Geotechnical Engineering

DHARM

COMPRESSIBILITY AND CONSOLIDATION OF SOILS 239

Secondary consolidation is believed to come into play even in the range of primary con-
solidation, although its magnitude is small, because of the existence of a plastic lag right from
the beginning of loading. However, it is almost impossible to separate this component from the
primary compression. Since dissipation of excess pore pressure is not the criterion here,
Terzaghi’s theory is inapplicable to secondary consolidation. The fact that experimental time-
compression curves are in agreement with Terzaghi’s theoretical curve only up to about 60%
consolidation is, in itself, an indication of the manifestation of secondary consolidation even
during the stage of primary consolidation.

Secondary consolidation of mineral soils is usually negligible but it may be considerable
in the case of organic soils due to their colloidal nature. This may constitute a substantial part
of total compression in the case of organic soils, micaceous soils, loosely deposited clays, etc. A
possible disintegration of clay particles is also mentioned as one of the reasons for this phe-
nomenon. Secondary compression is usually assumed to be proportional to the logarithm of
time.

Hence, the secondary compression can be identified on a plot of void ratio versus loga-
rithm of time (Fig. 7.31).

Void ratio

o Time(log scale) t

e

ei

eo

Initial

Primary

Secondary

Fig. 7.31 Void ratio versus logarithm of time
Secondary compression appears as a straight line sloping downward or, in some cases,
as a straight line followed by a second straight line with a flatter slope. The void ratio, ef, at the
end of primary consolidation can be found from the intersection of the backward extension of
the secondary line with a tangent drawn to the curve of primary compression, as shown in the
figure. The rate of secondary compression, depends upon the increment of stress and the char-
acteristics of the soil.
The equation for the rate of secondary compression may be approximated as follows:
∆e = – α. log 10 (t 2 /t 1 ) ...(Eq. 7.33)
Here, t 1 is the time required for the primary compression to be virtually complete, t 2 any
later time, and is ∆e is the corresponding change in void ratio. This means that the secondary
compression which occurs during the hydrodynamic phase is ignored, but the error is not
probably serious. α is a coefficient expressing the rate of secondary compression.