A study of the pro-water pressure for initiation of a large landslide triggered by a strong earthquake based on fluid-structure coupling

The Daguangbao landslide is the largest co-seismic landslide with a volume of approximately 12×10⁸ m³, which triggered by the Wenchuan earthquake (Ms 8.0) occurred on May 12, 2008. The slide zone is located in the Paleozoic carbonate strata and it is an interlayer structural dislocation zone with a depth of 400 m and a maximum thickness of 5 m inside the slope. A previous study concluded that the shear failure of the Daguangbao landslide developed within the bedding fault. Our latest investigation showed that this bedding fault was under the water table, and its rock mass was saturated before the landslide. To reveal the initiation mechanism of the Daguangbao landslide related to the groundwater, a simplified model of a hard carbonate slope with an inter-weak layer is given. The response characteristics of the model are stimulated by a fluid-structure coupling algorithm using the FLAC^3D. The results show that significant differences in deformation patterns during the earthquake, such as response time, wave shape, and amplitude exist between the upper and lower hard layers. Three uncoordinated deformation patterns, i.e., tension, compress, and shearing coming from the upper layer to the weak layer, are identified. The deformation pattern results in the amplification of compress and shear forces within the weak layer. The stress amplification subsequently induces excess pore water pressure with instant amplification and accumulation features. It is reasonably inferred that the excess pore water pressure essentially reduces the effective stress of the bedding fault. Our simulation confirms that the excess pore-water pressure can be generated within a saturated bedding fault deep situated in a slope during a strong earthquake. We propose that the excess pore water pressure within the saturated fault at 400 m underground is the cause of the Daguangbao landslide.