Geotechnical Engineering

DHARM

SHEARING STRENGTH OF SOILS 291

8.12.2 Shearing Strength of Clays

Shear behaviour of clays is influenced by the fact whether the clay is normally consolidated or

overconsolidated, by the fact whether it is undisturbed or remoulded, by the drainage condi-

tions during testing, consistency of the clay, by certain structural effects, by the type of test

and by the type and rate of strain. The following discussion relates to the shearing strength of

saturated clays which are in a normally consolidated state; the modifications that may be

expected in case the clay is in an overconsolided state are indicated at the appropriate places.

Unconsolidated Undrained Tests

It is difficult, if not impossible, to utilise the concept of effective stress in connection with the

shearing strength of saturated clays. It is difficult to imagine that any substantial part of the

normal stress is transmitted through particle contacts when grain-to-grain contacts are rela-

tively infrequent or when the solid phase is weak in itself. For this reason, it is common prac-

tice to consider only total stresses in the case of saturated clays. The results of unconsolidated

undrained tests in direct shear are indicated in Fig. 8.34.

t

Shearing strength

O Normal pressure s

cu

Fig. 8.34 Unconsolidated undrained tests in direct shear on saturated clays
It is seen that the total normal pressure does not influence the shearing strength of a
saturated clay from undrained tests; the intercept of the horizontal plot on the shear strength
axis gives the cohesion cu. The strength of a clay is often reported simply in terms of unit
cohesion, regardless of the overburden pressure.
The results of such tests in triaxial compression are indicated in Fig. 8.35.

s 31 s 32 s 33 s 1 s 11 s (^12)
t
cu
Effective stress circle Total stresses envelope
O s 3 s 13 s
u 2
u 3
u 1 u 1
u 2
u 3
Total stress
circles
Fig. 8.35 Unconsolidated undrained tests in triaxial
compression on saturated clays