Time dependent behavior and capacity assessment of open-end pipe piles

摘要

Three piers of the new 1-494 Mississippi River bridge are supported on 1.067 m diameter open-end steel pipe piles. At time of drive, and after pile re-strikes, high-strain dy-namic capacity values at plan tip elevations, were as low as 23% of design requirements. Pile lengths were extended 20% to 40% deeper than planned at two piers, driving to rock, impacting both the project cost and schedule. The significant discrepancy between predicted and observed capacity was investigated through a series of coordinated dynamic tests; a full-scale test pile was installed and monitored over 221 days by both dynamic and quasi-static methods. Unusu-ally slow strength gain, assessed by dynamic monitoring, was observed at all piles installed at the site. CPT soundings were advanced at the test pile and through the pile interior to investigate the soil plug. High-strain dynamic methods were found to have significant limitations in prop-erly assessing the long-term capacity of large-diameter open-end pipe piles during driving and short-term pile re-strikes. Although the full-scale test pile never exhibited the required ultimate capacity by high-strain dynamic methods, the quasi-static test indicated the test pile had suffi-cient capacity at the design tip elevation. Design and construction issues, development of a long-term testing program in a construction setting, test program implementation, results, and recommendations based on the findings are presented. The need for an improved understanding of the physical effects of pile installation and their impact on soil strength assessment in both design and monitoring procedures to avoid unsafe or needlessly expensive designs is discussed.