Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 576

w

d

d

y

y

(a)

(a)

w

d

w

d

f

(b)

f

(b)

Figure 11.75 Undrained triaxial and unconfined compression
tests on clayey sand compacted to low density: (a) deviator stress
versus strain for various confining pressures; (b) total stress point
envelope (after Chantawarangul, 1983).

Figure 11.74 Undrained triaxial and unconfined compression
tests on clayey sand compacted to high density: (a) deviator stress
versus strain for various confining pressures; (b) total stress point
envelope (after Chantawarangul, 1983).

maximum dry density and the optimum water content are
1815 kg/m 3 and 16%, respectively.
The shear strength parameters c and φ were obtained
from several single-stage and multistage direct shear tests on
compacted specimens that were saturated. The Indian Head
glacial till was found to have an effective cohesion inter-
cept of 10 kPa and an effective angle of internal friction of
25.5 ◦ . Multistage direct shear test results on saturated speci-
mens showed that a total shear displacement of 1.2mm was
sufficient to mobilize the peak shear strength (Gan, 1986).
The specimens were 50

The specimens were sheared under undrained conditions
at a constant strain rate of 0.0017 %/ s. Test results are
presented in Figs. 11.74 and 11.75 for the high- and low-
density specimens, respectively. The results show a curved
total stress point envelope which becomes a horizontal
envelope at high confining pressures. The total stress point
envelopes for specimens at various water contents are
plotted in Fig. 11.76a for the high-density specimens and in
Fig. 11.76b for the low-density specimens. The envelopes
also show a decrease in shear strength as the water content
of the specimens increased.

50mm and a total displacement
of 1.2mm was selected as the failure displacement for the
multistage direct shear tests.
Five multistage consolidated drained direct shear tests
were performed on five compacted specimens. The initial
volume-mass properties of the five specimens are tabulated
in Table 11.6. The tests were performed using the axis trans-
lation technique on a modified direct shear apparatus (Gan
and Fredlund, 1988). A displacement rate of 1.7

×

11.8.5 Direct Shear Test Results
Multistage direct shear tests have been performed on sat-
urated and unsaturated specimens of compacted glacial till
by Gan et al., (1988). The glacial till was sampled from
the Indian Head area of Saskatchewan, and material passing
the No. 10 sieve was used to form specimens for test-
ing. The soil consisted of 28% sand, 42% silt, and 30%
clay. The liquid and plastic limits of the soil are 35.5 and
16.8%, respectively. The soil specimens were prepared by
compaction in accordance with the AASHTO standard. The

10 -4
mm/s was selected. Each specimen had six or seven stages
of shearing. The tests were performed by maintaining a
constant net normal stress σ n −

×

u a of 72 kPa while varying