1109
Combination of Shrinkage Curve and Soil-Water Characteristic Curves for Soils that Undergo
Volume Change as Soil Suction is Increased
Combinaison des courbes de retrait et des courbes des propriétés hydriques des sols pour les sols
subissant un changement de volume avec une augmentation de la succion
Fredlund D.G.
Golder Associates Ltd., 1721 – 8
th
Street East, Saskatoon, SK., Canada
Zhang F.
University of Alberta, Edmonton, AB., Canada
ABSTRACT: The soil-water characteristic curve, SWCC, is commonly used for the estimation of unsaturated soil property functions,
USPF, in geotechnical engineering practice. The indiscriminate usage of the estimation techniques for unsaturated soil property
functions can lead to erroneous analytical results and poor engineering judgment. Essentially all estimation procedures for unsaturated
soil property functions, USPFs, make the assumption that the soil will not undergo volume change as soil suction is increased. The
evaluation of the correct air-entry value has a significant effect on the estimation of subsequent USPFs. This paper describes how the
SWCC laboratory results can be properly interpreted with the assistance of a shrinkage curve. Laboratory data sets are then used to
illustrate how the test data should be interpreted for high volume change soils.
RÉSUMÉ : La courbe des propriétés hydriques des sols, SWCC, est communément utilisée afin d’estimer les fonctions des propriétés
des sols non saturés, USPF, en géotechnique. L’usage abusif des techniques d’estimation des fonctions des propriétés des sols non
saturés peut mener à des résultats analytiques erronés et à un mauvais jugement au point de vue ingénierie. Toutes les procédures
d’estimation pour les fonctions des propriétés des sols non saturés, USPF, font l’hypothèse que le sol ne subira aucun changement de
volume avec une augmentation de la succion. L’évaluation d’une valeur d’entrée d’air correcte a un effet significatif sur l’estimation
de subséquents USPF. Cet article décrit comment les résultats de laboratoire des SWCC peuvent être correctement interprétés avec
l’utilisation d’une courbe de retrait. Des résultats d’essais enlaboratoire sont alors utilisés pour illustrer comment les données d’un
essai doivent être interprétées pour des sols subissant d’importants changements de volumes.
KEYWORDS: soil-water characteristic curves, shrinkage curve, volume change, soil suction, unsaturated soil property functions.
1 INTRODUCTION
The soil-water characteristic curve, SWCC, provides vital
information for applying unsaturated soil mechanics in
engineering practice. Much of the information regarding the use
of SWCC originated in soil physics and agriculture-related
disciplines. With time, information regarding the use of the
SWCC has been embraced for geotechnical engineering
applications (Fredlund, 2002; Fredlund and Rahardjo, 1993).
A common difficulty arises when large volume changes
occur in the soil as soil suction is increased. Sludge material and
slurry material may be deposited at water contents above the
liquid limit of the material. The material is deposited in ponds
and allowed to dry in order to increase its shear strength. The
geotechnical engineer may be called upon to undertake
numerical modeling simulations of the drying process.
2 SIGMOIDAL EQUATIONS FOR SWCCS
There are several sigmoidal type equations that have been
proposed to mathematically describe the water content versus
soil suction relationship (e.g., van Genuchten, 1980; Fredlund
and Xing, 1994). The S-shaped sigmoidal equations have the
appearance of being able to fit SWCC data regardless of the
measure that is used to represent the amount of water in the soil
(e.g., gravimetric water content, volumetric water content, or
degree of saturation). The Fredlund and Xing, (FX), (1994)
SWCC equation can be used to illustrate the usage of a
sigmoidal equation for various designations of water content.
The FX (1994) equation uses a correction factor that allows all
SWCCs to go to zero water content as soil suction goes to
1,000,000 kPa. The FX (1994) equation is first written in terms
of gravimetric water content and can then be used to best-fit the
SWCC.
w
(
)
w
s
1
ln 1
r
ln 1
10
6
r
1
ln exp(1)
a
f
n
f
m
f
(1)
where:
w(ψ)
= gravimetric water content at any specified
suction,
ψ
;
w
s
= saturated gravimetric water content; ;
=
residual soil suction;
a
f
,
n
f
, and
m
f
= the fitting parameters for
the SWCC equation. Equation 1 is written for the gravimetric
water content designation; however, the equation could also be
best-fit volumetric water content or degree of saturation versus
soil suction. The gravimetric water content SWCC can be used
in conjunction with a shrinkage curve to more accurately
interpret the parameters required for the estimation of
unsaturated soil property functions.
The degree of saturation versus soil suction can be computed
by combining Eq. 1 with the shrinkage curve data. The
volumetric water content versus soil suction SWCC is also
required to obtain the water storage coefficient for the soil. The
volumetric water content must be related to the instantaneous
overall volume of the soil mass in order to obtain the correct
value for numerical modeling purposes. Volume change of the
overall soil specimen can be taken into consideration if a
“shrinkage curve” is measured. The shrinkage curve is generally
measured under conditions of zero net normal stress.
bination of Shrinkage Curve and Soil-Water Characteristic Curves for
S ils that Undergo Volume Change as Soil Suction is Increased
les sols subissant un chang ment d volume avec un augmentation de la succion
