Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 582

(a)

(a)

(b)

(b)

Figure 11.84 Effect of strain rate on consolidated drained tests on
Dhanauri clay: (a) effect of strain rate on deviator stress; (b) effect
of strain rate on water content change (after Satija, 1978).

(c)

Figure 11.85 Undrained triaxial test on compacted clay shale
specimen tested at strain rate of 6 . 9 × 10 − 4 % /s : (a) shear stress
versus strain curve; (b) pore-water pressure versus strain at two
points of measurement; (c) total volume change versus strain (after
Bishop et al., 1960).

Gibson and Henkel (1954) used the theory of consolida-
tion to formulate a theoretical method for approximating the
time required to fail a specimen under drained conditions.
The theory is applicable for both triaxial and direct shear
tests. The strain rate used in the triaxial testing of saturated
soils has commonly been estimated using the Gibson and
Henkel (1954) procedure (Bishop and Henkel, 1962). For
unsaturated soils, it is necessary to consider the possibility
of impeded flow through the high-air-entry disk at the base
of the specimen as well as the physical properties of the soil
(Ho and Fredlund, 1982c). The high-air-entry disk has a low
coefficient of permeability k d . The disk not only prevents the
passage of air but also impedes the flow of water in and out
of the specimen during shear. Another factor affecting the
time to failure t f is the low coefficient of permeability of
the unsaturated soil.
Analytical considerations of the impeded flow problem
for an unsaturated soil specimen can be solved in a manner
similar to the problem of impeded flow caused by plugged
porous stones placed next

(Bishop and Gibson, 1963). The solution gives the time
required to fail the specimen when using a drained test. A
somewhat smaller value for time to failure t f should be
used for an undrained test. The high-air-entry disk may
control pore pressure equalization in the soil. It is suggested
that the time-to-failure value computed for a drained test
should be considered as a conservative estimate for the
time to failure for an undrained test.
The ability of an unsaturated soil specimen and the
measuring system to dissipate the excess pore pressures
developed during drained shear is the main consideration in
computing the time to failure. The excess pore pressure
dissipation is essentially a one-dimensional consolidation
process. Generally, the coefficient of permeability with
respect to the air phase, k a , is much larger than the
coefficient of permeability with respect to the water phase,

to saturated soil specimens