大直径钻孔灌注桩负摩阻力试验研究

 针对大面积堆载情况下，周边土体的沉降使桩基产生负摩阻力从而导致桩基承载力特性变化的问题，以宁海电厂工程2组冲孔灌注桩的现场负摩阻力试验为例进行讨论。通过对原位试验结果的全面分析，探讨桩周土体固结沉降对桩身所受下拉荷载和中性点位置的影响。根据实测桩土沉降曲线确定的中性点与根据桩身轴力沿深度变化曲线确定的中性点位置大体相一致，位于可压缩土层下部，桩身最大轴力随固结时间而增大，中性点位置也随时间略有上移；分析桩侧摩阻力系数的大致范围以及施工工艺对负摩阻力的影响，现场试验得到的桩侧摩阻力系数为0.3～0.4，由于桩基施工的影响导致该值与规范相比略大，工程中应充分考虑成桩工艺对负摩阻力的影响；指出负摩阻力桩基的设计分析中沉降计算至关重要。得出的结论可指导同类工程的设计和施工。

EXPERIMENTAL RESEARCH ON NEGATIVE SKIN FRICTION OF LARGE-DIAMETER BORED PILES

Due to the large-scale vertical loads，a relative movement can be induced when the soil settles in relation to the pile. Thus，the negative skin friction occurs if the movement of pile is upward.Taking the in-situ test of negative skin friction on punched cast-in-place pile of Ninghai Power Plant Project for example，the influence of the soil consolidation around pile on the downdrag forces and the position of neutral point are studied. Different subsurface soil settlements are measured within and outside the two sets of piles，suggesting different influential degrees of the piles on the surrounding soil. The positions of the neutral point obtained from the measured settlement curves of soils and piles agree with the ones from the variable curve of pile axial force with depth. The measured maximum coefficients of side skin friction around piles are consistent and they vary between 0.3 and 0.4. Furthermore，based on the test results，the effect of pile construction technology on negative skin friction is analyzed. It is commended that the calculation of the settlement distribution is emphasized in the design for the negative skin friction piles. These conclusions are useful for engineering practice，which may guide the design and construction of similar projects.