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Factors affecting compaction of soil
The important factors affecting compaction are
1. Water Content
2. Amount of Compaction
3. Type of soil
4. Method of compaction
5. Thickness of layer
6. Saturation line
8. Stone content
Water content: At low water content, the soil is stiff and offers more
resistance to compaction.
As the water content is increased the soil particles get lubricated.
The soil mass becomes more workable and the particles have closer
Admixtures: The compaction characteristics of the soils are
improved by adding some admixtures.
The most commonly used admixtures are lime, cement and bitumen.
Stone content: With addition of aggregates of 20 to 30 mm size up
to 40% volume, the density of soil increases.
Method of compaction: The dry density achieved also depends
upon the method of compaction.
For the same amount of compactive effort, the dry density will
depend upon whether the method of compaction utilizes kneading
action, dynamic action.
Thickness of layer: In the compaction of soils the thickness of soil
layer also play an important role.
Normally the thickness of layer in the laboratory is kept 30 to 40 mm,
while in the field the thickness of soil layer is kept about 200 to
300mm. The lesser thickness of soil layer gives higher dry density.
Saturation line: If all the air voids are removed from the soil by
compaction, the soil becomes fully saturated and higher max dry
density is achieved.
Saturation below 100% results in lower max dry density.
Type of soil : The maximum dry density achieved depends upon the
Coarse grained soil can be compacted to higher dry density than fine
A well graded sand attains a much higher dry density than a poorly
graded soil .
Cohesive soils have high air voids.
Methods of field compaction
The various method of field compaction
Different types of rollers are used for compaction of soils.
The compaction depends upon the following factors.
Number of passes (normally 5 to 15)
Layer of thickness (less than 15cm)
Speed of roller
Different types of rollers used are
i. Smooth wheel rollers
ii. Pneumatic tyred rollers
iii. sheep foot rollers
iv. Track laying vehicles
A hand operated rammer consists of a block of iron,
about 3 to 5 kg in mass attached to a wooden rod.
The rammer is lifted for about 0.30m and dropped on
the soil to be compacted.
Tampers are used to compact soils adjacent to
existing structures such as trenches, within plinth of
In vibratory compactors, vibrations are induced in the soil
during compaction. The compactors are available in a
variety of forms.
When the vibrator is mounted on a drum, it is called a
These rollers are available both as pneumatic type and
the smooth wheel type.
Another form of vibratory compactor is a vibrating plate
Hand operated vibrating plates are also available.
The effect of the vibrating plates is limited to small depths
Different types of rollers
Sheep foot Rollers: The sheep foot roller
consists of a hollow drum with a large number of
small projection on its surface.
These projections penetrate the soil layers during
the rolling operations and cause compaction.
The drum can be filled with water to increase the
The sheep-foot rollers are ideally suited for
compaction of cohesive soils.
Vibratory rollers: The rollers are available both as pneumatic
type and smooth wheel type.
In a smooth wheel type, as separate motor drives an arrangement of eccentric
weight to create high frequency, low amplitude, up and down oscillations of the
These rollers are suitable for compacting granular soils.
In a pneumatic tyred vibratory compactor, a separate vibratory unit is attached
to the wheel axle.
Pneumatic tyred Rollers: Pneumatic tyred Rollers use
compressed air to develop the required inflation pressure.
The roller generally consists of 9 to 11 wheels fixed on two axles, with the
pneumatic tyres so spaced that a complete coverage is obtained with each
pass of the roller.
These rollers are effective for compacting cohesive as well as cohensionless
Smooth wheel rollers
Types of smooth wheel rollers
1. Conventional three wheel type having weight 2 to 15 ton.
2. Tendem rollers weight having 1 to 14 ton.
3. Three axle tendem roller having 12 to 18 ton.
These rollers are useful for compacting granular base courses of
highway and runways.
These are not effective for compaction of deep layers of soils.