邊坡深層滑動破壞之情境模擬－鹿場案例

Abstract

大規模崩塌為具有潛在毀滅性之坡地災害，相關研究對於防災與減災深具重要性，莫拉克風災時震撼國人的小林村滅村事件即為典型案例。根據過往研究指出:大規模崩塌之全面破壞經常是深層邊坡重力變形的發展終點，若能掌握深層邊坡重力變形的變化趨勢，將有助於防範大規模崩塌與其導致的災損。因此，本研究嘗試藉由數值模擬分析方法，探討一邊坡由深層邊坡重力變形轉變為大規模崩塌的過程。案例場址為苗栗縣南庄鄉鹿場區域，此區域存在明顯崩崖、坡趾鼓起、厚崩積層覆蓋等地質或地貌特徵，並於民國93年艾利颱風過後，不斷出現局部邊坡滑動跡象。為瞭解鹿場區域發生大規模崩塌的可能性，本研究蒐集地形、地質、水文等相關資料，使用地工分析軟體FLAC3D建立三維模型，並依現地監測資料以逆向分析模擬場址現況，繼而進行一系列誘發邊坡失穩的情境模擬，包括地水位抬升、地震以及河道下切等。情境模擬結果顯示:1.地水位抬升將大幅提高大規模崩塌發生的可能性；2.當發生五級以上地震時，鹿場臺地東側邊坡為發生大規模崩塌之高風險區；3.河道下切導致邊坡側撐喪失，使得邊坡局部不穩定性增加，因此淺層崩塌更容易發生。但單就鹿場區域而言，深層崩塌機制與此效應的關聯並不明顯。

Large scale landslides may lead to catastrophic consequence.Researches relating to large scale landslides are important for hazard prevention and mitigation.Often, a potential deep seated landslide displays signs of deep seated gravitational slope deformation (DSGSD) before it leads to a final overall failure. Knowing the possible development in DSGSD will be helpful for preventing potential casualty and damage. This thesis attempted to investigate a selected site of potential deep seated landslide by means of numerical simulations. The studied site is at Luchan, in Miaoli, Taiwan; the hillslope in Luchan shows clear sign of DSGSD including extending scarp, bulge at toe, thick colluvium, etc. Local slope failure also occurred in the studied area. Various Information including terrain, geology and hydrology data were collected first. On the basis of the relevant information, the study made use of the software FLAC3D to construct a three-dimensional model and to conduct reverse analyses to estimate the in-situ groundwater surface under normal condition on the basis of monitored water-level data.Strength parameters of the colluvium were also estimated. Subsequently, a series of numerical simulations regarding various scenarios possibly triggering overall deep-seat landslide failure were conducted to assess the potential of catastrophic slope failure in the studied site. The results of these scenario simulations reveal the following. (1) A significant raise of ground water surface within the slope may largely increase the possibility of large scale landslide. (2) There will be a high potential of large-scale slope failure when the site is subjected to an earthquake ground motion up to an intensity V (0.10-0.25g). (3) River channel incision tends to reduce the lateral buttress at the toe of a hillslope so that may lessen the stability of the slope. However, the influence of river channel incision on the potential large scale landslide in Luchan appears insignificant.