本研究旨在探討台南市市道172線42K+300處邊坡於不同降雨強度與地震載重條件下之三維邊坡穩定性反應。研究以鑽探岩心資料建構擬真三維地質模型，使GMS (Groundwater Modeling System)建立地層分佈並設定2024年2月5日之地下水位面為初始條件，後續匯入FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions)，進行不飽和土壤之降雨入滲與地震動態模擬。
在降雨模擬部分，設定四種日降雨強度情境(80、200、350、500 mm/day)，透過FISH語言編寫飽和-不飽和滲流模型，考量基質吸力調整不飽和區之滲透係數與有效應力場，以模擬真實降雨入滲行為。模擬結果顯示，隨降雨強度增加，地層飽和度與孔隙水壓上升，導致有效應力降低，安全係數呈現遞減趨勢，滑動潛勢由淺層擴展至深層，反映降雨入滲為誘發滑動之關鍵因素。空間分布上，其中以N塊體邊坡最明顯，安全係數顯著低於其他區段；相對地，S、M與L塊體區域穩定性良好，滑動潛勢較低。
地震模擬則以2025年1月21日臺南楠西地震(M_L=6.4)為討論重點，採用沄水國小測站(C087)所測之三向地表加速度歷時資料，模擬近場之真實地震事件。模擬採用地震歷時中之十秒資料進行逐秒分析，結果顯示各塊體對地震載重之反應存在差異，其中S、M與L塊體雖於模擬中觀察到局部剪應變率變化與應力重分布，但整體安全係數維持穩定，顯示其崩塌潛勢較低；相對而言，N塊體在模擬期間安全係數明顯下降，顯示其對地震擾動敏感，為本區潛在滑動風險最高之區段。

This study presents a three-dimensional slope stability assessment of the slope located at 42K+300 along County Road 172 in Tainan City under various rainfall intensities and seismic loading conditions, utilizing the strength reduction method. A realistic subsurface geological model and corresponding groundwater table were constructed using the Groundwater Modeling System (GMS), and subsequently imported into FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) to perform coupled simulations of rainfall infiltration and seismic response in unsaturated soil conditions.
Simulation results under rainfall conditions indicate that increasing rainfall intensity leads to elevated saturation levels and pore water pressure, which in turn reduce effective stress and significantly lower the slope’s safety factor. This promotes the deepening of potential sliding surfaces and highlights rainfall infiltration as a critical triggering mechanism for slope instability. Under seismic loading, slope blocks exhibit differential responses; while S, M, and L blocks show localized changes in shear strain and stress redistribution, their overall safety factors remain relatively stable, suggesting limited failure potential. Conversely, the N block demonstrates a marked decline in safety factor throughout the simulation period, underscoring its heightened susceptibility to failure under both rainfall and seismic influences.

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