Hydrological behavior of unsaturated soils in gullied slopes during rainfall

[[abstract]]許多專家學者利用人造邊坡與人工降雨，了解雨水入滲土層內部變化之行 為，進而得知不飽和土壤邊坡破壞機制。影響邊坡破壞之因素，除降雨因素 之外，邊坡地形因素亦為影響邊坡穩定之重要因素，溝狀邊坡或凹谷地形因 地勢低窪，當發生降雨時，雨水及地表逕流易往此處匯集，且會經由表層土 壤或張力裂縫入滲邊坡土層，因地形因素，溝狀邊坡基質吸力低於其他邊坡 類型，基質吸力相當程度影響不飽和土壤之剪力強度，因此，溝狀邊坡相對 較易發生邊坡破壞之問題。 本研究針對溝狀邊坡進行長期監測土壤基質吸力與體積含水量隨環境變化之 行為，探討溝狀邊坡不同高程及不同深度於降雨期間不飽和土壤力學隨時間發展 行為與機制。本試驗地點位於高雄市甲仙區未經人為開發之自然溝狀邊坡，本試 驗縱向坡度約 35 ，橫向坡度約 25 -40 之間，於不同高程埋設土壤基質吸力及 土壤水分感測器，採用自動化進行量測數據，並利用遠端傳輸數據。除基質吸力 與體積含水量歷時數據外，並且分析單一降雨事件不同高程及不同深度基質吸 力與體積含水量隨時間變化之行為，本研究由基質吸力及體積含水量與時間變化 之行為定義出時間因子(Ts、Te)，Ts 代表任一深度濕鋒到達之時間，Te 代表不 飽和土壤貢獻剪力強度最低值之時間，除溝狀邊坡外，本研究也將分析非溝狀邊 坡之時間因子，藉由時間因子了解溝狀邊坡與非溝狀邊坡於降雨期間基質吸力與 體積含水量時間與空間分布之行為。 本研究結果顯示:(1)溝狀邊坡於降雨期間發展顯著之地下滲流，導致深層土 壤部分含水量增加幅度高於淺層部分。(2)於降雨期間，高程低處之監測點，濕 鋒到達之時間(Ts)隨著深度增加而減少，代表地表下存在著滲流水層，當邊坡發 生坡壞時，先由坡趾處開始破壞。(3)溝狀邊坡有匯集雨水及地表逕流之特性， 導致 Ts 及 Te 發展機制隨不同高程有不同之發展。(4)當降雨強度愈高，濕鋒到 達之時間與不飽和土壤剪力強度貢獻最低值之時間也將愈快。(5)非溝狀邊坡 Ts II 與 Te 發展行為隨著深度增加而增加，顯示地下滲流之現象並不顯著。(6)由溝狀 邊坡與非溝狀邊坡之時間因子整體發展趨勢，溝狀邊坡 Ts 及 Te 快於非溝狀邊坡， 溝狀邊坡發生邊坡破壞機率高於非溝狀邊波。[[abstract]]Abstract Many scholars and experts use the artificial scope and artificial rainfall in order to understand the internal changes such as rainfall water infiltration behavior inside the soil itself which came out that the failure mechanism was unsaturated soil slope The factors which caused the affect in slope failure besides rainfall the terrain of the slope stability is also important when a rainfall occurs in a slope gully or low-laying valley the rainfall water runs within surface runoff then collected also runs through surface soil or inner soil caused by tension cracks due to these terrain factors gully slope with matric suction are lower than other types of slopes the considerable influence of matric suction in unsaturated soil shear strength therefore the gully slope is relatively more likely to occur slope failure problems in this study long-term monitoring slope gully soil matric suction and volumetric water content changes with the environment's behavior discussing the behavior in unsaturated soil mechanics and mechanisms on slopes at different elevations and different depths during rainfall The test site is located in Kaohsiung District Jiasian Area undeveloped and naturally without artificial gully slopes the test is vertical slope about 35 degrees and cross slope between about 25-40 degrees at different altitudes buried soil matric suction and soil moisture sensors automated measures data and use the remote to transmission the data Beside the volumetric water content and matric suction historical date also to analysis of single rainfall events in different altitudes with different depths suction and volumetric water content behavior changes from time to time the studies was conducted in suction and volumetric water content changes behavior caused in time difference and this is the time define factors (Ts Te) Ts represents any depth of wetting front arrival time Te represents unsaturated soils IV contribution to a minimum value of time due to shear strength besides gullied slope the study will also analyze the non-gullied slope time factor understanding gullied and non-gullied slope in the matric suction during rainfall and volumetric water content timing in the space distributed behaviors The results showed: (1) slope gully during rainfall develops significant underground seepage this cause part of moisture rate in deep soil increases higher than the shallow part (2)during rainfall the lower altitude of the monitoring points reaches wetting front time (Ts) which decrease with increasing depth means under the surface layer seepage water exists when the slope failing occurs destruction starts at the toe of the slope (3) The rain water are gathered within grooved slope and characteristics of surface runoff causes Ts and Te to development different mechanism developments within different altitudes (4) When the rainfall intensity becomes higher wetting front arrival time and the shear strength of unsaturated soils is contribute to minimum value of time which will also become faster (5) Non-gullied slope Ts and Te’s behavior increases with the depth also indicating the phenomenon of underground seepage is not significant (6) By The overall trend of the gullied slope with the non-gullied slope time factor Ts and Te gullied slope faster than non-gullied slope gullied slope probability of occurrence of failure is higher than non-gullied slop