Environmental Engineering Reference
In-Depth Information
equation of the three straight lines of the SWCC can be
written as
Parameter
Value
Specific gravity,
G
s
2.65
0.981
Void ratio,
e
⎧
⎨
S
1
log
ψ
ψ
s
Dry density,
g
d
(kg/m
3
)
1338
w
s
+
when
ψ<ψ
aev
Water contents in decimal from
0.37
Saturated water content,
w
s
ψ
ψ
aev
0.1
w
aev
+
S
2
log
when
ψ
aev
<ψ<ψ
r
Saturated suction,
y
sat
(kPa)
w
=
0.365
⎩
Air-entry water content,
w
a
5.0
S
3
log
ψ
ψ
r
Air-entry suction,
y
aev
(kPa)
w
r
+
when
ψ
r
<ψ
0.1
Residual water content,
w
r
100
Residual suction,
y
r
(kPa)
(5.93)
−
0.003
Slope
S
1
where:
Slope
S
2
−
0.204
Slope
S
3
−
0.025
w
s
=
gravimetric saturated water content (decimal),
Suction increment ratio
2
ψ
s
=
low (arbitrary) suction corresponding to saturated
conditions, kPa,
Soil suction
(kPa)
q
w
w
S
0.1
0.370
0.495
1.000
w
aev
=
gravimetric water content at the air-entry value,
0.2
0.369
0.494
0.998
ψ
aev
=
suction at air-entry value, kPa,
0.4
0.368
0.493
0.995
w
r
=
gravimetric residual water content, and
0.8
0.367
0.492
0.993
ψ
r
=
residual suction.
1.6
0.366
0.490
0.990
3.2
0.366
0.489
0.988
6.4
0.343
0.459
0.927
12.8
0.282
0.377
0.762
The slope variables in the above equations can be defined
as follows:
25.6
0.221
0.295
0.596
51.2
0.159
0.213
0.430
102.4
0.100
0.133
0.270
w
aev
−
w
s
204.8
0.092
0.123
0.249
S
1
=
(5.94)
409.6
0.085
0.113
0.229
log
(ψ
aev
)
−
log
(ψ
s
)
819.2
0.077
0.103
0.209
w
r
−
w
aev
1,638.4
0.070
0.093
0.188
S
2
=
(5.95)
log
(ψ
r
)
−
log
(ψ
aev
)
3,276.8
0.062
0.083
0.168
6,553.6
0.055
0.073
0.148
−
w
r
13,107.2
0.047
0.063
0.127
S
3
=
(5.96)
log
(
10
6
)
−
log
(ψ
r
)
26,214.4
0.040
0.053
0.107
52,428.8
0.032
0.043
0.087
104,857.6
0.024
0.033
0.066
Typical input data for a spreadsheet are as follows: specific
gravity
G
s
=
209,715.2
0.017
0.023
0.046
2
.
65; saturated gravimetric water content,
419,430.4
0.009
0.013
0.025
w
s
=
0
.
1 kPa; gravimetric
water content at the air-entry value,
w
aev
=
37%; saturated suction,
ψ
s
=
838,860.8
0.002
0.003
0.005
1,000,000.0
0.000
0.000
0.000
36.5 %; air-entry
value for the soil,
ψ
aev
=
5.0 kPa; gravimetric water content
at residual conditions,
w
r
Figure 5.115
Spreadsheet showing computed SWCC data set
based on estimations of air-entry value and residual conditions.
=
10%, and residual suction
ψ
r
=
100 kPa. Other values shown on the spreadsheet are
calculated from the basic volume-mass information. The
suction increment ratio controls the spacing of the data
points along the SWCC (e.g., a value of 2 results in a
doubling of suction between each data point).
The spreadsheet can generate three representations of the
SWCC (Fig. 5.115). Figure 5.116 shows gravimetric water
content versus soil suction; Fig. 5.117 shows volumetric
water content versus soil suction; and Fig. 5.118 shows
the degree of saturation versus soil suction for the same
soil. Values can be cut and paste from the spreadsheet. It is
possible to best fit the data set with any of the commonly
used mathematical equations for the SWCC. Unsaturated
soil property functions can also be computed using the soil
suction versus water content data along with the saturated
soil properties.
5.14 CORRELATION OF FITTING PARAMETERS
TO SOIL PROPERTIES
The fitting parameters for any of the proposed SWCC
equations can be correlated with the classification properties
of a soil. Each SWCC equation has either two or three
fitting parameters and each parameter can bear a correlation
with one or more of the soil classification properties.
Unfortunately, independent correlations are required for
each of the proposed empirical SWCC equations.
Several researchers have undertaken correlations between
fitting parameters for the SWCC equation and the classifi-
cation properties of the soil (Ghosh, 1980; Williams et al.,
1983; Ahuja et al., 1985; Rawls et al., 1992; Cresswell and
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