The Geometric Characteristics and Initiation Mechanisms of the Earthquake-Triggered Daguangbao Landslide

The Daguangbao (DGB) landslide which triggered by the 2008 Wenchuan earthquake is one of the largest earthquake-triggered landslides in the world over the past century. In this study, remote sensing images analysis, field investigation, laboratory experiment and slope stability analysis were adopted to characterize this DGB landslide. We speculated the dominating structures of this landslide are folded bedding plane and a zigzag stepping-out joint system, which outcropped at the south and north of the landslide site, respectively. Accordingly, this landslide is a gigantic, atypical wedge failure. With the inferred slip planes, the intersection line is curved and counterclockwise rotated, which fit the trajectory of a mining tent pre and post this DGB landslide with a travel distance of 1.9 km. Meanwhile, the intersection line will be daylighted and rock mass shear-off on the toe of this landslide, as assumed by most of the researchers, is no more required. Based on the proposed wedge shape and DTMs, the volume of moving mass is about 10.51 × 10⁸ m³. The characteristics of the sliding surface on the south part of the landslide site were carefully investigated. The identified slip zone was composed of breccia and gouge layers of several centimeters thick. The intact dolomite rocks adjacent to the slip zone and the thin gouges were sampled. The peak friction coefficient of the tested dry dolomite discontinuities and wet gouges are 0.52–0.96 and 0.73–0.86 and the steady-state friction coefficients of the two samples are 0.1–0.57 and 0.16–0.63, respectively. The wedge analysis shows that the slope is quite stable (F.S. = 4.14). However, the gigantic wedge can be triggered by the Wenchuan earthquake based on the pseudo-static wedge stability analysis. Moreover, the friction coefficient of the gouges under large shear displacement will drop below 0.25 (~tan(14°); the intersection line plunged 14°) when the shear velocity exceeds 1.3 m/s. That is, the gigantic wedge can be speeded up by the inertial force generated by the earthquake and keep moving rapidly with long run-out. Based on a simple one dimensional particle motion model, the DGB landslide traveled 52–67 s with a maximum velocity of 57–74 m/s.