Facts about Soil Compaction.

What is Compaction of Soil?

Compaction of soil is the pressing of soil particles close to each other by mechanical methods. Air during compaction of soil is expelled from the void space in the soil mass and therefore the mass density is increased.

Compaction of soil is done to improve the engineering properties of the soil. Compaction of soil is required for the construction of earth dams, canal embankments, highways, runways and many other structures.

Methods of Testing Compaction of Soil

Standard Proctor's Test for Compaction of Soil

To assess the amount of compaction of soil and water content required in the field, compaction tests are done on the same soil in the laboratory. The test provides a relationship between the water content and the dry density.

The water content at which the maximum dry density is attained is obtained from the relationship provided by the tests. Proctor used a standard mould of 4 inches internal diameter and an effective height of 4.6 inches with a capacity of 1/30 cubic foot.

The mould had a detachable base plate and a removable collar of 2 inches height at its top. The soil is compacted in the mould in 3 layers, each layer was given 25 blows of 5.5 pounds rammer filling through a height of 12 inches.

IS: 2720 part VII recommends essentially the same specification as in Standard Proctor test, some minor modifications. The mould recommended is of 100mm diameter, 127.3 mm height and 1000 ml capacity.

The rammer recommended is of 2.6 kg mass with a free drop of 310mm and a face diameter of 50mm. The soil is compacted in three layers. The mould is fixed to the detachable base plate. The collar is of 60mm height.

Procedure of Proctor’s Test for Compaction of Soil

About 3kg of air dried soil is taken for the test. It is mixed with 8% water content and filled in the mould in three layers and giving 25 blows to each layer. The volume of the mould and mass of the compacted soil is taken. The bulk density is calculated from the observations. A representative sample is placed in the oven for determination of water content. The dry density id found out from the bulk density and water content. The same procedure is repeated by increasing the water content.

Compaction curve

A compaction curve is plotted between the water content as abscissa and the corresponding dry density as ordinate. It is observed that the dry density initially increases with an increase in water content till the maximum density is attained.

With further increase in water content the dry density decreases. The water content corresponding to maximum dry density is known as the optimum water content (O.W.C) or the optimum moisture content (O.M.C).

At a water content more than the optimum, the additional water reduces the dry density as it occupies the space that might have been occupied by the solid particles.

For a given water content, theoretical maximum density is obtained corresponding to the condition when there are no air voids (degree of saturation is 100%). The theoretical maximum density is also known as saturated dry density. The line indicating theoretical maximum density can be plotted along with the compaction curve. It is known as the zero air void line.

Modified Proctor Test for Compaction of Soil

The modified Proctor test was developed to represent heavier compaction than that in the standard Proctor test. The test is used to simulate field conditions where heavy rollers are used. The test was standardized by American association of State Highway Officials and is, therefore also known as modified AASHO test.

In this, the mould used is same as that in the Std Proctor test. However, the rammer used is much heavier and has a greater drop than that in the Std Proctor test. Its mass is 4.89 kg and the free drop is 450mm. The soil is compacted in five equal layers, each layer is given 25 blows. The compactive effort in modified Proctor test is 4.56 times greater than in the Std Proctor test. The rest of the procedure is same.

Factors Affecting Compaction of Soil

- 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 packing. The dry density of the soil increases with an increase in the water content till the O.M.C is reached.

- Amount of compaction

The increase in compactive effort will increase the dry density at lower water content to a certain extent.

- Type of soil

The dry density achieved depends upon the type of soil. The O.M.C and dry density for different soils are different

- Method of compaction

The dry density achieved depends on the method of compaction.

Effect of Compaction on Properties of Soil

1. Effect of Compaction on Soil Structure

Soils compacted at a water content less than the optimum generally have a flocculated structure. Soils compacted at water content more than the optimum usually have a dispersed structure.

2. Effect of Compaction of Soil on Permeability

The permeability of a soil depends upon the size of voids. The permeability of a soil decreases with an increase in water content on the dry side of optimum water content.

3. Swelling

4. Pore water pressure

5. Shrinkage

6. Compressibility

7. Stress-strain relationship

8. Shear strength

Methods of Compaction of Soil used in Field

Several methods are used in the field for compaction of soils. The choice of method will depend upon the soil type, the maximum dry density required and economic consideration. The commonly used methods are

1. Tampers

2. Rollers

3. Vibratory compactors

The compaction depends upon the following factors:

- Contact pressure

- Number of passes

- Layer thickness

- Speed of roller

- Types of rollers

- Smooth Wheel rollers

- Pneumatic tyred rollers

- Sheepsfoot rollers

- Controlling Compaction of Soil

Compaction control is done by measuring the dry density and the water content of compacted soil in the field

- Dry density

The dry density is measured by core cutter method and sand replacement method

- Water content

For the measurement of water content, oven drying method, sand bath method, calcium carbide method etc are used. Proctor needle is also used for this.