Influence of pilot hole depth on construction parameters and bearing behavior of pipe piles in saturated clay sites

Taking the saturated clay site as the background, the influence of pilot hole depth on pile installation, soil plugging effect, and bearing performance was systematically analyzed by comparing the construction parameters and single pile static load test results of prefabricated pipe piles (pile length of 9.0 m) with different pilot hole depths (3.0, 4.5, 6.0 m). Key findings include: (1) For jacked pile installation, the final pressure showed minimal variation when pilot hole depths were less than half the pile length (4.5 m), but significantly reduced at two-thirds pile length (6.0 m). In hammer-driven piles, the blow count decreased substantially with pilot hole depth, particularly for the final 80 cm penetration at 6.0 m depth. (2) Increasing pilot hole depth reduced soil plug length in jacked piles but increased it in hammer-driven piles, indicating greater soil intrusion into pile cavities during dynamic installation. (3) The ultimate bearing capacity of single piles decreased with pilot hole depth. Shaft resistance dominated capacity reduction at depths 4.5 m, while both shaft and tip resistances significantly diminished at 6.0 m depth. (4) The experimental ratio of the characteristic vertical compressive bearing capacity to the final jacking force averaged 0.293, approaching the lower bound of 0.30 stipulated in Clause 4.3.3 of the Technical Specification for Static Pressing Precast Concrete Pile Foundations (DBJ/T 15—94—2013). This indicates that the bearing capacity can still be estimated from the final pressure under pilot-hole conditions in saturated clay. In the absence of regional experience, the safety factor should be amplified by 11%~26% relative to the code value. (5) Considering both construction feasibility and bearing-capacity loss control, the pilot hole depth for pipe piles in saturated clay sites is recommended to be 0.4~0.5 times the pile length, which effectively mitigates extrusion effects while limiting capacity reduction. These findings provide a reliable reference for the design and construction of pilot hole techniques for pipe piles in saturated clay strata.