Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 217

equation was the van Genuchten (1980) equation where the
Fredlund and Xing (1994) “correction function” was applied
beyond residual suction, giving an Akaike number of

relationship. There are an infinite number of intermediate
drying and wetting scanning curves that bridge between the
wetting and drying curves, as shown in Fig. 5.41. The dry-
ing and wetting scanning curves become asymptotic to the
bounding curves.
An engineer cannot determine whether the soil is on a
drying path or a wetting path when the soil is sampled in
the field. The difference between the drying and wetting
branches of the SWCC may be as much as an order of
magnitude in terms of soil suction. This means that water
content could correspond to a soil suction ranging between,
for example, 10 and 100 kPa. Slight changes in the nature
of the soil can also produce a lateral shift in the SWCCs.
Consequently, it is difficult to use the SWCC for the esti-
mation of in situ soil suctions, other than to obtain a wide
range of approximate values. On the other hand, the SWCCs
have proven to be useful for the estimation of unsaturated
soil property functions in geotechnical engineering practice.

824.
The Akaike number was poorer when the Fredlund and
Xing (1994) correction function was not added to the van
Genuchten equation.
The Sillers (1997) study also showed that there was no
advantage associated with fixing the relationship between the
m vg and n vg variables in the van Genuchten (1980) equation
[i.e., using the fixed relationships suggested byBurdine (1953)
and Mualem (1976a)]. The best-fit analysis showed that the
same quality of fit was obtained by leaving the m vg variable
at 1.0 in the van Genuchten (1980) equation. The study also
showed that the Fredlund and Xing (1994) “correction factor”
could be applied to other SWCC equations and in each case
there was an improvement in the quality of fit.
An analysis of the SoilVision Systems database of results
showed that neither the Mualem (1976a) nor the Burdine
(1953) relationship between the m and n variables was close
to values computed using the van Genuchten (1980) equation
with independent m and n values. The results showed that the
n parameter generally varies between a small number and 10
with most of the n values being less than 3. The best-fit m
parameters appear to extend beyond values of 50, but most of
the values were less than 20. The data presented in Fig. 5.39
suggest that it is best to use m and n as independent variables.

−

5.5.1 Hysteresis in SWCC
The drying and wetting SWCCs are significantly different,
and in many cases it becomes necessary to differentiate
the soil properties associated with the drying curve from
those associated with the wetting curve. This means that the
geotechnical engineer must make a decision regarding the
process that is to be simulated (i.e., the drying or wetting
process) and then use the appropriate estimated unsaturated
soil property function (Tami et al., 2004). More elaborate
soil models that include scanning curves and hysteresis have
been developed (Mualem, 1974, 1976a, 1976b; Pham et al.,
2003a, b) but it might be more practical at present to use
simpler unsaturated soil property models in geotechnical
engineering practice. It might also be appropriate in some
cases to use an average SWCC (i.e., between the drying and
wetting SWCCs) when estimating unsaturated soil property
functions.

5.5 HYSTERESIS, INITIALIZATION, AND
INTERPRETATION OF SWCC

Typical features of the drying and wetting portions of the
SWCCs are shown in Fig. 5.40. The hysteresis loop indicates
that there is no single or unique SWCC. Consequently, spe-
cial precautions must be taken when applying the SWCC in
geotechnical engineering. The wetting and drying branches
form extreme bounds of the water content versus soil suction

100

80

60

m = 0 for n < 0.5
m = 22 n − 10 for 0.5< n <100

40

20

0

2 4 6 8 10

n

Figure 5.39 Best-fit regression n and m parameters for range of soils best fit with van Genuchten
(1980) SWCC equation.