BASIC CHARACTERISTICS AND FORMATION MECHANISM OF |THE LARGEST SCALE LANDSLIDE AT DAGUNGBAO OCCURRED DURING THE WENCHUAN EARTHQUAKE

The landslide at Daguangbao and Hongdongzigou site is the largest among all landslides occurred during the Wenchuan Earthquake. Its volume is up to 742 Mm3, and its debris dam height reaches 690 m. So it is also the most largest scale landslide triggered by earthquake among all investigated landslides in China. Additionally, it has the highest debris dam originating from landslide in China. It is even one of a few super large-scale landslides known all over the world, whose individual volume is over 500 Mm3. The debris dam formed by this landslide can be the highest around the world. The landslide is located on the upper plate of the Causative Fault. It is nearly 7 km away from the triggering seismic fracture of Yingxiu-beichuan fault. The slope was an isolated ridge before the Earthquake. it's the ridge had its 3 sides cut. Its height difference reaches 1500 m. The strike of clinothen was almost vertical to the slope surface. the clinothen had perfect extension. They composed together the basic conditions of sliding surface formation. Our investigation and analysis show that the free surface conditions of the slope and the surface connectivity of limestone bedding plane were the base of landslide formation. And the ground movement resulted from the intensive quaking with large intensity and long duration was the radical triggering factor of the landslide. The mechanism and process of the landslide could be divided into 4 stages as follows: (a) the stage of slope body shatter, relaxation and disintegration, (b) the stage of high speed sliding, 9c) the stage of accumulation with shock, and (d) the stage of the second projectile and debris flow deposition. Because of destabilization and high speed sliding down, the slope mass composed a deposit body along main direction of sliding, 4.2 km long and 2.2 km wide. High-speed clastic flow crossed downstream Fengboyan, and formed a deposit zone of debris flow along Hongdongzigou, 1 km long.