Christoph Monsberger

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 1 µm / m and a spatial resolution of 10 millimetres or even better. Thus, novel applications in several scientific fields may be realised, e. g. 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 800 kN. 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.

Monitoring of soil nailed slope stabilizations using distributed fiber optic sensing

Soil nailing systems are a common way to stabilize slopes and construction pits. Their design is usually based on the mechanical equilibrium of a rigid body motion and therefore, only tensile stresses are considered and accompanying forces like bending (shear stresses) in the soil nails are neglected. Continuous strain measurements along nails could verify this assumption, but may not be performed using conventional sensing technologies. This paper reports about monitoring of a soil nailed slope stabilization using distributed fiber optic sensing. Soil nails in different anchoring horizons were instrumented and autonomously monitored over several weeks, in which the construction pit was excavated continuously. After the excavation, the final load bearing capacity of one selected nail was determined within a classical geotechnical load test. In addition to the field measurements, the bending behavior of the instrumented nail system was analyzed under laboratory conditions. The presented studies demonstrate the high potential of distributed fiber optic sensing systems and their capability to extend traditional measurement methods in foundation engineering applications.

Performance assessment of a fiber Bragg grating sensor network inside a hydro power dam using optical backscatter reflectometry

In recent years, many hydro power plants were modified to pump storage operation. This changes the loading conditions and new monitoring concepts are required. We developed a fiber Bragg grating based monitoring system which was installed inside a hydro power dam in 2013. This paper reports on detailed investigations of this network using an optical backscatter reflectometer, which allows distributed strain sensing with a very high spatial resolution up to millimeters. Therefore, an analysis of the strain profile between two anchoring points of a FBG sensor can be performed. In addition to distributed sensing, the different wavelengths of the FBGs are determined in the frequency domain to verify the results of a classical FBG interrogator. These comparisons and further laboratory studies prove the suitability of the fiber optic system and demonstrate that a detailed analysis of FBG networks using optical backscatter reflectometry can provide valuable insights.

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.