加勁擋土牆以含細粒料之回填土在降雨作用下之行為及改善方法評估

Abstract

加勁擋土牆相較於傳統重力式擋土牆為柔性的擋土結構物，因此被視為邊坡穩定中最好的方法之一。然而為因應台灣地方法規，富含細粒料的現地土壤常被利用為背填土。當降雨入滲時，背填土基質吸力的喪失以及土壤剪力強度的弱化都是導致以含細粒料的現地土背填的加勁擋土牆破壞之主因。因此本研究致力於了解以富含細粒料的土壤為背填土的加勁擋土牆在降雨情況下的行為以及破壞模式。 本研究進行了一系列以富含細粒料的土壤為背填土的加勁擋土牆之縮尺試驗，除了了解其在降雨情況下的行為之外，也提出改善的方法供實際應用並探討其改善之效益。改善方法包含了縮小加勁材間距、使用高滲透係數的回填土、以及在加勁材界面鋪設薄砂層。試驗中降雨的延時設定使模型加勁擋土牆達到穩定狀態，或是直到擋土牆完全破壞為止。而在每組試驗中均紀錄了體積含水量、孔隙水壓、及牆變位；發展出的加勁材應變及土壤剪力也利用實驗後的影像處理來分析。 研究結果顯示縮小加勁材間距有助於維持模型加勁擋土牆之穩定，並使牆變位最小化。此外，使用高滲透係數的回填土雖然能有效的避免基質吸力的喪失，但仍會發展出張力裂縫，同時也使破壞機制轉為較於瞬間的表面淺層破壞。至於薄紗層的設計則是對以富含細粒料的土壤為背填土的加勁擋土牆在降雨情況下有相當優良的成效。除了加勁材與背填土之間的摩擦力增加之外，牆體本身受覆土壓力而變形的程度也因為採用了薄紗層而大幅改善。由於變形量為影響加勁擋土牆表現之關鍵因素，因此本研究認為在降雨情況下的設計安全係數須因應最大容許之牆變為來提升。

Geosynthetic reinforced soil walls (GRS walls) are considered as one of the best among all slope stabilization methods. However, in-situ soil (marginal backfill) was often adopted to adhere to a local regulation which specifies that the excavated and backfilled soils should be balanced. Loss of matric suction and soil shear strength due to rainfall infiltration is a main cause of the failure of GRS walls with marginal backfill. A series of reduced scale model tests was performed to investigate the performance of GRS walls with marginal backfill under rainfall conditions and to propose improved methods for practical design. The effects of the three improved methods, namely, reduction of reinforcement spacing, selection of better quality backfill, and adoption of sand cushions, were evaluated. Rainfall duration was set such that the wall model became fully wet or the monitored value reached steady state. The volumetric water content and the pore water pressure were monitored and the wall displacement was recorded throughout the tests. Mobilized reinforcement tensile strain was evaluated and the strain distribution in the wall models was analyzed via post image processing. The test results indicated that the reduction of reinforcement spacing can effectively improve the stability of the wall; meanwhile, the displacement was diminished. In addition, applying granular backfill can indeed avoid the loss of matric suction; however, tensions cracks or critical failure surface developed suddenly. Finally, the adoption of sand cushions is complementary beneficial to the performance of GRS walls with marginal backfill under rainfall condition. Not only was the interface friction enhanced but the deformation characteristic was improved. The sand cushions also accelerated the dissipation of water.