我國現行之耐震設計規範(內政部，2024)與基礎構造設計規範(內政部，2023)所採用之土壤參數折減係數(DE)，係參考日本建築學會(1988, 2001)建議值，該折減係數係反映地盤反力係數之弱化行為，但其較過去工程界慣用之日本道路協會(1996)建議值為保守，適用性尚待探討。為此，本研究以循環應力控制之動態三軸試驗對土壤隨超額孔隙水壓激發而弱化之現象進行定量探討，利用越南石英砂製作三組不同相對密度(Dr)之試體(30%、40%、50%)，在兩個有效覆土應力(50 kPa、100 kPa)之下進行1 Hz頻率之循環應力加載，除了建立液化阻抗曲線，亦即反覆剪應力比(cyclic stress ratio, CSR)與加載週數之關係外，亦觀察過程中超額孔隙水壓激發與剪力模數折減之趨勢。藉此，可建立抗液化安全係數FL與超額孔隙水壓比ru之關係，同時獲得弱化勁度比(δD，代表剪力模數之折減程度)隨孔隙水壓變化之趨勢。結果顯示，不同Dr下FL與ru之關係接近，當ru達0.6以上時FL已小於1。在ru<0.2之加載初期階段，δD會先隨加載週數略增；其後才轉為隨ru上升而降低，並於ru達0.7~0.8時降至近乎零。以上反映出土壤隨著孔隙水壓激發而弱化之行為，並以Dr較小時，δD降低趨勢較明顯。結合上述結果，進一步建立FL與土壤折減參數(DE)之關係，發現在FL小於0.98後DE大致小於0.01，與我國現行設計規範建議值比較後，可知日本建築學會之建議相對於本研究所得結果仍偏不保守。本研究基於一系列動態三軸試驗歸納液化地層之土壤參數折減特性，將可供現行規範後續檢討精進之參考。

This study investigated the degradation behavior of saturated sandy soils under cyclic loading using cyclic stress-controlled dynamic triaxial tests. In addition to establishing the liquefaction resistance curves representing the relationships of cyclic stress ratio (CSR) versus numbers of cycles, the development of excess pore water pressure and the reduction of shear modulus during loading were also observed. Through these tests, the relationship between the factor of safety against liquefaction (FL) and the excess pore pressure ratio(ru), as well as the variation of the degradation stiffness ratio δD (representing the reduction of shear modulus) with ru, can be established. The results show that the FL~ru relationships are similar at different relative densities (Dr); when ru > 0.6, FL generally falls below 1. In the early stage of cyclic loading (ru < 0.2), δD increases slightly with loading cycles; afterwards, δD decreases, as ru grows, eventually approaching zero when ru reaches 0.7~0.8. This indicates significant soil weakening due to excess pore pressure buildup, which is even more significant for a lower Dr . By integrating the above findings, the relationship between FL and the reduction factor of soil mechanical parameters (DE) can be constructed. It is observed that DE generally drops below 0.01 when FL< 0.98. Comparing the obtained DE values with the recommendations in current design codes in Taiwan, the latter are relatively unconservative.

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