竖向加载方式对桩侧向应力及侧阻性能的影响

为了研究竖向加载方式对桩侧阻性能的影响,采用数值模拟分析方法,分析了在桩顶下压、桩顶上拔和桩底上顶3种加载方式下由竖向荷载引起的附加的桩侧向应力.结果显示:在桩顶下压加载方式下附加的桩侧向应力为压应力,在桩顶上拔和桩底上顶两种加载方式下附加的桩侧向应力为拉应力;产生附加的桩侧向应力的主要部分位于桩顶下15～20m范围内及各土层分界面上下2.5m范围内;附加的桩侧向应力的最大值可达土自重产生的侧向应力的±80%以上,但沿桩长变化的梯度很大;桩周土类型、桩长和桩径对附加的桩侧向应力的数值及沿桩长的分布有不可忽略的影响.根据桩侧向应力与桩侧阻力之间的定性关系,讨论了竖向加载方式对桩侧阻力的影响.指出:通常不考虑土层的部位、不考虑桩长和桩径将桩顶下压加载方式下的桩侧阻力乘以一个折减系数作为桩顶上拔或桩底上顶加载方式下的桩侧阻力的做法是不适宜的.

Influence mechanism of vertical loading mode on pile lateral stress and pile side resistance behavior

Additional pile lateral stress induced by the vertical load of pressing down and pulling up at pile top and pushing up at pile bottom are analyzed with numerical simulation method. It is shown that additional pile lateral stress under pressing down at pile top is compressed stress, and under pulling up at pile top and pushing up at pile bottom are tensile stress; at 15~20m under pile top and ±2.5m from interfaces of soil layer are the mainly distribution parts of additional pile lateral stress. The max reach more than ±80% of pile lateral stress due to soil self-weight, but stress gradient along the depth is great. Influence of soil kind, pile length and pile diameter on the additional radial stress can not be ignored. According to the relation between pile lateral stress and pile side resistance, the influence of vertical loading mode on pile side resistance is discussed. It is pointed out unreasonable that the pile side resistance under pressing down at pile top multiplied a coefficient is regard as that under pulling up at pile top and pushing up at pile bottom in spite of soil location, pile length and pile diameter.