Many large-scale slopes failed due to the heavy rainfall brought about by a typhoon in September of 2011 in a mountainous area of the Kii Peninsula, Japan. The colluvial deposit caused by the failures dammed up a major river and became unstable. Then, a debris flow occurred due to the erosion of the colluvial deposit.

A GPS automatic monitoring system has been installed on the colluvial deposit to assess the validity of disaster recovery works aimed at stabilizing the deposit. The system is composed of three segments, namely, the measurement segment, the server segment and the user segment. The data received from satellites are transmitted to the server computer via a mobile phone. The server computer analyzes the data automatically and provides the three-dimensional displacements of the measurement points to the client over the Internet. The accuracy of the measurements is 1–2 mm for horizontal displacements and 2–3 mm for vertical displacements, although the accuracy of a conventional GPS is 5–10 mm.

This paper describes the functions and the advantages of the GPS automatic monitoring system. The system was used to monitor displacements in a large-scale colluvial deposit due to slope failures brought about by heavy rainfall. The stability of the colluvial deposit is discussed on the basis of the measurement results, and disaster recovery works aimed at stabilizing the colluvial deposit were designed.

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