Environmental Engineering Reference
In-Depth Information
and then gradually flattens (or even decreases) at high
suctions (i.e., suctions approaching residual conditions).
The shear strength of an unsaturated soil has been shown
to bear a relationship to changes in the degree of saturation
(or water content) of a soil. Therefore, it is not surprising
that proposed shear strength functions for an unsaturated
soil have been related to the SWCC. The shear strength
function for an unsaturated soil appears to adhere to the fol-
lowing limiting conditions. The shear strength of an unsat-
urated soil increases in response to the effective angle of
internal friction,
φ
, for matric suctions up to the air-entry
value of the soil. Once the air-entry value is exceeded, the
increase in shear strength responds to soil suction at a con-
tinuously decreasing rate throughout the transition region.
There appears to be no significant increase (or decrease) in
shear strength for most soils once residual suction conditions
are exceeded.
40
AEV
30
20
10
(
q
r
,
y
r
)
0
10
6
0.1
1
10
100
1000
10,000 100,000
Soil suction, kPa
(a)
200
150
f′
100
12.2.1 Observed Relationship between SWCC
and Shear Strength
Donald (1956) performed a series of direct shear tests on
various gradations of Frankston sand. This is probably the
earliest set of shear strength tests performed where the
associated SWCC desorption curves were also measured.
Figure 12.2b shows the change in strength that occurred for
the Frankston sand soils when soil suction was increased
past the air-entry value through the transition zone and
beyond residual suction conditions. Each shear strength plot
shows that the shear strength rises quickly as the negative
pore-water pressure is increased. However, in each case
there is a point at which shear strength levels off and even
decreases as matric suction is further increased.
Changes in the shear strength with matric suction bear
a relationship to the corresponding SWCCs for each of the
sand soils. Figure 12.2a shows the SWCCs corresponding to
the Frankston sand specimens. In each case, there appears to
be a relationship between the changes on the shear strength
envelope and key points on the SWCC, namely, the air-entry
value and residual suction conditions.
The first deviation from the effective angle of internal
friction,
φ
, occurs just prior to the air-entry value. There
is a leveling off of the shear strength envelope just after
residual suction conditions are exceeded. The peak values
of shear strength and the decrease in shear strength occur
in the transition zone of the SWCC. The test results on
the sand specimens show a relationship between the SWCC
and shear strength. These are some of the earliest results that
form the basis for the development of estimation procedures
for the unsaturated shear strength envelope. The saturated
shear strength parameters and the SWCC provide the basic
information required for formulating a shear strength func-
tion. Consequently, there is continuity between the shear
strength equations for saturated and unsaturated soils.
Numerous shear strength testing programs have been per-
formed over the years, and each time, when shear strength
AEV
50
Estimated shear strength function
0
0
50
100
150
200
250
300
350
Soil suction, kPa
(b)
200
150
f′
100
50
Fitted shear strength equation
0
0
50
100
150
200
250
300
350
Soil suction, kPa
(c)
Figure 12.1
Fitting experimental shear strength data and estimat-
ing a soil suction shear strength function: (a) fitting and (b) esti-
mation procedures to obtain shear strength function.
(Escario and Saez, 1986; Fredlund and Rahardjo, 1993a;
Vanapalli et al., 1996; Cunningham et al., 2003):
1. Higher matric suctions result in higher shear strengths
under the same confining pressures.
2. Higher confining pressures result in higher shear
strengths under the same matric suction.
3. The relationship between shear strength and soil suction
is nonlinear when the applied soil suctions are increased
beyond the air-entry value of the soil. The shear strength
increases most rapidly in the low-matric-suction range
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