回填土種類對勁擋土牆受降雨作用影響之模型試驗研究

Abstract

降雨長久以來除了對自然邊坡及擋土結構物造成偌大威脅外，亦被視為導致加勁擋土牆破壞的主要原因之一。縱使加勁擋土牆的概念與技術已有一定程度的發展，受降雨作用時仍不斷發生破壞；因此，降雨作用對加勁擋土牆之影響仍須研究及探討。本研究旨在探討回填土種類對加勁擋土牆受降雨作用之影響。 本研究進行了一系列加勁擋土牆之縮尺試驗，其中擋土牆的幾何參數及加勁材之強度根據模型相似性縮小，而回填土的性質則保持與現地相同。實驗中降雨是利用兩排懸吊在模型擋土牆上方的高壓噴嘴來模擬。此外，實驗中加勁擋土牆之變形及破壞模式均以相機錄影拍照，並在後續進行影像分析。 以富含細粒料土壤回填之加勁擋土牆之破懷模式為加勁材層間滑動或是過大變形。實驗結果顯示水能夠累積至含細粒料土壤之加勁擋土牆的頂部，牆面則呈現懸臂式變形，導致加勁材在頂部發展出最大的張力。以純砂土回填之加勁擋土牆整體來說有較良好的表現：僅出現層間的張力裂縫，牆面之變位也明顯較小；此外，牆面變位及發產出之加勁材張力在各層中的分布較為平均。本研究認為往後工程設計時應提高針對加勁材拉出及斷裂破壞之安全係數，並可在最上層鋪設阻水層及配置適當排水設施以確保上層土壤保持在不飽合狀態。

Rainfall infiltration continually causes serious threats on the stability of natural slopes and earth retaining structures and has been identified as one of the main causes for failures of geosynthetic-reinforced soil (GRS) walls. The fact that failure cases continue to occur despite existing established knowledge means there is a need to reevaluate what is understood of the effect of rainfall on GRS structures. This study therefore aims to investigate the performance of geogrid-reinforced walls with different backfill qualities subjected to rainfall. Experiments were conducted on reduced-scale GRS walls in accordance to the similitude law: wall geometry and reinforcement properties were scaled down, while backfill properties remain unchanged. Rainfall was introduced into the model by means of a suspended irrigation system. The wall deformation and failure process of the GRS wall model were also observed with a camera and analyzed via digital image analysis. For model walls built with silty sand, failure occurred in the form of interlayer sliding or excessive deformation. Water infiltration data showed water can accumulate up to the top of the wall. Wall deformations were large and had a cantilever shape, and the mobilized tensile load was concentrated at the top of the wall. For a model wall built with clean sand, the performance is overall better: failure was limited to vertical interlayer cracks, wall deformation was smaller and more uniform, water infiltration data showed good patterns, and distribution of mobilized tensile load was uniform across the reinforcement layers. These results will give better insights on design by increasing the required factor of safety against breakage and pullout for routine design calculations involving the simplified method, and by ensuring the top of the wall stays unsaturated through proper drainage measures or by applying an impermeable layer.