Understanding Soil Compaction: Importance, Factors, and Testing Methods

Soil Compaction

In the construction of highway embankments, earth dams, and many other engineering structures, loose soils must be compacted to increase their unit weights. Compaction increases the strength characteristics of soils, which increase the bearing capacity of foundations constructed over them. Compaction also decreases the amount of undesirable settlement of structures and increases the stability of slopes of embankments. Smooth-wheel rollers, sheeps foot rollers, rubber-tired rollers, and vibratory rollers are generally used in the field for soil compaction. Vibratory rollers are used mostly for the densification of granular soils. Vibroflot devices are also used for compacting granular soil deposits to a considerable depth. Compaction of soil in this manner is known as vibroflotation. This chapter discusses in some detail the principles of soil compaction in the laboratory and in the field.

General purpose of Soil Compaction

In general compaction is the densification of soil by removal of air that requires mechanical energy. The compaction of soil is measured in terms of its dry unit weight. When water is added to the soil during compaction, it behaves as a softening agent on the soil particles. The soil particles slip over each other and move into a densely packed position. 

Factors Affecting Soil Compaction

There are some factors that affect soil compaction. Now we will briefly look at these factors how they affect soil compaction.

  1. Moisture Content: Strong influence on the degree of compaction.

  2. Soil Type:

• Grain-size distribution.
• Shape of soil grains.
• Specific gravity of soil solids.
• Amount and type of clay minerals present.

1. Moisture Content

Water content plays a major role in determining the level of soil compaction. Low moisture content of soil does not lubricate soil particles in an appropriate way hence increasing friction among particles and hence making the particles to be compacted with more difficulty. This leads to reduced level of compaction and dry unit weight. The optimum moisture content (OMC) ensures the soil is at its highest dry unit weight because it is at this level that the moisture content gives the correct lubrication to the soil particles and hence maximum compaction is achieved. But when moisture content is excessive, more water in the gaps between particles will fill the soil hence less compaction is effective. The pressure of the water does not allow the particles to move towards each other and the dry unit weight and general compaction reduces. The moisture contents are thus of importance and the best moisture content is the most efficient to attain maximum compaction.

2. Soil Type

·       A smaller particle, such as clay, compact faster as compared to a bigger particle, such as sand.

·       Angular grains interlock more efficiently than smooth and rounded grains.

·       The soil particles that are of high specific gravity are heavier and can be compacted better.

·       Clay minerals are helpful in compaction, but when it is too much, it might swell or shrink.

·     Small and irregular particles of soil are easier to compact than large and round ones or excessive clay content.

 

For the construction of highways, airport and other type of pavement it is essential to compact soil. Compaction of soil increases the shear strength of soil. It is also decreasing the permeability and future settlement. We will see the difference between these tests in detailed. In compaction test the optimum moisture content and maximum dry density is determined. Following two basic type of test is used for this.

Standard Proctor Test (Standard AASHTO)

Modified Proctor Test (Modified AASHTO)

 

Standard Proctor Test

    Proctor Test Setup:

• Mold volume of proctor test is 944 cm³.
• The diameter of mold is 101.6 mm.
• Mold is attached to a baseplate at the bottom and an extension at the top.

  Compaction Process:

• Soil is mixed with varying amounts of water.
• The soil is compacted in three equal layers using a hammer that delivers 25 blows per layer.
• The mass hammer is 2.5 kg, and drop height is 30.5 mm.

    Dry Unit Weight Calculation:

• Moisture content is determined in the laboratory.
• Dry unit weight can be calculated with the known moisture content.

Standard Proctor Test Setup

Moisture can & other apparatus

Use of Standard Proctor Test

• Light construction projects (e.g., residential buildings, low-traffic pavements).
• Moderate compaction requirements.
• Fine-grained soils (silts and clays) passing through U.S. No. 4 sieve.
• Less intense compaction effort is sufficient for the project.

 

  Modified Proctor Test:

• Developed to better represent field conditions with the use of heavy rollers.
• ASTM Test Designation: D-1557, AASHTO Test Designation: T-180.

 Test Setup:

• Modified Proctor Test has the same mold volume (944 cm³) as the standard Proctor test.
• In modified proctor test soil is compacted in five layers.
• The mass of hammer is 4.54 kg, with a 457 mm drop.
• Number of blows per layer is 25, same as in the standard Proctor test.

  Compaction Energy:

• Calculated as 2700 kN-m/m³.

  Results:

• Maximum dry unit weight of soil increases.
• Optimum moisture content decreases due to increased compactive effort.

 

Use of Modified Proctor Test

• Heavy construction projects (e.g., highways, runways, large commercial buildings).
• High compaction energy needed to support heavy loads.
• Soils exposed to higher traffic or heavy machinery.
• Materials requiring higher load-bearing capacity (e.g, foundations, embankments).
• More intense compaction required for coarse-grained soils or high load-bearing applications.

standard & modified hammer