<p>Landfilled waste exhibits strain-softening behavior at large deformations due to fiber failure and fluid-like transitions, complicating the identification of trigger mechanisms for landfill flowslides. This study presents an advanced numerical approach to address strain-softening and state transitions in landfill flowslide disasters. A solid-viscous constitutive model, which decomposes effective stress into solid and viscous stress components, is introduced. The solid-liquid interaction is modeled through coupled displacement-pore pressure formulations. Validation of the proposed model extends beyond prior field data to include ring shear tests, flowslide model tests and centrifuge experiments, ensuring robust assessment of viscous stress and the solid-viscous constitutive formulation. Two case studies involving heavy rainfall and high leachate level demonstrate the approach’s advantages. Under heavy rainfall, failure initiates above the wetting front due to the dissipation of matrix suction and weakening fiber embedment. In contrast, high leachate level-induced flowslides exhibit deeper failure surfaces and longer sliding distances. Complete slope failure occurs when the unsaturated waste reaches its peak strength, and the failure surface fully penetrates. These findings provide critical insights into the mechanisms driving landfill flowslides and offer guidance for risk mitigation strategies in waste management practices.&#xa0;</p>