崩積土有效內摩擦角估算之研究─以砂崙仔崩塌地為例

Abstract

台灣山坡地占國土三分之二以上，加上颱風、豪雨與地震的影響，使山坡地邊坡穩定成為重要之工程和環境問題。而在進行穩定性評估時，需要先了解邊坡之有效內摩擦角(ϕ’)等土壤力學性質，才能進行有效且經濟的工程設計。目前國內邊坡穩定工程設計所採用之有效內摩擦角大多經由實驗室中的直接剪力儀或三軸試驗儀分析而得，但這些方法因受限於實驗試體尺寸的限制，並無法適當反映崩積土中礫石與土壤混和所產生的力學性質，因此可能會造成邊坡穩定度誤判之情況出現，使工程設計失其有效性及經濟性。 本研究先根據前人研究選定影響內摩擦角之關鍵因子，再以重模土壤試體之直接剪力結果建立有效內摩擦角複迴歸推估式及此式之68%信賴區間範圍，推估式如下： ϕ’ = tan-1 [ 1.18 × G^0.15 × ϒ_r^0.11 ×(D*)0.17 ] 式中G為200#篩以上顆粒含量，ϒr為相對乾單位重，D*為相對粒徑。 本研究並以石門水庫上游集水區中的砂崙仔區域作為研究對象，根據此地區之地質、地形及土壤粒徑分布等資料，分別以傳統實驗方式以及複迴歸模式進行現地有效內摩擦角推估，再以其結果帶入STABL模式中進行邊坡穩定分析。將分析結果與現實情況對照後，發現本研究建立之複迴歸模式所得之有效內摩擦角較傳統實驗合理，證明以複迴歸模式推估崩積土之有效內摩擦角較符合崩積土之力學特性且較具有代表性，可修正傳統實驗可能發生之有效內摩擦角低估之情況，並可作為往後研究及工程設計在推估有效內摩擦角時更合理且迅速之參考依據。本研究並發現STABL模式因僅就斷面之地形、地下水位、土壤性質等影響因子進行分析，忽略其他邊坡崩塌之影響因子(如道路、水系分布及坡面曲度等)對於安全係數的影響，導致無法正確預測較緩坡面發生崩塌之現象，建議未來應針對此方面進行相關修正之研究，以期達到更準確之分析結果。

The slopeland accounted for more than two-thirds of area in Taiwan, coupled with the impact of typhoons, heavy rains and earthquakes, the slope stability become important issues of engineering and environment. In order to carry out an effective and economical engineering design, it’s necessary to understand the soil mechanical properties such as the effective internal friction angle (ϕ') during the stability assessment. Nowadays, most of the slope stability projects estimate the effective internal friction angle by direct shear test or triaxial test instrument in laboratory. But these methods don’t reflect the mechanical properties of colluvium properly due to the restrictions of the specimen sizes. It may cause misjudgments of slope stability, losing effectiveness and economy of engineering design. This study selected the key factors which affect the angle of effective internal friction according to previous studies. Then establish the multiple regression equation and its 68% confidence interval by the results of direct shear test which using re-mold soil specimens: ϕ’ = tan-1 [ 1.18 × G^0.15 × ϒ_r^0.11 ×(D*)0.17 ] In which, G is the particle content over # 200 sieve, ϒr is the relative dry unit weight, D* is the relative diameter. In this study, we did the research in Shaluntz watershed which belongs to the Shihmen Reservoir catchment. According to the data of geology, topography and soil particle size distribution, this study estimated the effective internal friction angle by both the traditional experimental methods and, and analyzed the slope stability by STABL model with estimating results. After comparing the stability results with the real condition, we found the multiple regression model which established by this research was more reasonable than traditional experimental methods. It proved that the estimating values of colluvium’s effective internal friction angle by multiple regression model was more matching the mechanical properties of colluvium and could correct underestimate values estimated by traditional experiment. So the multiple regression model in this study can be a more reasonable and faster way to find out the effective internal friction angle and provide reference to the future research and engineering design in the future. This study also found that STABL model couldn’t predict the landslide of moderate slope correctly, because it only considered the effect of the topography, groundwater and soil properties, and ignored the effect of other factors (such as the distribution of road and river, and slope curvature, etc) for the factor of safety. In order to achieve more accurate results, it’s necessary to do further research in the future.