Liquefaction within a bedding fault: Understanding the initiation and movement of the Daguangbao landslide triggered by the 2008 Wenchuan Earthquake (Ms=8.0)

The Daguangbao landslide was the most catastrophic mass movement triggered by the 2008 Wenchuan earthquake with a magnitude scale of Ms. 8.0. The landslide, which was 4.6 km long and 3.7 km wide, had a volume of approximately 1.2 x 10(9) m(3). Since its occurrence, many assumptions regarding its initiation and movement mechanisms have been made; however, the mechanisms remain unclear. Our recent field evidence suggested that the Daguangbao landslide occurred along a saturated fault parallel to the bedding in the Paleozoic carbonate strata. We therefore examined whether the shear behavior of the fault material could have favored the initiation and movement of the Daguangbao landslide. First, we performed monotonic and cyclic loading tests on samples taken from the bedding fault breccia using ring-shear apparatus. We then conducted Newmark displacement analysis to examine the initiation and motion of the landslide. The laboratory results showed that the carbonate fault breccia on the sliding layer of the landslide has a high liquefaction potential and the friction coefficient at its steady-state under undrained condition could be as small as 0.04. We also found that with increase of shear displacement, the friction coefficients can at first exponentially increase to the peak-failure value and then exponentially decrease to the steady-state value. These relationships between the friction and shear displacements were incorporated in the Newmark analysis of landslide initiation and motion. The numerical calculation results showed that the landslide occurred 36 s after the 2008 Wenchuan earthquake origin time (at its hypocenter), and the landslide mass, with a speed of 94 m/s, collided with a riverbank in the 76th second. We infer that, in addition to the strong seismic force, pore-water pressure built up within the bedding fault during the seismic shaking, enhancing the instability of the Daguangbao slope, and a further increase of pore-water pressure with progress of sliding elevated the mobility of the landslide.

Automated summary

The Daguangbao landslide, triggered by the 2008 Wenchuan earthquake, was a catastrophic mass movement with a volume of approximately 1.2 x 10(9) m(3). Laboratory tests revealed that the carbonate fault breccia on the sliding layer had a high liquefaction potential and low friction coefficient, which contributed to the initiation and movement of the landslide. Numerical calculations showed that the landslide occurred 36 seconds after the earthquake and collided with a riverbank in the 76th second, indicating that pore-water pressure during seismic shaking intensified the instability and mobility of the slope.