坡地型廢棄物掩埋場邊坡穩定分析

Abstract

台灣地區由於人文社經條件的考量，掩埋場往往只能興建於城鄉邊緣的山坡地，目前台灣山坡地掩埋場營運中尚約有53場。由於掩埋場相關法令以及設計規範尚不完善，因此目前台灣的掩埋場設計之時大多未曾做過詳盡之邊坡穩定分析。此外，操作單位也並未根據工程計畫進行廢棄物的填埋，導致最終掩埋面往往超過預期的高度。上述情形導致坡地型掩埋場的破壞潛能提高，其穩定性值得深入探討。此外，掩埋場底部因地工膜布與相接觸之材料間的界面剪力強度頗低，形成最易發生滑動的弱面，因此本研究以掩埋場底部弱面破壞為對象，利用二維極限平衡之邊坡穩定分析法，探討山坡地掩埋場幾何設計參數安全係數之關係。 研究方法採用邊坡穩定分析程式SLOPE/W，首先以程式內建的五種破壞面搜尋形式探討各切片分析法之適用性，以決定後續參數變異性分析，進行的搜尋方式及分析法，再針對下列掩埋場穩定設計因子：掩埋面坡高(H)、掩埋場底部長度(L)、掩埋場底部邊坡角度(α)、掩埋面邊坡角度(β)、廢棄物剪力強度及地工合成材料界面剪力強度(ca、δa)，進行一階與二階等一系列組合的分析。同時，考慮地震力作用下，一階及二階廢棄物掩埋場之界面摩擦角參數變異性分析。 邊坡穩定分析結果顯示，界面摩擦角愈大、坡地型掩埋場之高度愈高、掩埋體長度愈短、背靠之開挖山壁坡角愈平緩、掩埋完成面之坡角愈平緩，則界面滑動之安全係數愈小。平台寬度對於二階掩埋場之邊坡滑動安全係數，影響不大，但仍可看出平台愈寬安全係數愈高之趨勢。掩埋體長度小於60m時，破壞模式由滑動界面控制，掩埋體長度大於70m時，破壞模式由淺層圓弧破壞控制。 分析結果顯示，坡地型廢棄物掩埋場之標準斷面( δ=15°、α= 60°、β = 45°、H = 10 m、L = 30 m、B1 = 3 m)，考慮界面滑動破壞，則可提出下列建議，以提供設計及分析之參考: (1)界面摩擦角(δ)至少大於19°，(2)掩埋高度需小於15 m或每階高度不得大於8 m，(3)掩埋長度(L)建議大於40 m，(4)背靠的邊坡之平台寬度(B1)建議不得小於3 m，(5)則背靠的坡角(α)可建議30° ~ 45°或60° ~ 75°，(6)掩埋面坡角(β)建議介於30° ~ 60°。 考慮地震力作用下，與未考慮地震力作用的分析結果比較。在界面滑動破壞模式時，安全係數平均下降約36%~59%，抵抗界面滑動之界面摩擦角提高8°~12°。在淺層圓弧破壞模式時，安全係數平均下降約25%~39%。由分析結果建議，一階坡地型廢棄物掩埋場，界面摩擦角必需大於21°；二階坡地廢棄物掩埋場，界面摩擦角必需大於25°

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. As a result, the potential risk of slope failure of these landfills is very high. Furthermore, although the interfacial shear strength between the waste and geosynthetics is very important to landfill slope stability, it has seldom been considered in the design. The objective of this study is to perform stability analyses on block failures through weak interface in the bottom lining system for typical landfills on slopes. Two-dimensional slope stability analyses were conducted with SLOPE/W 2004. Geometric 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. In addition, slope analyses are also performed for earthquake conditions. The results of 2-D stability analyses show that the factor of safety increases with lower height of wastes, longer length of waste body, higher slope angle of the back of the excavation, steeper face slope of waste body and higher interfacial friction angle. On the other hand, the factor of safety is only hardly affected by the berm width. While the length of waste body is smaller than 60m, failure is dominated by the interface sliding. While the length of waste body is greater than 70m, failure is governed by the rotational sliding within the waste body. According to analysis, the standard section of MSW Landfills on Slope ( δ = 15°、α = 60°、β = 45°、H = 10 m、L = 30 m、B1 = 3 m). When it concerned to interface slide, we can provide the following suggestions in order to offer references of design and analysis: (1) the interfacial friction angle (δ) more than 19° at least, (2) the height of the wastes must less than 15 m or the height of every layer not more than 8 m, (3) the length of the base (L) should more than 40 m, (4) the width of the berm (B1) should not less than 3 m, (5) slope angle of the back (α) should 30° ~ 45° or 60° ~ 75° (6) the waste should be filled to a slope angle between 30°~60° When the effect of earthquakes is taken into account, the decrease of safety factor ranges from 36% to 59%, which corresponds to a increase of the required minimum interface angle against translation slope failure from 8 degrees to 12 degrees. According to the results of this study, it can be concluded that the interfacial friction angle between the materials of the lining system should be carefully determined and stability analysis must be performed when designing the landfills on slopes. Furthermore, it is suggested that the minimum friction angle of the interfaces must be larger than 20 degrees to prevent translational slope failures.