Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 230

Table 5.5 Curve-Fitting Parameters for Seven Soils Using Meaningful Parameter SWCC Equation

Curve-Fitting Parameters

ID Soil w s S 1 S 2 ψ aev , kPa ψ r , kPa

Regina clay b 1 1.042 0.256 0.677 2310 2780

2 Jossigny silt 0.463 0.089 0.169 1010 4820

3 Kaolin 0.881 0.168 0.303 883 12,500

4 Beaver Creek sand 0.239 0.003 0.652 2.46 5.33

5 Saskatchewan silty sand 0.241 0.010 0.200 8.01 40.2

6 Processed silt 0.255 0.044 0.140 33.7 118

Indian Head till c

7 0.387 0.088 0.498 801 958

a Parameters t 1 and t 2 are chosen as constant values prior to the fitting procedure with t 1 =

8.
b Several empirical data points were added along the desaturation part for the Regina Clay to assist in curve fitting.
c Data points for the SWCC at suctions higher than 1500 kPa were measured by Sillers (1997).

4 and t 2 =

140 Measured data for Jossigny loam (Fleureau et al., 1995)

Measured data for kaolinite (Fleureau et al., 2004)
Measured data for Regina clay (Fredlund, 1964)
Best-fitted curve for jossigny loam
Best-fitted curve for kaolinite
Best-fitted curve for Regina clay

120

100

80

60

40

20

0

10 6

0.1 1 10 100

Soil suction, kPa

1000 10,000 100,000

Figure 5.61 Best-fit SWCCs using simplified SWCC along with published data sets for three
soils (after Pham, 2005).

has an air-entry value greater than 1 kPa). For soils with an
air-entry value less than 1 kPa, w s represents the water content
at a soil suction of 1 kPa on an extension of the line through
a suction value less than the air-entry value. The curve-fitting
parameter S 1 represents the slope of the gravimetric SWCC
at suction values less than the air-entry value.
Figures 5.59-5.62 show that the two SWCC equations
proposed by Pham and Fredlund (2009) result in close fits
of measured SWCC data for a wide range of soil types
(i.e., from sand to clay). Both equations make use of the
curve-fitting parameters w s and S 1 (i.e., representing the
gravimetric water content at 1 kPa soil suction and the slope
of the SWCC at soil suctions less than the air-entry value).
The meaningful parameter SWCC equation requires five
curve-fitting parameters (i.e., in addition to the two transi-
tional parameters for curvature at the air-entry and residual
points along the SWCC). The five curve-fitting parameters

50
45
40
35
30
25
20
15
10
5
0

Measured data for Beaver Creek sand
Measured data for processes silt
Measured data for Sask. sity sand
Measured data for Indian Head Till

Best-fitted curve for Beaver Creek sand

Best-fitted curve for sask. silty sand

Best-fitted curve for processed silt
Best-fitted curve for Indian Head till

1000 10,000 1000,000 10 6 0.1 1 10 100

Soil suction, kPa

Figure 5.62 Best-fit SWCCs using simplified SWCC equation
along with measured data sets for

four soils (after Pham,

2005).