高陡逆向坡岩體特性調查及軟硬岩層形成承載力破壞機制探討

Abstract

綜觀國內過去曾發生於岩石邊坡之災害案例，可發現逆向坡亦常有各種形式的邊坡破壞，造成之潛在危害並不亞於順向坡，值得深入探究其致災機制。例如1993年北二高新店隧道北口邊坡破壞、2008年貓空纜車T16塔基邊坡滑動、2013年康芮颱風造成萬里七甲尾地區崩塌等，皆發生於逆向坡區位。而高雄柴山之巨型古地滑案例、以及2007年國立海洋大學內一處邊坡破壞案例亦顯示，逆向坡有可能發生承載破壞。 本研究第一部分為「高陡逆向坡岩體特性調查」，係探討如何克服在逆向坡調查困難之課題。由於沉積岩區的逆向坡往往受差異侵蝕而形成陡峭的崖面，人員無法輕易接近調查節理面等岩體不連續面。又逆向坡常形成倒懸地形，在傳統測量技術與地形圖表現方法上，亦無法正確顯示倒懸之地形特徵。因此本研究運用近年發展迅速之無人飛行載具（UAV）攝影測量技術，嘗試以點雲資料轉換立面地形圖來判斷分析倒懸地形之範圍、體積，以及節理組之走向、間距。 本研究第二部分為「軟硬岩層形成承載力破壞」，分別於東北角八斗子、萊萊海岸地區，發現在逆向坡區位中，當節理發達之硬岩層覆蓋於較厚軟岩上時，硬岩層因差異侵蝕而倒懸於下方軟岩之上，其受節理切割的硬岩塊體可以於下方軟岩中發展出承載力破壞，並呈現弧形滑動的特徵。本研究運用傾斜平台製作縮尺物理試驗模型，初步觀察此種破壞之定性運動機制。同時利用PFC3D離散元素法設計一系列數值模擬，釐清發生此種承載破壞的因素。 研究成果顯示，逆向坡承載破壞受到兩種因素主控。硬岩塊體之幾何形狀主要決定承載破壞的最終形式為滑動破壞或傾覆破壞，而硬岩與軟岩之強度比例主要影響破壞是否發生，至於促崩因子則有軟岩差異侵蝕、軟岩材料風化兩項。

Rockfall failure and toppling failure are usually considered as the typical types of obsequent slopes failure. However, some field investigation cases on coastal area in northern Taiwan show that bearing failure could also be found in obsequent slopes. These cases are composed of a competent layer lying on an incompetent layer, or called the cap rock structure. The first part in this research is developing a method to describe overhang topography and estimate the size of overhang part. The contours are computed based on dip direction of cliff from point cloud produced by UAV photogrammetry. Therefore, the position and volume of overhang part can be calculated. Also, joint sets are identified by point cloud. The second part is to perform a small scale sandbox experiment to simulate bearing failure mechanism of the obsequent slope. The sandbox experiment demonstrates that bearing failure on obsequent slopes could be classified into two types, sliding (type I) and toppling (type II). The PFC3D model is used to analyze the factor, including the dimension of cap rock block and strength of rock layers, of bearing failure on obsequent slopes. The results show that the dimension of cap rock block and the dip angle of bedding are key factors of the failure type, sliding (type I) or toppling (type II). In addition, the ratio of normal stress of cap rock block to strength of incompetent layer is the key factor to decide whether the failure will happen or not.