GPS is now being widely used for monitoring rock displacements, and it has been a useful tool for various rock engineering projects. The most important technical issue for the practical use of GPS in monitoring displacements is to perform real-time and precise monitoring even under adverse observation conditions, i.e., steep slopes, the existence of trees and walls above/around the sensors, tropospheric delays, weather conditions, etc.

In this research, the GPS displacement monitoring system developed by the authors is applied to assess the stability of an unstable steep slope along a national road in Japan. Since local slope failures have occurred in the slope several times over the last 20 years, displacement monitoring has been conducted by borehole inclinometers and surface extensometers. Some of the instruments, however, have occasionally not worked due to large deformations, and it has become difficult to perform the monitoring continuously. In order to overcome such trouble, the GPS monitoring system has been applied. The monitoring procedure is based on the ISRM suggested method.

It has been confirmed that three-dimensional displacements could be continuously measured beyond the accuracy of conventional GPS monitoring. Moreover, the GPS monitoring was available even under conditions of heavy rain, with 30–45 mm/hour.

The GPS monitoring results provided useful data for assessing the stability of the slope, and effective information for managing traffic safety.

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