以某围堤穿越深厚软土区大桥群桩基础为工程实例,采用三维有限元数值计算手段,分析工程实施过程中附加荷载施加对既有桩基工作环境影响。研究结果表明①大面积堆载对桩基产生不利影响,并显著受控于桩-土摩擦系数,鉴于工程中取值较困难,通过对比数值计算桩周负摩擦力与原位试验地基土侧摩阻力标准值,确定本工程桩-土摩擦系数在0.2～0.3;②填筑会对邻近桩基产生负摩擦力,进而增加基桩轴力,但不引起桩身破坏;③对称而均匀的填筑可最大程度降低桩基不均匀沉降及附加荷载向地基深部水平向的扩散,基桩弯矩影响可控;④群桩中各基桩空间分布不同,附加荷载引起同一承台下基桩-土变形协调存在差异,对桥桩安全监测应具有针对性。该研究成果可有效克服规范法的一些局限,对于指导实际工程方案的制定以及工程施工过程中合理布置监测措施有一定参考价值。

Under large-area loading around existed pile group foundation, the pile shaft was not only loaded by negative friction, but also by extra horizontal force. With the pile group foundation of bridge passing through deep soft soil in a newly built cofferdam project for example, a 3-D finite element model was established to simulate the loading process. The simulation results indicate 1) large-area loading has negative effect on the pile foundation, and the effect is significantly dominated by pile-soil friction coefficient which is different to be determined in actual engineering; the pile-soil friction coefficient in the study case was determined in the range of 0.2-0.3 through comparison between negative friction around pile foundation in numerical simulation and standard lateral friction resistance of foundation soil in in-situ test; 2) negative friction around pile foundation can be generated by the filling materials, hence increasing the axial force of pile foundation, yet with no damage; 3) filling symmetrically and evenly could alleviate to the largest extent the uneven settlement of pile foundation and confine the horizontal diffusion of additional loads to the deeper soil; the influence on the bending moment of pile shaft can also be controlled; 4) due to the difference in spatial distribution of pile group, the compatibility of pile-soil deformation below the same cushion cap induced by the additional loads is different, hence the safety monitoring for bridge pile should be targeted.