Michael Iten

BOTDA road-embedded strain sensing system for landslide boundary localization

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 accuracy.

Monitoring of stress distribution along a ground anchor using BOTDA

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.

Distributed fiber optic sensor development, testing, and evaluation for geotechnical monitoring applications

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.

Study of a progressive failure in soil using BEDS

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.