Deformation monitoring and structural reliability assessment are key components in modern conventional tunneling. The state-of-the-art monitoring design is usually based on displacement measurements of geodetic targets using total stations paired with pointwise geotechnical sensors inside the tunnel lining. In recent years, distributed fiber optic sensing (DFOS) has become more popular in tunneling applications. DFOS measurements basically deliver internal strain and temperature distributions, but no direct relation to the tunnel shape’s behavior. This paper introduces a novel sensing and evaluation concept, which combines DFOS strain measurements and geodetic displacement readings for distributed shape assessment along curved structures, such as tunnel cross-sections. The designed system was implemented into shotcrete tunnel cross-sections as well as shaft linings and enables the determination of displacement profiles with high spatial resolution in the range of centimeters. Evaluations of continuous monitoring campaigns over several weeks as well as epoch-wise measurements performed by different DFOS sensing units in combination with stochastic analysis demonstrate the high potential of the developed approach and its capability to extend traditional monitoring methods in tunneling.

Risse und deren Veränderung sind wesentliche Indikatoren für Betonbauten. Konventionelle Risssensoren können nur bekannte Risse punktuell erfassen bzw. deren Weitenänderung bestimmen. In diesem Beitrag wird ein neues Rissmesssystem vorgestellt, welches mit nachträglich an der Betonoberfläche verklebten kostengünstigen Glasfasersensoren über weite Strecken Risse erfasst, ohne deren Lage zuvor zu kennen. Mit verteilten faseroptischen Messungen (engl. Distributed Fibre Optic Sensing „DFOS“) können bis zu einer Einzelmesslänge von 70,0 m neue Risse identifiziert, auf 3,5 cm verortet sowie deren Weitenänderungen bestimmt werden. Die erreichten Genauigkeiten betragen 0,035 mm (Labormaßstab) und 0,15 mm (reale Anwendungen). Mit DFOS werden somit Einzelrisse bei im Stahlbetonbau üblichen Rissabständen von 15 cm messbar, auch wenn keine direkte Sichtverbindung vorliegt. Es wird ein neu entwickelter Auswertealgorithmus vorgestellt, mit dem zusätzlich die historische Rissentwicklung erfasst wird. Bei Epochenmessungen kann mit diesem auf eine zwischenzeitlich aufgetretene maximale Rissbreite rückgeschlossen werden. Laborversuche und reale Messungen im Zuge einer Pilotanwendung von mehr als einem Jahr zeigen die Methodik sowie die Randbedingungen bei der Wahl der Faser und des Klebers. Gegenüberstellungen verschiedener Messsysteme ergeben eine erreichte Genauigkeit in ähnlicher Größenordnung wie konventionelle Risssensoren.

Civil structural health monitoring (CSHM) has become significantly more important within the last decades due to rapidly growing construction volume worldwide as well as aging infrastructure and longer service lifetimes of the structures. The utilization of distributed fiber optic sensing (DFOS) allows the assessment of strain and temperature distributions continuously along the installed sensing fiber and is widely used for testing of concrete structures to detect and quantify local deficiencies like cracks. Relations to the curvature and bending behavior are however mostly excluded. This paper presents a comprehensive study of different approaches for distributed fiber optic shape sensing of concrete structures. Different DFOS sensors and installation techniques were tested within load tests of concrete beams as well as real-scale tunnel lining segments, where the installations were interrogated using fully-distributed sensing units as well as by fiber Bragg grating interrogators. The results point out significant deviations between the capabilities of the different sensing systems, but demonstrate that DFOS can enable highly reliable shape sensing of concrete structures, if the system is appropriately designed depending on the CSHM application.

Predictive maintenance and safety assessment during the construction and operational phase are becoming more and more important in modern tunnelling. However, traditional measurement methods are often time-consuming, expensive and partially require an interruption of the tunnel traffic. In this article, we present a tunnel monitoring approach based on distributed fibre optic sensing (DFOS), which delivers hundreds of strain and temperature sensing points inside the structure and gives completely new information about the behaviour of the tunnel lining. Measurements can be performed automatically without the need of access to the tunnel and hence, monitoring can be made without disturbing the tunnel construction or operation. The developed system was installed within the shotcrete tunnel lining of a railway tunnel under construction in Austria. In addition to the critical installation process, this article discusses the main monitoring results and compares them to conventional measurements.

Deformation measurements of tunnel segments at a newly developed test rig Deformationsmessungen an Tübbingen mit einem neu entwickelten PrüfstandWith the use of tunnel boring machines under different geological and geotechnical conditions, the support with precast elements has been state of the art for decades. For the verification of the load-bearing and deformation behaviour of these high-strength reinforced concrete elements under exact known loading conditions, a test rig has been developed and implemented in a cooperation between experts of Austrian Federal Railways (ÖBB-Infrastruktur AG, SAE Fachbereich Bautechnik/Tunnelbau) and the Montanuniverität Leoben (Chair of Subsurface Engineering). The newly developed segment test rig allows biaxial tests on real segments. Due to the modular construction it is feasible to test segments with different geometries and dimensions. The obtained findings should be included in the optimization of these elements. Furthermore, the optimisation of the reinforcement design and replacement of steel bar reinforcement with fibre reinforcement is being investigated. With this test rig, an important step has been taken towards new technologies and materials in the field of segment linings. Currently, tests with distributed fibre optic sensing systems are carried out in cooperation with the Graz University of Technology (Institute of Engineering Geodesy and Measurement Systems -IGMS). A fibre optic sensing cable can be embedded in the segment during production and used for lifelong in-situ deformation measurements of the segment.