强震作用下群桩基础抗液化性能的振动台试验

为研究强震作用下群桩基础抗液化性能优于单桩基础的具体表现形式，依托海南省海文大桥工程，采用振动台模型试验开展单桩、四桩、六桩基础处理液化地基的差异性研究，分析了3种不同工况下饱和粉细砂土层中孔压比、桩身加速度和弯矩时程响应差异及其三者相互关系。研究结果表明:0.35g地震动荷载作用下，3种工况均产生液化现象，饱和粉细砂土层深处的孔压比开始增长时刻及稳定时刻均滞后于浅层；六桩基础完全液化耗时比四桩基础延缓4.41~4.82 s，四桩基础完全液化耗时比单桩基础延缓4.00~4.42 s；随着桩数的增加，同一深度处饱和粉细砂土层中桩身最大加速度及其放大系数均逐渐减小，桩身最大加速度出现时刻逐渐滞后，且随着孔压比的增大，桩身加速度逐渐减小；六桩基础最大弯矩较四桩基础小25.95%~43.50%，四桩基础最大弯矩较单桩基础小28.80%~33.10%，单桩基础最大弯矩出现时刻比四桩基础早1.22~1.27 s，四桩基础较六桩基础提前0.66~0.72 s，且桩身弯矩随孔压比的增大逐渐衰减，说明液化前饱和粉细砂土层具有软化减震作用。可见，六桩基础抗液化性能优于四桩及单桩基础，在液化土层桩基础抗震设计中，可通过群桩基础形式提高其抗液化性能。 

Shaking table test of liquefaction resistance of group piles under strong earthquake

To study the specific manifestation of the improved basic liquefaction resistance of group pile foundation in comparison with that of single-pile foundation under strong earthquake, considering the Haiwen Bridge Project in Hainan Province as an example, a shaking table model test was adopted to examine the differences in foundations comprising one, four, and six piles. The differences in the time-history responses of pore pressure ratio in saturated fine sand, pile acceleration, and bending moment under three different working conditions, and their relationships were analyzed. The results indicate that the liquefaction occurs under all three working conditions and a ground motion of 0.35g. The time when the pore pressure ratio begins to increase and that when the ratio becomes stable in the deep layers of saturated fine sand lag behind those in the shallower layers. The time required for the complete liquefaction of the foundation with six piles is 4.41-4.82 s longer than that for the foundation with four piles. The time required for the complete liquefaction of the foundation with four piles is 4.00-4.42 s longer than that for the single-pile foundation. With more piles, the maximum pile acceleration and its amplification factor in the saturated fine sand at the same depth decrease gradually, and the maximum pile acceleration gradually lags behind. In addition, as the pore pressure ratio increases, the pile acceleration decreases gradually. The maximum bending moment of the foundation with six piles is 25.95%-43.50% smaller than that of the foundation with four piles. Similarly, the maximum bending moment of the latter is 28.80%-33.10% smaller than that of the single-pile foundation. The maximum bending moment of the single-pile foundation appears 1.22-1.27 s earlier than that of the foundation with four piles, whereas that of the latter appears 0.66-0.72 s earlier than that of the foundation with six piles. Furthermore, the bending moment of pile gradually attenuates as the pore pressure ratio increases, indicating that the saturated fine sand provides softening and damping effects before liquefaction. In summary, the liquefaction resistance of the foundation with six piles is better than those of the foundations with four piles and one pile. Thus, in the antiseismic design of pile foundations for liquefaction-prone soil layers, the liquefaction resistance of foundations can be improved by using group pile foundations. 10 tabs, 12 figs, 32 refs.