降雨及洪水條件下加勁擋土牆之表現評估

Abstract

加勁擋土牆，作為一種柔性擋土結構，已在全球範圍內得到廣泛應用，其環境友好性、經濟效益及視覺美觀等特性受到高度評價。然而，在考慮到成本效益及工程的土方挖填平衡時，工程實踐中常採用當地富含細粒料的土壤作為回填材料。這些材料通常具有較低的滲透性，尤其在颱風或豪雨期間或是洪水條件下，易因水分入滲導致土壤基質的吸力降低、孔隙水壓增加及剪力強度下降，進而可能引起擋土牆的過度變形甚至破壞。雖然此類結構具備多種優勢，但在使用含高量細粒的邊際回填土或設計不當時，其穩定性可能受到威脅。本研究針對極端氣候變化背景下的降雨和洪水事件，探討了加勁擋土結構的性能及其改進措施的效應。 研究方法包括物理模型實驗與數值模擬的結合，旨在評估面對不同氣候條件時加勁擋土結構的穩定性。實驗部分通過分析加勁材料的勁度、垂直間距及加入薄砂層厚度等變量對結構穩定性的影響，採用一個縮尺比例為5的模型，模擬3米高的原型加勁擋土牆在洪水或降雨情境下的表現，特別是當水位達到牆高的三分之二，或降雨強度達每小時75毫米。主要監控的參數包括潛水面水位、牆面變形和加勁材料的拉伸應變。在數值模擬方面進一步探討了細粒料含量對結構穩定性的影響，以及可能引發的急洩降風險。 研究發現，透過調整加勁材料間的間距或增加薄砂層的厚度，可以有效控制擋土牆的變形並改變其破壞模式。具體來說，薄砂層的加入不僅增強了拉拔阻力和結構強度，還改善了排水性能。在洪水條件下，提高加勁材料的勁度及減小垂直間距均顯著降低了牆面的變形。傳統上，基於土壤的力學破壞模式建議使用細粒料含量15%以上的回填土。然而，研究中發現，當細粒料含量6%時達到最低安全係數，這一發現表明，在水力條件下，破壞模式與傳統力學考量並不完全相同。此外，本研究還確定了牆面內部水位、安全係數與急洩降風險之間的相關性。 基於以上研究結果，本論文提出在降雨及洪水條件下加勁擋土牆設計之建議，並強調了為緩解氣候變化影響所需採取的措施。

This thesis explores the resilience and performance of geosynthetic-reinforced soil (GRS) structures under extreme weather conditions, such as heavy rainfall and floods, which are intensifying due to climate change. Renowned for their versatility, cost-effectiveness, and rapid construction, GRS structures are pivotal in civil infrastructure. However, failures often occur due to using marginal backfill soils with high fines content and suboptimal design practices. This study aims to fill a gap by assessing how severe weather affects GRS walls. A series of model tests and numerical simulations were conducted to examine the impact of rainfall and flooding on GRS wall stability. The experiments evaluated the effectiveness of different reinforcement stiffness, vertical spacing, and the presence of sand cushions in enhancing wall stability. A reduced scaled model (N = 5) was used to represent 3-meter prototype GRS walls exposed to flood conditions up to two-thirds of the wall height or rainfall intensities of 75mm/hr. Key metrics such as phreatic surface levels, wall deformations, and reinforcement tensile strains were monitored and analyzed. The result indicates that in rainfall simulations, reducing the spacing between reinforcements or incorporating sand cushions controlled wall deformations effectively and modified failure modes. Notably, the sand layer improved pullout resistance, strength, and drainage properties. During flood conditions, both increasing reinforcement stiffness and reducing vertical spacing were effective, with the latter proving more significant in mitigating wall deformations. Based on soil mechanics, regulations recommend using backfill with fines content more than 15%. However, numerical simulations showed that the relationship between fines content and safety factor is nonlinear, and the most critical safety factor occurs at a fines content of 6%. Demonstrating that the failure mechanism differs from the traditional mechanical mode under hydraulic conditions. Based on these findings, the thesis discusses design strategies for using GRS walls as effective waterfront protection structures, highlighting adaptations to counteract the challenges posed by climate change.