Geotechnical Engineering

DHARM

292 GEOTECHNICAL ENGINEERING

Since drainage is not permitted both during the application of cell pressure and during
the application of deviator stress (or additional axial stress), the increase in cell pressure or
axial stress automatically increases the pore water pressure by an equal magnitude, the effec-
tive stress remaining constant. In view of this, the diameter of the effective stress circle will be
the same as that of the total stress circles with mere lateral shifts. The total stress envelope is
thus a horizontal line, the intercept on the shearing strength axis being cohesion cu, and φu
being zero. It may also be easily understood that the effective stress envelope cannot be ob-
tained from these tests since only one circle will be obained for all tests. Consolidated undrained
or drained tests may be used for this purpose. Pore pressurements are not usually made in the
unconsolidated undrained tests as they are not useful.
It is common knowledge that the shear strength of clay varies widely with its consist-
ency, the shear strength being negligible when the water content is at liquid limit. This is
reflected in Fig. 8.36.

t

Shearing strength

Normal pressure s

Stiff clay (at SL)

Medium clay (at PL)

Very soft clay (at LL)

0

Fig. 8.36 Variation of shearing strength with consistency of saturated clays
The shearing strength of partially saturated clays is a more complex phenomenon and,
hence, is considered outside the scope of the present work.

Consolidated undrained tests

If consolidated undrained tests are conducted in direct shear on remoulded, saturated and
normally consolidated clay specimens with the same initial void ratio, but consolidated under
different normal pressures, and sheared under the normal pressure of consolidation, without
permitting drainage during shear, results as indicated in Fig. 8.37 are obtained.

t

Shearing strength

Normal pressure

s 1 s 2 s 3 s

fcu

0

Fig. 8.37 Consolidated undrained tests in direct shear on remoulded,

saturated, and normally consolidated clay (consolidated and

sheared under normal pressures σ 1 , σ 2 , and σ 3 )