The Omarugawa Site is the pure pumped storage station which the Kyusyu Electric Power Company of Japan aims to start operating in July 2008. There are landslides recognized geographically from aerial photographs of the slope. The distribution of the landslide cannot be clearly defined from aerial photographs because they show some indistinct geomorphologic features. So as to show that the rock mass of the slope is estimated to be stable during operation, some detailed geological investigations were carried out in this area. The geographically recognized landslides are distinguished into 3 blocks depending on the weathering grade of the rock mass and the continuity of the deteriorated boring core. From the viewpoint of engineering geology, the conclusion was reached that there was no need for the engineering countermeasure to improve the stability of the rock mass in the upper reservoir after the reservoir fills with water because these landslides are located several 100m from the upper reservoir and are not be distributed inside the reservoir.
The Omarugawa Site is the pure pumped storage station which the Kyusyu Electric Power Company aims to start operating in July 2008, and is located at Miyazaki Prefecture, Kyusyu district, Japan (Fig 1). There are landslides recognized geographically from aerial photographs of the slope. The slope was called the "Iwaidani side", which is behind the right bank of the upper reservoir. Therefore the necessity to investigate the stability of the landslide material after the reservoir fills with water during the operation arose. The distribution of these landslides cannot be clearly defined from aerial photographs because they show some indistinct geomorphologic features. So as to analyze the stability of the rock mass of the slope, detailed geological investigations were carried out on this area.
