The Global Positioning System (GPS) began to be used for displacement monitoring in the fields of Rock and Mining Engineering in the mid-1980s. Although it had the potential to measure displacements of the ground surface continuously over an extensive area, it was not applied much in practice. The reasons were uncertain accuracy, troublesome handling, and high costs at that time. In order to overcome those issues, an automatic displacement monitoring system using GPS was developed. The system can measure three-dimensional displacements automatically with mm accuracy; it is now being widely used in various rock and geotechnical engineering projects in Japan. The authors have applied the system to monitoring many slopes and have proved its validity through its practical use.

This paper describes three case studies of applications of the GPS displacement monitoring system to large landsides, namely, Choja, Shimekake, and Kokugawa. 1) Choja landslide began to occur due to the hard rain caused by a typhoon in 1963 and it still continues to move. The sliding area is 200 m in width and 900 m in length with an average slope angle of 15 degrees. The geology around the sliding plane is composed of serpentinite and slate. While the displacement was 2 m a year at the beginning, it has decreased to a few cm a year due to countermeasure works. Comparing the measurement results by GPS and by inclinometers and extensometers, the validity of GPS is discussed. 2) The sliding area of Shimekake landslide is 400 m in width and 700 m in length. Since a large displacement of 15 cm occurred in one day due to a thaw in 2009, real-time displacement monitoring has been conducted using GPS. The geology of the sliding plane is composed of tuff and mudstone. 3) Kokugawa landslide was brought about by a rapid thaw in 2012. The area of 200 m in width and 500 m in length was squeezed downward 250 m within a few days. The GPS displacement monitoring system was installed in the landslide area to observe the movement; it detected a displacement of 10 cm per hour and continuously measured the area until the landslide stopped.

Through these three case studies, it is proven that GPS is useful for the continuous and safe monitoring of large landslides over a long term.

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