應用地工合成材加勁擋土牆防治土石流之研究

Abstract

加勁擋土牆由設計方法不同，可分作Mechanically Stabilized Earth Walls（MSEW）與Geosynthetics Reinforced Soil Composites（GRSC）。前者之設計理念與背拉式擋土牆接近，後者則是將其視為複合材料進行設計。本研究應用PLAXIS之數值軟體，以數值模擬的方式，探討不同設計概念之加勁擋土牆，在受到以擬靜態方法模擬土石流衝擊力的側向荷載時，牆面與牆頂之變形行為、加勁材受力情形，牆背土壓力分佈情形、以及牆體穩定性之變化。內容共分為：（1）改變加勁區範圍（2）加勁材間距變化（3）單側與雙側疊塊式牆面之比較（4）使用不同剪力強度參數之內填土（5）疊塊式牆面與灌漿式牆面之比較（6）持續加載或間歇性加載之結果差異等六大主題進行討論。 由研究結果發現，若內填土壤的剪力強度較弱、或雙面疊塊式擋土牆之厚度較薄，在受到側向壓力時，牆面容易出現較大的變形，且無法承受過大的側向壓力即發生破壞。牆面構築型式亦會影響牆面變形，若牆面為連續體時，因剛度較高，故在相同的側向壓力作用下，灌漿式牆面擋土牆之牆面變形量較小，安全穩定性亦較高。加勁擋土牆之牆頂變形量會隨著加勁區範圍變大而減少。牆背之土壓力分佈與加勁材之間距有關，若間距愈大，則層間土壤承受之土壓力愈小。又當雙面式疊塊擋土牆受到側向壓力時，於後方牆趾處會有應力集中的現象。若改變加勁區範圍或加勁材間距改變，在施加小荷重的情形下，因側向壓力會抵消部分土壓力，反而有助於牆體的穩定；但當荷載逐漸增大，牆體開始出現被動式破壞，故安全係數會隨著施加的側向壓力變大而有先升後降的情形。若牆體受到間歇性荷載，會有部分土壓力蓄積於牆背無法消散，而牆面與牆頂變形以及安全係數等，在加壓前後則沒有太大變化。

Geosynthetic-reinforced soil walls can be classified into two types according to the design concept: Mechanically Stabilized Earth Walls (MSEW) and Geosynthetics Reinforced Soil Composites (GRSC). The former is designed and constructed in the same manner as a tied-back wall; while the latter is treated and designed as a composite material. In this study, the finite element software, PLAXIS, was used to analyze the behaviors of the aforementioned two types of walls under quasi-static lateral pressures induced by debris flows. The influencing factors investigated were as follows: (1)width of reinforced zone; (2) vertical spacing of reinforcement; (3) one-faced and double-faced walls; (4) shear strength of soil; (5) block-facing and grouted-facing walls; (6) continuous loading and periodic loading. The results showed that the deformation at the top of the walls decreased as the width of reinforced zone increased. The distribution of lateral earth pressure at the back of wall-facing varied with the vertical spacing of the reinforcement; as the spacing was increased, the earth pressure between two layers of reinforcement would be smaller than the earth pressure close to the reinforcement. In addition, stress concentration was observed at the heel of a double-faced wall when its front side subjected to lateral pressure. Moreover, the factor of safety tended to increase as the wall was subjected to small lateral pressure, but it then decreased with increasing lateral pressure. The factor of safety of the wall was also affected by the shear strength of soil, the thickness of double-faced wall, and the facing type of wall. In the case of periodic loading, the stresses behind the wall-facing could not be fully dissipated when unloaded. However, the periodic loadings did not affect much the deformation of the wall-facing, the settlement, as well as the factor of safety of the wall.