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Geotechnical News December 2010
Geotechnical Instrumentation News
the load distribution along the tendon
(Figure 5). This provides a better
understanding of the real strength
mobilization and progressive failure
than some currently commercially
available monitoring anchors.
Applications Requiring High
Spatial Resolution
The
novel
BEDS
technology,
allowing for measurements with a
spatial resolution of 5cm, is likely
to expand the applications for FO
sensing in geotechnical monitoring.
It becomes possible to detect single
cracks in structures affected by ground
movements, and gives a comprehensive
strain profile along geotechnical
structures such as the monitoring
anchor or a pile. At the laboratory
scale, two applications have been
explored. In the first one, the strain
profile evolution in a 2m long FO cable
pulled out of sand was successfully
monitored (Iten et al., 2009b). In the
second application a crack monitoring
was performed by fixing a FO cable at
both sides of the “crack” leaving 10cm
of the free cable length and moving one
fixation point by 0.1mm (simulating a
crack opening). The BEDS data clearly
detects the crack opening (Figure
6). With this technology becoming
commercially available during the next
years, hundreds
of
individual
cracks can be
monitored with
one single FO
cable.
Conclusions
and Outlook
In contrast to
structural health
monitoring, FO
g e o t e c h n i c a l
m o n i t o r i n g
a p p l i c a t i o n s
are
not
yet
very
common.
However, their
ability to provide
e n o r m o u s
amounts of data
at low cost per reading (in spite of
the relatively expensive measurement
units) is a convincing fact. The issues
that have to be handled with care are
(a) FO cable selection, (b) FO cable
integration and (c) data interpretation.
FO cables of a broad range of stiffness
and protection are now available. The
FO cable integration is project-specific
and methods have been outlined in
the references. The data interpretation
requires background knowledge of FO
technology. The authors are convinced
that for the applications described in
this article, FO technology is a valuable
alternative to conventional methods.
References
• Iten M., Schmid A., Hauswirth D.
and Puzrin A. M. (2009a), “De-
fining and monitoring of landslide
boundaries using fi-
ber optic systems”,
Int. Symp. on Pre-
diction and Simu-
lation Methods for
Geohazard Mitiga-
tion, Kyoto, Japan.
• Hauswirth
D.,
Iten M., Richli R.
and Puzrin A. M.
(2010). “Fibre optic
cable and micro-an-
chor pullout tests in sand” . Proc. of
Physical Modelling in Geotechnics,
ICPMG 2010, Zürich, Switzerland.
• Iten M. and Puzrin A.M. (2010),
“Monitoring of stress distribution
along a ground anchor using BOT-
DA”, Proc. of SPIE, Vol. 7647,
SSNDE Conference, San Diego,
USA.
• Iten M., Puzrin A.M., Hauswirth D.,
Foaleng-Mafang S., Beugnot J.C.
and Thévenaz L. (2009b), “Study
of a progressive failure in soil using
BEDS”, Proc. of SPIE, Vol. 7503,
20th International Conference on
Optical Fibre Sensors, Edinburgh,
UK.
• Thévenaz, L. (2010). “Brillouin
distributed time-domain sensing in
optical fibers: state of the art and
perspectives”, Frontiers of Opto-
electronics in China, Vol. 3, No. 1,
China.
Alexander Puzrin, Michael Iten,
Dominik Hauswirth
Institute for Geotechnical Engineering
ETH Zürich - Swiss Federal Institute
of Technology
Wolfgang-Pauli-Strasse 15, 8093
Zürich, Switzerland
Tel 41+44+633+6785
Editor’s Note
Some readers may want to know the
commercial sources of FO sensing
systems. Here’s a partial list. If you
know of others, please let me know,
and I’ll update the list in a later GIN.
Company Name and Country Website
FOS&S, Belgium
Inventec, The Netherlands
Micron Optics, USA
www.
micronoptics.com
Omnisens, Switzerland
OpSens, Canada
Sensornet, England
co.uk
Smartec, Switzerland
Tencate, The Netherlands
Figure 6 Monitoring of single crack opening with 5cm resolu-
tion (in collaboration with Foaleng Mafang S. and Thévenaz
L, EPF Lausanne, Switzerland).tif
