Model Test Study on the Jacked Piling Mechanism and Vertical Bearing Characteristics of Open-Ended Double-Wall Pipe Piles

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.