降雨过程对兰州黄土中桩基负摩阻力的影响
Effect of rainfall on negative frictional resistance of pile foundation in Lanzhou loess

为了模拟预压工况下非连续降雨对兰州湿陷性黄土对桥梁桩基摩阻力影响，以自然渗透方式进行了现场试坑浸水试验。通过埋设钢筋计测算桩侧摩阻力，埋设水分探头测浸水深度。结果表明，浸水最大深度在10 m左右，且在该深度处含水率增量很小，说明浸水主要影响区域已在10 m之上。在整个实验过程中，桩侧负摩阻力始终存在，且其峰值随着荷载递增而增大。加载阶段，负摩阻力分布区间长与桩长比值约为0.24，是一个变化的过程；卸载过程中，也存在负摩阻力，区间长与桩长的比值在0.28～0.45范围内波动。试验结果表明，首次降雨对桩基受力影响较大，较长时间的间断性二次降雨影响较小。负摩阻力区间比规范规定的小，在实际桩基设计时，既要充分考虑首次降雨的影响，又可以根据负摩阻力长度和适当调整桩长。
To simulate the influence of non-continuous rainfall on the friction of pile foundation in Lanzhou collapsible loess, field test was carried out via natural infiltration method. The project measures the pile side friction by embedding the steel bar and the immersion depth by water probe. The results show that the maximum water immersion is about 10 meters, and the increase of water content below this depth is marginal. During the test, the negative friction resistance is taking effect, and its peak increases when the load increases. At the loading stage, the length of the negative friction resistance is about 0.24 times the pile length, subjected to certain fluctuation. During the unloading process, there is still negative friction resistance, and the interval length fluctuates within the range of 0.28~0.45 times the pile length. The results show that the initial rainfall has a significant influence on the pile foundation, and the effect of continuous intermittent rainfall is marginal. The actual negative friction resistance is much smaller than the threshold. Consequently for pile design, it is necessary to fully consider the impact of the initial rainfall.The pice length should also be adjusted based on the negative friction resistance.