The compacting process increases the density of soil
by reducing air void space. Consolidation, on the other hand increases soil
density by reducing water voids. Consolidation is a long-term process spread
over years, whereas compaction can achieved in a few hours. Compaction improves
bearing strength, permeability and compressibility. Compacting equipment
combine their static weight with tamping, vibration, impact and kneading action
to produce the desired compacting effort. Compaction equipment requirement
varies with soil characteristics and compacting effort.
Sheep Foot Rollers
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Vibratory Rollers
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Steel Drum Rollers
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Multi-Tyred Pneumatic Rollers
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The compacting equipment can be broadly classified into tamping foot rollers, pneumatic typed rollers, vibratory rollers, impactors, plate vibrators, and smooth steel-wheel rollers.
Tamping Rollers
A tamping roller consists of one or more hollow steel cylindrical drums with rows of steel studs like sheep’s feet mounted on it. As the roller is towed with a crawler tractor, these studs punch into the soil and compact it by tamping and kneading action. Generally, the compaction gets carried out to a depth of 150 mm. The cylinder drum can also be filled with water or sand to add extra weight while compacting.
The compaction depends upon the nature of the soil and
the roller passes are continued till the feet do not dig into the surface being
compacted. There are many varieties of tamping foot rollers. These include
sheep’s foot rollers for compacting vary cohesive soils, tamping foot rollers
for compacting soil with low to medium cohesiveness and grid or mesh segmented
rollers for compacting granular soils, specially gravels. In general, the depth
of compaction achieved in layers with the sheep’s foot roller in nearly equal
to the depth of the stud. Tamping foot rollers are rated in terms of static
load or foot pressure (termed the ground contact pressure) on the soil surface
and unit area.
Smooth Wheeled Rollers
These rollers have one or more smooth steel wheels,
and the latest variety rollers are self-propelled. The self-propelled tandem
and 3-wheeled rollers are used for finishing compaction of layers up to 150 mm
of sand, gravel and water bound macadam used in base courses. Smooth wheeled
rollers are employed for compacting bituminous materials specially the top
layers in road surfacing operation. Smooth wheeled rollers are classified
either by type or weight or both. Various types of rollers include 3-wheel two
axles, 2-wheel tandem and 3-wheel tandem. The weight of rollers can also be
increased by ballasting with water, sand or pig iron. Rollers are designated in
terms of static weight and ballasted weight i.e. 15/20 tone means that the
static weight of the roller is 15 tone and the maximum weight when ballasted is
20 tone. In order to indicate the pressure exerted, these rollers are also
designated by specifying the minimum weight per linear width of roller, i.e. 60
kg/cm width.
Pneumatic Rollers
Pneumatic rollers are available in light, medium and
heavy weights. They compact soil by a kneading action. The weight of the
equipment can be nearly doubled with ballasting using water, sand or pig iron,
and the ground pressure can be maintained as desired by controlling the weight
of the ballast, the number of the wheels, the width of the tyres and the tyre
pressure. The pneumatic tyred rollers are rated in terms of tyre pressure
(ground contact pressure) per unit area. It may be noted that the load on the
tyres determine the depth to which compaction is possible, whereas both the
tyre pressure and the tyre load are important for achieving compaction near the
surface.
Load Requirements for Compaction of Different Depths
Passes
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Job Characteristics
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Maximum Depth of Layer (in mm)
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Load Desired (in tone)
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4 to 8
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Compaction of loamy sand
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300 mm
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1.5 to 1.7
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500 mm
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2.0 to 2.5
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700 mm
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4.0 to 4.5
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4 to 6
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Compacting bituminous material
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80 mm
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1.5
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130 mm
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2.5
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200 mm
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4.0
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Vibratory Rollers and Compactors
Vibration improve compaction and save time when
compared with the static weight method of compaction. Vibrations set the rim
roller in oscillation, and these in turn transmit vibration to the soil. Vibrations
are induced by installing a rotating eccentric weight inside the roller drum. Vibratory
rollers combine the static weight with dynamic forces. Maximum compacting
effort is produced when the resonance frequency of the roller and soil
coincide. Generally, the rating for the vibratory compactor is stated as total
applied force expressed in tone and it is the numerical sum of the dynamic
forces plus static weight. The vibrating frequency is specified as
cycles/minutes. Vibration frequencies range from 1400 to 3000 cycles per
minute. Further, a slow displacement speed of say 2.5 to 4 km/h produces a
better effect than speedier movement.
Vibratory compactors are of various types and sizes. These
include smooth drum vibratory rollers and tamping foot vibratory rollers. These
are widely used for compacting non-cohesive soils.
