Foundations for the New Mississippi River Bridge, St. Louis-Article

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The New Mississippi River Bridge in St. Louis is a cable-stayed bridge with a 1,500-ft main span supported by two river pylons. Through an Alternate Technical Concept (ATC), the winning construction team redesigned the pylon foundations using six 11.5-ft-diameter drilled shafts per pylon instead of fourteen smaller shafts. The change reduced pile-cap and cofferdam dimensions and was expected to lower project costs. A full-scale load test was included to verify the design and support higher resistance values.<br /><br />Subsurface investigations found sand and silt over hard, predominantly sound limestone. Core recovery and rock quality were generally high, and no significant solution cavities were identified at the final foundation locations. Acoustic televiewer logs helped locate rock seams and fractures, aiding both site assessment and shaft construction. Construction-phase rock-strength tests were substantially higher than pre-bid results, although the difference did not significantly affect the work.<br /><br />A non-production test shaft was built using the same equipment and methods planned for the permanent foundations. Four embedded hydraulic jacks applied a bi-directional load of about 72,000 kips. The test mobilized average side resistance exceeding 44 ksf and base resistance just under 460 ksf, with only about 0.14 in. of displacement in either direction. These resistances exceeded the ATC design values, but the loading system could not mobilize the rock socket’s full nominal capacity.<br /><br />The test confirmed that the foundation design was adequate and indicated that the shafts’ structural capacity, rather than the limestone’s geotechnical resistance, controlled design. Minimum socket lengths were nevertheless retained to meet lateral-load and reinforcement-development requirements. The study also found that measured behavior was broadly consistent with established design guidance for rock-socketed shafts. Overall, the ATC and load test enabled a more efficient foundation design, validated construction methods, and provided quality assurance despite minor shaft imperfections.

Keywords

Mississippi River Bridge

St. Louis

cable-stayed bridge

drilled shafts

pylon foundations

alternate technical concept

load test

limestone socket

rock resistance

construction quality assurance