降雨引致異質性邊坡破壞之模型試驗

Model Tests of Non-homogenous Slope Failures Induced by Rainfalls

童自裕(Chin-Yu Tung)

指導教授 ： 陳榮河

國立臺灣大學/工學院/土木工程學研究所/碩士(2013年)

https://doi.org/10.6342/NTU.2013.02565

摘要

紅土礫石台地頂部廣闊且地形平坦，易於開發利用，故常形成聚落發展或農地開墾的地區。然而，高陡台地邊緣於降雨後易發生崩塌破壞，往往危及台地上方已開發區之生命財產安全，所以紅土礫石邊坡之材料特性與崩塌機制，成為研究坡地安全的重要課題。
本研究選取曾發生多次邊坡破壞之雲林縣坪頂村地區為案例，進行降雨引致異質性裸坡(由上而下為紅土層、礫石層、泥岩層等)破壞及探討現地石籠護坡功效之模型試驗。探討之裸坡有30º緩坡及60º陡坡兩種，具保護工法之邊坡則為60º之石籠坡。坡內均埋設水份計與水壓計以觀察邊坡破壞過程中坡體內含水量與孔隙壓力變化，並利用攝影機對邊坡破壞歷程進行紀錄，嘗試瞭解邊坡崩塌之過程及原因，及對施作保護工法之有效性檢核;最後利用有限元素軟體PLAXIS做數值分析驗證。
試驗結果發現，本研究採用之新等重量替代法可使試驗土樣更符合模型相似性。而邊坡最上部的紅土層滲透係數較低，可減緩降雨入滲及坡內水位累積；其張裂縫亦會影響坡內水位分佈。裸坡頂之沉陷亦會隨著浸潤深度而改變，且因下層泥岩滲透係數較小，故水位累積在泥岩層上方之礫石層形成滲流，使兩土層之界面處逐漸不穩定，終致邊坡滑落破壞。石籠坡面上因有放置石籠所整成之階梯平台，故使坡體之浸潤速度較裸坡為快，但石籠可有效抑制邊坡變形，進而延緩邊坡破壞；加上泥岩因滲透性較差，故不致發生大規模的滑動。

關鍵字

邊坡 ； 模型試驗 ； 紅土 ； 礫石 ； 泥岩 ； 降雨入滲 ； 保護工法 ； 數值分析

並列摘要

Lateritic-gravel tablelands are wide, flat, and easy to be developed for land uses such as dwelling houses or agricultural lands. However, the high steep edges of tablelands are prone to collapse after heavy rainfalls, which will endanger the lives and properties in the nearby area. Therefore, studying the engineering properties of lateritic gravels and its landslide mechanism is an important topic for research in the aspect of slope stability.
This study selected an area nearby Ping-Ding Village of Yun-Lin County as the study site; for there were several severe landslides occurred in this area in the past. Mosel tests were employed and carried out in a sand box to explore the failures of nonhomogeneous slopes (composed of laterites, gravels, and mudstones; from top to bottom) induced by rainfalls as well as the efficacy of the gabions used for slope protection. The bare slopes investigated were a 30º gentle slope and a 60º steep slope; while the slope protected by gabions was investigated only for 60º. During the experiment, the variations in pore water pressure and volumetric water content in the soil were measured. The characteristics of the failure mechanism, the responses of pore pressure and water content in the model slopes, as well as the performance of protection method are compared and discussed as functions of the variables. The finite element program PLAXIS was also utilized for numerical analysis.
The test results showed that the proposed new weight-replacement method (NWR) resulted in suitable testing soil that was in accordance with the law of similarity. In addition, the less permeable laterites at the upper part of the slope reduced rainfall infiltration and water accumulation within the slope; the tension cracks in the lateritic soil also affected the distribution of water in the slope. The subsidence at the top of bare slopes changed with infiltration depth. The mudstone at the bottom has small hydraulic conductivity, so the water accumulated in the upper gravel layer and formed seepage, causing gradual instability at the interface of gravel and mudstone layers and ultimate destruction of the slope. The slope face where gabions were installed induced more rapid infiltration than the bare slopes due to the small steps provided for installing the gabions. Nevertheless, the gabions effectively prevented the slope to deform and thereby improved slope stability and delayed slope failure. In addition, the mudstone is unlikely to result is mass sliding because of poor permeability.

並列關鍵字

slope ； model test ； lateritic soil ； gravels ； mudstone ； rainfall infiltration ； protection works ； numerical analysis

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