Environmental Engineering Reference
In-Depth Information
from a variety of soils. The shear strength of all soil types
appears to respond as a saturated soil as long as the matric
suction is less than the air-entry value of the soil. There is
curvature in the shear strength envelope once the air-entry
value is exceeded. The shear strength envelope for most
soils with silt- and clay-size particles bends toward an
almost horizontal line near residual suction conditions. In
other words, the shear strength envelope shows an increase
in strength up to the residual suction for the soil. Sandy
soils generally show a leveling off in strength even prior to
residual suction being reached and tend to fall off in strength
at higher soil suctions.
50
Fine Frankston
Graded Frankston
Medium Frankston
Brown sand
40
30
20
10
0
0
5
10
15
20
25
30
35
Matric suction ( u a - u w ), kPa
12.2.1.1 Shear Strength beyond Residual Suction
The early shear strength tests by Donald (1956) showed that
it was possible for the shear strength of sand to decrease
once residual suction conditions were exceeded. It is also
known that clay soils generally continue to increase in
strength as soil suction is increased beyond residual suction
conditions. Some estimation models use the entire SWCC
when estimating the unsaturated shear strength function.
Other estimation models normalize water content between
saturated conditions and residual water content conditions.
There is little information to confirm the accuracy of any
of the estimation models in the high-soil-suction range.
Nishimura and Fredlund (2002) performed unconfined
compression tests on two soils that were conditioned to total
suction values well beyond the respective residual suctions.
The two soils were a nonplastic silt soil and kaolin. The
statically compacted soil specimens were 100mm in height
with a diameter of 50 mm. The specimens were placed
into a chamber that controlled the temperature and relative
humidity and allowed equilibration under the applied total
suction environment. Total suctions were controlled using
saturated salt solutions.
Some of the soil specimens were prepared dry of resid-
ual total suction and then wetted to equilibrium at various
relative humidity values (i.e., 80, 70, 60, 50 and 40). Other
soil specimens were prepared wet of residual total suctions
and allowed to dry to the equilibrium state. The specimens
remained in the relative humidity environment for about 1
month. Unconfined compression tests were then conducted
at an axial strain rate of 0.5 mm/min.
The SWCCs for each of the soils were measured using
a pressure plate apparatus, salt solution desiccators, and a
relative humidity chamber. The SWCCs for the silt soil and
kaolin are shown in Fig. 12.5. The air-entry value and the
residual conditions were selected from the SWCC. The esti-
mated air-entry value for the silt soil and kaolin were 10
and 100 kPa, respectively. The residual state for the silt soil
occurred at a water content of 2.5% and the residual suction
was 2000 kPa. The residual suction for the kaolin appears
to be around 5000 kPa and the corresponding water content
was around 18%.
(a)
25
20
15
10
Fine Frankston
Graded Frankston
Medium Frankston
Brown sand
5
0
0
5
10
15
20
25
30
35
Matric suction ( u a - u w ), kPa
(b)
Figure 12.2 Result of series of direct shear tests on sands sub-
jected to range of matric suction (Donald, 1956, with modifications
by Vanapalli, 2009).
was measured over a wide range of soil suction values, it has
been clear that there was a relationship between the SWCC
and the shape of the unsaturated shear strength envelope.
Figure 12.3 shows the results of a series of direct shear tests
performed on glacial till. The SWCC shows that the air-
entry value of the soil is approximately equal to 60 kPa.
The shear strength envelopes show that the air-entry value
is near the point where shear strength starts to deviate from
the effective angle of internal friction, φ . It is difficult to
observe a distinct break in the SWCC that represents the start
of residual conditions; however, residual suction appears to
be beyond 10,000 kPa for the glacial till soil. Residual suc-
tion is well beyond the range of the shear strength tests. The
shear strength measurements extended to a matric suction of
500 kPa and the envelope continued to increase at a reduced
rate as soil suction increased.
The above-mentioned results along with other laboratory
test results have shown that there is a clear relationship
between the unsaturated shear strength of a soil and the
corresponding SWCC. Figure 12.4 summarizes the general
anticipated shear strength responses that might be anticipated
 
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