1. Landslide Monitoring by Optical Fiber Sensor SUGIMOTO, Hiroyuki Erosion and Sediment Control Research Group Public Works Research Institute 1.Principle 2.Basic Experiment 3.Field Experiment Contents July / 2001

2. What’s Optical Fiber Sensor ? Refractive indices : n 1 > n 2 Incident light is reflected at the boundary between core and cladding. Basic structure of fiber for telecommunication and for sensor are the same. Core, n 1 Cladding, n 2 Coating

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

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

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

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

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

8. 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. Wavelength Intensity Incident light Reflected light Transmitted light Wavelength Reflection Wavelength Transmission Periodic modulation of index of refraction Fiber core

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

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

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

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

13. Experimental banking Deformation

14. Deformation of Optical Fiber Sensor Optical Fiber Sensor Deformation

15. Strain Distribution Deformation SandGravel -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 43m42m41m40m39m38m37m36m35m34m Distance from instrument (m) Strain (%) 20mm 40mm 60mm 80mm 100mm 120mm 140mm 160mm 180mm 200mm Deformation

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

17. FBG Comparison with Strain Gage 0 50 100 150 200 250 300 05001,0001,5002,0002,5003,000 Strain （ μ ） Load （ kN ） FBG type strain meter Strain gauge type strain meter

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 Sensor for landslide movement (B-OTDR) Three anchors (FBG)

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

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

23. Anchor Tendon (Length : 40m)

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

25. Finished Thank you for your attention.