應用分離元素法探討紅菜坪地區地滑演化

Abstract

位於南投縣中寮鄉的紅菜坪地區，於1999年集集地震時發生了地表位移的情形。經前人之研究成果指出其具有滑動範圍大但位移量小、位移方向不一致、為崩積層再度滑動及其滑動面為曲面等特性，與一般常見之山崩有所不同，因此值得探討其地滑行為。 由於紅菜坪崩積層之產狀和堆積形貌跟岩層之破裂程度和滑動時之運動行為息息相關，而這些皆無法由極限平衡法與連續體變形分析法等一般常用之邊坡穩定分析方法模擬分析。因此可探討元素分離與大變形之分離元素法將更適用於探討紅菜坪地區之地滑演化。 本研究藉由蒐集資料並以鑽探所得之岩心進行試驗，並利用Hoek-Brown準則推估現地岩體強度，以建立地滑模型與獲得分析時材料參數，而後利用分離元素分析軟體PFC2D模擬紅菜坪古地滑與集集地震時之地滑運動行為。 研究結果顯示前人針對地滑方向而對紅菜坪地滑地之分區應屬同一滑動塊體，主要構成紅菜坪崩積層之材料中，砂岩之岩體強度大於頁岩，因此崩積層之產狀中，顆粒支持之崩積層多為砂岩質，而基質支持之崩積層多為頁岩質。而相較於其他分析方法，本研究藉由合理的分析假設，可以重現紅菜坪地區於集集地震前之崩積層範圍與特性。亦可由於邊界施加集集地震之地表加速度進行邊坡受震模擬，使其產生之地表位移場與前人利用震前後航照分析相符合之結果。而經由參數分析，初步推論主要影響紅菜坪古地滑與集集地震地滑運動行為之變因為發生初始滑動後之滑動面摩擦特性，摩擦係數越大者，不論其滑動材料為岩層或崩積層，其滑動速率與位移量皆會隨之下降。

In 1999, a landslide took place at Hungtsaiping, Nantou, because of Chichi earthquake. By former researches, the landslide has several unique, such as few displacements in a large region, the different direction of displacement in different area, colluvium reactive slide and a curved slip surface, etc. The landslide behavior is different from custom, so it could be a topic for discussion. The texture of colluvium at Hungtsaiping is related to the fractures of slide mass. Equally, the deposition of colluvium is related to the kinematics of landslide. These behaviors can not be analyzed by limit equilibrium method or continuum deformation analysis. Therefore, distinct-element method (DEM), which can discuss the separation of element and large deformation, is suitable to infer the landslide evolution of Hungtsaiping area. In this study, we collect the geological and geomorphical data to set the landslide model. To investigate the mechanical behavior of material, the cores were used to conduct laboratory experiments. We use the Hoek-Brown failure criterion to represent the strength of the insitu rock mass with the intact rock strength obtained from laboratory experiments and the geological strength index (GSI). Finally, we use Particle Flow Code in 2 Dimension (PFC2D) to simulate the landslide kinematics of Hungtsaiping area in two events, old landslide and Chichi earthquake landslide. The results show that the three zones of slide mass due to the sliding direction by former researches should be the same slide mass. In the major materials which compose the colluvium of Hungtsaiping, the sandstone rock mass has higher strength than that of shale rock mass may explain that the shale rock mass is prone to disintegrate after slide and the causes for the matrix as well as the granular supported colluviums. With some reasonable assumptions, we can reproduce the region and characteristics of colluvium in Hungtsaiping area before Chichi earthquake. It is possible to evaluate the similar displacement field to that with aerial photo by applying the strong ground motion of Chichi earthquake to the boundary of model.