Stability and movement characteristics of isolated boulders under delayed cyclic rainfall: an experimental and analytical study

Instability of isolated boulders during rainfall poses a major geological hazard, yet predicting it remains challenging due to delayed infiltration effects. This study integrates theoretical modeling, laboratory experiments, and numerical simulations to systematically analyze stability evolution and post-failure motion under cyclic rainfall. A dynamic safety factor model is innovatively proposed to quantify the time-dependent weakening of soil-rock interface strength during rainfall and lag phases. Experimental results across various rainfall intensities (33.7–104.24 mm/h) and slope angles (30° and 60°) identify a four-stage instability progression: stable → critically stable → unstable → collapse. Rainfall intensity and infiltration depth are primary controlling factors, with slope angle secondary. Notably, boulders located near the slope crest exhibited a markedly higher tendency toward instability than those positioned at the slope toe under identical rainfall-lag conditions. Motion simulations reveal boulder mass and slope angle dominate trajectories, with impact energy exceeding 6.9 × 107 J. The coupled experimental-simulation approach provides a systematic framework for hazard assessment and critical insights for early warning systems and protective design in mountainous regions.