Landslide Monitoring by Optical Fiber Sensor

1. Landslide Monitoringby Optical Fiber Sensor SUGIMOTO, Hiroyuki Erosion and Sediment Control Research Group Public Works Research Institute Contents • Principle • Basic Experiment • Field Experiment July / 2001

2. Coating Cladding, n2 Core, n1 What’s Optical Fiber Sensor ? • Refractive indices : n1 > n2 • Incident light is reflected at the boundary between core and cladding . • Basic structure of fiber for telecommunication and for sensor are the same.

3. Why is Optical Fiber Sensor ? Features • Insensitive to electromagnetic interference • Durable and chemically inert • Low transmission loss • Distributed sensing

4. Measurand field M(z,t) Instrument Fiber M(t) M(z,t) z Distributed Sensing For example • B-OTDR（Brillouin Optical Time Domain Reflectometer) • Strain, Temperature

5. Measurand field M(z,t) Instrument Fiber M(t) M(zj,t) Sensitized regions z Quasi-distributed Sensing For example • FBG （Fiber Bragg Grating) • Strain, Temperature

6. Incident light (Pulse) Laser Transmitted light Detector Fiber core scattering light Back scattering light Principle of B-OTDR Distancez = tV / 2 t : two-way propagation delay time V : velocity of light in the fiber z

7. Back Scattering Light Incident light wavelength intensity Rayleigh scattering Brillouin scattering Raman scattering (Anti-stokes) Raman scattering (stokes) Wavelength Depending on temperature Depending on strain and temperature

8. Transmission Reflection Wavelength Wavelength Intensity Wavelength Principle of FBG • Wavelength of reflection peak is depend on the modulated period of index of refraction. • A large number of FBGs place on a single fiber to create an array of quasi-distributed sensors. Incident light Transmitted light Fiber core Reflected light Periodic modulation of index of refraction

9. Application • B-OTDR • Movement of landslide • Surveillance of river embankment • FBG • Stress of anchor, pile works • Movement of rock slope River embankment Landslide Rock slope

10. Sensor Work Office Landslide Sensor Sensor Road Movement of landslide • B-OTDR • Real-time Monitoring • Distributed Sensing (10～20km)

11. Anchor tendon Anchor Body FBG Stress of Anchor Works • Long-term behavior (maintenance) • Durable and chemically inert • Strain of anchor tendon, anchor body

12. Target for Measurement Extensometer Experimental banking chain block Fixed Point load cell Moving Plate Optical Fiber Sensor BOTDR Basic Experiment (B-OTDR) • Detecting deformation of banking • Move “Moving plate” (1cm step)

13. Experimental banking Deformation

14. Deformation of Optical Fiber Sensor Deformation Optical Fiber Sensor

15. 1.6 Deformation 1.4 20mm 1.2 40mm 1 60mm 80mm 0.8 Strain (%) 100mm 0.6 120mm 0.4 140mm 0.2 160mm 0 180mm -0.2 200mm 43m 42m 41m 40m 39m 38m 37m 36m 35m 34m Distance from instrument (m) Strain Distribution Deformation Gravel Sand

16. Basic Experiment (FBG) • Unconfined compression test Strain gauge type strain meter Load cell grout specimen (φ125mm×250mm) FBG type strain meter

17. 300 250 200 Load（kN） 150 100 50 0 0 500 1,000 1,500 2,000 2,500 3,000 Strain（μ） Strain gauge type strain meter FBG type strain meter FBG Comparison with Strain Gage

18. Field Experiment • Okimi Landslide (Niigata Prefecture) • Displacement : about 1 meter / year • B-OTDR • Displacement of the landslide • Fiber sensor : total about 2 km • FBG • Strain of anchor tendon, anchor body • Three anchors (length : about 40m)

19. Okimi Landslide Test Site Landslide

20. Fiber Sensor Setting Three anchors (FBG) Sensor for landslide movement (B-OTDR)

21. Fiber Sensor Setting Sensor for landslide movement (B-OTDR)

22. Fiber Sensor Setting Optical Fiber Sensor (depth : 50cm)

23. Anchor Tendon Anchor Tendon (Length : 40m)

24. Anchor Body Strain gauge type strain meter FBG type strain meter

25. Finished Thank you for your attention.