Performance assessment of geotechnical structural elements using distributed fiber optic sensing

In civil engineering, geotechnical structural elements like micro piles, anchors or rock nails are used to underpin heavy structures or to stabilize slopes and embankments. On large construction sites hundreds of these components may be installed to provide a suitable and safe subsurface. Geotechnical load tests are usually performed on a number of these elements to optimize their length and to proof their bearing capacity. Currently, the deformations during these investigations are mostly measured with electronic sensors at the surface. However, these measurements at the top of the component do not deliver information about the load distribution along the object. IGMS developed a distributed fiber optic sensing system to monitor the strain distribution of structural elements during geotechnical tests. The designed setup enables strain measurements with a very high precision of about 1µm/m every 10 millimeters or even better. In cooperation with Keller Grundbau Austria GmbH, various load tests of driven piles and anchor pull-out tests are already carried out in order to assess the suitability of the system for geotechnical applications. Test results show that the developed fiber optic system delivers completely new information, which cannot be measured using other techniques. Due to the high spatial resolution of the system, cracks in the grout material of the structural elements can be detected and the progressive failure of the interface between the grout and the object, e.g. anchor or pile, could be observed. Current research is focused on the interaction between the grout material and the optical fiber. In addition, further field tests with the fiber optic measurement system in different soils are planned to generalize the gained knowledge and to provide a powerful tool for the testing of geotechnical structural elements in the future.

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Acknowledgements
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This project is funded by the Austrian Research Promotion Agency (FFG), project nr. 858505.
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