中文摘要：

研究目的：基桩负摩阻力对桩产生下拉荷载，增大桩身轴力和端阻力，甚至导致桩身破坏。由于桩周土和桩体承受荷载后的沉降发展过程不同，桩载与堆载施加顺序必然对桩体负摩阻力产生较大影响。本文通过模型试验，研究不同桩端持力层条件下堆载和桩载施加顺序对单桩负摩阻力的影响。研究结论：（1）先堆载后桩载工况下，堆载完成后，中性点离桩顶最远，随着桩载增加，中性点逐渐上移，最终黄土和粗砂持力层中性点位置分别在桩顶下0．491和0．561处；桩身轴力呈先增加后减小的趋势，单桩承载力发挥系数分别为0．69和0．57；（2）先桩载后堆载工况下，施加桩载时，桩身轴力沿深度逐渐减小，无中性点，施加堆载时，轴力呈先增加后减小的趋势，中性点出现并逐渐下移，最终黄土和粗砂持力层的中性点位置分别在0．411和0．50／附近，单桩承载力发挥系数分别为0．86和0．69；（3）同种持力层情况下，先桩载后堆载的承载力发挥系数较先堆载后桩载的大，安全储备小，实际工程中应严格控制堆载宽度、集度及堆载边缘距桩中心的距离；（4）在实际工程中应综合分析地质条件、桩基的受力特点及承载要求，选取合适的加载顺序来减小桩身负摩阻力；（5）该研究成果可为堆载条件下桩基的设计提供参考。

英文摘要：

Research purposes： The pile negative friction leads to the drop load on pile, which increases the axial force and end resistance of the pile and even leads to pile failure. Because of the different settlement development process of the soil around the pile and the pile, the sequence of the pile loading and the surcharge loading must have a great influence on the negative friction of the pile. In this paper, through model tests, the effects of the sequence of the pile load and the surcharge loading on negative friction of single pile under the condition of different bearing stratum at pile - end were studied. Research conclusions： （ 1 ） On the condition of pile - surcharge loading sequences, after the applying of the surcharge load, the neutral point is furthest away from pile top; With the increase of pile load, the position of neutral point is gradually moved up, eventually, the neutral point positions of loess and coarse sand bearing stratum are 0.49l and 0.56l far from pile top, the axial force of the pile increases first and then decreases, the exert coefficient of bearing capacity of single pile is 0.69 and 0.57, respectively. （2） On the condition of surcharge - pile loading sequences, when first applying pile load, axial force of the pile gradually decreases along the depth, no neutral point, then while applying surcharge load, the axial force increases first and then decreases, and the neutral point appears and gradually moves down, finally the neutral point position of the loess and the coarse sand beanng tayer t~ ,,~,~ , exert coefficients of beating capacity of single pile are 0.86 and 0.69, respectively. （3） Under the same bearing stratum, there is the greater exert coefficient of beating capacity at the pile - surcharge loading sequence, the safety reserve of the beating capacity of the pile is smaller. （4）In practical engineering, comprehensive analysis of geological conditions, stress characteristics and bearing requirements of the pile foundation should be conducted and appropri