以振動台試驗探討液化地盤中單樁受震樁土互制關係

Abstract

本研究使用國家地震工程研究中心的振動台與大型雙軸向剪力試驗盒進行1-g飽和砂土中模型樁振動台試驗，探討可能液化地盤中單樁受震之樁土互制作用。模型樁固定於剪力盒底部以模擬基樁貫入岩盤或是埋置於堅實地層之受振行為，並在樁頂裝設質量塊模擬上部結構載重之反應。另外在樁身黏貼應變計以及微型加速度計量測基樁受振之反應，同時也對於土壤試體之位移、加速度與孔隙水壓反應進行連續監測，以完整紀錄地盤受振過程之動態反應。 為探討上部結構與地盤變位作用於樁身之效應以及液化後樁土互制作用行為，本研究進行水平地盤與傾斜地盤之振動台試驗。根據試驗結果顯示樁基礎受振反應及樁土互制作用，主要受土層與基樁之顯著頻率、上部結構慣性力及地震之頻率分布的影響。此外，本研究利用雙向剪力試驗盒之機制與垂直坡面方向的輸入運動方式，確實能分別考量地盤側潰與上部結構物慣性對樁基礎的作用力。 本研究採用時頻分析與系統識別方法判識試驗過程中樁土系統頻率變化，探討地盤受震期間樁土系統頻率與超額孔隙水壓變化之關係，並提出簡化數學模式評估樁土系統頻率與水平地盤反力係數之關聯性，進而建立正規化水平地盤反力係數因超額孔隙水壓激發而折減的關係。其結果顯示在液化期間，土壤勁度幾乎消失，但隨著超額孔隙水壓消散，土壤勁度會漸次回復。

Series of shaking table tests on a model pile within a saturated sand specimen using a large bi-axial laminar shear box were conducted at the National Center for Research on Earthquake Engineering (NCREE), to study the soil-pile interaction in a liquefiable ground during earthquake. The shaking table tests included a single pile within the level ground and inclined ground with a slope angle of 2。. The pile tip was fixed at the bottom of the shear box to simulate the condition of a pile foundation embedded in a firm stratum. The pile top was mounted with steel disks to simulate the superstructure. In addition, strain gauges and mini-accelerometers were placed on the pile surface to observe the behavior of the pile under shaking. The near- and far-field soil responses, including pore water pressure changes, accelerations, and settlements were also measured. According to analyses of the dynamic responses of the soil-pile system, it was found that the behavior of the model pile and soil-pile interaction under shaking was affected by the dynamic characteristics of the piles and the surrounding soil, the mass of the superstructure and the frequency content of earthquake shakings. In addition, the kinematic and inertial loadings on the model pile due to lateral spreading during shaking can be evaluated independently with the input motion imposed in the direction perpendicular to the slope direction. These experimental data were analyzed to identify the time-dependent predominant frequency of soil-pile system during generation and dissipation of excess pore water pressure under the shaking by performing time-frequency analyses and system identification. The relation between the predominant frequency of soil-pile system and the coefficient of horizontal subgrade reaction can be obtained by the proposed mathematical model of soil-pile system. Hence, the relation between the normalized coefficient of horizontal subgrade reaction and the pore pressure ratio was established based on the experimental data and the proposed mathematical model of soil-pile system. It can be seen that the stiffness of the soil almost vanished during the period of liquefaction and the stiffness of the soil would increase with the dissipation of pore water pressure. The trend of the stiffness reduction is found close to the reduction of soil parameter proposed by Architectural Institute of Japan (AIJ, 1998).