臺灣清境地區板岩邊坡重力變形特徵與過程研究

Abstract

斜坡深層重力變形是分佈在山脈間的一種岩體變形行為，其特徵是變形速度極緩慢，且多發生在具葉理狀構造的岩坡當中。儘管斜坡深層重力變形的速度緩慢，但該變形行為將持續很長的時間，過程中所累積的變形位移仍對人工構造物的性能帶來很大的考驗，此外，由於岩坡長期受斜坡深層重力變形的影響，導致部分岩體高度破碎、風化，甚至在坡體內部形成剪裂帶，當有極端事件(如豪雨、地震等)發生時，可能驅動斜坡深層重力變形演變成災難性的岩體滑動。在臺灣，由於板岩中受存在發達的葉理面(劈理)，斜坡深層重力變形經常被記錄在臺灣板岩帶的範圍，位於南投清境地區的廬山北坡和廬山聚落便是其中的著名案例，由於清境地區同時是臺灣長年的一處旅遊勝地，因此自2009年莫拉克颱風以來，便陸續有大型的調查及監測計畫在此展開，也累積了相對臺灣其他山區有更為豐富的地表與地中資料，為本研究欲探討斜坡深層重力變形行為的一處絕佳場域。 本研究以清境地區為例，著重於存在順向坡當中的斜坡深層重力變形的演育過程和相關特徵，並區分出三個部分來達成研究目標。第一部分透過整合地表地質調查和地中鑽井資料，重建數個具斜坡深層重力變形行為的場址的二維地質剖面，包含博望地區、定遠地區與壽亭地區，上述地質剖面強調了在不同板岩邊坡中所觀察到的重力變形特徵與剪裂帶分佈，以及推測的深層滑動面；第二部分基於離散元素法模擬進行一系列數值力學分析，釐清一理想的斜坡深層重力變形自幼年階段到成熟階段的發生機制與演化過程，並提出幾何和運動學模型來說明當斜坡深層重力變形來到成熟階段，在不同位置能夠觀察到的地表地貌與內部結構特徵，以及利用垂直鑽井或沿地表佈設的監測儀器會展現的變形位移模式；第三部分利用雷達持久散射體干涉技術解算2018年至2020年的Sentinel-1 A雷達影像，瞭解近期清境地區內的邊坡運動和活動性，其加值產品也幫助斜坡深層重力變形的圈繪和其演育階段的評估。最終，結合所有的主要發現，本研究提出了一個基於地質調查、力學分析和地表監測的地質力學模型，該模型描述了清境地區斜坡深層重力變形從幼年階段到相對成熟階段的演化過程，也反映由斜坡深層重力變形所造成的岩坡漸進式破壞的可能發展。 總的來說，本研究整合多個調查數據集和不同分析方法，建立針對斜坡深層重力變形的地質剖面、幾何和運動學模型和地質力學模型，有助於解釋緩慢變形中的板岩邊坡當前地表和地中監測資料，增進針對斜坡深層重力變形的調查監測作業的規劃與其有效性，同時也為未來評估板岩邊坡具有斜坡深層重力變形和岩體滑坡共存機制的潛在災害打下基礎。

Deep-seated gravitational slope deformation (DSGSD) is a rock mass deformation process of mountain hillslopes, featuring a slow movement rate and has often been observed in foliated slopes. Although DSGSD movement is slow, it can continue for a long period, producing large cumulative displacements and could transform into catastrophic rockslides. In Taiwan, DSGSD has often been reported in the slate belt of Taiwan because of the inherent cleavage characteristic. The Chinjing region in this geological environment is recognized as a popular summer resort as well as notorious because of several large landslides such as the Lushan North Slope. Because of the important protected objects and high disaster risk in the Chingjing region, a series of investigations and monitoring tasks have been launched to observe the landslide activity within the DSGSDs. This provides a chance to study the DSGSD characteristics and its process with the relative completeness observation data in the Chingjing region. This study focuses on where the cleavage dip direction is parallel to the topographic downslope direction and is divided into three parts to achieve the goals. The first part aims to reconstruct the geological cross-sections of the DSGSDs by integrating surface geological investigations and subsurface borehole data. The geological cross-sections highlight the structural styles of the cleavages and potential shear planes in different slate slopes in the Chingjing region. The second part performs a series of two-dimensional numerical simulations based on the DEM to observe the DSGSD evolution from young to old stages. Based on mechanical modeling, the proposed geometric and kinematic model sheds light on the DSGSD occurrences and the corresponding geomorphological features and internal structure characteristics. Lastly, the third part retrieves present-day slope kinematics and activity by processing the 2D decomposition of PS-InSAR products derived from Sentinel-1 A radar satellites acquired in ascending and descending orbits. Combined with all the major findings, a conceptual geomechanical model is proposed for the DSGSD evolution from a younger to a more mature stage in the Chingjing region. Overall, this study assembled multiple investigation datasets into the geological cross-sections of the DSGSD in the Chingjing region, providing a basis for the interpretation of present-day slope activity from PS-InSAR measurements. Combining with mechanical modeling, we proposed a geometric and kinematics model and a geomechanical model that address the idealized and realized DSGSD behaviors, respectively. In addition, this thesis demonstrates an integrated approach that can be further used to understand the role of DSGSD on rock slope instabilities and assess present-day hazards of mass movements in which DSGSD and rockslide mechanisms coexist in the future.