with a characteristic inverted V-shape. At low water contents the soil
is stiff and difficult to compact, producing low dry densities: as the
water content is increased compaction is made easier and higher dry
densities are obtained. At high water contents the water occupies an
increasing volume within the soil void space, and the dry density is
diminished. For a given compactive effort there is therefore an optimum
water content, wopt, at which a maximum value of dry density, (^) d max, is
achieved. If all the air in the soil could be expelled by compaction, the soil
would be in a state of full saturation (Sr100%) and (^) dwould attain the
maximum possible value for the given moisture content. In practice this
degree of compaction, analogous to 100% ‘efficiency’, cannot ever be
achieved.
Increased compactive effort displaces the dry density–moisture
content curve to give a higher maximum dry density at a lower optimum
water content (Fig. 2.7). For a constant compactive effort, different soil
types yield different dry density–water content curves, and in general
coarse soils can be compacted to higher dry densities. For effective com-
paction, the soil layer thickness should be as low as is economically viable.
Specified maximum layer thicknesses for effective field compaction gener-
ally lie in the range 150–250 mm.
ENGINEERING CHARACTERISTICS OF SOILS 55
Fig. 2.7 Soil compaction relationships