Geotechnical Engineering

DHARM

424 GEOTECHNICAL ENGINEERING

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 compactive effort or energy
expended and the nature of the soil. A change in moisture content or compactive 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 :
(i) Compaction increases the dry density of the soil, thus increasing its shear strength
and bearing capacity through an increase in frictional characteristics ;
(ii) Compaction decreases the tendency for settlement of soil ; and,
(iii) Compaction brings about a low permeability of the soil.

12.3 Compaction Test

To determine the soil moisture-density relationship and to evaluate a soil as to its suitability

for making fills for a specific purpose, the soil is subjected to a compaction test.

Proctor (1933) showed that there exists a definite relationship between the soil mois-

ture content and the dry density on compaction and that, for a specific amount of compaction

energy used, there is a particular moisture content at which a particular soil attains its maxi-

mum dry density. Such a relationship provides a satisfactory practical approach for quality

control of fill construction in the field.

12.3.1Moisture Content—Dry Density Relationship

The relation between moisture content and dry density of a soil at a particular compaction

energy or effort is shown in Fig. 12.1.

Optimum moisture content

Dry side Wet side

Dry density kN/m

3

Maximum drv density

100% compaction

Moisture content %

Fig. 12.1 Moisture content versus dry density at a particular compactive effort

The addition of water to a dry soil helps in bringing the solid particles together by

coating them with thin films of water. At low water content, the soil is stiff and it is difficult to

pack it together. As the water content is increased, water starts acting as a lubricant, the

particles start coming closer due to increased workability and under a given amount of

compactive effort, the soil-water-air mixture starts occupying less volume, thus effecting gradual

increase in dry density. As more and more water is added, a stage is reached when the air

content of the soil attains a minimum volume, thus making the dry density a maximum. The