Helmut Woschitz

Design and Calibration of a Fiber-Optic Monitoring System for the Determination of Segment Joint Movements Inside a Hydro Power Dam

In hydro power dams with arch dams as water storage, the movements of the concrete segment joints are monitored. New monitoring concepts are required as most dams were updated to more efficient pumped-storage power stations within the last years, which significantly changed the loading conditions. Inside one specific dam, the largest segment joint movements were up to 2.5 mm in the past. We developed a fiber Bragg grating (FBG)-based automatic monitoring system for the simultaneous measurement of concrete segment joints. Using our fiber-optic calibration facility, several commercial FBG-based strain transducers were tested. The facility and its capabilities are described. For the investigated sensors, nonlinearity and hysteresis effects of more than 20 μm/m were found, which is much more than expected. The results clearly show the need for standards in fiber-optic sensing where minimum requirements for sensor specifications should be defined. In an extended field trial, several appropriate sensors were installed in a hydro power dam. First results show an increased number of load cycles and smaller movements for the dam in pumping mode. With the new system, these small deformations can be detected, and it will be investigated in future how they contribute to the aging of the structure.

Deformation Measurement of a Driven Pile Using Distributed Fibre-optic Sensing

Abstract New developments in distributed fibre-optic sensing allow the measurement of strain with a very high precision of about 1u2009µmu2009/u2009m and a spatial resolution of 10u2009millimetres or even better. Thus, novel applications in several scientific fields may be realised, e.u2009g. in structural monitoring or soil and rock mechanics. Especially due to the embedding capability of fibre-optic sensors, fibre-optic systems provide a valuable extension to classical geodetic measurement methods, which are limited to the surface in most cases. In this paper, we report about the application of an optical backscatter reflectometer for deformation measurements along a driven pile. In general, pile systems are used in civil engineering as an efficient and economic foundation of buildings and other structures. Especially the length of the piles is crucial for the final loading capacity. For optimization purposes, the interaction between the driven pile and the subsurface material is investigated using pile testing methods. In a field trial, we used a distributed fibre-optic sensing system for measuring the strain below the surface of an excavation pit in order to derive completely new information. Prior to the field trial, the fibre-optic sensor was investigated in the laboratory. In addition to the results of these lab studies, we briefly describe the critical process of field installation and show the most significant results from the field trial, where the pile was artificially loaded up to 800u2009kN. As far as we know, this is the first time that the strain is monitored along a driven pile with such a high spatial resolution.

Fibre optic monitoring system for the determination of segment joint movements inside a hydro power dam

In recent years hydro power stations were rebuilt to more efficient pumped storage power stations. This changed the loading conditions and new monitoring concepts are required. We developed a FBG based automatic monitoring system for the simultaneous measurement of concrete segment joints. With a fibre optic testing facility sensor nonlinearity and hysteresis effects were investigated. Using selected sensors the requested accuracy of 0.02 mm for deformations up to 3 mm can be achieved. Several sensors were installed in a hydro power dam. First results show an increased number of load cycles and smaller movements for the dam in pumping mode.

Performance assessment of geotechnical structural elements using distributed fiber optic sensing

Geotechnical structural elements are used to underpin heavy structures or to stabilize slopes and embankments. The bearing capacity of these components is usually verified by geotechnical load tests. It is state of the art to measure the resulting deformations with electronic sensors at the surface and therefore, the load distribution along the objects cannot be determined. This paper reports about distributed strain measurements with an optical backscatter reflectometer along geotechnical elements. In addition to the installation of the optical fiber in harsh field conditions, results of investigations of the fiber optic system in the laboratory and the most significant results of the field trials are presented.

Distributed fibre optic strain measurements on a driven pile

In civil engineering pile systems are used in unstable areas as a foundation of buildings or other structures. Among other parameters, the load capacity of the piles depends on their length. A better understanding of the mechanism of load-transfer to the soil would allow selective optimisation of the system. Thereby, the strain variations along the loaded pile are of major interest. In this paper, we report about a field trial using an optical backscatter reflectometer for distributed fibre-optic strain measurements along a driven pile. The most significant results gathered in a field trial with artificial pile loadings are presented. Calibration results show the performance of the fibre-optic system with variations in the strain-optic coefficient.

Field examples for optical fibre sensor condition diagnostics based on distributed fibre optic strain sensing

Fibre optic sensors for monitoring in safety-relevant structures have to be validated in order to proof their reliability under typical structural load conditions. The reliable use of optical fibre sensors depends strongly on an appropriate and qualitative application. Diagnostics of the physical condition of embedded and surface-applied fibre optic strain sensors are demonstrated on field examples. Distributed strain measurement based on Rayleigh backscattering is used to determine breakage of the fibre, interface adhesion problems and to identify application related strain transfer mechanisms.