Geotechnical Engineering

DHARM

COMPACTION OF SOIL 425

water content corresponding to this maximum dry density is called the ‘optimum moisture
content’. Addition of water beyond the optimum reduces the dry density because the extra
water starts occupying the space which the soil could have occupied.

The curve with the peak shown in Fig. 12.1 is known as the ‘moisture-content dry den-
sity curve’ or the ‘compaction curve’. The state at the peak is said to be that of 100% compaction
at the particular compactive effort; the curve is usually of a hyperbolic form, when the points
obtained from tests are smoothly joined.

The wet density and the moisture content are required in order to calculate the dry
density as follows :

γd =

γ

()1+w

, where

γd = dry density,

γ = wet (bulk) density,

and w = water content, expressed as a fraction.

12.3.2 Effect of Compactive Effort

Increase in compactive effort or the energy expended will result in an increase in the maxi-
mum dry density and a corresponding decrease in the optimum moisture content, as illus-
trated in Fig. 12.2.

Dry density kN/m

3

Moisture content %

Higher

compactive

effort

Lower

compactive

effort

Fig. 12.2 Effect of compactive effort on compaction characteristics
Thus, for purposes of standardisation, especially in the laboratory, compaction tests are
conducted at a certain specific amount of compactive effort expended in a standard manner.

12.4 Saturation (Zero-air-voids) Line

A line showing the relation between water content and dry density at a constant degree of
saturation S may be established from the equation:

γd =

G

wG

S

γw

F 1 +

HG

I

KJ