加载次序对湿陷性黄土中基桩承载力检测的影响
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摘要
湿陷性黄土地区基桩承载力检测中桩顶荷载是在土体沉降后施加的,桩基使用条件下是桩顶荷载施加之后再出现土体沉降。简要分析了这两种加载次序之间桩土相互作用差异的机制。采用有限元方法分析了这种差异。计算结果表明,在这两种加载次序中,桩顶荷载的增加都会引起沉降增加,中性点下降,下拉力减小。结果还表明,与桩顶荷载先施加的相比,桩顶荷载后施加得到的中性点的位置比较低,桩基沉降比较小,下拽力也较小,这些差异随着桩顶荷载的增大而更加明显。在两种加载次序的各个加载阶段,桩周摩阻力的分布与发挥也会出现细微的变化。这会使湿陷性黄土地区桩的承载力检测结果略微偏于不安全。
Load on pile top is acted after the settlement of soil surrounding the pile in bearing capacity test of piles in collapsible loess,and it is acted before the settlement of soil on general working condition of pile foundation.Different mechanisms of pile-soil interaction between such two loading sequences are simply analysed.Finite element method is used to calculate the differences.Analysis results show that settlement of pile increases,neutral point goes down and dragload induced by negative skin friction reduces as the load on pile top increases in both loading sequences.It also shows that comparing with load on pile top acted before in general working condition,loading sequence of bearing capacity test in collapsible loess would make the neutral point position in pile lower,settlement of pile smaller and the dragload in pile smaller.The differences of neutral point position,settlement and dragload in the two loading sequences increase gradually as working load increases.At every stage in both loading sequences,there exists small differences on distribution and development of skin friction along the piles.This will cause the testing result of bearing capacity of piles relatively unsafe in collapsible loess zone.
Load on pile top is acted after the settlement of soil surrounding the pile in bearing capacity test of piles in collapsible loess,and it is acted before the settlement of soil on general working condition of pile foundation.Different mechanisms of pile-soil interaction between such two loading sequences are simply analysed.Finite element method is used to calculate the differences.Analysis results show that settlement of pile increases,neutral point goes down and dragload induced by negative skin friction reduces as the load on pile top increases in both loading sequences.It also shows that comparing with load on pile top acted before in general working condition,loading sequence of bearing capacity test in collapsible loess would make the neutral point position in pile lower,settlement of pile smaller and the dragload in pile smaller.The differences of neutral point position,settlement and dragload in the two loading sequences increase gradually as working load increases.At every stage in both loading sequences,there exists small differences on distribution and development of skin friction along the piles.This will cause the testing result of bearing capacity of piles relatively unsafe in collapsible loess zone.
引文
[1]FELLENIUSB H.Results from long-term measurement in piles of download and downdrag[J].Canadian Geotechnical Journal,2006,43:409-430.
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[8]高绍武,王建华,毛娜.层状地基中单桩负摩擦问题积分方程解法[J].岩土力学,2005,26(9):1456-1460.
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[10]齐静静,徐日庆,龚维明.湿陷性黄土地区桩侧负摩阻力问题的试验研究[J].岩土力学,2006,27(增):881-884.
[11]夏力农,王星华,雷金山.加载次序引起的负摩阻力桩摩阻力分布的变化[J].岩土力学,2006,27(增):784-787.
[12]黄雪峰,孙树勋,徐毅明,等.大厚度自重湿陷性黄土场地人工成孔灌注桩负摩阻力沿桩身传递特征[C]//韩丽霞,莫庸编.甘肃省土木建筑学会建会五十周年[S.l.]:[s.n.],2002,144-150.
[2]FELLENIUS B H.Downdrag on piles in clay due to negative skin friction[J].Canadian Geotechnical Journal,1972,19(4):323-337.
[3]POULOS H Q,MATTES J L C.The development of negative skin friction with time in end-bearing Pilesp[J].Australian Geomechanics Journal,1972,G2(1):11-20.
[4]孙军杰,王兰民.桩基负摩阻力研究中几个基本理论问题的探讨[J].岩石力学与工程学报,2006,25(1):211-216.
[5]夏力农.桩基负摩阻力特性的理论与试验研究[博士学位论文D].长沙:中南大学,2007.
[6]陈福全,龚晓南,马时冬.桩的负摩阻力现场试验及三维有限元分析[J].建筑结构学报,2000,21(3):77-80.
[7]夏力农,雷鸣,聂重军.桩顶荷载对负摩阻力性状影响的试验研究[J].岩土力学,2009,30(3):664-668.
[8]高绍武,王建华,毛娜.层状地基中单桩负摩擦问题积分方程解法[J].岩土力学,2005,26(9):1456-1460.
[9]孙军杰.黄土地基沉降时桩的负摩阻力计算方法研究[硕士学位论文D].兰州:中国地震局兰州地震研究所,2004.
[10]齐静静,徐日庆,龚维明.湿陷性黄土地区桩侧负摩阻力问题的试验研究[J].岩土力学,2006,27(增):881-884.
[11]夏力农,王星华,雷金山.加载次序引起的负摩阻力桩摩阻力分布的变化[J].岩土力学,2006,27(增):784-787.
[12]黄雪峰,孙树勋,徐毅明,等.大厚度自重湿陷性黄土场地人工成孔灌注桩负摩阻力沿桩身传递特征[C]//韩丽霞,莫庸编.甘肃省土木建筑学会建会五十周年[S.l.]:[s.n.],2002,144-150.
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