Assessment of Rainfall Induced Landslide Potential

由於降雨入滲使得孔隙水壓上升，可能引發邊坡不穩定，因此降雨引致崩塌與水文反應有密切的關係。然而，過去探討山區邊坡穩定分析大都是建立數值分析模式進行災害案例分析，缺乏針對研究崩塌地進行現地試驗、監測、分析，無法將實際水文地質參數整合於邊坡穩定分析模式。因此，為瞭解不同研究崩塌地進一步探討降雨引發坡地崩塌機制時，水文地質特性對崩塌地之影響，以及不同降雨特性包含降雨雨型、降雨延時、累積雨量對坡定穩定性影響；本研究於研究崩塌地設置監測儀器、觀測崩塌地地下水位、分析現地試驗參數，將現地監測資料整合於研究崩塌地降雨、入滲、地下水位變化乃至邊坡穩定性變化之分析模式。 本研究首先利用一系列現地水文地質試驗包括地表地電阻剖面影像調查、雙環入滲試驗、孔內攝影調查及封塞水力試驗，以獲得崩塌地之地層分布、滑動模式及水力參數等資訊來協助水文地質概念模式的建立。而後再進一步將水文地質參數及經由長期監測而得之水文紀錄資料，整合於GeoStudio程式，以進行降雨入滲、滲流及邊坡穩定暫態耦合分析(transient coupled analysis)。在水文地質參數率定過程中，建立滲流分析模式、穩定分析模式以及應力-應變分析模式，以探討崩塌地與水文地質之關聯性，以及瞭解水文地質特性對崩塌地之影響。 本研究將不同雨型、強度及累積量等降雨條件代入模式進行分析，探討崩塌地降雨特性與坡地穩定性間之關聯性。降雨雨型分析結果顯示，尖峰值最快達到之前峰式雨型對崩塌地穩定性影響最小，而尖峰值最後達到的後峰式雨型則對坡地的影響較大。降雨強度分析結果顯示，義興崩塌地的安全係數隨降雨強度的增加而降低，當降雨強度超過某門檻值時，坡地的穩定性開始明顯受到影響。而一旦降雨強度達到某一高值時，安全係數的下降規模更大幅減少；另在降雨強度超過此一高值時時發現安全係數下降率趨於減緩，說明了降雨強度對本崩塌地穩定性的影響仍存有一上限值。最後，累積雨量分析結果亦顯示崩塌地的穩定性隨累積雨量的增長而呈一線性的下降趨勢，惟其趨勢並非顯著。The pore pressure increases due to rainfall infiltration, which may result in the slope instability. Thus, the rainfall-induced landslide is closely related to the hydrological response. Regarding the slope stability analysis in the mountain area, usually numerical models were developed and then case studies of disasters were performed. It lacked field testing, monitoring and analysis, and hence the actual hydrogeological parameters could not be integrated into the numerical models. The purpose of this study is to further understand the mechanism of rainfall-induced landslides, the influences of hydrogeological and rainfall characteristics on landslides are investigated. Therefore, instruments are installed, groundwater levels are observed, and field parameters are analyzed. These field data are integrated into the model. First, a series of field hydrogeological tests including the ground resistivity image profiling, the double-ring infiltration test, the borehole camera investigation and the double-packer test are performed to obtain the strata distribution, slip mode and hydrological parameters to help develop the hydrogeological conceptual model. Furthermore, the hydrogeological parameters and the long-term hydrological records are compiled with the GeoStudio software to perform the transient coupled analysis. The conceptual model and the corresponding parameters applied in the model are based on a series of in-situ investigations and laboratory experiments. A seepage analysis is conducted, and the model is calibrated and verified using the field monitoring data in order to investigate the relationship between landslides and hydrogeology and to understand the influence of hydrogeological characteristics on landslides. In order to understand the stability of landslide sites under different rainfall characteristics, three types of deign rainfall conditions including different patterns, intensity and cumulative amount are introduced into the model. The design rainfalls are obtained based on the data of a rain gauge near the landslide site. The coupled stability analysis is then conducted to establish the relationship between the rainfall characteristics and the slope stability. The advanced rainfall pattern has the least influence on the factor of safety while the delayed rainfall pattern has the greatest influence. It can be concluded that the delayed rainfall pattern threatens the slope stability the most among all the rainfall patterns. As to the influence of the rainfall intensity, the factor of safety decreases as the rainfall intensity increases. When the rainfall intensity is more than a threshold value, apparent decrease in factor of safety is found. Most noticeably, the factor of safety yields a significant drop when the rainfall intensity reaches a certain high value, and the drop slows down when the intensity is over that high value. It shows that the influence of rainfall intensity on the slope stability has an upper limit. Finally, as to the influence of cumulative rainfall, the factor of safety linearly decreases with increasing cumulative rainfall, but the decreasing trend is not significant