Heavy rainfall has long been recognized as a key factor in the triggering of landslides, as large amounts of precipitation generally have strong weakening effects on anti-sliding forces along slip surfaces. However, it remains to be understood why the catastrophic Xintan landslide in China was triggered by only moderate rainfall rather than the area's heaviest historical downpour. In this study, we demonstrate that a locked segment (a natural sustaining arch) at Jiangjiapo on the Xintan slope strongly restrained the movement of the sliding mass and was the key internal factor governing the slope stability. When such a locked segment is damaged enough to reach its volume-expansion point, slip displacement starts to accelerate. When its peak-stress point is attained, this displacement decelerates due to the homogenization effect of resistance. Once the homogenization process concludes, the displacement acceleration resumes, and slope instability occurs soon after reaching its residual-strength point. A post-factum analysis of the Xintan landslide indicated that the damage behavior of a locked segment from the volume-expansion point follows specific mechanical rules. The evolution of a locked-segment-dominated slope toward instability is caused by the progressive damage of the locked segment, which is promoted by various external factors including rainfall. Such factors can neither alter the inherent mechanical rules nor trigger a landslide before the locked segment fails. This mechanism explains the seemingly paradoxical relationship between precipitation and the occurrence of landslides, and these findings can greatly improve our understanding of landslide mechanisms.

Heavy rainfall has long been recognized as a key factor in the triggering of landslides, as large amounts of precipitation generally have strong weakening effects on anti-sliding forces along slip surfaces. However, it remains to be understood why the catastrophic Xintan landslide in China was triggered by only moderate rainfall rather than the area's heaviest historical downpour. In this study, we demonstrate that a locked segment (a natural sustaining arch) at Jiangjiapo on the Xintan slope strongly restrained the movement of the sliding mass and was the key internal factor governing the slope stability. When such a locked segment is damaged enough to reach its volume-expansion point, slip displacement starts to accelerate. When its peak-stress point is attained, this displacement decelerates due to the homogenization effect of resistance. Once the homogenization process concludes, the displacement acceleration resumes, and slope instability occurs soon after reaching its residual-strength point. A post-factum analysis of the Xintan landslide indicated that the damage behavior of a locked segment from the volume-expansion point follows specific mechanical rules. The evolution of a locked-segment-dominated slope toward instability is caused by the progressive damage of the locked segment, which is promoted by various external factors including rainfall. Such factors can neither alter the inherent mechanical rules nor trigger a landslide before the locked segment fails. This mechanism explains the seemingly paradoxical relationship between precipitation and the occurrence of landslides, and these findings can greatly improve our understanding of landslide mechanisms.