Geotechnical Engineering

DHARM

COMPRESSIBILITY AND CONSOLIDATION OF SOILS 235

excess pore pressure is not shown or is ignored; this effect is known as ‘Secondary consolida-
tion’, which is briefly presented in the following section. The curves of Figs. 7.26 and 7.24 bear
striking similarity; in fact, one should expect it if Terzaghi’s theory is to be valid for the phe-
nomenon of consolidation. This similarity becomes more apparent if the curves are plotted
with square root of time/time factor as the function, as shown in Fig. 7.27 (a) and (b).

The theoretical curve on the square root plot is a straight line up to about 60% consoli-
dation with a gentle concave upwards curve thereafter. If another straight line, shown dotted,
is drawn such that the abscissae of this line are 1.15 times those of the straight line portion of
the theoretical curve, it can be shown to cut the theoretical curve at 90% consolidation. This
may be established from the values of T at various values of U given in Fig. 7.24 for case I; that

is, the value of T at 90% consolidation is 1.15 times the abscissa of an extension of the
straight line portion of the U versus T relation. This property is used for ‘fitting’ the theo-
retical curve to the laboratory curve.

Sample thickness/Dial gauge reading

Th (d ) 90 90

Th 100 (d 100 )

Th (or d )ff

0

Th (d )ii

Th (d ) 0 0

Öt 90

ÖTime,Öt

Degreo ofconsolidation U %

0

ÖT 90

ÖTime factor,ÖT

(^50) x
60
70
80
90
100
0.15 x
(a) Sample thickness/Dial gauge reading
versus square root of time (Laboratory curve)
(b) Degree of consolidation versus
square root of time factor (Theoretical
curve from Terzaghi’s theory)
0 0
Fig. 7.27 Square root of time fitting method (After Taylor, 1948)
The laboratory curve shows a sudden initial compression, called ‘elastic compression’
which may be partly due to compression of gas in the pores. The corrected zero point at zero
time is obtained by extending the straight line portion of the laboratory plot backward to meet
the axis showing the sample thickness/dial gauge reading. The so-called ‘primary compres-
sion’ or ‘primary consolidation’ is reckoned from this corrected zero. A dashed line is con-
structed from the corrected zero such that its abscissae are 1.15 times those of the straight line
portion of the laboratory plot. The intersection of the dashed line with the laboratory plot
identifies the point representing 90% consolidation in the sample. The time corresponding to