Strength Gain Trends of Large Diameter Open-End Pipe Piles in Cooper Marl-PPT Presentation

PPT Presentation

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The Port Access Road project in Charleston, South Carolina, used large-diameter open-end steel pipe piles to support a major elevated roadway over highly variable ground. The piles were founded in Cooper Marl, a thick, cemented, fine-grained formation with consistent bearing-layer elevation but unusual behavior: driving generates high excess pore-water pressures, causing piles to rebound and develop greater resistance over time as pressures dissipate (“setup”).<br /><br />The project installed 30-, 36-, and 42-inch piles, typically 70–100 feet long, to achieve ultimate capacities of about 800–1,200 kips. Open-end pipes were selected to limit displacement and rebound. Because plugging and end-bearing contribution during testing were uncertain, the design conservatively assumed little end bearing and increased pile embedment by approximately 6–9 feet.<br /><br />Dynamic testing included 56 restrikes. Results confirmed substantial, time-dependent setup, with resistance increasing roughly 2.5 to 4.5 times relative to end-of-driving measurements. Deeper pile tips generally developed greater restrike resistance. Some piles required a second restrike: resistance below the requirement after seven days exceeded it at later tests. Across the project, measured resistance averaged 115% of the required bearing capacity, and estimated tip elevations proved adequate.<br /><br />Testing presented challenges, including selecting hammer energy to mobilize both shaft friction and end bearing, deciding how long to wait before restriking, and interpreting blow-by-blow measurements. Restrikes after about 17 days were considered reasonable and conservative.<br /><br />The project demonstrated that large open-end pipe piles can perform effectively in Cooper Marl. Vibrating piles to the marl, then impact-driving them to design tip, limited installation difficulties; subsequent restrike testing documented setup and verified capacity without requiring deeper-than-planned tips.

Keywords

Port Access Road

Charleston South Carolina

open-end steel pipe piles

Cooper Marl

elevated roadway

pile setup

dynamic testing

restrike resistance

bearing capacity

pile driving