三軸壓縮試驗探討蜂巢格網的圍束效應

Abstract

加勁土壤結構近三十年來已發展成熟，在經濟性及安全性上皆已獲得肯定，也能符合現今各界所提倡之生態工法，故已廣泛地應用於大地工程上。其中，蜂巢格網之應用在國內外之案例雖不少，但對蜂巢格網的尺寸、形狀、格數對整體土壤加勁效果的影響仍未盡了解，又因大型蜂巢格網亦不易於實驗室進行試驗，故於設計分析蜂巢格網加勁結構時，應如何適切地選取力學參數，常令人困惑。 本研究以尼龍製成的蜂巢格網內填砂土或礫石，進行三軸壓縮試驗，試體的直徑有2.8吋及6.0吋兩種。研究中分別考慮土壤有無加勁、以及蜂巢格網的格室形狀(圓形、方形和六角形)、大小、格室數等各項變因對加勁土壤之應力-應變行為的影響。 試驗結果顯示，加勁土壤之摩擦角與未加勁土壤之摩擦角差異不大；其中以六角形格室之摩擦角較其他形狀稍高。視凝聚力則隨格室大小、形狀及格數之不同有較大的差異。最顯著者為單格之格室越小，視凝聚力越大；其中以平滑之圓形格室的視凝聚力最大，方形次之，而六角形最小。當格室大小及形狀固定而格數增加時，視凝聚力反而略為降低。此外，隨著圍壓增加，加勁效果愈不明顯；換言之，覆土壓力較小時，加勁成效較為顯著。至於土壤粒徑方面，對於礫石之加勁效果優於砂土，此亦顯示粒徑較大之土壤加勁時較能顯現成效。最後，利用理論式計算由加勁材圍束效應所增加的軸差應力增量與環向應力增量，並由分析與試驗結果比較顯示，加勁土壤於低圍壓時之行為較符合環向伸張模式(hoop tension theory)，而於高圍壓時之行為則較類似薄殼壓縮模式(compression shell theory)。

Reinforced soil structures have been developed for more than three decades. They have the merits in the aspects of economy and safety, and they also are considered as an ecological engineering method. Among them, geocells has been widely applied to geotechnical engineering; nevertheless, understanding about the effects of the size, shape, as well as number of the pocket of geocell is still unclear. Furthermore, conducting laboratory test on geocell of large size is difficult. Therefore, selecting appropriate parameters for designing geocell-reinforced structures remains a difficult task. This study used a geocell made of nylon and filled within it with gravels or sands to conduct triaxial compression tests in two different sizes of triaxial cells, 2.8” and 6.0” in diameter, respectively. The factors considered were soil type, soil with or without reinforcement, as well as the shape (circle, block and hexagon), size and number of the pocket. The effects of these variables on the stress-strain behavior of geocell-reinforced soils were investigated. Test results showed that the difference in friction angle between reinforced soil and unreinforced soil was not much. For various pocket shapes, hexagon pocket showed the highest friction angle. On the other hand, the apparent cohesion of reinforced soil depends largely on the size, shape, and number of the pocket, of which the most significant factor is the pocket size. In other words, the soil reinforced with geocell of smaller pocket exhibited greater apparent cohesion. Moreover, the smooth circular pocket induced the highest apparent cohesion among all shapes, while that of hexagon pocket is the lowest. Remarkably, for same size and same shape of pocket, the apparent cohesion decreased with increasing number of pocket. In addition, as the confining stress on the reinforced soil increased the reinforcing effect became less significant, meaning that the effectiveness of reinforcement is more significant when under small overburden pressure. As to the grain size of soil, reinforced-gravel displayed better reinforcing effect than reinforced-sand. This result suggests that geocell is more effective to soil with larger grain sizes. Theoretical formulae are also used to estimate the increases in deviatoric stress and hoop tensile stress that induced by geocell, respectively, and the comparison is made between analytical and test results. It was found that the behavior of reinforced soil under low confining stress is similar to the prediction by hoop tension theory. When under high confining stress the compression shell theory is appropriate to predict the behavior of reinforced soil.