Environmental Engineering Reference
In-Depth Information
Figure 11.89 Time to failure t f for drained triaxial test specimen
140mm in height.
Figure 11.90 Time to failure t f for drained direct shear tests with
specimen height of 12.7 mm.
in the impedance factor λ ). The increase in time to failure
due to a decrease in the coefficient of consolidation c v is
uniform at various λ values. However, the increase in time
to failure due to a decrease in the impedance factor becomes
more significant for impedance factors less than 10. When
the impedance factor is greater than 10, the time to failure is
almost constant for a particular coefficient of consolidation.
An impedance factor of 10 corresponds to a specific
permeability ratio k w / k d (Fig. 11.88). As an example, an
impedance factor of 10 corresponds to a k w / k d ratio of 0.7
when the L d / d ratio is 0.16. Impedance values less than
10 correspond to k w / k d ratios greater than 0.7 (Fig. 11.88).
A comparison between Figs. 11.89 and 11.90 indicates that
the time to failure t f is significantly affected by impeded
flow when the impedance factor is less than 10. In other
words, the impeded flow problem exists even when the
high-air-entry disk has a coefficient of permeability equal
to the coefficient of permeability of the soil specimen. In
many cases, the coefficient of permeability of the disk is
lower than that of the soil. On the other hand, the effect of
impeded flow on the time to failure t f remains essentially
unchanged when the impedance factor is greater than
10. This means that little improvement can be made in
shortening the time to failure t f when the impedance factor
has reached a value greater than 10.
It is evident from the above discussion that impeded flow
due to the presence of the high-air-entry disk is often the
governing factor influencing the time to failure. One way
to reduce the time to failure is by using a thin high-air-
entry disk. High-air-entry disks generally range in thickness
from 3.2 to 9.5 mm. For a specific k w / k d ratio and a spe-
cific specimen height, a decrease in the thickness of the
high-air-entry disk causes an increase in the impedance fac-
tor (Fig. 11.88). An increase in the impedance factor reduces
the time to failure t f (Fig. 11.89). However, thicker disks
are superior due to their ability to reduce the diffusion rate
of air through the ceramic disk.
Thestrainrate
ε f for triaxial testing of unsaturated soils
can be computed in accordance with Eq. 11.34. The com-
puted strain rate is only an estimate because of the assump-
tions involved in the theory and the difficulties in accurately
assessing relevant soil properties. Nevertheless, the theory
does provide a somewhat rational approach to obtaining the
strain rate for unsaturated soil testing. Typical strain rates
that have been used for triaxial tests for unsaturated soil
testing are tabulated in Table 11.7. The strain rate for test-
ing should be relatively slow, even when the soil appears to
be quite pervious, since the strain rate is largely controlled
by the high-air-entry disk. Equation 11.34 can also be used
to obtain a conservative estimate of strain rate for undrained
shear strength testing.
It appears that water can more readily flow out from a
soil specimen through the high-air-entry disk than into the
specimen through the high-air-entry disk. If a soil tends to
dilate during shear, it may be difficult to ensure the upward
flow of water through the low-permeability disk into the
soil specimen. This problem can be somewhat alleviated by
using the axis translation technique and periodically flushing
diffused air from below the high-air-entry disk.
˙
11.9.3 Displacement Rate for Direct Shear Tests
The rate of horizontal shear displacement in a direct shear
test is analogous to the strain rate in a triaxial test. The hori-
zontal shear displacement rate can be defined as the relative
 
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