Environmental Engineering Reference
In-Depth Information
Table 4.17 Suggested Lateral Shifts of Inflection Point
between Drying and Wetting Curves for Various Soils
As an example, a water content corresponding to the
inflection points would have soil
suctions written as
5
.
62
<
7
.
5
<
10
.
0 and read as
Soil Type
Range of Typical Shifts
Average Shift
=
7
.
5 kPa,
Most likely or median suction
(% of log cycle)
(% of log cycle)
Maximum estimated suction
=
10
.
0 kPa,
Minimum estimated suction
=
5
.
62 kPa, and
Estimated range of suctions
=
5
.
62
−
10 kPa.
Sand
15-35%
25%
Silt and loam
35-60%
50%
The above format provides a representation of the range and
central tendencies that can be anticipated. The geotechnical
engineer should view all three suction values and ask, “Does
an understanding of the suction range and central tendencies
assist me in making engineering judgements?” In some case,
the answer may be “yes” while in other cases it may be “no.”
Clay
—-
Up to 100%
published data from various researchers. Experimental results
showed that the largest shift between the drying and wetting
curves occurred for clay soils and the smallest shift occurred
for uniform sand soils. Table 4.17 summarizes typical (aver-
age) lateral shifts at the inflection point of the SWCC for
various soils (Pham et al., 2002, 2003a).
Let us assume that the soil under consideration is sand
with the
a
parameter on the drying curve equal to 10.0 kPa.
Let us also select a lateral shift of 25% of a log cycle to
move to the wetting curve. The
a
parameter for the wetting
SWCC can be calculated using Eq. 4.55:
4.6.2.5 Example Problem to Illustrate Typical Values
and Ranges of Values from the Proposed Procedure
to Obtain Soil Suction
Let us assume the soil parameters shown in Table 4.18 for
sand soil, silt, and clay soil. The Fredlund and Xing (1994)
equation is used to illustrate typical differences between var-
ious soil classifications.
Drying, wetting, and median SWCCs are computed for
three different soil types. The soil parameters are typical
values for a sand, silt, and clay but should not be taken as
fixed values for each soil classification. Figure 4.104 illus-
trates the relationship among the three SWCCs (i.e., drying
curve, median curve, and wetting curve) for a soil with a
lateral shift of 25%. The
n
parameter was set to 4, which
represents rather steep SWCCs typical of uniform sand.
Figures 4.105 and 4.106 illustrate the relationship among
the three SWCCs for soils with a lateral shift of 50 and
100%, respectively. The
n
parameters were reduced to 2.0
and 1.5, respectively. All three sets of SWCCs are shown
in Fig. 4.107 to show the wide range of SWCCs that can
occur for various soils.
log
ψ
a
w
0
.
25
=
log
(
10
)
−
(4.55)
Therefore, the suction corresponding to the
a
parameter
at the inflection point on the wetting curve is
10
0
.
75
ψ
a
w
=
=
5
.
62
(4.56)
The suction at the inflection points went from 10 kPa on
the drying curve to 5.62 kPa on the wetting curve. The suc-
tion at the inflection point on the median SWCC can be
calculated by using a lateral shift of 12.5% (i.e., 0.125).
The
n
fitting parameter remains the same for all SWCCs
because of congruency.
Three soil suction values can be computed for any mea-
sured water content. The following format can be used to
represent soil suction values corresponding to the measured
water content:
4.6.2.6 Suggestions and Recommendations
for Using the SWCC to Estimate In Situ Soil Suction
in Geotechnical Engineering Practice
The following guidelines are suggested for usage of the
SWCCs for the estimation of in situ suction: (1) The
Wetting curve suction
<
median curve suction
<
drying curve suction
(4.57)
Table 4.18 Soil Properties Associated with Drying and Wetting SWCCs for Three Soils
Saturated
a
d
Drying
n
d
Drying
a
w
Wetting
a
m
Median
Soil Type
Water Content
SWCC
SWCC
% Shift
SWCC
SWCC
Sand
30.0
10.0
4.0
25
5.623
7.500
Silt
40.0
200
2.0
50
63.25
112.5
Clay
60.0
3000
1.5
100
300.0
948.7
Note
: SWCC variable
m
=
1
.
0 and correction factor
C(ψ)
=
1
.
0
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