Geotechnical Engineering

DHARM

COMPRESSIBILITY AND CONSOLIDATION OF SOILS 237

Tangent

Asymptote

0.008 0.01 0.1 1 2 3

Time factor T

(log scale)

2

20

40

60

80

100

Degree of consolidation U%

(b) Degree of consolidation versus

logarithm of time factor

(Theoretical curve from Terzaghi’s theory)

Fig. 7.28 Logarithm of time fitting method (After A. Casagrande, 1939)

Since the early portion of the curve is known to approximate a parabola, the corrected

zero point may be located as follows: The difference in ordinates between two points with

times in the ratio of 4 to 1 is marked off; then a distance equal to this difference may be stepped

off above the upper points to obtain the corrected zero point. This point may be checked by

more trials, with different pairs of points on the curve.

After the zero and 100% primary compression points are located, the point correspond-

ing to 50% consolidation and its time may easily be obtained and the coefficient of consolida-

tion computed from:

Cv =

TH

t

50

2

50

...(Eq. 7.32)

where t 50 is read off from Fig. 7.28(a)

T 50 = 0.197 from Terzaghi’s theory, and

H is the drainage path as stated in the previous subsection.

The primary compression ratio may be obtained as given in the previous subsection.

7.7.3 Typical Values of Coefficient of Consolidation

The process of applying one of the fitting methods may be repeated for different increments of

pressure using the time-compression curves obtained in each case. The values of the coeffi-

cient of consolidation thus obtained will be found to be essentially decreasing with increasing

effective stress, as depicted typically in Fig. 7.29.

This is the reason for the caution that, for problems in the field involving settlement

analysis, the coefficient of consolidation should be evaluated in the laboratory for the particu-

lar range of stress likely to exist in the field.

The range of values for Cv is rather wide—5 × 10–4 mm^2 /s to 2 × 10–2 mm^2 /s. Further, it

is also found that the value of Cv decreases as the liquid limit of the clay increases. This should

be expected since, in general, clays of increasing plasticity should be requiring more time for a

particular degree of consolidation, as is evident from Eq. 7.25.