Managing the preventive maintenance of slopes along national expressways is a fundamentally important issue. A rational and effective management system, including monitoring methods, is required. The West Nippon Expressway Company gives priority to potentially unstable slopes, while considering geographical and geological features. It continuously monitors deformations, such as rises in underground water levels or rainfall, for the design of countermeasures or mitigation to prevent slope failures.

This paper describes a case study of monitoring the displacements of a cut slope, composed of pelitic schist, along an expressway over a period of a few years. The slope had been unstable due to localized torrential rain. A GPS slope movement monitoring system continuously monitored the slope displacements at several measurement points, and it was able to detect the initial signs and processes leading up to slope failure. When the velocity of the displacements measured by GPS exceeded the criteria for assessing stability, counterweight fill work was conducted to increase stability. The three-dimensional displacements continuously monitored by GPS provided useful data for estimating the extent of the area with large deformations caused by the rainfall. Then, cut-and-removal work at the top of the slope was designed as a permanent measure for the stabilization of the slope. At the same time, it was decided that the expressway would be temporarily closed to traffic based on the measurement results by GPS. It was proven that GPS slope movement monitoring is effective for managing the preventive maintenance of expressways.

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