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New devices for water content measurement
Les appareils nouveaux pour la mesure de la teneur en eau
Toll D.G., Hassan A.A., King J.M., Asquith J.D.
School of Engineering and Computing Sciences, Durham University, Durham, UK
ABSTRACT: Two new devices for water content measurement are described: (i) an automated multi-electrode resistivity system and
resistivity probe and (ii) a coiled TDR probe that can be used in conjunction with a high suction tensiometer. A flexible resistivity
data acquisition system has been developed to acquire resistivity data using different arrays and which automatically switches
electrodes interchangeably. A wide range of high precision reference resistors and soils have been used to test the system and the
output data have been compared with a commercial resistivity system. The system developed has been used to investigate wetting
and drying of clay using a new resistivity probe with a square electrode configuration that can be used for localised water content
determination. The novel coiled TDR device uses a two-pronged TDR wrapped around the body of the Durham University high
capacity tensiometer. The calibration of the device takes account of the contact with the tensiometer body. The device can be used
with a steel bodied tensiometer and provides accuracy in volumetric water content of ±0.075 over a range of volumetric water
contents of 0 to 0.9. A ceramic bodied device has also been investigated that does provide improved accuracy of ±0.047.
RÉSUMÉ : Deux appareils nouveaux pour la mesure de la teneur en eau sont décrites: (i) un système automatique multi-électrode de
résistivité et une sonde de résistivité et (ii) une sonde TDR qui peut être utilisée en parallèle avec un tensiomètre de forte capacité de
succion. Un système d'acquisition de données de résistivité a été développé pour acquérir des données de résistivité en utilisant des
tableaux différents et qui commute automatiquement par électrodes interchangeables. Une large gamme de résistances de référence de
haute précision et les sols ont été utilisés pour tester le système et les données de sortie ont été comparées à partir d'un système
commercial de mesure de résistivité. Le système mis au point a été utilisé pour étudier l’humidification et le séchage de l'argile à
l'aide d'une nouvelle sonde de résistivité avec une configuration d'électrodes carrée qui peut être utilisée pour déterminer la teneur en
eau localisée. Le dispositif de TDR utilise un TDR à deux volets enroulé autour du corps d’untensiomètre de grande capacité de
l’Université de Durham. L'étalonnage du dispositif tient compte du contact avec le corps de tensiomètre. Le dispositif peut être utilisé
avec un tensiomètre en acier et offre une précision de la teneur en eau volumétrique de ± 0,075 sur une plage de teneurs en eau
volumétriques de 0 à 0,9. Un dispositif en céramique a également été étudié qui donne une meilleure précision de ± 0,047.
KEYWORDS: Resistivity, TDR, water content
1 INTRODUCTION
An accurate knowledge of soil water content is crucial to
understanding the impact of climate change on engineered earth
structures. However, quantifying water content in unsaturated
soils is difficult due to the complexity of unsaturated soil
systems and the difficulties associated with gathering
representative measurements. A large spectrum of techniques
has been developed to measure soil water content. These
include; neutron scattering, dielectric methods such as Time
Domain Reflectometry (TDR) and Frequency Domain
Reflectometry (FDR), capacitance probes and remote sensing
techniques that provide measurements at regional scale.
Robinson et al. (2008) and Vereecken et al. (2008) have
presented detailed reviews of these techniques.
In geotechnical testing there is an increasing demand to
develop efficient techniques to measure soil water content.
Among the options available, TDR is becoming more widely
used in geotechnical testing and electrical resistivity has also
emerged as a cost effective and non-invasive tool to map the
spatiotemporal variability of water content that cannot be
provided by more traditional techniques (Zhou et al., 2001). In
this paper, two new systems are described: (i) an automated
multi-electrode resistivity system and resistivity probe (ii) a
coiled TDR probe that can be used in conjunction with a high
suction tensiometer to provide measurements of water content
and suction at the same position. The devices have been
developed to carry out experimental studies to monitor water
content changes in unsaturated soil specimens submitted to
drying and wetting cycles.
2 ELECTRICAL RESISTIVITY
2.1
Theoretical Background
An unsaturated soil is a multi-phase system consisting of air,
water and soil grains. Electrical resistivity (the reciprocal of
electrical conductivity)
is an intrinsic physical property of a
material that describes its ability to resist the ionic mobility in
pore water. Since electrical conduction is mainly electrolytic
and takes place through the pore water (Bryson, 2005),
electrical properties of soils are mainly controlled by water
content. A traditional four-electrode resistivity system therefore
is based on the principle that the potential drop across a pair of
electrodes due to a direct (DC) or low frequency current
injected via another pair of electrodes is proportional to the
electrical resistivity, that is:
* /
K V I
(1)
Where,
ρ
is resistivity (Ohm.m), the ratio of
V
, the potential
drop (Volts), and
I
, the current (Amps), is the material
resistance (Ohm).
K
is a geometric factor (m) representing the
electrode arrangement. For 2D and 3D resistivity studies,
traditional four-electrode systems are time consuming and
impractical. Therefore, the development of automated multi-
