Geotechnical Resistances of Drilled Shafts in Triassic Basin Sedimentary Rocks-Article

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The case study examines axial load tests on rock-socketed drilled shafts in Triassic Basin sedimentary rocks at three Mid-Atlantic projects: the Goethals Bridge in the Newark Basin, the Silver Line in the Culpeper Basin, and the Susquehanna River Bridge in the Gettysburg Basin. The sites share similar “red bed” geology, chiefly siltstone, sandstone, and shale formed in rift basins during the breakup of Pangaea.<br /><br />Load tests at Goethals and the Silver Line showed unit side resistance of roughly 1.3–1.7 MPa and base resistance approaching or exceeding 14–17 MPa. These results broadly support the FHWA design method for predicting side resistance and show that side and base resistance can be combined when both are mobilized at acceptable displacements.<br /><br />Construction and inspection quality proved especially important for base resistance. Effective base cleaning and quality assurance at Goethals and the Silver Line enabled high resistance at small movements. At Susquehanna, where water seepage and other construction or testing problems were reported, measured side resistance was generally lower than predicted. Compressible material beneath test shafts delayed mobilization of base resistance, although substantial resistance was eventually achieved in one test.<br /><br />The authors conclude that FHWA methods can provide design-level estimates for these rocks when supported by appropriate geological characterization, load testing where warranted, and rigorous construction quality control and assurance. Rock strength, discontinuity orientation, socket stability, groundwater control, and base cleanliness all affect performance.

Keywords

axial load tests

rock-socketed drilled shafts

Triassic Basin

sedimentary rocks

mid-Atlantic projects

side resistance

base resistance

FHWA design method

construction quality control

geological characterization