Geotechnical Engineering

DHARM

SHEARING STRENGTH OF SOILS 289

If the sand is moist, the failure envelope does not pass through the origin as shown in
Fig. 8.31. The intercept on the shear stress axis is referred to as the ‘apparent cohesion’, attrib-
uted to factors such as surface tension of the moisture films on the grains. The extra strength
would be lost if the soil were to dry out or to become saturated or submerged. For this reason
the extra shear strength attributed to apparent cohesion is neglected in practice.

Failure envelope

Shearing strength

Value of apparent cohesion

Normal stress
Fig. 8.31 Failure envelope for moist sand indicating apparent cohesion
In the case of triaxial compression tests, different tests with different cell pressure are
to be conducted to evaluate the shearing strength and the angle of internal friction. In each
test, the axial normal stress is gradually increased keeping the cell pressure constant, until
failure occurs. The value of φ is obtained by plotting the Mohr Circles and the corresponding
Mohr’s envelope.

The failure envelope obtained from a series of drained triaxial compression tests on
saturated sand specimens initially at the same density index is approximately a straight line
passing through the origin, as shown in Fig. 8.32.

O s

t

s =

sftan

Mohr envelope

(common tangent)

Fig. 8.32 Drained triaxial compression tests on saturated sand
Similar results are obtained when undrained triaxial compression tests are conducted
with pore pressure measurements on saturated sand samples and Mohr’s circles are plotted in
terms of effective stresses. However, if Mohr’s circles are plotted in terms of total stresses, the
shape of envelopes will be similar to those for a purely cohesive soil. The failure envelope will
be approximately horizontal with an intercept on the shearing stress axis, indicating the so-
called ‘apparent cohesion’, as shown in Fig. 8.33.