以加勁擋土牆作為臨水護岸之模型試驗

Model Tests on Geosynthetic-Reinforced Soil Walls as River Banks Protection

林佳靜(Jia-Jing Lin)

指導教授 ： 楊國鑫

國立臺灣大學/工學院/土木工程學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202202130

摘要

加勁構造物為俱有良好施工性、經濟性及環境友善之柔性結構，近年來廣泛使用於國內之防災工程、邊坡整治、交通工程及環境保育。實務上，加勁結構物透過將加勁材料鋪設於回填土中以提升結構體的穩定性。然而，國內考量經濟成本及挖填方平衡，常使用富含細粒料之現地土壤做為回填材料。此回填土滲透性低且多為不飽和土壤，遇水後容易累積孔隙水壓力，喪失基質吸力造成土壤剪力強度下降，並伴隨結構體過分變形及破壞的發生。因此，採用富含細粒料回填土之加勁結構物是否能做為臨水護岸使用仍然存疑。
本研究進行一系列縮尺模型試驗，探討加勁擋土牆受洪水作用之力學行為及破壞機制。實驗縮尺率為 N = 5，模擬3公尺高之現地擋土牆。試驗分為五組，評估不同加勁材材料勁度及間距對於擋土牆穩定性的影響。在原型尺度下，以加勁間距75公分作為基本案例，探討在同樣間距下使用加倍勁度與三倍勁度之加勁材料，以及不改變加勁材勁度並採用37.5及25公分加勁間距，是否能有效提升整體穩定。本試驗透過量測水分及水壓分布情形、牆體變形及加勁材應變發展，結合影像分析技術，討論加勁擋土牆於水分入滲的過程中所產生的破壞機制及改善成效。
實驗結果顯示擋土牆發生三階段漸進式破壞：水分入滲引致土壤有效應力下降，牆面隨之變形形成剪力裂縫；牆體發生主動破壞並且過分變形，隨後水面上之塊體因重力作用傾覆，形成完整滑動面及牆頂張力裂縫；急洩降使得變形及破壞弧發展加劇。此外，分析結果指出，以縮小間距作為改善方式可於第一階段隨即抑制牆面變形，而加強勁度則是於第二階段提供較顯著的改善。顯示出在妥善設計下，加勁擋土牆可做為臨水護岸使用。在研究的最後更以實務常使用之土壓力法評估各層加勁材之最大張力發展，發現土壓力法有低估的現象。

關鍵字

臨水護岸 ； 加勁擋土牆 ； 破壞機制 ； 縮模實驗 ； 邊際回填土

並列摘要

Geosynthetic reinforced soil (GRS) retaining walls have been extensively adopted as alternatives to conventional gravity walls in earthwork construction due to their flexibility, eco-friendliness, ease of construction, and higher differential settlement tolerance, etc. In engineering practice, marginal soil is commonly used as a backfill for GRS walls, but the strength of marginal soil can be substantially reduced during water seeping, results in reduced wall stability. Thus, the application of GRS walls as waterfront structures subject to rising water level still requires further investigation.
This study performed a series of model tests to investigate the performance of flooded GRS walls. The influence of various reinforcement stiffnesses and vertical spacings in improving wall stability was evaluated. Test models with a scaling factor N = 5 were used to model 3-meter prototype GRS walls subjected to flood up to two third of the wall height. The phreatic surface level, wall deformation, and mobilized reinforcement tensile strain were monitored and analyzed. The test results indicate the GRS wall developed a progressive failure: (1) The soil lost strength and formed shear cracks near the wall face during flood level rising. (2) The soil actively failed and generated excessive wall deformation. (3) The wall deformation further increased due to rapid drawdown. The test results found that decreasing reinforcement spacing has better efficiency in enhancing wall stability and reducing wall deformation than increasing reinforcement stiffness. Based on the test results, this research discusses the design implications for GRS walls as waterfront protection structures.

並列關鍵字

GRS Walls ； Waterfront protection structure ； Failure mechanism ； Model test ； Marginal backfill

參考文獻

AASHTO (2002). Standard Specifications for Highway Bridges, 17th edn. American Association of State Highway and Transportation Officials, Washington, DC, USA.

ASTM D3080. (2011). Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions”, ASTM International, USA.

ASTM D4595. (2017). “Standard Test Method for Tensile Properties of Geotextiles by the Wide-Width Strip Method“, ASTM International, USA.

ASTM D5084. (2008). Standard Test Methods for Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter. ASTM International, USA.

ASTM D5321. (2008). Standard Test Method for Determining the Coefficient of Soil and Geosynthetic or Geosynthetic and Geosynthetic Friction by the Direct Shear Method. ASTM International, USA.