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Methods of compaction of soil used in field

What is Compaction of Soil?

In the construction of fills and embankments, loose soils are placed in layers of 200 mm to 300 mm in thickness. Each layer is compacted to a specified standard by means of compaction equipment. The various equipments used in the compaction are:

  • Smooth wheeled roller
  • Sheep foot roller
  • Pneumatic roller
  • Vibratory roller
  • Impact rammers
Smooth Wheel Roller

A smooth-wheel roller generally consists of three wheels. There are two large wheels in the rare part and one small in the front. The weight of smooth wheel roller varies from 2 to 15 tons. The roller is operated by internal combustion engine. There is another version of smooth wheel roller known as tendem smooth wheel roller. It consists of only two drums: one at front part and another at rear part. Smooth wheeled rollers mostly suited to seal the surface of the fill at the end of day’s work to provide a smooth surface to quickly drain the rain water.

They are not suitable for deep layers of soil such as uniform sands and silty sands because the resulting compaction pressures induced are low. In addition these rollers cause stratification in deep layers due to none-uniform compaction.

They are usedto compact well-graded  coarse grained soilsfor use in  highway base courses and non – plastic fine-grained soils.  

Sheep Foot Roller

Sheep foot rollers are named from sheep because, in ancient times, a flock of sheep were used to compact a newly formed fill. The roller is similar to the smooth wheel roller except that it contains numerous projecting feet about 200 mm to 250 mm long with an end area of 40 to  65 cm2.  The wheels of the roller consist of hollow drums and can be filled with water and ballasts to increase their weight.  The drums are mounted on a steel frame. 

Sheep foot rollers are available both as a self-propelled unit or a towed unit.  The projections of the roller penetrate the soil layers during rolling and cause compaction.  During rolling the weight of roller is transmitted to fill through projecting feet.

The pressure imposed range from 7 to 42 kg/cm2. These rollers are suitable for both plastic and nonplastic fine-grained soils and for coarse-grained soils with more than 20% fines.  

When the roller is used for the first time,  the projection penetrates the soil layer below and the lower portion is compacted. In successive passes compaction is achieved at the middle and top portion of the soil layer.

The depth of the layer being compacted depends on the length of projection and weight of the roller.  Depending upon the  size of the  roller  the  layer thickness varies from 15  cm to 30  cm. In general, the thickness of the  layer  compacted should  not be 5 cm more than the length of the projection. 

Vibratory Rollers

In vibratory rollers a vibrator mounted on a drum. These rollers are available both in the form of pneumatic tired type of smooth wheel type.  In smooth wheel type, a  separate motor drives an arrangement of eccentric weights to create high frequency, low amplitude and up and down oscillations of the drum.

The roller are suitable for compacting granular soils with no fines where layers up to 1 m in thickness can be contacted. If the soil contains appreciable fines, the layer thickness is to be reduced. In the pneumatic type of vibratory compactor, a separate vibrating unit is attached to the wheel axle.

The ballast box suspended separately from the axle so that it does not vibrate. These compactors are suitable for compacting granular soils with thickness of layer about 30 cm. Vibractory compactors can compact the granular soil to a very high maximum dry density.


Vibrating Plates

In this system, there are numbers of smaller plates, each plate are vibrated by a separate vibrating unit. Hand operated vibrating plates are also available. these compactors are suitable for compacting soils having smaller thickness.

The main use of vibrating plates is to compact granular base courses for highways and runways where the thickness of layers is small. Vibrating plates are suitable to coarse-grained soils containing up to 12 % fines.

Impact Rammers

Impact rammers consist of a block of iron about 3 to 5 kg wooden rod. The rammer is lifted  by about 30  cm and dropped on the soil to be compacted.   A mechanical rammer is operated by compressed air gasoline power.  It is much heavier and weighs 30 to 150 kg.

Mechanical rammers have been used up to a weight of 1000  kg.  Power controlled or manually operated impact rammers are used where the area to be compacted are small, the area is inaccessible and use of large equipment is not justified. 

They are also used for compacting backfill behind retaining walls or small trenches.  They can be used for all types of soils.

Factors affecting field compaction

Contact Pressure

The main  aim  of compaction  is to make a soil capable of resisting higher contact pressure imposed on it. The contact pressure depends up n the load per unit width of compacting equipment and its diameter.

Number of Passes

The number of passes made increases the compactin of a fill. Beyond a certain limit,  the dry density does not increase with an increase in numbers of passes.  For economic considerations, the number of passes is restricted to 5 to l 5.


Layer Thickness

The compaction of soil is inversely proportional to layer thickness. For economic consideration, the layer thickness cannot be less than 15 cm.

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