Recent years have seen reports of rock collapses and rock-falls from steep slopes, even if such points have not been extracted as those requiring inspection. In the maintenance of steep slopes, it is important to confirm the presence of rock-falls and source rocks corresponding to the outlined survey; however, it can be difficult to investigate from the target structure to the vicinity of the ridge because of cost concerns and vegetation overgrowth. It is important to obtain detailed geographical information for an efficient overview survey; therefore, laser measurement technologies with significant performance improvements have been used in the field of slope disasters including public surveys and the study of landslides, and it is expected that the utility of these technologies will further increase.

Herein, multiple combinations of airborne and ground-based laser measurement technologies were used to study steep slopes facing a river. Fluctuation analysis was performed using three-dimensional (3D) point cloud data obtained via laser measurements. As a result, airborne laser measurements deciphered 1.5-m high scarplets. Furthermore, by conducting displacement vector analysis, we grasped the tendency of fluctuation that gradually increased from the mountain to the river. In addition, for the overhanging blocks closest to the river, we detected an average variation of 141 mm via displacement vector analysis using a terrestrial laser scanner (TLS) with a 2-m mesh. We report the evaluated behavior of steep slopes depending on the type of topography and geology and the effectiveness of the technology as a periodic measurement method for maintenance purposes.

1 Introduction

Japan has experienced frequent slope disasters caused by heavy rains and earthquakes in recent years. Although the geological conditions associated with slope disasters do not significantly change over short periods, statistics show that the probability of heavy rainfall over 80 mm/h has increased by 10% over the past 10 years (Japan Meteorological Agency 2017). In addition, large-scale earthquakes and epicentre earthquakes have been increasing.

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