COMPACTION OF SOIL

COMPACTION OF SOIL

‘ Compaction’ of soil may be defined as the process by which the soil particles are artificially rearranged and packed together into a state of closer contact by mechanical means in order to decrease its porosity and thereby increase its dry density. This is usually achieved by dynamic means such as tamping, rolling, or vibration. The process of compaction involves the expulsion of air only.

In the natural location and condition, soil provides the foundation support for many structures. Besides this, soil is also extensively used as a basic material of construction for earth structures such as dams and embankments for highways and airfields. The general availability and the relatively low cost are the chief causes for using soil as construction material.

Properly placed and compacted, the resulting soil mass has better strength than many natural soil formations. Such soil is referred to as a ‘compacted earth fill’ or a ‘structural earth fill’.

For the purpose of supporting highways or buildings or for retaining water as in earth dams, the soil material must possess certain properties while in-place. These desirable features can be achieved by proper placement of an appropriate soil material. Most of these desirable qualities are associated with high unit weight (or dry density), which may be achieved by compaction.

Virtually any soil can be used for structural fill, provided it does not contain organic matter. Granular soils are capable of achieving high strength with relatively low volume changes. Properly compacted clay soils will develop relatively high strengths and low permeabilities which may be desirable features as for earth dams.

. Compaction Phenomenon

The process of compaction is accompanied by the expulsion of air only. In practice, soils of medium cohesion are compacted by means of rolling, while cohesionless soils are most effectively compacted by vibration. Prior to the advent of rolling equipment, earth fills were usually allowed to settle over a period of years under their own weight before the pavement or other construction was placed.

The degree of compaction of a soil is characterised by its dry density. The degree of compaction depends upon the moisture content, the amount of compact effort or energy expended and the nature of the soil. A change in moisture content or compact effort brings about a change in density. Thus, for compaction of soil, a certain amount of water and a certain predetermined amount of rolling are necessary.

The following are the important effects of compaction:

. Compaction increases the dry density of the soil, thus increasing its shear strength and bearing capacity through an increase in frictional characteristics;

. Compaction decreases the tendency for settlement of soil; and,

. Compaction brings about a low permeability of the Soil