APPLICATION OF NEW LANDSLIDE MONITORING TECHNIQUE USING OPTICAL FIBER SENSOR AT TAKISAKA LANDSLIDE, JAPAN K. Higuchi 1 , K. Fujisawa 1 , K. Asai 1 , A. Pasuto 2 , & G. Marcato 2 , Society for study of Optical Fiber Sensors 3 1 Public Works Research Institute, Japan (e-mail: fujisawa@pwri. go. jp) 2 Istitute di Recerca per la Protezione Idrogeologica, Consiglio Nazionale delle Ricerche, Italy (e-mail: alessandro. pasuto@irpi. pd. cnr. it) Abstract: A joint research project on new techniques of landslide monitoring is being carried out between the Public Works Research Institute in Japan and Institute di Recerca per la Protezione Idrogeologica, Consiglio Nazionale delle Ricerche in Italy. As one of the new techniques, an optical fiber sensing system was installed in the Takisaka Landslide, which was selected as a test site in Japan. The Takisaka Landslide is located in Fukushima Prefecture in eastern Japan, at north latitude 37° 38 – 39’ and east longitude 139° 38 – 39’. It is 2,100 m long (south-north direction) and 1,300 m wide (east-west direction). The volume of the moving soil mass is 4.8×10 7 m 3 , and the maximum depth of the moving soil mass is about 140 m. The optical fiber sensing system has 18 sensors and was installed at the end of the Takisaka Landslide facing the Aga River. Of the various methods of optical fiber sensing, the OTDR (Optical Time Domain Reflectometry) method was selected at this site. The sensor is a mechanical device in which part of an optical fiber bends in response to landslide displacement. Several sensors are installed along the optical fiber measurement line, and the OTDR detector detects the transmission loss of the light caused by bending of the optical fiber, at the locations of several sensors simultaneously. The landslide displacement is calculated from the change of transmission loss. Measurement was started manually in December 2004, but is now controlled automatically by a computer. As a result of measurement, tensile displacement was detected; this is similar to the tensile displacement of an adjacent extensometer, but an error of several mm is also observed. Therefore, ways of improving the measurement precision are now being studied. INTRODUCTION Electrical sensors are commonly used for monitoring devices of landslides such as extensometers, although they are easily damaged by lightning and electromagnetic noise. In contrast, optical fibers are free from such flaws, and devices using optical fibers have been developed in recent years. The sensors basically work by detecting changes in transmission light due to bending or warping of the optical fiber when the ground moves. In recent years, the Public Works Research Institute (PWRI) in Japan has been developing a sensor to detect landslide displacement using the OTDR method. The applicability of the developed sensor was tested in the Takisaka Landslide area, as reported in this paper. This study was conducted under the Joint Laboratory on “Research, Training and Documentation on Figure 1. Logo 1074