地面横向往返运动下可液化土层中桩基响应机理

Mechanism of pile foundation response in liquefiable soils under seismic cyclic ground motion

通过非液化和液化土层中桩基础宏观震害现象以及等幅波与真实地震波振动台模型实验中桩和土层的加速度、位移、桩土相互作用力、桩动力p-y曲线、桩身弯矩与孔压发展过程对比,研究地震引起的地面横向往返运动下可液化土层中桩基响应机理。结果表明:非液化土层中上部结构惯性力控制着桩的反应性态,桩头加速度和桩身弯矩与土层加速度时程基本保持一致;液化过程中桩土相互作用力呈现明显增大现象,土体侧向刚度虽然衰减,但同时土层相对位移和桩土相对位移增大的影响更为强烈,即土层和桩土相对位移对桩土相互作用力增大的作用明显大于土体刚度衰减引起桩土相互作用力减小的作用;液化土层中桩土相互作用最大反应不是在土层加速度峰值时刻,而是土体相对位移达到最大时响应最大,此时土层孔压比为0.8左右;非液化土层中桩土相互关系为桩推土,惯性力是控制因素,液化土层中则为土推桩,土体位移起主要作用,而液化发展是这一转变决定性因素;常规仅考虑土体刚度衰减的拟静力方法不适合液化土层中桩基础地震响应计算分析。

The mechanism of the dynamic response of pile foundations in liquefiable soils under excitation of seismic cyclic ground motion is investigated through study of the actual behavior of pile foundations in earthquakes as well as comparison of the accelerations, displacements, interaction forces between pile and surrounding soil, dynamic p-y curves of pile, bending moments of pile and pore water pressures between shaking table tests of pile foundations in non-liquefied and liquefied soil layers under uniform and realistic seismic excitations. The results indicate that the inertia force of the superstructure controls the behavior of pile foundation. The pile bending moments and the pile-head acceleration basically follow the acceleration history. Although the lateral rigidity of the soil layer decreases with the liquefaction process, the pile-soil interaction increases significantly due to the increase of the soil displacement and the relative displacement between pile and soil during liquefaction, i.e., the effect of the increase in soil relative displacement on the increase of pile-soil interaction force is significantly larger than the effect of the decrease in soil lateral rigidity on the decrease of pile-soil interaction force. The maximum pile-soil interaction force appears at peak soil displacement rather than at soil acceleration peak. In the non-liquefied soil, soil pushes the pile and inertia force is the dominant factor, whereas in the liquefied soil, pile pushes the soil, and soil displacement is the dominant factor. The existing quasistatic method considering only soil rigidity deterioration is not suitable for simulating the response of pile foundations in liquefiable soils.