蜂巢格網加勁砂土力學行為之三向度分析

Abstract

加勁土壤結構目前發展成熟，在經濟性及安全性上皆獲得肯定，也符合生態工法之概念，故廣泛地應用於大地工程上。其中，蜂巢格室之應用在國內外案例雖不少，但對於其蜂巢格室單元的力學性質了解並不多，常將整個加勁結構視為均質且等向之複合材料，以進行整體穩定分析，卻忽略了隨著格室形狀的不同，格室單元各個方向的性質也有所差異。 本研究使用有限元素軟體PLAXIS分析蜂巢格室的加勁行為。首先，以蜂巢格室加勁土試體之三軸壓縮試驗結果為依據，針對土壤參數、加勁材參數與邊界條件設置等進行驗證與比對，藉以驗證數值模式的正確性。 數值模型中加勁材之形狀設為鐘形曲線(Bell-shape curve)，且前人研究發現因格室高徑比為1時之加徑效果不錯，因此選定單格尺寸為0.2m*0.2m*0.2m，再透過單格蜂巢格室加勁土受到不同方向的加載，求出各方向加壓後的加勁土壤之力學性質。蜂巢格室加勁土受壓後的彈性行為，進行彈性模數與波松比的求取與回歸分析；塑性部分則考量加勁土的剪力強度參數。此外，亦觀察試體受壓後，內部加勁材的變形與張力發揮。 為了解實際之應用，另將單格蜂巢格室加勁砂土的格數增加，形成加勁層並應用於路基之加勁。以評估加勁前後的加勁效果，模擬情況包含未加勁路基、三向度蜂巢格室加勁路基。其中，路基的加勁層有兩種高度，分別為0.2 m與0.4 m，藉以比較加勁層高度對加勁效果的影響。

Geosythetic-reinforced soil structures have been widely applied in geotechnical engineering. Among them, geocells are applied in enhancing bearing capacities of pavements and footings, reinforcing soft grounds and slopes, and so on. In the practice, the reinforced soil is often regarded as a composite material that is uniform and isotropic. In other words, the effect of the geometry of geocell is ignored, and the behavior of reinforced soil is assumed to be the same irrespective of loading directions. The main objective of this study was to analyze the anisotropic behavior of sand reinforced with geocell by using the finite element software PLAXIS. Verification of the numerical model was based on the results of triaxial compression test on geocell-reinforced-sand samples with appropriately selected material properties, boundary conditions, and strength parameters. In the numerical model, the geometry of the geocell was assumed to be composed of two bell-shaped curves. The aspect ratio of single cell was chosen 1.0, and therefore the dimension of a unit of reinforced sand was 0.2m x 0.2m x 0.2m. The unit was then subjected to compressive pressures from three mutually perpendicular directions (x-, y-, z-directions), and the mechanical behaviors of the reinforced sand were obtained accordingly. The deformations of the unit and the reinforcement, as well as the tensile stress distribution in the geocell were examined. To understand the practical applications, the number of unit was increased to form a reinforced layer in soft ground, which was subjected to vertical surcharges at the ground surface.