<p>A weathered solution-collapse breccia slope forms from the weathering of solution-collapse breccia, which originates from the dissolution of underlying soluble bedrock, typically limestone or dolomite. Despite numerous prevention measures, these slopes often undergo repeated failures due to excavation and rainfall. The Songkan slope is a typical example, where two failures occurred despite extensive efforts in design and construction. This study investigates the causes of these failures through onsite investigations and monitoring. Additionally, a new FLAC3D-TOUGH hydromechanical coupling framework that considers rainfall-induced strength degradation is constructed to reveal the failure mechanism by examining the spatiotemporal evolution of pore pressure and effective stress, as well as changes in overall and local stability. These findings suggest that the primary intrinsic direct factor of both failures was the rainfall-induced degradation effect. In the first failure, the designers underestimated the sensitivity of weathered solution-collapse breccia to excavation-induced unloading and rainfall-induced degradation effects. In the second failure, a late repair of the site drainage system resulted in persistent water accumulation at the slope toe. Under the continued influence of the rainfall-induced degradation effect, the reduction in sliding resistance was not alleviated. Moreover, the delayed reinforcement failed to provide the necessary sliding resistance promptly, and its resistance was insufficient to prevent slope movement. This study revealed that both types of failure were related to identifying and controlling causative factors, timing, and quality control of slope drainage engineering. Consequently, relevant engineering insights have been integrated to provide substantial theoretical contributions and practical guidance for addressing similar geotechnical challenges.</p>