Effect of Topography on the Mechanical Behavior of Unsaturated Residual Soil Slopes and the Technique for Analyzing the Stability of Slopes with Different Topography(Ii)

[[abstract]]本研究計畫(100-101 學年度計畫)為延續99 學年度專題研究計畫「暴雨下非飽和土壤公路邊坡之破壞機制監測與分析－子計畫:地形效應對不飽和風化土壤邊坡力學行為之影響及邊坡穩定分析技術研究(I)」。研究地形因素對不飽和土壤邊坡力學行為(土壤基質吸力、含水量及抗剪強度)與安全穩定性之影響，於現地不同邊坡地形情況，執行於雨季及乾季期間不飽和土壤邊坡之現場實驗與監測。本研究對於不飽和土壤邊坡穩定分析之合理性有其重要意義。於工程實務上，一般邊坡穩定分析多以二維方式進行，絕大多數自然邊坡土壤多為不飽和情況，而降雨時不同地形邊坡之地表逕流及地下滲流均不相同，因此，不同地形邊坡土壤之含水飽和度亦受到不同程度之影響，導致不同地形邊坡土壤之基質吸力於單一降雨事件中有不同之變化行為與分佈。土壤基質吸力直接影響不飽和土壤之抗剪強度，因此，不同地形邊坡於單一降雨事件之安全穩定性亦應不相同，即邊坡穩定性之三維(3-D)地形效應必須納入考慮。於傳統邊坡安全評估方式多未考慮邊坡區域地形因素之影響，即水文條件(地表逕流及地下滲流)未於邊坡穩定分析中加以考慮，可能導致邊坡安全分析評估結果之不合理，此一情況可經由本計畫之執行獲得進一步探討。本計畫主要研究課題包括：(1)探討不同地形邊坡土壤基質吸力及含水量於乾季及降雨期間之歷時行為；(2)探討台灣南部地質條件(主要為頁岩)邊坡土壤之 SWCC 曲線；(3)探討不同地形邊坡土壤於降雨及乾季期間之不飽和土壤剪力強度之變化情況；(4)探討不同地形情況不飽和土壤邊坡穩定分析方法與技術；(5) 評估不同地形邊坡於降雨期間之邊坡破壞致災敏感性(Landslide susceptibility)。本研究計畫之成果可支援於傳統二維邊坡穩定與分析之思維架構下，加入考慮邊坡穩定分析之三維效應，可增進現有二維邊坡穩定分析之合理性。台灣山坡地面積佔約70%，台灣中南部山坡地於莫拉克颱風後嚴重受創，更突顯台灣山坡地保育之重要性。本計畫課題過去國內外之研究資料有限，期藉由本研究計畫之執行，除可就不同地形邊坡土壤於降雨及乾季期間之基本力學機制進行探討，並藉由本計畫成果之獲得，進一步提昇傳統二維邊坡穩定分析模式轉變為可考慮地形效應之擬二維邊坡穩定分析(Pseudo-two dimensional slope stability analysis)所需之分析技術。[[abstract]]This research project (the academic year of 100~101) is an extension of the project entitled “Effect of topography on the mechanical behavior of unsaturated residual soil slopes and the technique for analyzing the stability of slopes with different topography”which was endorsed in the academic year of 99. This research aims to investigate the effect of topography on the mechanical behavior of unsaturated soil slopes and on the aspect of slope stability. In-situ instrumentation and testing program with automatic logging system are used to obtain the behavior of unsaturated soils in soil slopes with various topography. This study plays an important role in the rationalness of the technique for analyzing the stability of slopes currently used. Two-dimensional slope stability analysis is usually adopted in engineering practice. Most of the slopes are in unsaturated conditions. The topography strongly affects the surface runoff and seepage in slopes. Thus, soil moisture contents in the slope are affected by the topography in the same environmental condition. In turn, soil matric suctions in the slope are affected by the topography in a given rainfall event. The shear strength of unsaturated soils is related to the soil matric suction. Thus, the stability of the slope at various topography is different. The effect of 3-D topography on the stability of the slope needs to be taken into account, and the 3-D topography is normally not considered in the slope stability analysis normally used in the engineering practice. That is to say, the hydrological condition of the slope in a rainfall activity is not taken into account in the normally-used slope stability analysis. The factor of safety of the slope may not be reasonable in currently used methods of slope stability. These issues will be investigated in the research proposed. The topics in the proposed research project include: (1) investigation of the effect of topography on the soil matric suction and soil moisture content during dry and rainy seasons; (2) investigation of the SWCC curve for the weathered soil of the geological formation in southern Taiwan; (3) investigation of the shear strength of unsaturated soils in dry and rainy seasons; (4) investigation of the technique of slope stability analysis for 表C011 共3 頁第3頁 unsaturated soil slopes with consideration of topography; (5) assessment of the landslide susceptibility in different topography in rainy season. The outcome of this research can be helpful in improving the reasonableness of the currently used 2-D slope stability analyses by incorporating the soil matric suction in different topography in rainy condition into the analysis. Almost 70% of the land area is mountain in Taiwan. The mountainous region in southern Taiwan was devastated severely by Typhoon MORAKAT in August, 2009. Thus, the conservation of the mountainous region is extremely urgently important in Taiwan. The literature on this topic is limited. The research project proposed herein can investigate the mechanical mechanism of unsaturated slopes in dry and rainy seasons. Additionally, the technique needed for considering the effect of topography on the slope stability analyses can be provided through the results of this research.[[note]]NSC100-2625-M327-003-MY