地下水位升降與坡頂加載對崩塌後邊坡穩定性之影響

Abstract

由於台灣大部份邊坡災害多由颱風與豪雨所引發，因此本研究選定湖口台地南緣崩塌地作為研究之對象，從事現場勘察、鑽探、儀器裝設與監測（如地表傾斜儀、地表伸縮計、管式應變計、地下水位觀測井、雨量計），且以觀測所確認之滑動面位置，進行邊坡穩定分析反求土壤強度參數，並探討因降雨所引致之地下水位升降與坡頂加載對崩塌後邊坡穩定性之影響。 本研究區域靠近竹九－竹十四聯絡道路之邊坡，每當豪雨過後皆有明顯變動，更於民國八十六年六日連日豪雨過後發生大規模崩塌滑動。管式應變計測得本研究區域有地下六米與地下十二米兩個滑動面，兩者分別位於同一泥岩層之上方與下方和砂岩層交界處。邊坡穩定分析結果顯示，坡頂加載與地下水位上升皆會使邊坡穩定度大幅下降。歸納觀測與穩定分析之結果，湖口崩塌地之破壞機制為：降雨入滲，使邊坡原有土壤與坡頂棄土土體重量大幅增加，增加下滑動剪應力；同時地下水位因降雨上升，導致泥岩層發生土壤強度軟化現象，減低土層之抗剪強度，使邊坡沿泥岩與砂岩層界面破壞。研究結果提出由地下水位與坡頂加載高度以評估邊坡穩定性之準則（F.S. =1），供為工程設計之參考與利用，該評估模式如下：（圖 4.32） Y≧ -0.36X -3，Y地下水位高度（M），Ｘ坡頂覆土高度（Ｍ）。

In Taiwan, most of the slopes was failed after typhoons and the pouring rain. For this reason, we choose the collapsed area at the southern Hu-ko Plateau as the research area. In this researching, we prospected and explored the land, set up the instruments (including inclinometer, extensometer, strain pipe, groundwater level measurement, rain gage), kept records, observed the movement of the sliding surface and analyzed the stability of the slopes. We discuss the effects of the groundwater and the surcharge on the top of slop in Hu-ko landslide. The slopes slid and collapsed after pouring rain in June 1997,especially the area where people dump the dirt at the tops of the slopes The strain pipe shows that there are two sliding surfaces (6m and 12m underground), located between the mud layer and the sand layer. By analyzing the stability of slopes, we find that the surcharge on the slope surface and the rising level of ground water will decrease the stability of slopes. The leading cause of collapse areas at Hu-ko is because of the increasing weight of the discarded dirt on the top of the slopes by pouring rain, and enlarges the shear strength of the downward sliding. At the same time, the level of ground water risen by rainfall leads the strength softening in the mud layer, and finally the slopes failure between the mud layer and the sand layer. The judgment equation is (Ref. to Fig 4.32) Y≧-0.36X-3 where Y: ground water table (meter) X: Surcharge height on the top of slop (meter)