崩塌為土體沿著坡面向下運動的過程，也是集水區常見的侵蝕行為，其破壞而崩落的鬆散土方會危及人民生命財產安全，亦會在進入河道後，影響河川下游的水流含砂濃度，進而產生河口三角洲和棲地的演化、水庫淤積、水質等牽涉到生態與經濟層面的問題，因此對於崩塌之發生有必要進行深入的了解，以提供防災和減災等相關作業之需要。本研究以Rivendell study site, USA為研究區域，利用監測資料探討降雨入滲下土壤水分的傳遞行為，並提出土壤孔隙內壓力變化的解析解，然後配合無限邊坡理論進行邊坡的穩定分析。監測的資料結果顯示，土壤內的水份傳輸接近動態波傳行為，因而造成入滲進土壤內的水份還未到達時，原先儲存在土體內的水份便已經向下傳遞。接著本研究以孔彈性理論為基礎，提出兩個參考的時間尺度tk和tep，可以分別用來表示壓力波和消散行為在土體內的傳遞時間。隨著深度的變化，當飽和度越小的時候tk的差異會越來越顯著，在接近乾土的時候差距可以到10倍左右，然而在飽和度較高的時候tk幾乎沒有什麼變化，主要集中在0-5秒，另一方面，tep也會隨著飽和度的增加而逐漸減少，在土壤接近飽和時可以達到一天以內，因此結果顯示，降雨入滲下土層內的壓力傳遞行為主要是由動態波來控制，但是當土體接近飽和的過程中，壓力的消散作用會逐漸明顯。本研究同時利用解析解對降雨入滲下的邊坡穩定進行時變性分析，結果顯示壓力主要集中在土壤與岩層之接的交界面，且土壤性質對邊坡的穩定分析有很大的影響。

Landslide is a natural geomorphic erosion process on hillslopes, and can be threat to people's life and property. This paper aims to investigate shallow landslide triggering on hillslopes response to rainfalls. We discuss the mechanism of water movement advancing through an unsaturated soil profile during rainfall by hillslopes hydrology monitoring in the Rivendell study site and propose a hydrologic response function combining the analysis of slope stability to clarify how does rainfall trigger landslide. Instrumental records of soil moisture dynamics show very fast response to rain infiltration that suggests pressure wave propagation predominates, leading to the release of old soil water downward into deep zones before new infiltrating water arrival. Based on poroelsticity theory, we propose two reference time tk and tep that can represent the arriving time at a certain depth of wave propagation and dissipation, respectively. Form ground surface to a depth of 1 m, tk has ten-fold differences under nearly unsaturated conditions for diverse soil properties; however, no significant variations in tk can be observed under nearly saturated conditions, which just varies from 0 sec to 5 sec. Values of tep for coarse sand is much greater but decreases to the smallest one (within 1 day) than those for other soil properties under a nearly saturated condition. Results indicates that transient pore pressure transmission is mainly dominated by dynamic wave propagation but the effect of dissipation could become more important with increase in water saturation. Analysis of slope stability for hypothetical sandy and silty soils with the function of pressure head evolution induced by rain infiltration derived from poroelasticity theory indicate that positive pressure heads concentrates on soil bedrock boundary, leading to the generation of shear zone at this boundary. Slope failures always occur with positive pressure heads. Results also show that soil properties play an important role on shallow landsliding triggered by rainfall. Use of those response function in conjunction with topographic data, soil properties, rainfall intensity and duration information, and an infinite-slope failure criterion can evaluate the timing and depth of shallow, rainfall-triggered landslide, hereby refining hazard prevention and mitigation.

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