Geotechnical Engineering

DHARM

218 GEOTECHNICAL ENGINEERING

consolidation of the field deposit in-situ, is also a straight line tending to pass through c. Since
the point f, representing the original conditions, should also lie on this line, the curve F, that
is, ef fc represents the compression curve for the consolidation of the field deposit. The portion
fc is referred to as the “field consolidation line”.

Let the straight portion of plot U be produced backwards and upwards to meet ef f line

in u and let the corresponding pressure be σu. σu will be less than σf for all clays except extra-

sensitive ones. The ratio σσu/ f indicates the degree of disturbance during sampling. An aver-

age value for this ratio is 0.5.

Terzaghi and Peck (1948) recommend that the field consolidation line F be taken as the
basis of settlement computations. The reconstruction of this line is possible by procedures
suggested by some workers, e.g., Schmertmann (1955).

7.2.9 Relationship between Compressibility and Liquid Limit

A.W. Skempton and his associates have established a relationship between the compressibility
of a clay, as indicated by its compression index, and the liquid limit, by conducting experi-
ments with clays from various parts of the world. The relationship was found to be linear as
shown in Fig. 7.17.

0 20 40 60 80 100 120 140

1.0

0.8

0.6

0.4

0.2

0

Compression inde

xC

c

Liquid limit w %L
Fig. 7.17 Relationship between compression index and liquid
limit for remoulded clays (After Skempton)
The equation of this straight line may be approximately written as:
Cc = 0.007 (wL – 10) ...(Eq. 7.7)
wL being the liquid limit in per cent.

It has also been established that the compression index of field deposits of clays of low
and medium sensitivity is about 1.30 times that of their value in the remoulded state. There-
fore, we may write for consolidation of field deposits of clay:
Cc = 0.009 (wL – 10) ...(Eq. 7.8)
This equation is observed to give a satisfactory estimate of the settlement of structures
founded on clay deposits of low and medium sensitivity.

These two equations are, as reported by Terzaghi and Peck (1948), based on the work of
Skempton and his associates.