蘭台地區降雨引致地下水位上升對邊坡穩定之影響

Abstract

臺灣之地理位置特殊，山脈高聳而降雨豐沛，每逢夏季總有颱風頻繁侵擾，颱風挾帶之強降雨常導致山坡地發生崩塌與土石流等災害，危及生命和財產之安全。 本研究以蘭台大規模崩塌潛勢區為研究區域，整理2019至2022年之降雨資料，並根據中央氣象局有發布警報之颱風紀錄，篩選出於此區域帶來明顯降雨之颱風事件，以回歸方式探討降雨量對蘭台地區地下水位變化之關係，同時蒐集彙整現地之地表及地中監測資料，比對颱風事件時間，選定有造成邊坡滑動之三筆颱風案例，結合蒐集之材料參數及實驗室室內試驗結果，利用數值分析程式SEEP/W及SLOPE/W模擬三個颱風案例於蘭台地區主崩區之降雨造成之地下水位變化以及對邊坡穩定之影響。 研究結果顯示，依據李明熹 (2006) 土石流降雨警戒指標中之降雨參數-總有效累積雨量與此區域之地下水位變化呈現高度正相關，滲流分析結果雖無法完整反應不飽和區域之滲流情形，但經過扣除地下水位延遲之時間調整後，亦可推測出邊坡安全係數接近1之時刻，將上述之時刻與現地監測資料比對，驗證數值分析結果之合理與代表性；將邊坡穩定分析得到之滑動面與鑽孔破碎帶位置進行比對，可對應到相對破碎之區段。根據三筆案例之分析結果，此區域之臨界滑動地下水位約位於地下20.22m至地下20.04m間。

Taiwan's unique geographical features, characterized by high mountain ranges and abundant rainfall, make it highly susceptible to typhoons, particularly during the summer months. The intense rainfall brought by these typhoons frequently triggers landslides and debris flows, posing significant risks to lives and properties. This study focuses on the Lantai area, a region prone to large-scale landslides. Rainfall data from 2019 to 2022 were compiled and significant typhoon events were identified using records from the Central Weather Bureau (CWB). Through regression analysis, the correlation between rainfall and groundwater level fluctuations in Lantai was examined. Additionally, ground surface monitoring data were gathered and three typhoon cases that displayed landslides movement were selected. The numerical analysis programs SEEP/W and SLOPE/W were used to simulate the changes in groundwater level caused by rainfall and the effects on slope stability in the Lantai study area for the identified events. The analysis incorporated collected material properties and parameters from laboratory tests. The results indicate a strong correlation between the total effective cumulative rainfall, as defined by Ming-Hsi Lee (2006), and groundwater level changes. Although seepage analysis could not fully capture the conditions in the unsaturated zone, the time variation in the factor of safety reduction was deduced by accounting for groundwater response delays. These findings were validated against on-site monitoring data. The sliding surfaces were compared to the borehole logging to validate the simulation results. The threshold groundwater levels for the Lantai area were found to range between 20.22m and 20.04m below ground surface.