FeatureOptical Fiber (2)
Effective utilization in efficient maintenance and management of infrastructure
as well as in the Business Continuity Plan (BCP)
Unlike conventional electrical sensors, optical-fiber sensors do not rust and can be used for a long time even in outdoor environments. If optical-fiber sensors are left installed in infrastructure for safe and high-quality construction management, they can also be used for efficient maintenance and rapid response according to the BCP.
Bridges
Having excellent long-term durability,
optical fiber can be useful for improving the efficiency of inspection work
and making swift BCP decisions regarding bridges in service.
Bridges are long, linear structures, and for their maintenance, it is important to detect all deformations that occur during their long service life. Optical-fiber sensors are durable and can measure strain distribution, and those features make them one of the most suitable methods for measuring such deformations. For example, in some cases, optical fiber-embedded pre-stressed-concrete (PC) cables installed during construction management are set up to continue measurements even while the bridge is in service. They can thus help to detect long-term deformations and deformations caused by earthquakes in a manner that contributes to bridge maintenance and BCP decisions.
Example of a bridge fitted with optical-fiber sensors
Example results of measurement of PC-cable tension force during construction management and service
Successful applications
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- Tsukidate Viaduct, Yoshino River Sunrise Bridge, etc.
List of published papers
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- X. Yamashita, et al., Measurement of PC Tension of Long-span Bridges by Distributed Optical Fiber Strain Sensors and Its Application to Health Monitoring,
76th Annual Meeting of Japan Society of Civil Engineers, 2021, VI-350. - X. Yamaguchi, et al., Health Monitoring System for PC Cables of Long Span Bridges Using Optical Fiber,
77th Annual Meeting of Japan Society of Civil Engineers, 2022, VI-824.
- X. Yamashita, et al., Measurement of PC Tension of Long-span Bridges by Distributed Optical Fiber Strain Sensors and Its Application to Health Monitoring,
Tunnels
Optical fiber makes it possible to monitor the long-term behavior of abnormalities
that cannot be confirmed visually.
As for tunnels currently in service, long-term deterioration of the ground around the tunnel, excavation of a new tunnel nearby, and alteration of the ground directly above the tunnel, for example, can cause damage in the tunnel such as heaving of the roadbed or deformation of the roadbed lining. However, it is difficult to predict the occurrence location and timing as well as the extent of such deformations, and it is difficult to visually confirm them until they become serious.
Optical-fiber measurement makes it possible to monitor such deformations over a long period of time and determine the location and extent of deformation at an early stage. In addition, by measuring the stress state of the tunnel structure, it is possible to make decisions based on actual conditions and consider rational countermeasures in terms of possible construction work.
Measuring ground heaving
(example of measuring underground displacement in a bullet-train tunnel)
- Determine not only the amount of uplift in the track bed but also the areas where major deformation has occurred
Example of measurement of strain occurring in tunnel lining
(results of loading test in a simulated tunnel)*
- High-precision measurement of strain on the lining surface when load is applied from the top of the tunnel
*This measurement case study was conducted under the “Research and Development for Productivity Improvement of Tunnel Maintenance and Management by Optical Fiber Measurement Technology Capable of Wide-Area and Comprehensive Monitoring” program funded by the Ministry of Land, Infrastructure, Transport and Tourism (JPJ000094) in FY2021-2022 with input from a committee of experts in the fields of tunnel engineering and measurement engineering.
List of published papers
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- X. Fukushima, et al., Measuring Underground Displacement using Optical Fibers for Mountain Tunnel Maintenance,
77th Annual Meeting of Japan Society of Civil Engineers, 2022, VI-789. - X. Kawabata, et al., Research on Improving Efficiency of Tunnel Maintenance Using Optical Fiber Measurement Technology (Part 1) Research Objectives and Implementation Tests on Linings and Inverts,
58th Geotechnical Engineering Research Conference, 11-3-2-05 - X. Miyajima, et al., Research on Efficiency of Tunnel Maintenance Management Using Optical Fiber Measurement Technology (Part 2) Study on Efficiency of Design for Countermeasures against Tunnel Deformation,
58th Geotechnical Engineering Research Conference, 11-3-2-04, Kajima Corporation
- X. Fukushima, et al., Measuring Underground Displacement using Optical Fibers for Mountain Tunnel Maintenance,
Introduction to measurement technology
Distributed optical-fiber sensors can even detect
the slightest changes in structures,
even if they occur at an unknown location.
Optical-fiber sensors analyze scattered light that occurs when light propagates through an optical fiber; therefore, they can comprehensively collect strain and temperature information over the entire length of the optical fiber. However, while the Brillouin scattered light that was previously used had a wide measurement range, its accuracy was insufficient, so its range of application was limited. Recently, a high-speed, high-precision measurement technology utilizing Rayleigh scattered light has been practically applied. As a result, it has rapidly expanded the applications range of infrastructure monitoring using optical-fiber sensors.
Features of optical-fiber sensors
Long distance (up to km)/high resolution (up to cm)
Rayleigh measurement is comparable to that of a strain gauge
Overview of optical-fiber measurement system
Scattered light generated in optical fiber
When light is incident on an optical fiber, several types of scattered light are generated. The types with the greatest optical power and next-greatest optical power are respectively Rayleigh scattering and Brillouin scattering.
A property of the scattered light is that it changes in response to strain and temperature in the optical fiber.
By analyzing these changes, optical-fiber sensors can calculate changes in strain and temperature.
*Rayleigh scattering: A scattering phenomenon caused by particles smaller than the wavelength of light
*Brillouin scattering: A scattering phenomenon caused by sound waves in optical fibers
Differences in the properties of Rayleigh-scattered light and Brillouin-scattered light
When strain in an optical fiber varies, the spectrum of both types of scattered light shifts in the frequency-axis direction.
Rayleigh-scattered light forms a finer scattered-light spectrum than that of Brillouin scattered light, so the former can detect a frequency shift even for slight changes in strain.
