Preliminary study on the 2003 slope failure in Woo-wan-chai Area, Mt. Ali Road, Taiwan

Mt. Ali Road (Province Road 18) is a primary access to Alishan National Scenic Area and Yushan National Park of Taiwan. Woo-wan-chai, an old landslide area, covers an area of about 50 ha and locates at a mileage of about 30 km along the road. On June 26, 2003, a major slope failure occurred in the area with estimated volume of about 50,000 m3 and resulted in a serious damage to the road and the local traffic. The paper herein discusses results of preliminary study of the failure based upon observations of the in-situ ground monitoring data. The study found appreciably increases in the groundwater level and the slope deformation during the intense rainfalls of June which lasted approximately 3 weeks prior to the major slide. The triggering factor of the failure appeared to be the attainment of a critical groundwater condition in the slope. Stability analysis results confirmed a marginal stability of the slope with the groundwater condition and material strength properties at the time the slide started. Historic monitoring data of the site had shown delayed relationships existed among the precipitation, groundwater level and flow, and slope deformation. The significant surge in groundwater level prior to the failure was thought to be in relation to some other water sources as well as the on-site precipitation.