
 Laboratory Compaction – The variation in compaction with
water content and compactive effort is first determined in the
laboratory. There are several tests with standard procedures
such as:
1. Indian Standard Light Compaction Test (Similar to
Standard Proctor Test)
2. Indian Standard Heavy Compaction Test (Similar to
Modified Proctor Test)
 Indian Standard Light Compaction Test – Soil is
compacted into a 1000 cm3 mould whose internal diameter is
100 mm & internal effective height of 127.5 mm. (IS : 2720
Part VII 1980/87) in 3 equal layers, each layer receiving 25
blows of a 2.6 kg rammer dropped from a height of 310 mm
or 12 inches above the soil. Compactive Energy used for the
Test is 6065 kg.cm/1000 ml. The compaction is repeated at
various moisture contents.

6. STANDARD PROCTOR COMPACTION TEST

7. STANDARD PROCTOR COMPACTION TEST

9. CONT…
 3 kg of Air-dried & Pulverized Soil – Passing 4.75 mm
Sieve – Mixed thoroughly with a Small Quantity of Water.
 Leave the Mixture by covering it with Wet Cloth for about 5
to 30 mins (more for clay soil).
 Water to be added – Probable.
 Initial Water Content – 4% for Coarse Grained Soils & 10%
for Fine Grained Soils.
 Weigh Empty Mould with Base Plate without the Collar.
 Attach the Collar to the Mould – Place the Matured Mixture
into the Mould upto 1/3rd of its height and provide 25 No. of
Blows.
 Similarly, fill the 2nd and 3rd part into the mould with 25
blows each. Final layer should project 6 mm higher than the
collar.
 Volume of Mould – 945 ml.

10. CONT…
 Remove Excess Soil by trimming off.
 Weigh the Mould with Base Plate & Soil.
 Take Representative Sample from the Centre of the
Compacted Specimen and Determine its Water Content.
 Calculate the Bulk Density & Dry Density.
 Remove Soil from the Mould – Break it with Hand and
Remix with raised Water Content (2 or 4 %).
 Allow to Mature – Repeat the Process – Determine Bulk,
Dry Density and Water Content.
 Plot Compaction Curve – Water Content in the Abscissa
and Dry Density as Ordinate.
V
M
ρ 
w1
ρ
ρd



13. CONT…
 Air Voids Line – Line showing Water Content-Density
Relation for the Compacted Soil Containing a Constant % Air
Voids.
 Zero Air Voids Line/Saturation Line – Line showing Water
Content Density Relation for the Compacted Soil Containing
0 % Air Voids or No Air Voids. Theoretical Maximum
Compaction for any given Water Content.
 
s
wsa
d
wG1
ρGn1
ρ



s
ws
d
wG1
ρG
ρ


0na 
S
wG
e s
 1S

14. CONT…
 Indian Modified Heavy Compaction Test – It was found that
the Light Compaction Test (Standard Test) could not
reproduce the densities measured in the field under heavier
loading conditions, and this led to the development of the
Heavy Compaction Test (Modified Test).
 Heavier Transport & Military Aircraft – Modified Proctor
Compaction Test.
 Standardized by AASHO.
 Volume of Mould – 945 ml.
 No. of Layers – 5 Nos.
 No. of Blows/Layer – 25 Nos.
 Weight of Rammer – 4.5 kg.
 Drop Height – 18 in.
 Compactive Energy – 27260 kg.cm/1000 cm3. 4.5 times >
Std. Proctor Test.

16. FIELD COMPACTION CONTROL
 Consists of (i) OMC Determination (ii) Dry
Density & Degree of Saturation
Determination achieved.
 Rapid Method – used.
 Rapid Method of Water Content
Determination – Calcium Carbide & Proctor
Needle Method (Field).
 Placement Water Content can be Found.

17. FIELD COMPACTION CONTROL

18. FACTORS AFFECTING COMPACTION
 Effect of Increasing Water Content
As water is added to a soil at low moisture contents, it
becomes easier for the particles to move past one another
during the application of compacting force.
 The particles come closer, the voids are reduced and this
causes the dry density to increase. As the water content
increases, the soil particles develop larger water films
around them.
 This increase in dry density continues till a stage is reached
where water starts occupying the space that could have
been occupied by the soil grains. Thus the water at this
stage hinders the closer packing of grains and reduces the
dry unit weight.
 The maximum dry density (MDD) occurs at an optimum
water content (OMC), and their values can be obtained
from the plot.

20. CONT…
 Effect of Increasing Compactive Effort
The effect of increasing compactive effort is shown. Different
curves are obtained for different compactive efforts. A
greater compactive effort reduces the optimum moisture
content and increases the maximum dry density.
 An increase in compactive effort produces a very large
increase in dry density for soil when it is compacted at water
contents drier than the optimum moisture content.It should
be noted that for moisture contents greater than the
optimum, the use of heavier compaction effort will have only
a small effect on increasing dry unit weights.
 It can be seen that the compaction curve is not a unique soil
characteristic. It depends on the compaction effort. For this
reason, it is important to specify the compaction procedure
(light or heavy) when giving values of MDD and OMC.

23. CONT…
 Method of Compaction – Weight of Compacting
Equipment, Mode of Operation (Dynamic or Impact, Static,
Kneading or Rolling), Time and Area of Contact.
 Type of Soil – Well Graded Coarse Grained Soil – High Dry
Density & Low OMC – Fine Grained Soil – More Water –
Lubrication – Greater Specific Surface.
 The water content of a compacted soil is expressed with
reference to the OMC. Thus, soils are said to be compacted
dry of optimum or wet of optimum (i.e. on the dry
side or wet side of OMC). The structure of a compacted
soil is not similar on both sides even when the dry density is
the same, and this difference has a strong influence on the
engineering characteristics.

25. EFFECT OF COMPACTION ON SOIL PROPERTIES
 Soil Structure – For a given
compactive effort, soils have a
flocculated structure on the dry
side (i.e. soil particles are oriented
randomly), whereas they have a
dispersed structure on the wet side
(i.e. particles are more oriented in a
parallel arrangement perpendicular
to the direction of applied stress).
This is due to the well-developed
adsorbed water layer (water film)
surrounding each particle on the wet
side.
 Swelling – Due to a higher water
deficiency and partially developed
water films in the dry side, when
given access water, the soil will soak
in much more water and then swell
more.

26. EFFECT OF COMPACTION OF SOIL PROPERTIES
 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.
 Construction Pore Water Pressure
– The compaction of man-made
deposits proceeds layer by layer, and
pore water pressures are induced in
the previous layers. Soils compacted
wet of optimum will have higher
pore water pressures compared to
soils compacted dry of optimum,
which have initially negative pore
water pressure.

27. CONT…
 Permeability – The randomly oriented soil
in the dry side exhibits the same
permeability in all directions, whereas the
dispersed soil in the wet side is more
permeable along particle orientation than
across particle orientation.
 Compressibility – At low applied
stresses, the dry compacted soil is less
compressible on account of its truss-like
arrangement of particles whereas the wet
compacted soil is more compressible. The
stress-strain curve of the dry compacted soil
rises to a peak and drops down when the
flocculated structure collapses. At high
applied stresses, the initially flocculated
and the initially dispersed soil samples will
have similar structures, and they exhibit
similar compressibility and strength.

28. CONT…
 Stress-Strain Characteristics
– Soil Compacted Dry Side of
Optimum – Steeper Stress-
Strain – Higher Modulus of
Elasticity. Soil Compacted Wet
Side of Optimum – Gradual
Stress-Strain Brittle Failure.
 Shear Strength – Depend on
Dry Density, Moulding Water
Content, Soil Structure, Method
of Compaction, Strain used to
define Strength, Drainage
Condition and Type of Soil.