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

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

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

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

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

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

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

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

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

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

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

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

Experimental banking Deformation

Deformation of Optical Fiber Sensor Optical Fiber Sensor Deformation

Strain Distribution Deformation SandGravel m42m41m40m39m38m37m36m35m34m Distance from instrument (m) Strain (%) 20mm 40mm 60mm 80mm 100mm 120mm 140mm 160mm 180mm 200mm Deformation

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

FBG Comparison with Strain Gage ,0001,5002,0002,5003,000 Strain （ μ ） Load （ kN ） FBG type strain meter Strain gauge type strain meter

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)

Okimi Landslide Test Site Landslide

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

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

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

Anchor Tendon (Length : 40m)

Anchor Body FBG type strain meter Strain gauge type strain meter

Finished Thank you for your attention.