降雨入滲作用下非飽和邊坡之破壞行為分析 －以高雄六龜邊坡為例

Abstract

臺灣山區地形陡峭、地層破碎，且強降雨事件頻繁，導致邊坡災害潛勢極高，尤以崩積層引發之淺層滑動最為常見。然傳統穩定性分析多假設土壤為飽和狀態，未能合理考量非飽和條件下基質吸力消散所致之抗剪強度下降，亦忽略水力傳導係數與含水量之非線性關係，限制其模擬降雨誘發破壞的能力。 本研究以高雄六龜地區一實際崩塌案例為對象，建立具備三維地形與水力－力學耦合之數值模型，探討極端降雨條件下非飽和邊坡之破壞行為。模擬採用 PLAXIS 3D，結合 van Genuchten–Mualem 模型描述非飽和滲流行為，並以 Hardening Soil 模型處理崩積層力學性質。模擬過程考量現地地層配置與2024年凱米颱風降雨歷程，並輔以監測資料進行模型參數校正與驗證。 模擬結果成功重現滑動範圍與破壞時機點，顯示強降雨造成之水分自地表滲入，蓄積於崩積層與破碎硬頁岩交界處，因滲透率差異形成瞬時高飽和區與正孔隙水壓，導致有效應力驟降與剪應變集中，最終沿層間界面發生滑動破壞。 研究結果證實，三維之水力－力學耦合模擬可有效掌握非飽和條件下破壞機制，對邊坡潛勢區辨識、災後成因分析具實務工程應用價值。

Taiwan’s mountainous regions are characterized by steep topography, fractured geological formations, and frequent heavy rainfall events, resulting in a high susceptibility to slope hazards. Among these, shallow landslides triggered within colluvial deposits are particularly common. However, conventional slope stability analyses often assume fully saturated soil conditions, which fail to reasonably account for the reduction in shear strength caused by the dissipation of matric suction under unsaturated conditions. In addition, the nonlinear relationship between hydraulic conductivity and volumetric water content is typically neglected, limiting the ability to accurately simulate rainfall-induced failures.

This study investigates an actual landslide case in Liouguei District, Kaohsiung, and establishes a three-dimensional hydro-mechanical coupled numerical model to examine the failure behavior of unsaturated slopes under extreme rainfall conditions. The PLAXIS 3D software is employed for the simulation, integrating the van Genuchten–Mualem model to characterize unsaturated seepage behavior and applying the Hardening Soil model to represent the mechanical properties of the colluvium. The model setup considers the actual subsurface stratigraphy and the rainfall history associated with Typhoon Kompasu (2024), and is calibrated and validated using site monitoring data.

Simulation results successfully reproduce the observed sliding extent and failure timing. The analysis reveals that intense rainfall leads to downward infiltration and water accumulation at the interface between the colluvium and underlying fractured shale. Due to contrasting permeability between the two layers, a transient high-saturation zone and positive pore water pressure develop, resulting in a rapid drop in effective stress and localized shear strain concentration, ultimately inducing sliding along the interlayer boundary.

The findings confirm that a three-dimensional hydro-mechanical coupling approach provides effective insights into failure mechanisms under unsaturated conditions, and offers practical value for slope hazard zoning and post-failure forensic analysis.