Geotechnical Engineering

DHARM

COMPRESSIBILITY AND CONSOLIDATION OF SOILS 213

1.2

1.0

0.8

0.6

0.4

Void ration e

0

0.2

400 800 1200 1600

PressureskN/m (log scale)^2

Virgin compression curve

Original compression

curve (if continued)

Recompression curve

Rebound curve

Fig. 7.11 Virgin compression, rebound and recompression

curves for a clay (Arithmetic scale)

1.2

1.0

0.8

0.6

0.4

Void ratio e

10 20 30 40 50 100 200 300 400 500 1000

Rebound curve

PressureskN/m (log scale)^2

1600 2000

Recompression curve Virgin compression curve

0.2

60 600

Fig. 7.12 Virgin compression, rebound and recompression
curves for a clay (Pressure to logarithmic scale)
It may be noted from Fig. 7.12 that the curvature of the virgin compression curve at
pressures smaller than about 200 kN/m^2 resembles the curvature of the recompression curve
at pressures smaller than about 800 kN/m^2 from which the rebound occurred. This resem-
blance indicates that the specimen was probably subjected to a pressure of about 150 to 200
kN/m^2 at some time before its removal from the ground. Therefore, the initial curved portion
of the so-called virgin curve can be visualised as a recompression curve; it may also be con-
cluded that a convex curvature on this type of semi-logarithmic plot always indicates
recompression.
This past maximum pressure to which a soil has been subjected is called “Preconsolidation
pressure”; usually this term is applied in conjunction with the virgin curve, although it can
also be used in conjunction with a laboratory recompression curve.
As an example let us consider a soil sample obtained from a site from a depth z as shown
in Fig. 7.13 (a). The ground surface has never been above the existing level, and there never