1909
New Sensing Technology and New Applications in Geotechnical Engineering
Nouvelle technologie de détection et nouvelles applications à l
’ingénierie
géotechnique
Wang Y.H., Ooi G.L., Gao Y.
Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong
ABSTRACT: Soils are inherently a particulate medium, and relevant physical principles behind the macro-scale engineering
properties originate from particle interactions. However, it is difficult in general to conduct measurements which can monitor soil
particle movement and even characterize micromechanics behind different soil behaviour. Advancement of sensing technologies in
recent years offers us the opportunity to do so. Two examples are presented in this paper. The first is on using the tactile pressure
sensor (film-like sensor) to monitor the evolution of contact normal forces among particles in aged sand. The measurement reveals
that the contact forces are continuously redistributed during aging. This ultimately strengthens the soil structure and therefore
increases the associated small-strain shear modulus. The second is on using the miniature MEMS accelerometer to characterize the
soil movement in a laboratory flow landslide. The MEMS sensors demonstrate promising results in describing the rich features of
local responses of soil movement in the shear zone, e.g. liquefaction, deceleration, contraction and dilation.
RÉSUMÉ : Les sols sont intrinsèquement un milieu particulaire et les principes physiques pertinents derrière les propriétés
mécaniques à macro-échelle proviennent d'interactions entre particules. Cependant, il est difficile en général d'effectuer des mesures
qui peuvent suivre le mouvement des particules du sol et même de caractériser la micromécanique derrière différents comportements
du sol. L'avancée des technologies de détection ces dernières années nous offre la possibilité de le faire. Deux exemples sont présentés
dans cet article. Le premier utilise le capteur de pression tactile pour suivre l'évolution des forces de contact normales entre des
particules dans du sable âgé. La mesure révèle que les forces de contact sont continuellement redistribuées au cours du vieillissement.
Cela renforce finalement la structure du sol et augmente donc le module de cisaillement associé à petites déformations. Le second
utilise l'accéléromètre miniature MEMS pour caractériser en laboratoire le mouvement du sol dans un glissement de terrain. Les
capteurs MEMS démontrent des résultats prometteurs pour la description des caractéristiques abondantes des réponses locales du
mouvement du sol dans la zone de cisaillement, par exemple, la liquéfaction, la décélération, la contraction et la dilatation.
KEYWORDS: tactile pressure sensor; aging mechanism; MEMS
•
landslide initiation mechanism.
1 INTRODUCTION
Measurements of micromechanical interactions among soil
particles are invaluable to constitute insights into the underlying
physics of different soil behavior encountered in geotechnical
engineering. However, it is difficult in general to conduct
measurements which can monitor soil particle movements and
even characterize micromechanics involved. In recent years,
manufacturing industries witness breakthroughs in both
miniaturization of sensors and improvement of sensing
technologies. Therefore, we have an opportunity to carry out
measurement at the particulate scale. In this paper we would
like to introduce two applications in laboratory testing using
such new technologies: (1) tactile pressure sensor to
characterize the underlying mechanisms of aging in sand, and
(2)
3D Micro-Electro-Mechanical-Systems (MEMS)
accelerometers to capture soil movement in the initiation
process of a laboratory flow landslide.
Aging can occur in all types of soils: for instance, increases
in their shear strength and shear modulus are observed as time
elapses. Such aging effects in sands have been reported not only
from laboratory tests but also via field observations (e.g., see
review in Schmertmann 1991, Mitchell and Soga 2005, Wang
and Tsui 2009; Gao et al. 2013). Nevertheless, at present, the
associated underlying mechanisms remain inconclusive. In the
first study, the tactile pressure sensor installed in a tailor-made
oedometer was used to characterize the evolution of contact
normal force among particles in dry sand during aging. The
bender element sets were also utilized in parallel to monitor the
associated changes in the small-strain shear modulus, G
max
. The
ultimate goal of this experiment is to provide evidence that
could account for the underlying mechanisms of aging effects.
Initiation mechanisms of flow landslides are likewise still an
unsettled open discussion. There had been published studies
linking the initiation process of flow landslides to pore pressure
rise and presence of fines in soil (Iverson et al. 2000, Wang and
Sassa 2003); however, exactly how the shear zone develops,
liquefies or decelerates due to dilation prior to complete
fluidization is not measured. The miniature size of 3D MEMS
accelerometers (in the range of
mm
) makes the measurements of
localized soil responses inside slope possible since they can
move like a soil particle without any pronouncing inertial effect.
Reported MEMS sensor responses shed lights on a variety of
soil movement involved in the initiation process of flow
landslide, which will be discussed in detail in the second study.
2 USING TACTILE PRESSURE SENSOR FOR THE
STUDY OF AGING MECHANISMS
2.1 The I-scan system
Tactile pressure sensors are ultra-thin and flexible and comprise
numerous individual tiny sensing elements, called sensels.
These features resolve the problems associated with the
conventional load cells and enable us to accurately measure
stress inside soils. Fig. 1 presents the pressure mapping system
used in the first study, i.e., the I-Scan® system (Tekscan Inc.,
MA., USA). This system consists of software, scanning
electronics (called a handle), and a tactile pressure sensor. There
are 1936 sensels in the sensor adopted in this study (model
5076). The sensel is a force-sensitive resistor, whose impedance
