土壤水分移動與植生根系對邊坡穩定之研究

Abstract

本文乃利用相關理論與模式分析去探討在不同自然環境條件下，植生的存在對於邊坡穩定上實際的效益，並建構了一套部分飽和植生邊坡穩定分析模式，不僅將未飽和層之土壤水分傳輸對土壤邊坡安全係數的影響納入傳統分析法中，而且將植生根系調查資料量化成根力模式，適當地整合生物、自然環境與工程設計相互作用之機制。在邊坡穩定分析方法中，使用無限邊坡切片法、Bishop修正法及Janbu簡化法；在土壤水分傳輸方面則加入遲滯效應之影響；在根力模式中，我們利用吳正雄（1990）對台灣杉及山黃麻的根系分析結果作為基礎。進而評估不同濕鋒入滲型態與植生條件下，通過不同特性之破壞面的安全係數變化。 分析結果發現，不論邊坡坡度或破壞面的形狀特性，不同的濕鋒入滲型態或植生條件會明顯地影響安全係數的改變，故在使用傳統之邊坡穩定分析法時，應考量土壤水分傳輸與植生所產生的效應。由植生效益與破壞面深度之分析得知，台灣杉及山黃麻的根系對於較深之破壞面並無顯著的正面效益，只對淺層破壞有所助益，而山黃麻又較台灣杉能提供更多的穩定性，深破壞面反而會因為植生地上部之荷重增加導致安全係數降低。由植生設計間隔與位置之分析結果來說，植生種植密度越低，其根系對於邊坡穩定之正面效益越不明顯，而種植密度過於密集又會增加邊坡荷重與影響作物正常生長等問題。此外，增加或減少植生種植間距對於深層不穩定來說並無法有效地提升安全係數，只能藉由降低土壤含水量（地表水與地下水排水工程）或坡度來增加其穩定性；而淺層破壞則可利用適當間距的植生，提升單位面積的根系數量，有效地達到增加邊坡的穩定性。由本研究模擬結果可知，並非每種植生都適用於每個邊坡的生態工法設計上，必須經由更細密的研究分析，才能夠找出真正適用於特定邊坡上的生態工法。

This research established a partially saturated vegetated slope stability model combined the transportation of soil water content and root model. The paper discussed the actual benefits of the root element of the vegetation offered to the slope stability under different environment and integrated the mechanism of biomechanics, environmental, and engineering properly. In the methods of slope stability, we modified the slice method of infinite slope, Bishop’s modified method and Janbu’s simplified method. In the transportation of soil water content, the hysteresis effect is considered in the simulator. Besides, the root system of the vegetated element in this study is based on “Relations of Root System Mechanics and Slope Stability” (Wu, 1990) in which investigated root system mechanics of Taiwania cryptomerio ides and Trema orientalis (L.) Blume. Finally, the present model in this research calculated the safety coefficients of the different destruction surface in accordance with different soil water content conditions and the kinds of vegetation. The results indicated that different distributions of soil water content and the kinds of vegetation would change the safety coefficient apparently regardless of the slope gradient and the patterns of destruction surface. Therefore, the engineers should consider the transportation of soil water content and the vegetated elements when using the traditional analysis methods of slope stability. The root model of Taiwania cryptomerio ides and Trema orientalis (L.) Blume had no remarkable benefits to deeper destruction surface, but raised the safety coefficient of shallow destruction surface obviously. On the contrary, the weight of vegetative body reduced the safety coefficient of deeper destruction surface. The root system of Trema orientalis (L.) Blume is better than Taiwania cryptomerio ides in the shallow slope stabilization. The simulative results of different vegetative arrangements also showed that the planting intervals are sparser and the benefits offered to the slope stabilization are more unapparent. On the other way, the planting intervals are too close to grow normally, and the vegetation increases the loading of the slope. Besides, decreasing the soil water content of the slope or cutting down the slope gradient are effective strategy to raise the stabilization of the deeper destruction surface. Utilizing the suitable planting intervals to increase the root amount of the unit area can enhance the slope stability effectively. In conclusion, not all kinds of vegetation are suitable for some particular slopes in the design of the ecological engineering. Detailed researches and analysis are required to identify the suitable ecological engineering for a particular slope.