1719
Field Monitoring of Shield Tunnel Lining Using Optical Fiber Bragg Grating Based
Sensors
Surveillance de doublure d’un tunnel au bouclier utiliser les capteurs optiques de fibre-Bragg-
grating
Huang A.B., Lee J.T., Wang C.C., Ho Y.T., Chuang T.S.
Department of Civil Engineering, National Chiao Tung University, Taiwan
ABSTRACT: The design of soft ground shield tunnel lining for Taipei Mass Rapid Transit (MRT) system has been relying on semi-
empirical procedures. The earth pressures involved in tunnel lining structural design has been determined based on experience. It is
not certain if the design is adequate. Also, occasionally new MRT tunnel or other forms of underground constructions can be located
dangerously close to an existing tunnel. An effective means of monitoring the behavior and integrity of the tunnel lining is
imperative to the future design and safety of the MRT tunnel system. The authors developed techniques to attach optical fiber Bragg
gratings (FBG) in the reinforcement as a means to monitor the strains experienced by the shield tunnel lining. Readings were
recorded from pre-cast concrete section production through field installation and continued after field installation. The paper
describes the techniques of FBG field monitoring, their available records and discusses implications in the design and safety
monitoring of shield tunnel linings.
RÉSUMÉ : Le projet du rêvetement de tunnel bouclier au terrain moule pour la système du Masse Transit Rapide de Taipei (MRT)
avait eu en s'appuyant sur les procedures semi-empiriques. Le sol impliqué dans la construction de MRT a été alluvial dans la nature
avec l’argile limoneuse et le sable limoneuse intercouché avec le dépôts de gravier occasionel. Les pressions de terre impliqué dans le
projet structural du rêvetement de tunnel avait eu détérminées basées sur l'expérience. Il n'est pas certain si le projet est trop
conservateur. Également, parfois le tunnel nouveau de MRT ou less autres formes des constructions sousterrain peuvent être situées
dangereusement près d’un tunnel existant. Le moyen efficace de surveillance du comportement et l’intégrité du rêvetement de tunnel
est impératif du projet avenir et la sécurité de système de tunnel de MRT. Les auteurs ayant développés les techniques à attacher les
grilles de Bragg à fibre optique (FBG) dans l’renforcement comme un moyen de surveiller les deformations expérimentées par le
rêvetement de tunnel bouclier. Les FBG détectés le rêvetement de tunnel sont instalés dans la ligne Shin-Yi MRT de Cité de Taipei en
2007. Les lectures sont enregistrés de la section de béton pre-contriante pendant l’installation des chantier et continués jusqu'à
aujourd’hui. Cet article décrit les techniques de FBG la surveillance de chantier, leur enregistrements disponibles et discute influences
dans le projet et la sécurité de surveillance des rêvetements de tunnel bouclier.
KEYWORDS: fiber Bragg grating, shield tunnel lining, monitoring
1 INTRODUCTION
The design of shield tunnel lining has been based on semi-
empirical procedure and may be rather conservative. The
conservatism is mainly due to lack of field measurement as to
the stresses or strains that the tunnel linings are actually
experiencing during different stages of construction and
operation. For municipal subway tunnels, there may be threats
from other construction activities such as those for new building
basements or other tunnels in close proximity. Earlier attempts
of using electrical strain gauges for field monitoring in shield
tunnel had the drawbacks that include the lack of longevity and
signals prone to electromagnetic interference (EMI). There
have been reports on the use of optical fiber sensors to monitor
subway tunnel lining deformation using the fully distributive
Brillouin Optical Time Domain Reflectometry (BOTDR)
method (Mohamad et al. 2007). In their report, optical fiber was
mounted inside the existing tunnel lining. The BOTDR signals
are immune to EMI and optical fibers are durable, and thus are
clearly more desirable than the conventional electrical sensors.
There were no baseline readings when the tunnel lining was
completely free of exterior earth pressure. The strain readings
were therefore, relative to the post tunnel-construction
conditions. While the BOTDR readings serve the purpose of
revealing the effects of nearby constructions on the existing
tunnel lining, the results had little value in evaluating the
current design procedure. For the latter purpose, it is necessary
to measure the absolute strains within the tunnel lining.
Optical fibres are made of silica, with a diameter about the same
of a human hair, and can transmit light over long distances with
very little loss of fidelity. Optical fibres comprise two essential
components: a core surrounded by an annular cladding. The
core of the optical fibre serves to guide light along the length of
the optical fibre. The cladding has a slightly lower index of
refraction than the core. Its primary function is to ensure total
internal reflection within the core and that very little light is lost
as it propagates along the core of the optical fibre. The typical
combined diameter of core and cladding is 125
m. The silica
core/cladding is protected by an acrylic coating. The total
outside diameter of an optical fibre with the acrylic coating is
250
m. By adopting technologies from telecommunication,
many fibre optic based sensing techniques have been developed.
The fibre optic Bragg grating (FBG) is one of the many
available forms of optical fibre sensors. An FBG is made by a
periodic variation of fibre core refractive index. The typical
length of an FBG is 1 to 20 mm long. When the FBG is
illuminated by a wideband light source, a fraction of the light is
reflected back upon interference by the FBG. The wavelength
of the reflected light, is linearly related to the longitudinal
strains of the FBG, thus making FBG an ideal strain gage. By
monitoring the temperature induced strain in a loose FBG, it is
possible to use FBG as a temperature sensor with a resolution
on the order of 0.1
o
C. The returned signal from every FBG
carries a unique domain of wavelength, making it possible to
have multiple FBG elements on the same fibre. The
