开口双壁管桩静压沉桩机理及竖向承载特性模型试验研究

    Model test investigation into jacking mechanism and bearing characteristics of open-ended double-wall piles

    • 摘要: 深入认识开口管桩在沉桩过程的桩土作用机理对准确计算管桩的竖向承载力十分重要。传统的开口管桩无法将内、外阻力分离,设计了能将内、外阻力分离的开口双壁管桩,在砂土地基中(相对密度Dr≈50%)进行了开口双壁管桩循环静压过程的大比尺模型试验。采用光纤光栅应变传感器等监测手段,研究了循环压桩过程中的土塞特性、桩身内力、桩内外局部侧摩阻力和沉桩阻力(包括单位面积土塞阻力、环形阻力和桩端阻力)等变化规律。随后,进行了休止时间为10 d的受压静载试验。研究表明:①开口管桩在贯入初期会出现土塞高度大于贯入深度的情况,即土塞增长率(IFR)大于1。②内、外管局部侧摩阻力随着h/R的增加而逐渐衰减,存在明显的h/R效应,在h/R=2.3时,内管局部侧摩阻力约为外管的4倍。③土塞阻力与IFR之间存在显著的对应关系,IFR越大土塞阻力越小。归一化环形阻力(qann/qc)与IFR值无关(qc为静力触探试验的锥阻力),其基本维持在一个稳定值。归一化桩端阻力(qb/qc)随着IFR值增加而逐渐减小,当IFR接近0时,归一化桩端阻力(qb/qc)接近1。④通过归一化桩端阻力(qb/qc)与有效面积比Ar,eff的关系,建立了开口管桩静压沉桩的桩端阻力计算公式,验证了该公式的可靠性,建议在进行承载力计算时要区分沉桩方式的影响。

       

      Abstract: A comprehensive understanding of the pile-soil interaction mechanism during the jacking process of open-ended pipe piles is essential for accurately calculating their vertical bearing capacity. Traditional methods for assessing such piles are unable to distinguish between internal and external soil resistances. This paper introduces the design of an open double-wall pipe pile that enables the independent measurement of internal and external soil resistances. Large-scale model tests are conducted to investigate the cyclic penetration behavior of this pile type in sandy soil foundations with a relative density (Dr) of approximately 50%. This study employs monitoring techniques, including Fiber Bragg Grating (FBG) strain sensors, to examine the variation patterns of soil plug characteristics, internal pile forces, local lateral friction resistances both inside and outside the pile, and pile driving resistances—specifically soil plug resistance, annular resistance, and base resistance—during cyclic penetration processes. Subsequently, a 10-day static compression load test is conducted. Research indicates that ①In the initial penetration phase of open-ended pipe piles, the soil plug height may surpass the penetration depth, indicating that the incremental filling ratio (IFR) exceeds 1. ②The local shaft resistance of both the inner and outer pipes gradually decreases with an increasing h/R ratio, demonstrating a pronounced h/R effect. At an h/R ratio of 2.3, the local shaft resistance of the inner pipe is approximately four times greater than that of the outer pipe. ③There is a significant correlation between soil plug resistance and the incremental filling ratio (IFR), where higher IFR values correspond to lower soil plug resistance. In contrast, the normalized annular resistance remains largely independent of IFR and maintains a stable value. The normalized base resistance (qb/qc) gradually decreases as the IFR value increases. When the IFR approaches 0, the normalized base resistance (qb/qc) approaches 1, indicating that the base resistance qb becomes nearly equal to the cone resistance qc. ④Based on the correlation between normalized base resistance and the effective area ratio, a formula for calculating the base resistance during jacking of open-ended pipe piles was established and validated. It is recommended to consider the influence of different driving methods in bearing capacity calculations.

       

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