Geotechnical Engineering

DHARM

438 GEOTECHNICAL ENGINEERING

P(air-dry condition)

Q (Point of minimum density)

Water content %

R (100% saturation)

Dry density

Fig. 12.10 Typical moisture-density relationship for sand
Small moisture films around the grains tend to keep them apart and can decrease the
density up to a certain water content. The point Q on the curve indicates the minimum den-
sity. Later on, the apparent cohesion gets reduced as the water content increases and is de-
stroyed ultimately at 100% saturation of the sand. Thus, the point R on the curve indicates
maximum density. Thereafter, once again, the density decreases with increase in water con-
tent.

Increase of compactive effort has much less effect in the case of cohesionless soils than
on cohesive soils. Vibration is considered to be the best method suitable for densifying
cohesionless soils, which are either fully dry or fully saturated. This is because the stresses at
the soil water menisci tend to prevent full densification. Also, relative density or density index
is invariably used to indicate relative compaction or densification of sand.

12 .8 Compaction versus Consolidation

Compaction, as a phenomenon, is different from the phenomenon of consolidation of soil.

The primary differences between the two phenomena may be set out as given in
Table 12.1.

Table 12.1 Compaction versus consolidation

S.No. Compaction Consolidation

1. Expulsion of pore air Expulsion of pore water


2. Soil involved is partially saturated Fully saturated soil


3. Applies to cohesive as well as cohesionless Applies to cohesive soils only
   soils


4. Brought about by artificial or human Brought about by application of load or by
   agency natural agencies


5. Dynamic loading is commonly applied Static loading is commonly applied

(Contd.)...