加勁擋土牆受降雨入滲作用下之物理模型試驗研究

Abstract

加勁擋土牆為柔性擋土構造物，近年來常被廣泛的應用，具有環境友善、經濟及美觀等優點。然而，由於考量施工成本及挖填平衡之規定，在回填土選用上，常就近採用現地富含細粒料之土壤。此土壤一般具有低滲透性的特質，在颱風豪雨期間，受降雨入滲作用，造成土壤基質吸力喪失，孔隙水壓累積，土壤剪力強度下降，最終導致擋土牆過度變形，甚至產生破壞。因此本研究致力於瞭解以富含細粒料土壤為回填土之加勁擋土牆受降雨入滲作用下之力學行為與破壞機制。 本研究進行一系列縮尺模型試驗，縮尺率(Scaling law)為 N = 5，在原型尺度下，加勁擋土牆高為3 m，探討不同加勁間距之加勁擋土牆在降雨作用下的力學表現與穩定性，加勁間距分別有100、75、60與50 cm，除了了解降雨情況下之行為外，也以加勁間距75 cm為基本案例，評估以薄砂層做為改善工法，改變薄砂層厚度並探討其效益，薄砂層厚度分別為10、20與30 cm，而降雨強度為75mm/hr。在量測方面，加勁擋土牆內安裝水份計、水壓計，並透過影像分析技術分析牆面變位、牆頂沉陷量，破壞面上土壤剪應變發展，與加勁材應變量，藉此了解隨降雨入滲過程，加勁擋土牆變形與破壞發生之過程與機制，更利用水溶性螢光染劑試驗，了解模型內部真實流線分布。 試驗結果顯示縮小加勁間距有助於維持加勁擋土牆之穩定性，並使牆體變形在相同降雨條件下達到最小。增加薄砂層厚度，可以有效延緩水分入滲時間，降低孔隙水壓力累積，也可藉由薄砂層較高之介面摩擦係數提升加勁擋土牆之總體勁度，抑制牆體變形。透過螢光染劑試驗更證明薄砂層確實具由排水功能。最後依據試驗結果，提出在不同降雨期距下，抗降雨設計方法之相關建議。

Geosynthetic reinforced soil wall (GRS wall) is an ecology engineering and can against the heavy rainfall. Due to the economic consideration and local regulation that the GRS wall usually applies marginal backfill. However, marginal backfill has low permeability that easily accumulates the pore water pressure and decreases the soil shear strength. Finally, the wall will be deformed or collapsed. In this study, a series of reduced scale model tests were performed to investigate the performance of the GRS wall with marginal backfill subjected to rainfall. According to the scaling law N = 5, the experimental tests modeled 3 m geogrid-reinforced soil walls with various reinforcement spacing Sv = 100, 75, 60, 50 cm in prototype, and also based on the spacing 75cm to apply different thickness of sand cushion tsc = 10, 20, 30 cm subjected to the rainfall with an intensity of 75 mm/hr. The distribution of the volumetric water content and accumulation of the pore water pressure were monitored during the test by instrument. The development of the wall displacement and reinforcement tensile strain were observed through the digital image analysis to evaluate the failure process and failure mode of GRS walls. The test results indicated that decrease the reinforcement spacing can effectively reduce the wall displacement. Adding the sand cushion can delay the water infiltration and decrease the accumulation of pore water pressure and also due to the higher interface shear strength to restrict the deformation and enhance the stability of the wall. Based on the test results, some suggestions for the design method with different return periods are purposed.