為了評估地盤與結構物受震時的反應，土壤的動態性質隨剪應變之變化為重要分析參數。然而，常見的動態性質求取方法為參考ASTM-D3999規範進行之動態三軸試驗，但過程中往往忽略超額孔隙水壓激發對於動態性質的影響，故本研究以循環應變控制方式進行動態三軸試驗，採用特定的剪應變，將單一試體反覆加載至液化，可視為循環應變控制液化阻抗試驗，藉以探討土壤動態性質、剪應變與超額孔隙水壓間之關係。
試驗採用越南石英砂，配置三種不同相對密度之試體(30, 40, 50%)，於三種初始有效圍壓(20, 50, 100 kPa)，在指定的6個剪應變(γ=0.019, 0.038, 0.057, 0.076, 0.11, 0.15%)下，以加載頻率為1Hz及0.1Hz進行試驗，觀察到剪力模數與初始有效圍壓及相對密度成正比，且與加載頻率無關；阻尼比則與初始有效圍壓及相對密度成反比，與加載頻率略有關，而在超額孔隙水壓的激發與剪應變及循環加載週數均成正比。
將循環應變控制液化阻抗試驗所得動態性質與參考ASTM-D3999規範所得試驗結果進行比較，於剪力模數方面，在剪應變較小時，前者超額孔隙水壓激發尚不明顯，故兩者差異不大；隨著剪應變的遞增及循環加載週數的遞增，超額孔隙水壓的激發愈趨明顯，使前者所得剪力模數漸減。於阻尼比方面，在小剪應變下前者所得低於後者，可能為不同之試驗程序差異造成試體狀態有所差異；隨著剪應變的遞增及循環加載週數的遞增，超額孔隙水壓明顯的激發，造成前者之阻尼比大致上大於後者。從超額孔隙水壓與剪應變及循環加載週數之關係，若採用Vucetic and Dobry (1988)提出之模型，並利用Mei et al(2018)提出之建議率定模型參數，將可得到趨勢一致且數值接近之結果。

To reasonably evaluate the seismic site response, the changing of soil dynamic properties during earthquake excitation should be adequately considered. Therefore, this study aimed to investigate the variation of soil dynamic properties with shear strain (γ) and excess pore water pressure (EPWP) using dynamic triaxial tests. In each test, a specimen made of Vietnam silica sand was cyclically loaded with a specified γ amplitude at a constant frequency until liquefaction occurred; therefore, it can be regarded as a cyclic strain-controlled test for liquefaction resistance, yet the focus was on the change of dynamic properties during the loading process. Comparing the test results with the shear modulus degradation curves obtained using the ASTM-D3999 procedures, the difference was insignificant when γ was small and generation of EPWP in the former was insignificant. With the increase of γ and loading cycles, the EPWP was gradually increasing, so the shear modulus obtained by the former was getting lower than the latter. Regarding the damping ratio, that of the former was lower than the latter at small γ possibly due to different test procedures EPWP but was generally larger after notable build-up of EPWP. These results can be used to deduce the relationship between the shear modulus reduction and the excess pore water pressure ratio (r_u). However, it was also shown that soil dynamic properties, γ, and EPWP are closely dependent and their relationships may not be sufficiently interpreted by merely dynamic properties versus γ and dynamic properties versus r_u curves.

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