Slope stabilization and landslide size on Mt. 99 Peaks after Chichi Earthquake in Taiwan

The study focuses on the landslide characteristics of Mt. 99 Peaks in Nantou County, the most serious landslide prone area caused by Chichi Earthquake in Taiwan. Several investigations and field surveys were made on Mt. 99 Peaks for 5 years to research the landslide area and depth, rainfall trend, and slope stabilization. The total landslide volume caused by the earthquake on Mt. 99 Peaks was about 1.47×10⁶ m³ and the mean landslide thickness was about 0.22 m. Gravel layers with a volume of more than 80% of total soil profile dominated Mt. 99 Peaks. The landslide on Mt. 99 Peaks was induced by heavy rainfall from July to September because the rainfall on Mt. 99 Peaks had a nonuniform distribution in time. Although the vegetation recovery on Mt. 99 Peaks was in progress, the soil slope had remained unstable. As a result, Typhoon Mindulle occurred in July 2004 collapsed the hillslope again after 5 years of Chichi Earthquake. This study suggests that vegetation recovery on Mt. 99 Peaks for 5 years was insufficient to stabilize the landslide affected area.