DHARMSHEARING STRENGTH OF SOILS 295The effect of preconsolidation is to reduce the value of A-parameter and thus cause
higher strength. At higher values of over-consolidation ratio, A-factor may be even negative;
the effective stress circles will then get shifted to the right of the total stress circles instead of
to the left. This gives lower value of effective apparent cohesion and higher value of effective
angle of shearing resistance than those of total stress values.
The results from the second method appear some what as shown in Fig. 8.40.
The results indicate that, for a particular series, the deviator stress at failure is inde-
pendent of the cell pressure. The failure envelope will be horizontal for each series, the appar-
ent cohesion ccu being different for different series; the angle φcu is zero, as indicated by Fig.
8.40 (a), (b) and (c). The greater the effective consolidation pressure, the greater is the appar-
ent cohesion. This is indicated in Fig. 8.40 (d). If the clay is over-consolidated, the consolida-
tion pressure versus apparent cohesion curve will show a discontinuity at the pressure corre-
sponding to the preconsolidation pressure; below this pressure, the relationship is non-linear
and will show an intercept at zero pressure and, above this pressure, it is linear. If the clay is
normally consolidated for all the consolidation pressures used in the tests, this relationship
will be a straight line, which, when produced backwards, will pass through the-origin.
Drained tests
The specimen is first consolidated under a certain cell pressure and is then sheared suffi-
ciently slowly so that no pore pressures are allowed to develop at any stage. The effective
stresses will be the same as the total stresses. The results will be similar to those obtained
from the consolidated undrained tests, with the same modifications as for a clay in an
overconsolidated condition, as shown in Fig. 8.41.
fdt Envelope for
overconsolidated clayEnvelope for normally consolidated claysp s
Preconsolidation pressure
Fig. 8.41 Drained tests in triaxial compression on a remoulded saturated
clay sheared under cell pressure equal to the consolidation pressureStress-strain behaviour of clays
The stress-strain behaviour of clays is primarily dependent upon whether the clay is in a
normally consolidated state or in an overconsolidated state. The stress-strain relationships for
a normally consolidated clay and those for an overconsolidated clay are shown in Figs. 8.42
and 8.43 respectively.
The behaviour of a normally consolidated clay is somewhat similar to that of a loose
sand and that of an overconsolidated clay is similar to that of a dense sand. In the case of
plastic nature of stress-strain relationship with no specific failure point, an arbitrary strain of
15 to 20% is considered to be representative of failure condition.