Soil Compaction

Compaction refers to the pressing of soil particles near to each other by automatic techniques. The air throughout compaction is barred from the empty space in the soil mass and as a result the mass density is greater than before. Compaction is carried out to perk up the engineering characteristics of the soil. Compaction of soil is needed for the building of canal embankments, earth dams, highways, runways, railways and many other constructions.

Standard Proctor Test

To evaluate the quantity of compaction and water content needed in the turf, compaction tests are carried out in the same soil in the lab. The test gives a rapport between the water content and the dry compactness. The water content at which highest dry density is achieved is attained from the rapport given by the different tests. Proctor makes use of a set mould of 4 inches inner diameter and an efficient stature of 4.6 inches with a ability of 1/30 cubic foot. The mould had a removable base plate and a detachable collar of 2 inches stature at its top. The soil is compressed in the mould in 3 levels; each level has to be given 25 blows of 5.5 pounds rammer filling throughout a stature of 12 inches.

IS: 2720 part suggests basically the same conditions as stated by the Standard Proctor test, some slight changes. The mould suggested is of around 100mm span, 127.3 mm stature and 1000ml capacity. The rammer suggested is of 2.6 kg weight with a free drop of 310mm and a side span of 50mm. The soil is compressed in three levels. The mould is set to the separable bottom plate. The collar is of approximately 60mm height.


Around 3kg of dried soil is taken for the test. The soil has to be air dried. It is then blended with 8% water content and packed in the mould in three levels and 25 blows should be given to each layer. The size of the mould and weight of the compressed soil is then taken. The mass density is evaluated from the inspection. A representative model is then placed in the oven for calculation of the water content. The dry density is then figured out from mass density and water content. The same process is again carried out by increasing the proportion of the water content.

A compaction bend is taken out between the water content as abscissa and the relating dry thickness as ordinate. It is watched that the dry thickness at first increments with an expansion in water content till the greatest thickness is accomplished. With further increment in water content the dry thickness diminishes. The water content relating to most extreme dry thickness is known as the ideal water content (O.W.C) or the ideal dampness content (O.M.C).

At water proportion more than the ideal, the extra water diminishes the dry thickness as it involves the space that may have been possessed by the solid particles.

For a given water proportion, hypothetical maximum  thickness is gotten comparing to the condition when there are no air voids (level of immersion is 100%). The hypothetical maximum thickness is otherwise called saturated dry density. The line demonstrating hypothetical maximum thickness can be plotted alongside the compaction bend. It is known as the zero air void line.

Modified Proctor Test

The modified Proctor test was first formulated to signify weighty compression than that in standard Proctor test. The test is basically carried out to conjure up field conditions where weighty rollers are used. The test was prevailed and standardized by the American association of State Highway Officials and is, as a result also known as  the modified AASHO test.

In this method, the mould which is same as the one in the Standard Proctor test. Yet, the rammer used is much weight and has a bigger drop than that in Standard Proctor test. Its weight is 4.89 kg and the open drop is 450mm. The soil is compressed in five equal levels; each level is given minimum of 25 blows. The compressive effort in the altered Proctor test is 4.56 times bigger than in the Standard Proctor test. The remaining of the process is same.

Factors Affecting Compaction

  • Water content: At low water content, the dirt is firm and offers more imperviousness to compaction. As the water substance is expanded, the dirt particles gets more lubricated. The dirt mass turns out to be more workable and the particles have nearer pressing. The dry thickness of the dirt increments with an expansion in the water content till the O.M.C is achieved.
  • Amount of compaction: The raise in compressed effort will amplify the dry thickness at lower water proportion to a particular extent.
  • Types of soil: The dry density reached is based upon the kind of soil. The O.M.C and dry thickness for various soils differ.
  • Method of compaction: The dry density reached is based on the technique of compaction.

Effect of Compaction on Properties of Soils

  1. Soil Structure

Soils compressed at a water proportion lesser than the ideal mostl have a flocculated surface. Soils compressed at water proportion more than the ideal generally have a discrete structure.

  1. Permeability

The permeability of a soil is based on the size of voids. The permeability of a soil diminishes with a raise in water proportion on the dry side of the ideal water content.

  1. Swelling
  2. Shrinkage
  3. Pore water pressure
  4. Compact ability
  5. Stress-strain relationship
  6. Shear Strength

Methods of Compaction used in field

Different techniques are used in the field for the compression of soils. The choice of technique will be based on the type of soil; the greatest dry density needed and cost consideration. The popularly used methods are

  • Tampers
  • Vibratory compressors
  • Rollers

Compaction Control

Compaction control is carried out by calculating the dry thickness and the water proportion of compressed soil in the field.

  • Dry density: The dry density is calculated by core cutter technique and sand replacement technique.
  • Water content: For the calculation of water proportion, oven drying technique, sand bath technique, calcium carbide technique etc are carried. Proctor needle is also often used for this process.