In this paper, an overview of optical sensor development, testing and evaluation for several geotechnical monitoring
applications is presented. Additionally, sensor integration and data interpretation are addressed as key influences to the
overall success of the monitoring project. They should be taken into consideration already in the design stage.
Particular focus is given on strain sensor development to minimize the slippage of the fiber inside the protection. For the
first time, slippage progression monitoring by high spatially resolved Brillouin measurements is presented as a new tool
for sensor testing and evaluation for geotechnical projects.
The main findings of the study are that in a geotechnical monitoring project, special care has to be taken by choosing the
sensor slippage properties, longitudinal stiffness and robustness, as well as in the design of the sensor system itself
(fixation, gauge length and bond strength). With appropriate alignment of these factors, reasonable monitoring data can
be obtained, as shown in the applications proposed in this manuscript.
For the understanding of the bearing behavior of a loaded ground anchor, the measuring and monitoring of the stress
distribution in the anchor tendon is essential. This paper proposes a novel monitoring ground anchor using embedded
optical fibers for the continuous strain assessment along the anchor tendon. In a first step, optical sensors have been
integrated into short tendons using different methods and laboratory strain testing was performed on these instrumented
tendons. The evaluation of the laboratory testing enabled the design and development of an 8m long monitoring ground
anchor for field application. In 2009, this anchor has been placed into a wall supporting an excavation pit and
subsequently, anchor pullout test was carried out. The anchor was loaded stepwise up to 470kN, almost reaching its
ultimate bearing capacity. Optical measurements were taken successfully at each load step. Comparison of the optical
data with data acquired using conventional methods indicated good consistency of the results. To a geotechnical
engineer, this proposed monitoring anchor provides a powerful tool for the measuring of the pullout load, the anchor
head displacement and the load distribution in the anchor tendon.
In Geotechnical Engineering, progressive failure in soil-structure interaction is one of the least understood problems. It is
difficult to study this phenomenon at laboratory scale, because of the large amount of strain gages required per unit
length/area of the structure, which would interfere with the mechanical properties of both the structure and the soil. The
recently developed Brillouin Echo Distributed Sensor (BEDS) technology overcomes this dilemma by distributed
readings and 5cm spatial resolution. A laboratory pullout testing program has been carried out to verify applicability of
BEDS for the study of progressive failure in the soil-structure interaction.
The determination and monitoring of landslide boundaries is essential for analysis of creeping landslides. A novel
landslide boundary localization technique has been recently proposed and tested on two large creeping landslides in an
urban area. The technique uses asphalt road-embedded distributed fiber optic sensors. This paper deals with the issue of
interpretation of the monitoring records. It has been shown that an improved protection of the cable increases the
measurement strain range, but leads to non-linear strain-frequency response. Two methods of strain data interpretation
have been analyzed: the truncated average method (TAM) and the convolution product (CP). Advantage of the TAM is
in its simplicity; disadvantage is that the amount of the valid sampling points is significantly reduced, especially when
the fixed strain section lengths are close to the spatial resolution. The alternative CP method uses all sampling points in
the vicinity of the fixation point, but is rather complex, especially considering that a proper interpretation of the
measured data can be only achieved using a weighting function with parameters dependent on the strain step at the
fixation point. Further signal processing and data interpretation models should be encouraged to improve system
A novel technique for the determination of a creeping landslide boundary is demonstrated. It is based on application of
distributed optical fiber strain measurements using Brillouin Optical Time Domain Analysis (BOTDA) technology. A
road crossing the St. Moritz landslide boundary was instrumented with a fiber optic cable, which turned the road,
effectively, into a large scale strain gauge. The obtained monitoring data was in good agreement with visual observation
and also followed the trends of the geodetical data. The presented validation of this technology allows for a conclusion
that distributed fiber optic strain sensing is a promising new tool in landslide surveillance. At present, until methods and
standards in this field are established and reliable, combination with traditional methods is necessary. Ongoing
measurements during 2008 may strengthen the conclusions of this paper.