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 Study on the Jacked Piling Mechanism and Vertical Bearing Characteristics of Open-Ended Double-Wall Pipe 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。通过采用光纤光栅(Fiber Bragg Grating,简称FBG)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天的受压静载试验。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(Incremental Filling Ratio,简称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,且验证了该公式的可靠性,并建议在进行承载力计算时要区分沉桩方式的影响。

       

      Abstract: A thorough 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 open-ended pipe piles cannot differentiate between internal and external resistances. This paper presents the design of an open double-wall pipe pile capable of separating these internal and external resistances. Large-scale model tests were conducted to investigate the cyclic penetration behavior of open double-wall pipe piles in sandy soil foundations with a relative density (Dr) of approximately 50%. This study employed monitoring techniques, including Fiber Bragg Grating (FBG) strain sensors, to investigate 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 pile tip resistance—during cyclic penetration processes. Subsequently, a 10-day static load test under compression was conducted. Research indicates that: ① During the initial penetration phase of open-ended pipe piles, the soil plug height may exceed the penetration depth, meaning the soil incremental filling ratio (IFR) is greater than 1;②The local lateral friction resistance of both the inner and outer pipes gradually decreases as the h/R ratio increases, demonstrating a pronounced h/R effect. At h/R = 2.3, the local lateral friction resistance of the inner pipe is approximately four times that of the outer pipe;③A significant correlation exists between soil plug resistance and IFR, with higher IFR values corresponding to lower soil plug resistance. Normalized annular resistance remains largely independent of IFR, maintaining a stable value. The normalized pile tip resistance (qb/qc) gradually decreases as the IFR value increases. When the IFR approaches 0, the normalized pile tip resistance (qb/qc) approaches 1, indicating that the pile tip resistance qb becomes nearly equal to the cone resistance qc;④Based on the relationship between normalized pile tip resistance and the effective area ratio, a formula for calculating pile tip resistance during jacking of open-ended pipe piles was established and validated the reliability of the formula. It is recommended to account for the influence of different driving methods when performing bearing capacity calculations.

       

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