The 2009 Jiweishan rock avalanche, Wulong, China: deposit characteristics and implications for its fragmentation

In this study, field investigation and numerical modeling using Particle Flow Code (PFC) were conducted to investigate deposit characteristics and their implications for the fragmentation mechanisms of the 2009 Jiweishan rock avalanche in Wulong, China. The results show that average grain-size distribution of the debris diminishes both from the proximal to the distal end and from the surface to the bottom of the deposit. A carapace of large boulders was formed at the surface and passively carried by the underlying finely fragmented debris during runout. Stratigraphic preservation, directly aligned boulders on the surface, and jigsaw fracture patterns exclude fragmentation mechanisms involving collision. Fragmentation induced by normal stress also cannot explain the progressive reduction in grain size of the deposit from the proximal to the distal end. Loose soil on the transport path was bulldozed to the very front and pushed passively by the rock debris, rather than remaining at the bottom and reducing frictional resistance between the debris and the ground. Thus, the undrained-loading hypothesis widely used to explain the long-runout of rock avalanches is not applicable. Shear-induced fragmentation is the most probable mechanism that can adequately explain the characteristics of the rock avalanche deposit.