Geotechnical Engineering

DHARM

COMPRESSIBILITY AND CONSOLIDATION OF SOILS 217

7.2.8 Compressibility of Field Deposits

The compressibility characteristics are usually found by performing an oedometer test in the

laboratory on a “so-called” undisturbed sample of clay or on a remoulded sample of the same

clay. The pressure-void ratio diagrams for these will be invariably different. This difference is

attributed to the inevitable disturbance caused during remoulding. Extending this logic, the

disturbance caused during sampling in the field, as also that caused during the transfer of the

sample from the sampling tube into the consolidation cell, would naturally alter the

compressibility characteristics of the field deposit of clay. Also, depending on the depth of

sampling, a certain “stress release” occurs in the field sample by the time it is tested. For these

reasons it is natural to expect that the compressibility characteristics of the so-called undis-

turbed samples do not reflect the true characteristics of the field deposits of clay. Extending

the logic of comparison, the true compressibility of field deposit would be somewhat greater

than that displayed by laboratory samples. If this difference is not recognised, we would be

erring on the wrong side in so far as settlement computations are concerned.

A typical set of pressure-void ratio curves for undisturbed and remoulded samples of

clay in relation to that which may be anticipated for the corresponding field deposit is given in

Fig. 7.16.

Void ratio e

Pressures(log scale)

ruf( , e )sff

c

e=0

F-Field consolidation line

U-Undisturbed soil

R-Remoulded soil

ef

sr su sf

Fig. 7.16 Compression curves for undisturbed and remoulded
samples for a field deposit of clay
Let a sample of clay be taken from a depth z from the ground surface. For the overburden
pressure σf on it, the void ratio of the sample was, say, ef. The point f on the plot represents
these values, the pressure being plotted to the logarithmic scale. Let ef and f be joined by a
dotted line. The moment the undisturbed sample is taken out from the ground it gets freed of
the overburden pressure, the water content and void ratio remaining the same. The compression
curve for this sample will be curved until the pressure reaches σf and, later on, will be a
straight line, as shown by the plot U. If the sample is remoulded at the same water content,
the compression curve obtained will be as depicted by the plot R. If σr is the pressure

corresponding to the void ratio ef on this plot, it will be observed that this plot will be almost a

straight line at pressures greater than σr. If the plot U is produced downwards to meet the

pressure axis at c, the straight portion of the plot R also would almost pass through c, if

produced. It is, therefore, logical to assume that the compression curve corresponding to the