1137
Extension of measurement range of dew-point potentiometer and evaporation
method
Extension de gamme de mesure de potentiomètre de point de rosée et méthode d'évaporation
Maček M., Smolar J., Petkovšek A.
University of Ljubljana, Faculty of Civil and Geodetic Engineering, Ljubljana, Slovenia
ABSTRACT: There are numerous methods for measuring soil water retention curve (SWRC). With tensiometer device it is possible
to measure suction up to 85 kPa, but above this point measurements are not possible due to cavitation of water. To measure higher
suctions other methods should be used. One of very popular devices is dew-point potentiometer WP4-T (Decagon Devices), which
enables suction measurements above 1000 kPa. If high capacity tensiometers from HYPROP evaporation method device and dew-
point potentiometer are used for suction measurements, the measurements are only possible for low suction range between 0 and
approximately 100 kPa and for high suction range between 1000 and 300000 kPa. Between 100 and 1000 kPa suction could be
measured by other methods or some extension of these two methods should be used. This paper presents SWRC measurements with
HYPROP and WP4-T devices for different soils, the extension of suction range for both devices and comparison between measured
suctions in the extended measurement range.
RÉSUMÉ : Il y a de nombreuses méthodes pour mesurer la courbe de rétention d'eau de sol (SWRC). Il est possible de mesurer la
succion jusqu'à 85 kPa avec le dispositif de tensiomètre, mais au-dessus de ce point les mesures ne sont pas possibles en raison de
cavitation d'eau. Pour mesurer des succions plus élevées d'autres méthodes devraient être utilisées et l’un des dispositifs très connus
est le potentiomètre de point de rosée WP4-T (les appareils Décagone), qui permet des mesures de succion au-dessus de 1000 kPa. Si
des tensiomètres haute capacité de l’instrument HYPROP, la méthode d'évaporation et le potentiomètre de point de rosée sont utilisés
pour des mesures de succion, les mesures sont seulement possibles pour la gamme de succion basse entre 0 et environ 100 kPa et pour
la gamme de succion élevée entre 1000-300000 kPa. Entre 100 et 1000 kPa la succion pourrait être mesurée par d'autres méthodes ou
une certaine extension de ces deux méthodes devrait être utilisée. Cet article présente des mesures SWRC avec des dispositifs
HYPROP et WP4-T pour des sols différents, l'extension d’une gamme de succion pour les deux dispositifs et la comparaison entre des
succions mesurées dans la gamme de mesure prolongée.
KEYWORDS: soil suction, dew-point potentiometer, evaporation method, soil water retention curve.
1
INTRODUCTION
Soil suction and the soil water retention curve (SWRC)
influence many engineering properties of unsaturated soils and
their behaviour. Therefore, accurate measurements of soil
suction are important for modelling soil behaviour (Fredlund
and Rahardjo, 1993). Due to large differences in soils and their
SWRC different measuring techniques were developed and each
of them has its own limitations. A good review of measuring
techniques can be found in Tarantino et al. (2008).
Tensiometer was developed by Richards (1928) and it
measures matrix suction between 0 and 85 kPa. Suction
measurements above 85 kPa are not possible due to cavitation
of water inside the tensiometer. Special high capacity
tensiometer, which enables suction measurements up to 1500
kPa, was developed by Ridley (1993). This is done by using
deaired deionised water, small water volume behind porous cap,
smooth surfaces, as well as special materials and
preconditioning techniques.
In HYPROP evaporation method device (UMS) a continious
SWRC is obtained by simultaneously and continuously
measuring suction with two high capacity tensiometers installed
at different heights of the soil sample and weight change during
drying. A typical work range of HYPROP device is from 0 to
slightly above 100 kPa.
For soil suction measurements between 1000 and 300000
kPa a potentiometer WP4-T (Decagon Devices) could be used
(Operator’s manual, 2005). A dew-point potentiometer
measures relative humidity of air above soil sample and soil
suction is calculated trough Kelvin equation. This measurement
technique is an indirect measurement of total suction.
In the range of 100 and 1000 kPa the soil suction could be
measured using other methods or some extension of these two
methods should be performed. Using the potentiometer WP4-T
the accuracy of the measuring method can be increased, if
average of data are used instead of a single point measurement.
With this approach total suction as low as 300 kPa can also be
measured. In case of HYPROP device the suction range can not
be extended, but single value of suction could be estimated from
air entry value (AEV) of tensiometers porous ceramic caps.
In this paper the HYPROP device and potentiometer WP4-T
were used for soil suction measurements and an attempt was
made to connect the results within the grey zone, where the
limitations of both methods exist.
2
SOIL WATER RETENTION CURVE
Soil water retention curve (SWRC) is defined as relationship
between the water content and the soil suction and it can be
divided in 3 characteristic zones (Bardner, 1965) (Figure 1):
1.
Capillary saturation zone
where soil is fully saturated.
Changes in water contents result in volume deformations
without any decrease in the degree of saturation.
2.
Desaturation zone
: When the matrix suction exceeds the air
entry value (AEV) of the tested soil, the degree of saturation
decreases rapidly. Hysteresis between wetting and drying
curve is typical for the desaturation zone.
3.
Zone of residual saturation
: in this zone water content can
be changed only by vapour transport (Bishop, 1960). The
beginning of this zone is residual suction (s
r
), which is
