滲出水淤積對掩埋場穩定及掩埋場變形之影響

Abstract

台灣土地資源有限，人口密度又高，在人民自我意識高漲的今日，適當之垃圾衛生掩埋場用地實不易覓得及取得，地方政府所選之場址大多位於山坡地之河谷帄原區、山窪、坡腳等處。掩埋場周邊的排水系統及內部的滲出水系統是否能正常營運往往會影響掩埋場之穩定性。在國內由於掩埋場的設計或施工不良造成滲出水系統在營運期間損壞無法使用，導致滲出水淤積在掩埋場內部。本研究以掩埋場底部弱面為研究重點，利用傳統二維極限帄衡穩定分析，探討掩埋場滲出水淤積與形狀因子對掩埋場穩定安全係數之關係。另外，由於傳統極限帄衡法無法考慮材料本身之變形性，故利用有限差分軟體分析掩埋場之變形，探討掩埋場形狀因子與掩埋場變形之關係。 本研究利用SLOPE/W軟體分析掩埋場之穩定性，首先使用SEEP2D軟體計算掩埋場內孔隙水壓值，利用各不同地下水位高度來模擬滲出水淤積在掩埋場內之情形。掩埋場變形則是利用FLAC軟體分析，而掩埋場穩定與掩埋場變形分析則針對以下掩埋場之設計因子：掩埋面高度(H)、掩埋面長度(L)、掩埋場底部邊坡角度(α)、掩埋完成面邊坡角度(β)、掩埋場台階寬度(B1)進行一階與二階掩埋場參數變異性分析，而掩埋場穩定分析多了一項掩埋場底部界面摩擦角(δ)之參數變異。 掩埋場穩定分析結果顯示，當掩埋面高度(H)越高、掩埋面長度(L)越短、掩埋場底部邊坡角度(α)越小、底部界面摩擦角(δ)越小對於掩埋場內有地下水時，會大幅降低其掩埋場安全係數。掩埋場底部界面摩擦角對安全係數影響最大，當地下水位越高時，安全係數變化之幅度會越小。 掩埋場滲出水淤積對掩埋場變形之影響，滲出水淤積會增加一階掩埋場之水帄位移量，而二階掩埋場則會降低體積壓應變量。在不考慮滲出水淤積情形下，一階掩埋場背靠邊坡角度(α)影響剪應變量最大。掩埋面高度(H)對於垂直向位移量與體積壓應變量影響最大。而掩埋面長度(L)則是對於體積張應變量影響最大。另外，二階掩埋場之體積壓應變、水帄位移、垂直位移影響最大者為掩埋場背靠邊坡角度(α)。 此外值得關注的是，一階掩埋場安全係數隨著掩埋面長度(L)增加而遞增，但掩埋場之垂直位移量亦隨著掩埋面長度增加而遞增。而二階掩埋場安全係數隨著掩埋面長度(L)增加而遞增，但掩埋場之體積壓應變與位移量亦隨著掩埋面長度增加而增加。

Due to high population density in narrow coastal plains and the resistance of the public arisen from the not-in-my-backyard (NIMBY) attitude, a large portion of landfills in Taiwan are located in mountainous area. Current regulations on landfills do not require slope stability analysis while designing these landfills. In addition, the potential sliding of along the weak interfaces along the components of lining systems was not recognized. As a result, the potential risk of slope failure of these landfills is very high. On the other hand, the leachate collection and drainage systems in Taiwan’s landfills were poorly designed and operated such that it is not uncommon to find large quantity of leachate accumulate above the lining system and increase the risk of slope failure. The objective of this study is to evaluate the effect of leachate accumulation on the slope stability of municipal solid waste landfills in mountainous areas. Stability analyses on block failures through weak interface in the bottom lining system for typical landfills on slopes were performed while taking the pore water pressure generated by leachate into account. The pore water distribution under assumed maximum accumulated leachate head was generated with the computer software SEEP2D and then incorporated into analyses by the slope stability software SLOLE/W. Furthermore, the deformation of the landfills was also computed by the software FLAC to study the relative criticality of landfill deformation to the overall landfill stability. In these analyses, parameters such as height, slope angle of the back, the slope angle of the waste body, the length of the base, the width of the berm, and also the interfacial friction angle were varied to investigate their effect on the factor of safety against slope failure and deformation. The results of stability analysis show that leachate accumulation significantly reduces the factor of safety, especially when higher height of wastes, short length of waste body, lower slope angle of the back of the excavation, gentle slope of waste body and lower interfacial friction angle. The results of deformation analysis show that leachate accumulation significantly increase horizontal displacement for one-stage landfill and reduce volumetric compression strain for two-stage landfill. For one-stage landfills, the angle of the back of excavated slope has greatest effect on shear strain. On the other hand, the vertical displacement and volumetric compression strain was affected by the height most. For two-stage landfills, the angle of the back of excavated slope has the greatest effect on both the volumetric compression strain and the displacement. In addition, while the factor of safety increase with the length (L) of landfill, the vertical displacement increase with length (L) for one-stage landfill.Similarly, while the safety of factor increase as length (L) increases, the volumetric compression strain and displacement also increase with length(L) for two-stage landfill.