落石區崖線崩退與崖錐堆積形態之研究

Abstract

台灣山區落石災害層出不窮，每逢地震事件或颱風豪雨季節常引致山區落石災害。回顧台灣落石災害歷史，造成重大傷亡與財產損失的落石事件，往往屬於大規模岩塊群同時崩落所引致的災害。諸如中橫沿線、南雅里、基隆市麥金路及龍泉溪等，其落石群不僅嚴重威脅崖下保全對象，甚至可能形成堰塞湖，更擴大影響下游保全對象之安全。有鑑於此本研究針對崖錐堆積體保存完整之紅菜坪與象山等落石區進行深入調查，搭配歷史地形與航照影像資料，並藉由物理模型釐清崖線崩退與崖錐堆積形態之關連性。 本研究工作內容主要包含三大項，首先於室內進行各時期地形與航照影像定位、疊圖分析，以初步了解落石源頭、運動、堆積等區於各時期之地形變化。接著以現場調查搭配地形分析結果，釐清落石區源頭崩退類型與其崩退機制，並將其落石現象適度簡化反應於室內物理模型實驗設計之中。藉由物理模型實驗控制落石量、落距、地形坡度、源頭崩落形式等變因，來深入了解落石群形成崖錐堆積體之建立過程與堆積形態變化趨勢。另外，選用分離元素軟體(PFC3D)模擬室內物理模型實驗過程，待模擬結果比對正確後再推廣至全尺度現地模擬，以消除尺度效應之問題。最終將依其崖線崩退型式所對應之崖錐堆積特性，利用半錐形函數，建置其崖錐堆積形態與落石群影響範圍之形狀函數。 由歷年地形分析與現場調查結果顯示，紅菜坪落石區崖線崩退型式分成減坡崩退與平行轉減坡崩退等兩種型式，主要崩退機制包含節理弱面分佈、地表水與地下水滲流侵蝕、互層差異侵蝕等，其中以節理構造主控崖線崩退影響最甚。而象山落石區為典型單面山地形，西翼崩崖屬逆向坡地形，其崖線屬減坡崩退類型；東翼崩崖略偏順向坡地形，其崖線崩退則以平行崩退型式為主。象山主要崩退機制包含地表水與地下水滲流侵蝕、互層差異侵蝕、平行崖面之張裂縫與節理分佈、弱面位態偏順逆向坡型態等。 物理模型實驗與地形分析結果顯示，於研究區域中當落石群崩落於不同角度之運動地形，所形成的堆積形態差異甚大。運動坡度於75、90度之地形條件時，其崖錐堆積以縱向前端(L)發展為主，並呈上、中坡段緩、下坡段陡之堆積形態；運動坡度於45、60度之地形條件時，崖錐堆積以橫向及縱向後端發展最為顯著，其落石群影響分佈範圍也較高角度運動地形為大，並呈上坡段陡及中、下坡段緩之堆積形態。當崩落型式屬減坡崩退型式，落石群於75、90度之地形運動時，將使崖錐堆積上坡段逐漸趨緩，呈上坡段更緩，中、下坡段陡之堆積形態；而45、60度運動地形減坡崩退則容易形成上坡段緩、中坡段陡、下坡段緩之堆積形態，其落石群受崖下堆積體之影響，將使落石群分佈範圍大幅減小，其崖錐堆積以縱向後方及兩側發展為主。當落石群崩落型式屬平行崩退時，於75、90度地形運動條件下，將發展成長軸平行崩退方向之半橢圓狀堆積體，並呈上坡段陡、中坡段緩、下坡段陡之堆積形態；而於45、60度地形運動條件下，堆積發展則以縱向後方及兩側發展為主，並呈上坡段緩、中坡段陡、下坡段略緩之堆積型態。 另外，藉由全尺度落石區案例數值模擬與物理模型實驗，將各影響因子(包含坡度、落距、落石量、節裡間距、堆積區粗糙起伏程度、崖線崩退形式等)對於堆積形態之特性，反應於崖錐堆積與落石群影響範圍形狀函數之中。其將有助於落石區現場觀察與研判未來落石群可能影響分佈範圍及未來崖錐堆積之趨勢，以期作為後續落石區土地利用與防護工程配置之參考。

During storms or earthquakes, Rockfall is a frequent mishap in mountain areas. Reviewing the histories of Rockfall disasters in Taiwan, as the cluster of Rockfall consists of simultaneous movements, it might cause serious disasters, which has occurred in Central Cross-lsland highway, Nanyali, Maijin road of Jilong City, Taidong Zhiben river, and the landslide-blocked lake of Longquan river. Such disasters have threatened the safety of life and need to be concerned. Whereas, this research focus on talus deposits development induced by various cliff retreat types in Hungtsaiping and Xiangshan Rockfall area. In the present paper, we analyzed the geomorphologic changes of cliffs and talus deposits based on the findings from the topographic maps and interpreting aerial photos. The physical modeling are carried out by simplifying in-situ Rockfall behavior, which has helpful to understand the relationship of cliff retreat and talus deposition patterns. The results of physical modeling were compared with those produced by numerical analysis (Application of discrete element method by PFC3D program) so that the correctness of the numerical simulation could be justified. Subsequently, calibrated numerical methods adopted in the small-scale model were used to simulate the full-scale model. The simulation results should be as close to reality as much as possible. Finally, combining the results of physical model with numerical analysis to establish the shape function of talus deposition patterns and the cluster of Rockfall influence area. According to the results of geomorphologic analysis and field investigation, the cliff retreat types include central rectilinear slope retreat type and parallel rectilinear slope retreat transform central rectilinear slope retreat type in Hungtsaiping Rockfall area. Based on field observations, three typical cliff retreat profiles at Hungtsaiping area presented which are dominated by the orientation and spacing of the discontinuities, the influence of groundwater seepage, and the degree of differential erosion. Xiangshan belong to the typical cuesta, since the initial topographic surface at eastern cliff was formed by gentle cataclinal slope, the cliff retreat type tended to parallel rectilinear slope retreat type. On the other hand, the western cliff was formed by steep anaclinal slope, they tended to central rectilinear slope retreat type. Five typical cliff retreat profiles at Xiangshan are presented which are dominated by the orientation and spacing of the discontinuities, the influence of groundwater seepage, the thickness of interbeded layer crop out above the apex of the talus slope, and the degree of differential erosion. Base on the results of physical modeling and geomorphologic analysis, the main talus deposition patterns in study area could be categorized into six types, i.e., (1) simultaneous retreat type with steep slope(>60o), the talus deposits tended to lengthwise deposition developement, and appeared gently dipping in the upper, middle parts and steep in the lower part; (2) simultaneous retreat type with gentle slope(<60o), the talus deposits tended to spread wider, and appeared steep dipping in the upper part and gently in the middle, lower parts; (3) central rectilinear slope retreat type with steep slope(>60o), the talus deposits tended to width of deposit developement, and appeared gently dipping in the upper part and steep in the middle, lower parts; (4) central rectilinear slope retreat type with gentle slope(<60o), the talus deposits tended to deposit progressively backward towards the cliff line and width of deposit developement, and appeared gently dipping in the upper, lower parts and steep in the middle part; (5) parallel rectilinear slope retreat type with steep slope(>60o), the talus deposits tended to the half of ellipse form major axis paralleled the cliff retreat direction, and appeared steep dipping in the upper, lower parts and gently in the middle part; and (6) parallel rectilinear slope retreat type with gentle slope(<60o), the talus deposits tended to deposit progressively backward towards the cliff line and width of deposit developement, and appeared gently dipping in the upper, lower parts and steep in the middle part. Furthermore, the shape function reflects the deposits characteristic of main factors (the factor includes the slope angle of movement area, the fall height, Rockfall amount, the joints spacing, the mean roughness height of deposition area, cliff retreat type) through the full-scale model simulation and physical modeling tests. The shape function are helpful for estimated Rockfall hazard zonation and developing reasonable and scientifically sound guidelines while giving land use assessment and protection engineering sited in Rockfall area.