Although ground anchors are widely used in fields of civil engineering, as in soil and rock stabilization or anchoring structures, such as excavation pits, retaining walls or tunnel constructions, the load transfer behaviour has not been entirely investigated yet. Ground anchors are usually monitored by load cells at the anchor head or by strain gauges at selected points along the anchor, which both do not deliver reliable information along the entire anchor. Distributed strain sensing provides an opportunity to get the strain information along the entire length of the anchor, on the tendons as well as in the grout, as we have already shown in a preliminary anchor test. However, there were still lots of issues, which did not allow a general conclusion for all anchor types and ground conditions. Thus we investigated further ground anchors in different soil conditions (clay, rocklike material and gravel), within a research project. This paper gives an overview of the sensor systems used, the obtained results of the anchor pullout tests, gathered experiences and finally gives a brief concept of a monitoring anchor for long term monitoring.
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.
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.
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.
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.
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