液化侧扩流场地桥梁群桩效应分析

基于u-p有限元公式模拟饱和砂土中水和土颗粒完全耦合效应,建立液化侧向流场地群桩动力反应分析的三维数值模型。模型中,砂土采用多屈服面弹塑性本构模型模拟、黏土采用多屈服面运动塑性模型模拟,群桩在计算过程中保持线弹性状态;采用20节点的六面体单元和考虑孔压效应的20-8节点分别划分黏土层和饱和砂层;选用剪切梁边界处理计算域的人工边界,模拟地震过程中土层的剪切效应;应用瑞利阻尼考虑体系的阻尼效应。随后对比分析2×2群桩中各单桩的地震反应规律,结果表明,各单桩的弯矩、位移时程规律基本一致,峰值弯矩及峰值位移出现时刻滞后于输入加速度峰值时刻,上坡向桩的弯矩和位移峰值大于下坡向的桩的反应值。接着通过改变桩间距研究群桩效应,随着桩间距增加,群桩中各单桩的弯矩最大值均出现在土层分界处,且各单桩的弯矩、桩顶位移逐渐增大。最后给出液化侧向流场地群桩效应的基本原因,得出该类场地群桩抗震设计的基本认识。

Analysis of the Bridge Pile Group Effect in Liquefaction-induced Lateral Spreading Sites

The Finite Element method was used to analyze the dynamic response of pile groups in the ground subjected to the liquefaction-induced lateral flow of soils.The u-p Finite Element formulation was used to depict the coupling effect of water and sand soil particles in the Finite Element analysis.A 3D numerical model was developed to analyze the effect of a 2×2 pile group subjected to liquefaction-induced lateral spreading. In this model,sand was simulated using a pressure-independent multi-yield surface plastic model.Clay material served as a nonlinear hysteretic material with a multi-surface kinematic plasticity model,and the pile group maintained its linear behavior in the process of calculation.The clay layer and saturated sand layer were meshed in a 20-node brick element and separately in a 20-8 node element.The boundary of the numerical model was considered as the shear beam boundary,which simulated the shear effect of the soil layer during the earthquake.Finally,the Rayleigh damping method was used to model the damping of the system.The dynamic response of each pile in pile group was compared,and it showed that the bending moment and displacement time history of piles at different depths developed in the same way,and the time of maximum bending moment and displacement of the pile appears to lag behind the time of peak acceleration of the input seismic wave.The maximum bending moment and displacement of the leading pile were larger than the those of the back piles.By comparing the maximum bending moment and displacement,it can also be concluded that,as depth increases,the maximum bending moment first increases and then decreases.The bending moment of the pile at the 2.5 m depth was greater than those at other depths. In terms of displacement,as depth increased,the maximum pile displacement decreased,and the maximum displacement of the pile head was greater than other observed points on the pile.This demonstrated the different behaviors of the pile bending moment response.In order to consider the effect of pile spacing on the pile group effect,several Finite Element models were developed for different pile spacing.This modeling concluded that the maximum bending moment appeared to occur in the boundary of different soil layers.As pile spacing increased,the maximum bending moment and pile head displacement in the group increased.In the pile group with pile spacing equal to 7D (diameter),the maximum bending moment of the each pile was very close.The difference was about 3% when pile spacing was equal to 5D,and the difference was about 4%,when pile spacing was equal to 3D.The maximum bending moment of the first pile group was 10% larger than the bending moment of the second pile group.In the last part of the study,the cause of the pile group effect was analyzed and a basic understanding of the seismic design requirements for this type of pile group was obtained.