COMPACTION

Compaction means pressing of the soil particles close to each their by mechanical

methods. Air is expelled from soil mass and mass density id increased. It is done to improve the engineering properties like shear strength, stability etc. Reduce compressibility and permeability.

Laboratory Test on Compaction:

Compaction Energy per volume:

Compaction Curve:

Applications of Compaction

The following are the situations in which compaction will improve the existing field condition.

1. Compaction of foundation soil for house construction.

2. Compaction of soil/gravel/crushed rock/asphalt in road & pavement construction.

3. Compaction of soil in earth embankments.

4. Compaction of soil behind retaining walls.

5. Compaction of soil backfill in trenches.

6. Dam construction

7. Construction of clay liners for waste storage areas

8. Ground improvement

Factors Affecting Compaction:

The increase in dry density of soil by compaction depends upon the following

factors:

1. Water Content:

At low water content, the soil is stiff and offers more resistance tocompaction. As the water content is increased, the soil particles getlubricated.

The dry density of the soil increases with an increase in the water contenttill the optimum water content in reached.

At that stage, the air voids attain approximately a constant volume. Withfurther increase in water content, the air voids do not decrease, but thetotal voids (air plus water) increase and the dry density decreases.

Thus the higher dry density is achieved upto the optimum water contentdue to forcing air voids out from the soil voids. After the optimum watercontent is reached, it becomes more difficult to force air out and to furtherreduce the air voids.

2. Amount of Compaction:

The compaction of soil increases with the increase in amount ofcompactive effort.

With increase in comactive effort, the optimum water content required forcompaction also decreases.

At water content less than the optimum, the effect of increased compactionis more predominant.

At water content more than the optimum, the volume of air voids becomealmost constant and the effect of increased compaction on soil is notsignificant.

The maximum dry density does not go on increasing with an increase inthe compactive effort.

3. Type of Soil:

The compaction of soil depends upon the type of soil.

Coarse grained soils can be compacted to higher dry density than fine-grained soils.

With the addition of even a small quantity of fines to a coarse-grained soil,the soils attain a much higher dry density for the same compactive effort.

A well graded sand attains a much higher dry density than a poorly gradedsoil.

Cohesive soils have high air voids. These soils attain a relatively lowermaximum dry density as compared with the cohesionless soils. Such soilsrequire more water than cohesionless soils and therefore the optimumwater content is high.

Heavy clays of very high plasticity have very low dry density and a veryhigh optimum water content.

4. Method of Soil Compaction:

The dry density achieved depends not only upon the amount of compactiveeffort but also on the method of compaction.

For the same amount of compactive effort, the dry density will dependupon whether the method of compaction utilizes kneading action, dynamicaction or static action

Different methods of compaction curve give their own compaction curves.Consequently, the lines of optimums are also different.

Fig: Compaction curves for different soils

Effect of compaction on soil properties-

1. Density

2. Shear strength

3. Permeability

4. Bearing Capacity

5. Settlement

6. Soil Structure

7. Pore Pressure

8. Stress Strain characteristics

9. Swelling & Shrinkage

Solution: