摘要：研究目的:桩侧负摩阻力对桩产生下拽力，导致基桩沉降增大，竖向抗压承载安全等级降低，甚至造成桩 体破坏。为分析不同桩端下卧层（黄土和粗砂）对基桩沉降和负摩阻力的影响，本文通过室内模型试验，对比 研究两种下卧层情况下桩体、桩周土变形及桩身轴力分布的差异，明确基桩中性点位置的变化规律并给出建议值。研究结论：(1)下卧层对桩周土沉降的影响较小，但对桩身沉降的影响较大，相同堆载等级下粗砂下卧层 的中性点位置明显低于黄土下卧层'（2)两种下卧层的桩身轴力都随堆载等级的增大而增大，但粗砂下卧层 的下拉荷载和桩端反力明显大于黄土下卧层；（3)两种下卧层情况下，依据桩土相对位移和桩身峰值轴力确 定的中性点位置基本一致'（4)在实际工程中应根据桩端下卧层的地质情况，采取适当措施，尽量减小基桩负 摩阻力，充分发挥基桩的承载性能'（5)建议黄土下卧层中性点位置取0.51l~0.6l，粗砂下卧层可取0.6l~0.8l;(6)该研究成果可为类似工程堆载条件下桩基中性点的确定及设计提供一定的参考。

Abstract ： Research purposes： The negative skin friction could produce dragloads on pile，which will cause the increase of displacement and the decrease of vertical compressive security levels，even results in pile destruction. To analyze the influence of different underlying layers ( loess and coarse sand) on displacement and negative skin friction， through model tests，this paper compared the difference of pile and pile - surrounding soil displacement，and the distribution of axial load for these two underlying layers， then clarified the change regularity of neutral point position and gave the suggestion value. Research conclusions ：( 1) The influence of underlying layers on pile - surrounding soil disj)lacement is smaller，while on pile displacement is bigger. Compared witli loess underlying layer，the neutral point position of coarse sand is deeper under the same surcharge loading level. ( 2 ) The axial force of these two underlying layers increases with the increase of load level. But tiie dragloads and pile - end reaction of coarse sand are bigger than loess. (3) The neutral point positions which are determined by pile - soil relative displacement and peak axial force are basically the same. (4) Considering the geological conditions in actual engineering，some appropriate measures should be taken in order to minimize pile negative friction and make full use of tiie bearing capacity of pile. (5) This paper suggests that tiie neutral point position of loess underlying layer ranges from 0. 51 to 0. 61 while coarse sand underlying layer ranges from 0. 61 to 0. 81. (6) These results can provide some references to decision and design of the neutral point under surcharge loading in the similar projects.