- 學位論文
六角形金屬網(120 mm*150 mm)石籠破壞機制與強度特性
Hexagonal wire mesh (120 mm*150 mm) gabions failure mechanism and related strength characteristics
摘要
生態工法乃環境與工程共生共存的自然工法，其中石籠工法在台灣運用於護岸、堤防、擋土牆等工程上相當廣泛。石籠是由金屬線材編織成六角形網孔的網狀結構，網片經組裝後內部填入卵礫石料或土石籠袋而成；並具有良好透水性、柔性結構等優點。然而石籠雖有上述優點，但實務上常因自然外力導致金屬網破壞，並失去其功能性；故需對金屬網做相關力學破壞機制之測試。現行相關金屬網規範中，然美國材料試驗協會(ASTM)所制定的ASTM A975標準規格，有訂定較為完整的石籠六角形金屬網規格強度特性及測試機制；由於地形、地質與工程特性等因素；國內常用規格尺寸並未表列於ASTM A975標準規格中。 本研究共分兩階段，第一階段以現地踏勘方式，分析國內石籠破壞案例佐證A975力學測試機制；第二階段針對國內最常用之六角形金屬網規格(線徑ψ4.0mm，網孔規格為120mm×150mm)，依據A975力學特性測試法，評估不同編織法對網身縱橫向抗拉、邊框線接合抗拉及貫入強度等之影響。然而，石籠網身因有其焊線處、單線鉤接破孔等弱面缺陷及施工不當案例造成網身破孔之情形，亦為研究項目之一；故在金屬網身縱橫向抗拉及貫入試驗等項目，執行網片中央處剪斷後對強度折減之影響。網片間接合強度部分，以線徑2.3mm、3.0mm之雙股絞接接合法做接合強度分析。然而實務上，籠體網片與網片間必須接合組裝；故網身受力時，有側邊拘限力存在。然而ASTM A975並未對網身抗拉試驗時，側邊緊縮現象所產生之側邊抗張力有所訂定。為模擬現地網片固定後側邊拘限力對抗拉行為之影響，故本研究亦也執行側邊拘限-網身縱橫向抗拉試驗，針對網身不同編織結構在完整網片與中央剪斷網片狀態下，側邊拘限力對網身抗拉試驗之影響。 透過現地破壞踏勘結果發現，石籠現地破壞型式亦有脹裂及貫入等破壞行為；並多位於網片間接合處及邊框線接合處等結構弱面。由側限條件-縱向抗拉、非側限縱向抗拉與貫入試驗可見，完整網片狀態下；三圈直向編織(Type-A)與四圈斜向編織(Type-B)金屬網，兩者強度差異皆不超過5%。然而網身經中央剪斷後，Type-A抗拉強度皆折減至完整時之40~50%左右；反觀四圈仍有80~90%的強度保留率。由側限條件-橫向抗拉、非側限橫向抗拉試驗數據得知，Type-B金屬網橫向抗拉強度較Type-A金屬網高20~30%，兩者橫向網身抗拉強度皆是縱向時的50%左右。Type-A橫向網身經中央剪斷後，強度折減至完整時之50%左右，為各試驗項目中之最低值；反觀四圈仍有80~90%的強度保留率。三圈直向編織結構(Type-A)經網線斷裂後，斷線處會隨著拉伸力由扭轉軸反向退轉破壞，使破孔容易拓大且阻抗強度不易上升；反觀四圈斜向編織結構(Type-B)則屬扭轉軸節點斷裂破壞，網身受力行為較不受斷線影響。並由拘限-橫向完整抗拉試驗結果得知，橫向網身經側限儀固定後，兩者破壞均由邊框線接合處產生；亦此可證其邊框線接合對網身橫向抗拉強度影響甚大。由邊框線接合抗拉試驗結果得知，四圈斜向編織(Type-B)因網身結構使邊框線扭轉節點受力分散，其扭轉處不易退轉。反觀，三圈直向編織(Type-A)網身結構使扭轉節點受力集中較易退轉。故兩者於典型單邊纏繞固定法上；四圈網身邊框線接合強度較三圈高55.5%。由網片間接合強度試驗，絞接接合法於 ∅ 2.3mm 施作時，常因施作較易(線徑細)而導致扭轉緊實度較鬆散或扭轉過緊而使綑紮線斷裂，其上述之變異因素，亦影響試驗數據之重覆性。反之，∅ 3.0mm因施作較難(線徑粗)，較無此例。∅ 2.3mm絞接7處之接合抗拉強度三組數據平均值為23.76 kN；∅ 3.0mm絞接5處則為33.93 kN。 綜合以上，依本研究之成果，期望未來ASTM A05技術委員會，將台灣石籠金屬網實務常用之規格納入ASTM A975標準中及其規格之力學特性要求強度，並希望本研究成果有助於國內相關單位修訂規範及訂定金屬網力學特性測試法。 關鍵字：六角形金屬網、箱籠、蛇籠、護岸、邊坡穩定、落石防護。
並列摘要
Metal wire mesh is commonly practice for river bank protection, rock fall protection, and slope stabilization applications. Currently, the use of wire mesh for river bank protection and slope stabilization applications is more than 15 million square meters per year in Taiwan. Only the materials cost is more than 20 billion New Taiwan dollars per year. Due to natural disasters or human activities, the replacement of wire mesh for these applications is too often. Currently, ASTM A975 hexagonal wire mesh standard specification consist the test methods and material specifications for two general used wire mesh within US markets. However, the specifications for the commonly used wire meshes in Taiwan are included in the standard. The tasks of this study include two parts. The first task is to conduct a series field investigate to explore the gabion failure mechanism in practice applications. The second task is to evaluate the engineering property of a commonly used wire mesh (120mm x150mm, ψ=4.0mm) with triple-twist (Type A) and fourth-twist (Type B) weaving methods according to ASTM A975 test standard. Tensile strengths in longitudinal and transverse directions without and with lateral constrain conditions, panel connection strength using lacing wire or twisted wires, connection strength to selvedges, and punch strength are all evaluated. Test panel without and with one center cut were also tested to simulate the wire braking condition during penal service life. The penal to penal connections and penal connection to selvedge are the weakest locations for box and tube gabions. Burst and punch failures are the most common failure mechanisms. Weaving method (Type A vs. Type B) induces only 5% variance in the longitudinal tensile strength and punch strength for without center cut samples under lateral constrain or non-lateral constrain conditions. However, 15% and 24% variances were observed in transverse tensile strengths of non-lateral constrain and lateral constrain samples without a center cut due to weaving method (Type A vs. Type B). Weaving method (Type A vs. Type B) also has a great influence on the tensile strength and punch strength for panel with and without a center cut. In general, Type A samples would retained only about 40% to 50% strength after the sample with a center cut. However, Type B samples would retain about 80% to 90% strength for a center cut sample. In general, lateral constrain would slightly increase the longitudinal or transverse tensile strengths for samples without a center cut for type A and Type B wire meshes. However, lateral constrain would only improve the tensile strengths for Type B samples with a center cut. Lateral constrain would only induce a minor influence on the tensile strength on Type A wire meshes. Wire un-twisting behavior near the breaking wires was observed for most Type A sample tensile and punch tests. This behavior would cause widening the sample opening and reduce the strength of the wire mesh. However, wire breaking is the individual events and has a very influence for the adjacent wires for Type B sample during tensile and punch tests. Due to Type B panel is a diagonal weaving pattern mesh, the tensile load can be more evenly transfer to each wire perpendicular to loading direction. The panel connection to selvedge strength for Type B mesh is about 55% higher than that for Type A panel. Stress concentration and wire un-twisting behavior near breaking wires were observed for Type A panel sample during the connection to selvedge tensile tests. Seven diameter 2.3 mm twisted wire joints per meter can provide 23.76 kN/m connection strength. However, five diameter 3.0 mm twisted wire joints per meter would provide 33.93 kN/m connection strength. These connection schemes are all greater than the value shown in ASTM A975 specification. The engineering strength properties for diameter of 4.0 mm hexagonal wire mesh with opening of 120mm by 150mm obtained from this study would be valuable information for ASTM. The data would be valuable information for ASTM A05 committee for future revision of ASTM A975 standard specification. In addition, the results of this study can be adopted by different Taiwan government agencies to revise related handbooks or specifications. Keywords: hexagonal wire mesh, gabion, sack gabion, river bank protection, slope stabilization, rock-fall protection.