大規模崩塌地形衍育概念模型-鹿場案例

Abstract

潛在大規模邊坡破壞是威脅性重大之天然地質災害，許多學者深入探討此議題，主要探討的面向多在於那些邊坡有大規模崩塌潛勢，了解邊坡破壞的機制，或是預測何時可能發生全面性破壞。邊坡的發展可能經過一系列的地型演變所造成，若能重塑潛在大規模邊坡的衍育過程，不僅能加深對邊坡現況的瞭解，也可藉由此模型推測未來邊坡發展的可能。蒐集邊坡的現況殘留跡象，抽絲剝繭的回推過去可能發生的事件。 本研究選擇苗栗鹿場地區一潛在大規模崩塌邊坡場址為研究案例，該場址具明顯的崩崖以及厚層崩積層，根據調查推測該地區曾經發生過大規模崩塌。本研究依據工址調查成果建立地質模型，進而建構此地之地形衍育概念模型，再透過一系列假定情境下之邊坡穩定分析與運移模擬以檢討此地形衍育概念模型之可能性。 推測該邊坡原始地形為一上凸下凹之地形面，且破壞機制一開始為上部節理連通致底部層面造成大規模順層滑動，直到崩崖退至接近背斜軸軸部後轉為落石，形成崖錐堆積，經過多期破壞堆積終至現今地貌。本研究由蒐集場址現況的詳細資料開始，推定合理的破壞機制，尋找衍育的起點，以建立地形衍育的概念模型。合理的模型地形衍育過程中絕不應違背力學條件及運動條件，本研究透過極限平衡切片法與顆粒流運動堆積模擬以佐證衍育過程每一階段之合理性，從多期的破壞與堆積逐步衍育至現今之地貌。

It is important to investigate the potential for large-scale landslides for natural geo-hazard mitigation. Previous studies on this issue were majorly to identify potential landslide sites, to understand failure mechanism and to predict possible failure time. A hillslope with a potential for large-scale landslide could experience a series of geomorphological evolution in the past resulting from previous landslides and subsequent runout. Through exploring the past geomorphological evolution of a potential large-scale landslide site, it can be beneficial for a better understanding of the site condition at present, and can be helpful for predicting the potential slope failure in the future. A site in Luchang (in Miaoli County, Taiwan) is taken as a case study for studying the geomorphological evolution of a hillslope with large-scale landslide potential. This hillslope contains obvious scarps and thick colluvium which show evident signs of large-scale landslide in the past. Starting with a comprehensive site investigation, this thesis constructs the geological model of the studied area and proposes a conceptual model of geomorphological evolution at this site. Successively, a series of slope stability analysis under various scenarios and subsequent run-out simulation are conducted to support the possibility of the conceptual model of geomorphological evolution. With an initial convex ground surface, the hillslope first experienced a couple times of large-scale plane failure and its following runout. After the scarp retreated to near the axis of anticline, the mechanism of slope failure turned from plane sliding to lasting rockfalls; talus was thus formed under the scarp. Eventually, the hillslope turned into its current landform. This study simulated the evolution process of this hillslope stage by stage. The results of the numerical simulations demonstrate that each process in the conceptual model is possible in mechanics and kinematics.