垂直受荷桩负摩阻力时间效应的试验研究

EXPERIMENTAL RESEARCH ON TIME EFFECTS OF NEGATIVE SKIN FRICTION BEHAVIORS OF VERTICAL-LOADED PILES

在地面堆载作用下,对3根施加不同荷载桩的负摩阻力性状的时间效应进行现场测试。试验结果表明,负摩阻力随堆载作用时间的延续而变化;堆载使土体沉降,土体沉降就会引起负摩阻力。在堆载初期,负摩阻力引起的桩附加沉降速率随着桩顶荷载的增大而加快,且稳定的附加沉降随着桩顶荷载的增大而增加。在有无桩顶荷载的2种情况下,下拉力随着堆载作用的时间延续而增大,稳定的下拉力随着桩顶荷载的增加而减小。在堆载作用下,桩顶荷载推迟了负摩阻力出现的时间。桩顶荷载越大,中性点初次出现的时间越晚、位置越浅。中性点出现的时间随着桩顶荷载的增加而延后,桩顶荷载的增大会加剧这些现象。上述研究结果对于桩的负摩阻力性状的研究有一定参考价值。

Under the action of the ground surcharges, field measurements for time effects of negative skin friction behaviors of three piles with different vertical loads on the pile top are adopted. Test results show that negative skin friction behaviors change with the extension of time under the action of surcharges. Ground surcharges will cause the soil settlement surrounding the piles; and the soil settlement surrounding the piles can induce negative skin friction in piles. At the initial stage of surcharges, the additional settlement rate of piles caused by negative skin friction gets rapidly~ and the final additional settlement in piles increases with the increase of load on the pile top. Drag forces caused by negative skin frictions in piles increase gradually with the extension of surcharges loading time：and final drag forces in piles decrease gradually with the increase of load on the pile top in both vertically loaded and unloaded piles. Under the action of surcharges, the load on the pile top postpones the arrival of negative skin friction in piles. The larger of the load on the pile top is, the later the appearance of the neutral point in pile is, and the smaller the length between the neutral point and the pile top is. The time of the neutral point in piles is delayed with the increase of the load on the pile top. Increase of load acted on pile top would promote above-mentioned phenomena. It may be useful for the research of negative skin friction behaviors of piles.