强震作用下的砂土液化对桩基力学特性影响

为了提高位于液化土层桥梁桩基的抗震性能, 基于三向六自由度大型振动台模型试验, 分析了地震波作用下桩顶水平位移、桩身加速度及弯矩等动力响应, 并研究了地震波加载后桩基的损伤。试验结果表明: 在地震波作用下, 随着液化层埋深的增加, 土体液化后产生的侧扩效果逐渐减弱, 因此, 桩顶水平位移峰值逐渐减小, 但是当地震加速度超过0.6g时, 桩顶水平位移峰值不受液化层埋深的影响; 因地震荷载作用下粉细砂土层液化, 桩身加速度在该土层位置明显增大; 上部覆盖层压力作用使土层抗剪强度增大, 因此, 桩顶放大系数随着液化层深度的增加而增大, 且桩顶放大系数在Kobe波作用下最大, 5002波作用下最小, 砂土液化同时造成土层强度降低, 从而使桩身加速度在该土层出现放大效应; 桩身弯矩最大值均出现在液化层和非液化层分界处, 且在相同强度地震波作用下, 桩身弯矩最大值随着液化层埋深的增加呈增大趋势, 当地震加速度从0.30g增大到0.35g后, 桩身弯矩增幅为33.3%, 增幅最大; 不同类型地震波对桩基的破坏程度并无差异, 在加速度0.35g作用下, 桩基基频无变化, 但当地震波强度超过0.40g时, 桩基基频从1.65 Hz突降到0.45 Hz, 因砂土层液化产生侧向位移, 桩身剪切变形, 最终导致桩基损坏。综上所述, 当液化层较浅时, 应重点考虑地震波作用下过大的桩顶水平位移; 在桩基抗震设计时, 必须考虑液化层和非液化层分界处桩基的抗弯能力和液化层埋深的影响。 

Effect of sand liquefaction on mechanical properties of pile foundation under strong earthquake

In order to improve the seismic resistance behavior of bridge pile foundation located at the liquefied layer, the shaking table model test with three directions and six degrees of freedom was carried out, the dynamic responses of pile tops' horizontal displacements and piles' accelerations and bending moments were analyzed under the seismic waves, and the damages of pile foundations under the actions of seismic waves were studied. Experiment result shows that, under the actions of seismic waves, the lateral expansion effect gradually decreases with the increase of the depth of liquified layer. Therefore, the peak horizontal displacement of pile top gradually decreases. However, the peak horizontal displacement of pile top will no longer be affected by the liquefied layer depth when the seismic acceleration exceeds 0.6g. The pile accelerations increase significantly in the fine sand layer because of the liquefaction of fine sand layer under the seismic loads. The stress caused by the overburden soil can enhance the shear strength of lower layer, therefore, the amplification factor of pile top increases as the depth of liquefied layer increases. Moreover, the amplification factor is the largest under the action of Kobe wave, and the smallest under the action of 5002 wave. The sand liquefaction also causes the strength of soil layer to decrease, leading to the acceleration magnification in the soil layer. All the maximum bending moments of piles appear at the boundary between the liquefied layer and non-liquefied layer, and under the same seismic intensity, the maximum bending moment of pile increases with the increase of liquefaction layer depth. When the seismic acceleration increases from 0.30g to 0.35g, the bending moment of pile shows a maximum increase of 33.3%. The pile foundations experience no difference in the damages caused by different types of seismic waves. Under the acceleration of 0.35g, the fundamental frequency of pile foundation has no change. But when the seismic wave strength exceeds 0.40g, the fundamental frequency of pile foundation suddenly drops from 1.65 Hz to 0.45 Hz. The pile foundations in the sand layer laterally displace due to the liquefaction, and the piles deform due to the shear stress, eventually leading to the damages of pile foundations. In conclusion, when the liquefied layer is relatively shallow, the excessive horizontal displacements of pile tops under the actions of seismic waves should be fully considered. In the seismic design of pile foundation, the bending resistance of pile foundation at the boundary between the liquefied and non-liquefied layer, and the liquefied layer depth must be considered.