集集地震後，紅菜坪地區發生範圍涵蓋數百公頃之地層滑動，地 滑區崩積層最厚處達82 公尺。為瞭解紅菜坪地區滑動機制，紅菜坪 地區佈設了GPS、光纖感測器以及傳統傾斜計進行地層移動監測，並 裝設了開口式水壓計，利用人工以及自動紀錄水位指示器長期監測水壓變化。其中三處裝設於崩積層與岩盤交界面之水壓計，紀錄到暴雨期間孔隙水壓不昇反降之異常反應。本研究根據2005 年11 月1 日至2006 年11 月30 日降雨-孔隙水壓變化曲線，分別針對季節性孔隙水壓變化以及暴雨期間之反應進行分析與討論。就季節性變化而言，岩盤與崩積層交界面孔隙水壓乾季略降而濕季略昇，孔隙水壓變化不超過30 kPa，季節性孔隙水壓反應屬於單峰型至微變化型。降雨期間，孔隙水壓變化正常反應為隨降雨孔隙水壓逐漸上升，降雨減緩則孔隙水壓隨之下降，上邊坡崩積層厚度較薄者(30m 左右)，降雨孔隙水壓上升幅度較大(約10kPa)，邊坡中段崩積層厚度較厚者(60~80m 左右)，孔隙水壓較不受降雨影響而變化幅度較小(不到3kPa)，降雨孔隙水壓反應則有單峰對稱與非對稱型、多峰型與微變化型。除了降雨上昇雨後緩降正常反應之外，紅菜坪地滑區崩積層與岩盤交界面孔隙水壓會因降雨而發生孔隙水壓突然下降之異常反應。根據地表與地下位移觀測資料，推測此一降雨-孔隙壓力變化複雜之行為與崩積層受剪體積變化造成之超額孔隙壓力變化有關，降雨-孔隙壓力上升-變形-孔隙壓力下降之特性為紅菜坪地滑地為緩慢潛移的可能解釋。另外，本研究根據雨場分割結果，探討降雨特性與降雨-孔隙水壓異常反應之關聯性，結果發現當累積雨量超過150~200mm、降雨延時超過11hr 以及降雨強度超過20mm/hr 時，紅菜坪地區崩積層與岩盤交界面即可能產生驟降之異常反應。 The Hungtsaiping Landslide triggered by Chi-Chi earthquake covers an area of hundreds of hectors. The rock mass is covered by a thick colluvial that is as deep as 82m. To explore the sliding mechanisms, GPS, inclinometer and fiber optic displacement and water pressure sensing systems were used to monitor the status of the landslide. Among them, three piezometers installed in the interface of colluvial and bed rock automatically recorded lots of valuable data during July of 2005 to the end of 2006. Based on records of the pore pressure varied with rainfall, normal and abnormal responses were identified and discussed. The seasonal variation of pore pressure was less than 30 kPa. During the period of heavy rainfall, the variation of pore pressure was less than 5 kPa. Besides the normal response of pore pressure increased quickly after rainfall, abnormal response was observed which the pore pressure decreased during the period of heavy rainfall. In the same period of the pore pressure dropped suddenly, the surface and subsurface displacement was also observed. Accordingly, the decreasing of pore pressure could original from the dilation in the sliding surface which induced a negative excess pore pressure. The process of “rainfall - infiltration - water level raising – sliding – negative excess pore pressure develop” can be used to account for the the creep mechanism of Hungtsaiping Landslide.
