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Movements in geological areas are often caused by shear zones deep in the underground or by hardly observable slipping zones. Detection of movements reaching critical values is of crucial importance to avoid losses and harms. Optical sensor fibers with gauge lengths of several meters or even distributed fiber-optic sensors with gauge lengths of hundreds of meters allow detecting deformation of such zones with quite good resolution. The paper presents a new technology of detecting deformations and movements in soil or rock massifs. The core part of the sensing system is a tiny pultruded rod with an outer diameter of 3 mm to 6 mm. It contains two fiber-optic sensor systems. One sensor system is used for permanent online measurement and provides integral strain information along the whole length of the rod; in case of critical deformations, it provides a warning signal. The second sensing system uses fiber-optic Bragg grating (FBG) strain sensors located close to the expected shear zone. This measurement system will be activated after the warning signal has been initiated by the integral strain measurement system. The FBG strain sensor system records then resulting deformations in detail. The sensing rod containing both sensor systems can be installed in a borehole to detect beginning soil movements automatically. The electronic equipment is powered by a solar panel; the amount of deformation in soil or rock formations can be estimated from the strain change measured in the sensor fibers. The effectiveness of this warning system will be shown using the example of a critically sliding slope in an open brown coal pit in Germany.
Measurement and data recording systems are important parts of a holistic Structural Health Monitoring (SHM) system. They are often based on modern measurement and data communication technologies for which standards (or at least guidelines) do not exist yet. A lack in standardization, however, makes the establishment of new sensing and application technologies - and thus up-to-date SHM systems - substantially difficult. Successful commercialization and wide acceptance of SHM systems needs widely accepted standards and guidelines. Standards help to design SHM systems in an optimal way. Currently, standardization gets increasing attention in civil engineering field; there are some activities e.g. in ISHMII, IEC and ISO groups to develop standards for the most important components of SHM systems.
The paper gives an overview on current activities in standardization to propose sound and safe procedures for assessment of the existing structure's behavior and for estimation of its residual life. Special attention is paid in this paper to newly developed standards for fiber-optic measurement systems; important aspects of such sensor systems are mentioned.
The contribution is aimed at owners of structures that have to be assessed as well as at developers of sensors and monitoring systems who have to select appropriate effective sensing technologies. Using standards, both parties find recommendations how to proceed in development SHM systems to estimate the design or residual life, and how to find out the optimal maintenance strategy. The usefulness of standardized procedures is shown using the example of risk-based management of structures.
Standardization in fibre-optic sensing for structural safety - activities in the ISHMII and IEC
(2015)
Fiber-optic sensors are increasingly established in the sensor market. Their advantages have unquestionably been verified by numerous demonstrations to enhance the operational performance of aged structures or to monitor the structural behavior of safety-relevant structures or their components. However, there are some barriers in use due to a lack of extensive standardization of fiber-optic sensors. This leads very often to restraints in the users community. The paper shows the status in international standardization of fiber-optic sensors as well as current activities in leading institutions such as IEC and ISHMII and others with the purpose of providing relevant standards for a broader use of selected fiber-optic sensor technologies.
Reinforced concrete piles are used in case of structures that are constructed on soft ground to transfer the loads into deeper strata with sufficient bearing capacity. In order to determine the pile’s behavior and possible damage, static and dynamic pile tests are carried out. Dynamic measurements taken from the pile head can show the bearing behavior and structural integrity by using the theory of wave propagation. In order to receive more precise information about the pile features, now, a string of sensors is embedded at different levels of the pile. A fiber optic strain wave sensor, based on the extrinsic Fabry-Perot interferometer (EFPI), has already been developed and tested in full-scale field tests by Schallert (2010). It was possible to detect the introduced deformation caused by the static load and the dilatational wave during dynamic loading. Although the full-scale tests were successful, the engineering design of the sensor body left room - from the economical point of view - to be optimized. After laboratory tests with the optimized sensor, a cast-in-situ bored pile has been built at the BAM Test Site Technical Safety in Horstwalde, South of Berlin. Additionally to the EFPI sensors, fiber Bragg grating (FBG) sensors, temperature sensors and resistance strain gauge (RSG) sensors are embedded in order to compare the signals with each other. In this paper, the modified sensor and the Setup of the cast-in-situ bored pile along with results of dynamic tests are shown.
Fibre-optic sensors for early damage detection in plastic insulations of high-voltage facilities
(2011)
Fibre-optic sensors (FOS) have great potential as online damage detectors when integrated in HV accessories. Among their well-known use as temperature and strain sensors, there are some more opportunities of use, e. g. they can intimately be embedded in polymeric insulations of HV cable terminations and joints to detect and monitor partial discharges right at the location of their origin. Two FOS types for early PD detection were investigated: an embeddable fibre-optic acoustic sensor to measure acoustlc waves in polymeric insulations generated by PDs, and a fluorescent optical fibre to detect first optical effects during ionization processes in the insulation material. The paper descrlbes these methods, related monitoring Problems and shows first test results.
Different types of fiber-optic sensors based on glass or polymeric fibers are used to evaluate material behavior or to monitor the integrity and long-term stability of load-bearing structure components. Fiber-optic sensors have been established as a new and innovative measurement technology in very different fields, such as material science, civil engineering, light-weight structures, geotechnical areas as well as chemical and high-voltage substations. Very often, mechanical quantities such as deformation, strain or vibration are requested. However, measurement of chemical quantities in materials and structure components, such as pH value in steel reinforced concrete members also provides information about the integrity of concrete structures. A special fiber-optic chemical sensor for monitoring the alkaline state (pH value) of the cementitious matrix in steel-reinforced concrete structures with the purpose of early detection of corrosion-initiating factors is described. The paper presents the use of several fiber-optic sensor technologies in engineering. One example concerns the use of highly resolving concrete-embeddable fiber Fabry-Perot acoustic emission (AE) sensors for the assessment of the bearing behaviour of large concrete piles in existing foundations or during and after its installation. Another example concerns fiber Bragg grating (FBG) sensors attached to anchor steels (micro piles) to measure the strain distribution in loaded soil anchors. Polymer optical fibers (POF) can be — because of their high elasticity and high ultimate strain – well integrated into textiles to monitor their Deformation behaviour. Such 'intelligent' textiles are capable of monitoring displacement of soil or slopes, critical mechanical deformation in geotechnical structures (dikes, dams, and embankments) as well as in masonry structures during and after earthquakes.