竖向及水平荷载加载水平、顺序对单桩承载力的影响

Effect of loading level and sequence of vertical and lateral load on bearing capacity of single pile

在桩基础设计中通常分别确定单桩的竖向和水平承载力,而不考虑竖向荷载与水平荷载之间的相互作用对桩承载力的影响,这显然不能反映桩的真实受力状况。软土地基上建造超高层建筑有时需要采用长度达80 m甚至更长的超长桩,目前考虑超长桩的竖向荷载和水平荷载二者之间相互作用的研究还很少。利用有限元软件ABAQUS模拟天津市某工程的79 m长桩载荷试验结果,通过参数调整使有限元计算结果与载荷试验的结果一致,在此基础上进行仿真模拟,研究竖向及水平荷载作用下超长桩的荷载传递与变形特点。分析结果表明超长桩表现出与已有文献中的刚性短桩不同的变形特性:对于承受水平荷载与竖向荷载的桩,竖向荷载对水平荷载作用下桩的性状的影响与竖向荷载相对于水平荷载的施加顺序、竖向荷载的大小以及土质条件等均有关。就本文算例,先施加竖向荷载再施加水平荷载时,存在一个最优的竖向荷载,在最优竖向荷载作用下,竖向荷载减小桩顶侧移的有利作用最明显,且竖向荷载减小桩顶侧移的作用主要表现在水平荷载加载后期,即水平荷载较大时。在土质条件较好的情况下竖向荷载对桩顶侧移的有利影响不如土质条件较差时那样明显。先施加水平荷载再施加竖向荷载时,竖向荷载不再起到减小水平荷载作用下桩顶侧移的作用。加载顺序变化对桩水平承载力的影响要大于对桩竖向承载力的影响。

It was a common practice to determine the vertical beating capacity and lateral bearing capacity separately in the design of pile foundations without consideration of the interaction between the vertical load and the lateral load. It would certainly not reflect the real conditions. Piles with the length of 80 m or even longer were usually adopted in the foundations of super-high building in the soft soil area. At present, there were few documents about the interaction between the vertical load and the lateral load for the unusually long piles. Based on modeling the vertical and lateral load-test results of a pile with the length of 79 m, the necessary characteristics of soil for FEM analysis were obtained, and then the bearing capacity and deformation of extra-long piles under the vertical and lateral loads were studied. For piles under the vertical and lateral roads, effects of the vertical loads on the lateral bearing capacity and the deformation varied with the loading sequence of the vertical and lateral loads, the load level and the strength of soil. As far as the piles and loading condition being concerned, there was a critical vertical load under which the lateral deformation was reduced mostly at high loading level when the vertical load was applied before the lateral one. The weaker the soil, the greater the effect of the vertical load on the lateral displacement at the pile top. The vertical load did not help to reduce the lateral displacement at the pile top anymore when it was applied after the lateral load. The effect of loading sequence of the vertical load and lateral loads on the lateral bearing capacity was larger than that on the vertical one.