地表特徵與邊坡穩定性之關係－以鹿場邊坡為例

Abstract

本研究場址鹿場部落常見有多處崩塌現象產生，經現地調查發現此區上覆易受侵蝕之厚層崩積層。而在本研究區範圍內之道路常有龜裂、路基下陷、擋土牆損壞及建物傾斜等現象產生。因此，本研究將使用歷年航空照片與歷史地形圖來分析並說明鹿場部落因長期受到地質不穩定性之影響，進而產生崩塌、侵蝕等地形變遷之情形，及分析鄰近本研究區東側邊坡之鹿場溪歷年谿線高程變遷情形，並詳加敘述歷年河道對本研究區邊坡之影響性。 經歷年航空照片判釋結果顯示，本研究區近年來持續有新及二次崩塌之情形產生，其中陡邊坡處之局部崩塌範圍已影響至本研究區之道路，且鹿場台地下邊坡處疑似有張力作用所產生之裂隙；而於主稜線下之岩壁也隨時間之增加後退、崖錐堆積範圍明顯增加之情形產生；此外，於1980年之航空照片即可發現，本研究區內有多處下邊坡鼓脹區，可能於古老時期(無定年資訊)就已形成，顯示此區於早期即有地質不穩定之情形產生。基於以上之現象判定本研究區係屬不穩定之狀態。 其主因在於本研究區之東側邊坡係由材料性質較劣之厚層崩積層所堆疊而成，再受大雨滲入之影響下，抗剪強度變得更低，導致上部所覆蓋之厚層崩積材料部份流失；而於本研究區主稜線下之陡崖，長期不穩定之原因在於此區岩性係屬砂頁岩互層為主夾厚層砂岩，受差異侵蝕影響下產生較硬岩懸空，當豪雨來臨時易發生落石崩落之情形。

關鍵字：航空照片、歷史地形圖、地質不穩定性、地形變遷、谿線高程變遷。

One observes that many slumps usually happen in the tribal site, Luchang, Miaoli. Based on the results of the field investigation, we find that the top soil is composed of the eroded thick colluvium; the roads are cracked and subsided;the retaining walls are damaged; and the buildings are leaned in this site. In this study, the interpretation of aerial photographs and historical maps are adopted to analyze and show the impact of the geological on slope instability, the geomorphological change, and the rate of river incision of the Luchang River near the east slope of studied site. Also, the effect of this river incision on the slope stability of the study site would be investigated in detail. From the results of aerial photograph interpretation, we find that there are two major past slope failure events in the area between 1980-2009. Moreover, the events caused the road damages and the slope fractures under Luchang area reveals fractures that result from the effect of suspected tensions. In this situation, rocks under the major ridge would fall and taluses would increase proportional to time. Besides, it is found that, from the 1980’s aerial photographs, many colluviums under the slope of Luchang area have been probably formed in an ancient period (not an age dating of information). It could lead the area into instability. Based on the discussions above, the study site shows its instable status. Because the top of east slope is composed of thick colluvial deposits and the shear strength decreases due to the infiltration by heavy rain, this leads the cover of thick colluviums to erode away. For the steep cliff under the major ridge, it can make the whole situation instable because the lithology belongs to a kind of layered sedimentary rocks. According to the effect of differential erosion, hard rocks would be overhung and easily fall down when heavy raining.

Keywords: aerial photographs, historical maps, slope instability, geomorphological change, river incision.