Drilled Pier Load Tests on Dulles Metrorail Project - Overview, Results and Discussions-Paper

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The Dulles Corridor Metrorail Phase I project used drilled shafts to support aerial guideways across Northern Virginia’s Piedmont geology. Fourteen load tests using Osterberg cells (O-Cell) and Statnamic methods assessed shaft capacity and helped validate design assumptions.<br /><br />Subsurface conditions typically comprised fill and residual soils over increasingly dense soils, decomposed rock, and bedrock. Test results showed measured side shear of 0.2–4.6 ksf in soils and 2.3–8.0 ksf in decomposed rock. End-bearing resistance in decomposed rock ranged from 41 to 91.8 ksf, averaging about 55 ksf. Most tests were conducted on production shafts and did not reach ultimate capacity, so results may not represent maximum resistance.<br /><br />Design calculations using the AASHTO β-method and FHWA-1999 method generally agreed with test results. They tended to overestimate resistance in shallow, weaker soils and underestimate side shear in deeper, stronger decomposed rock. Calculated average end bearing was approximately 58 ksf, slightly above the measured average. Overall, the design approach was somewhat conservative, a prudent choice given construction variability.<br /><br />The paper also documents practical lessons. Excessive casing vibration disturbed a thin silt layer and prevented one O-Cell test from being completed; a subsequent Statnamic test confirmed the shaft’s acceptability. The contractor later adopted slurry for ground support and water control. Load-test findings also led designers to reconsider conservative reductions for group effects in strong decomposed rock and bedrock.<br /><br />The authors conclude that these findings can inform preliminary drilled-shaft design in similar Piedmont conditions, while noting that newer FHWA and AASHTO methods may better match measured capacities.

Keywords

Dulles Corridor Metrorail

drilled shafts

Osterberg cell

Statnamic load test

aerial guideways

Piedmont geology

side shear

end-bearing resistance

AASHTO beta method

FHWA design method