以共振柱試驗探討二元混和物之動態特性

Abstract

土壤之動態特性，像是動態剪力模數以及阻尼比，是土壤動態分析中不可或缺之參數，常被用於預測土壤中的波傳，以評估土壤結構物之動態行為。現地土壤通常是由不同粒徑大小之顆粒組合而成，並因蒸發與降雨，隨著不同深度，有著不同飽和度的變化。而不論是顆粒之排列或是飽和度，都被研究證實對於土壤之行為有著相當的影響。近年來，許多學者提出二元混和物的概念，試著以兩種粒徑大小之混和試體，解釋現地土壤中細粒料含量的對於其排列以及力學行為影響。 本研究以不同的細粒料含量以及不同的飽和度作為出發點，探討二元混和物之動態特性。採用非塑性的越南石英砂作為材料，其粗細顆粒有效粒徑比為12.07。從全粗粒料到全細粒料，對八種不同細粒料含量之二元混和物進行共振柱試驗。分別控制相對密度為50%、孔隙比為0.55，以及六種不同飽和度，對細粒料含量在動態特性上之影響做完整的比較，並套用Thevanayagam 在2007年所提出之分類，探討傳統孔隙比與等效粒間孔隙比在二元混和物上的適用性。另外，以流量幫浦試驗，量測0%與35%細粒料含量下之土壤水分特徵曲線，比較細粒料含量對其之影響，並將此試驗所得到之基質吸力結果，套用進不飽和共振柱試驗，試以基質吸力之概念，解釋不飽和土壤之動態特性變化。 根據本研究實驗結果顯示，細粒料含量對於最大剪力模數的影響是非線性的，共出現了兩次轉折點，探討其影響原因為粗細粒料本身之力學行為，以及粗細粒料交互作用之影響。對於正規化之剪力模數，細粒料含量位於15%到50%時，會有較快速的折減趨勢，因其狀態介於粗顆粒主控與細顆粒主控之間，較易因小應變而改變其排列並影響其強度。另外，傳統孔隙比被證實不適用於二元混和物，而等效粒間孔隙比則能更有效的描述二元混和物實際之顆粒接觸情形。由流量幫浦試驗所求得之土壤水分特徵曲線結果可發現，細粒料含量對於土壤之殘餘飽和度以及基質吸力上升之趨勢影響並不明顯，而是影響其基質吸力之大小，細粒料含量愈高其基質吸力隨之上升。並可以由基質吸力之角度，對於不飽和土壤之動態特性做出相對的解釋。

Dynamic properties, such as shear modulus and damping ratio, are significant and indispensable parameters in dynamic analysis of soils. They are widely used to evaluate wave propagation in soils to assess the dynamic response of structures above. In-situ soils are composed by different sizes of particle, and various degrees of saturation along the depth due to the evaporation and rainfall. Both of particles configuration and degree of saturation, have been studied to have immense effects on behaviors of soil. In recent years, the concept of binary mixtures is proposed by several researchers. It tries to illustrate the influence of fines content and the arrangement of particles on soils mechanical behaviors, by the mixtures of two different particle sizes. This study starts from the perspectives of fines content and degree of saturation, and focuses on investigation of dynamic properties of binary mixtures. Non-plastic Vietnam silica sand mixtures with effective size ratio of 9.66 are used. From pure coarse particles to pure fine particles, binary mixtures with eight different fines content are conducted resonant column test to comprehensively investigate the effects of fines content. The classifications of binary mixtures proposed by Thevanayagam (2007) are applied to discuss the applicability of global void ratio and equivalent intergrain void ratio on the binary mixtures. Furthermore, flow pump tests are conducted with 0% and 35% fines content mixtures, to measure the soil water characteristics curve and compare the influence of fines content on it. The measured matric suctions are applied to the results of unsaturated resonant column test, trying to interpret the change on dynamic properties of unsaturated soils by the concept of matric suction. According to the results, the effects of fines content on maximum shear modulus (Gmax) are non-linear, as two inflection points are observed. Two major factors are proposed, the original behavior of pure coarse and fine particles, and the interactive effect between coarse and fine particles, respectively. The normalized shear modulus (G/Gmax) shows that the mixtures with fines content between 15% to 50% reduce more obviously with shear strain increasing. Furthermore, global void ratio is validated not suitable in binary mixtures, and equivalent intergrain void ratio has a better description on the actual contact of binary mixtures. Soil water characteristics curve obtained from flow pump tests shows that fines content does not have obvious effects on residual degree of saturation and the increasing rate of matric suction, but affects the value of matric suction. Higher fines content induces higher value of matric suction. The dynamic properties of unsaturated soil can be illustrated from the perspective of matric suction.