In‐situ assessment of strain behaviour inside tunnel linings using distributed fibre optic sensors

Abstract

In modern tunnelling, deformation monitoring is an important component to ensure a safe construction. It is state of the art to measure displacements at the inner side of the tunnel lining using total stations. In addition, pointwise geotechnical sensors, e.g. electric strain gauges, may be installed in geological fault zones, which, however, do not deliver a complete picture of the internal deformations. The Institute of Engineering Geodesy and Measurement Systems (Graz University of Technology) supported by the Austrian Federal Railways (ÖBB‐Infrastruktur AG, SAE Fachbereich Bautechnik/Tunnelbau) developed a fibre optic sensing system, which realizes thousands of measurement points inside the tunnel lining. The distributed measurements can be used to assess the in‐situ strain behaviour as well as to localize failures (e.g. cracks) in the lining. This paper reports about the calibration of the fibre optic system under well‐known laboratory conditions and the practical utilization of the system in mechanized and conventional tunnelling. The results demonstrate the high potential of distributed fibre optic systems and their capability especially in the operational phase to extend classical measurement methods in tunnelling projects.

150 years reliable railway tunnels – Extending the hybrid method for the long‐term safety assessment

“…However, this time it will not be the week-to-months-long creep and stress redistribution phases during the construction phase, but the years-to-decades-long creep and redistribution phases during the operation phases. Several observations point towards the relevance of such (very) long time effects: Concrete is known to creep logarithmically over decades and centuries, without reaching any asymptote [31], and convergence measurements performed in tunnels with a pronouncedly rheological behaviour clearly show similar trends as well [13] (Fig. 5).…”

“…At the same time, displacement measurements at the surface of linings were complemented by strain measurements inside the linings, e.g. by means of embedded strain gauges or fibre-optical sensors [13,14]. Such methods provide quantitative access to strain distributions across a dense grid of up to thousands of measurement points spanning both the circumferential and longitudinal directions of a tunnel (Fig.…”

Distributed fiber optic shape sensing along shotcrete tunnel linings: Methodology, field applications, and monitoring results

“…5) to assess the structural integrity of individual construction parts and, finally, to increase the work safety on-site. These installations include monitoring of conventional tunnel cross-sections [19,28] and shaft linings [13] (SBT 1.1), reinforced earth structures [20] (SBT 2.1) as well as pipelines [11] (SBT 3.1).…”

“…Although the sensing cable installation procedure is critical due to the harsh tunnel environment, DFOS have already been successfully implemented inside shotcrete tunnel linings. These existing installations are mainly focused on investigations of mechanical stress as a result of creepage, shrinkage, and/or rock pressure [9,19,28], as well as convergence analysis [4,10], but do not deliver concepts for fully distributed shape analysis along the lining. This paper introduces a distributed fiber optic shape sensing and evaluation approach, which utilizes DFOS strain measurements along different sensing layers in combination with pointwise displacement readings for fully distributed shape assessment along curved structures, such as tunnels.…”

Distributed Fiber Optic Shape Sensing of Concrete Structures

“…The fiber optic connectors are stored in connection boxes for protection during the concreting process. Inside the tunnel, the individual concrete segments can be connected to one continuous sensing loop to evaluate the ring’s overall behavior [ 59 ].…”