Geotechnical Engineering

DHARM

216 GEOTECHNICAL ENGINEERING

R

M

E

S

B

a a/2

T

D

C

sRsE sC

Void ratio e

Effective stresss(log scale)

Fig. 7.15 A. Casagrande’s procedure for determining

preconsolidation pressure

3. A tangent MT to the curved portion is drawn through M.


4. The angle SMT is bisected, MB being the bisector.


5. The straight portion DC of the plot is extended backward to meet MB in E.


6. The pressure corresponding to the point E, σE, is the most probable past maximum

effective stress or the preconsolidation pressure.

Sometimes the lower and upper bound for the preconsolidation pressure are also men-
tioned. If the tangent to the initial recompression portion and the straight line portion of the
virgin curve DC meet at R, the pressure σR corresponding to R is said to be the minimum

preconsolidation pressure, while that corresponding to C, σc, is said to be the maximum

preconsolidation pressure.

7.2.7 Time-Lags During the Compression of Clay

Considerable time is required for the full compression to occur under a given increment of

stress for a clay soil. This is a well-known characteristic of clays. A typical time-compression

curve for clay has already been presented in Fig. 7.2. Although it may not take more than

twenty-four hours for the full compression to occur for a laboratory sample, it may take a

number of years in the case of a field deposit of clay. This is the reason for settlements continu-

ing to occur at an appreciable rate after many years for buildings founded above thick clay

strata, although, generally speaking, the rate should be steadily decreasing with time.

Two phenomena are responsible for this time-lag. The first is due to the low permeabil-

ity of clays and consequent time required for the escape of pore water. This is called the “hy-

drodynamic lag”. The second is due to the plastic action in absorbed water near grain-to-grain

contacts, which does not allow quick transmission of the applied stress to the grains and the

effective stress to reach a constant value. This is known as the “plastic lag”. The frictional lag

in sands may be thought of as a simple form of plastic lag.

The theory of one-dimensional consolidation of Terzaghi, presented in section 7.4, does

not recognise the existence of plastic lag, although it presents a good understanding of the

hydrodynamic lag and consequent rates of settlement. This may be the reason for the predic-

tion on the basis of the Terzaghi theory going wrong once in a while.