Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 264

Estimation of the SWCC

Correlation of soil
properties with
parameters of
SWCC equations

Database mining for
SWCCs from similar soils

Estimations based on the

grain-size distribution
curve,

PTFs *

Fredlund et al. (1997)

Comparison
of grain-size
or Atterberg

Comparison
of soil
classification

Correlation
with SWCC
parameters

Arya + Paris (1991) Average of

many past
results

limits

Vereecken et al. (1989)

Tyler + Wheatcraft (1989)

* PTF means pedo-transfer

Rawls + Brakensiek (1985)

functions (i.e., soil properties
determined from the soil

Scheinost et al. (1997) classification properties)

Figure 5.113 Procedures for estimation of the SWCC.

Let us suppose that a number of grain-size distribution
curves may have been measured on soil samples taken in the
field. It is possible to utilize one of several proposed pedo-
transfer functions for the estimation of the SWCC (M.D.
Fredlund et al., 2002a). An analysis of available information
allows the engineer to arrive at a “best guess” for the air-
entry value, the residual conditions, as well as the saturated
initial porosity for the soil.
A computer spreadsheet can readily be used to generate
a series of data points along an entire SWCC by simply
entering the soil suction and water content for two points
on the SWCC. The artificially generated data set can then
be used for the estimation of unsaturated soil property
functions.
Figure 5.114 shows the definition of variables used in the
calculation of the SWCC. The degree of saturation versus

soil suction graph provides the most definitive way to iden-
tify the air-entry value ψ aev and the residual conditions ψ r
for a soil. However, if the soil undergoes negligible volume
change as soil suction is increased, then the air-entry value
and residual suction occur at the same suction values on the
gravimetric and volumetric plots versus soil suction.
The SWCC consists of three straight lines with slopes of
S 1 ,S 2 , and S 3 on a logarithmic scale (Pham, 2005). The
slope S 1 refers to the slope between the saturated condi-
tions at a low suction (e.g., 0.1 or 1.0 kPa) and the air-entry
value computed on the gravimetric water content scale. The
slope S 2 refers to the slope of a line between the air-entry
value and residual conditions computed on the gravimet-
ric water content scale. The slope S 3 refers to the slope
of a line between residual conditions and completely dry
conditions corresponding to soil suction of 10 6 kPa. The

S 1

w sat

w aev

S 2

w r

S 3

ψ sat ψ aev ψ r ψ (kPa)

10 6

Logarithmic soil suction, log ( ψ )

Figure 5.114 Definition of variables used to compute three main branches of a SWCC presented
in terms of gravimetric water content (after Pham, 2005).