Dulles Metrorail/Silver Line (Phase 2) Drilled Shaft Lateral Load Testing Program-Paper

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The Dulles Silver Line Phase II includes elevated guideways supported by large-diameter drilled shafts socketed into siltstone. Three instrumented lateral load tests were conducted to verify the soil and rock resistance assumptions used to design these foundations, particularly under high combined production loads.<br /><br />The test program used three 6-foot-diameter shafts at sites with different depths to rock. Each was loaded laterally to approximately 1,100–1,126 kips, with a nearby shaft serving as a reaction. Instrumentation measured shaft deflections and strains. LPile models were developed to select test loads that would produce rock-socket responses comparable to those expected in the larger, heavily loaded production shafts.<br /><br />Measured deflections and bending-moment distributions generally agreed with LPile predictions. The estimated maximum mobilized resistance in the test sockets also closely matched the predicted response of production shafts. Deflections within the rock sockets were equal to or less than model predictions. The two shafts with shallow rock showed greater-than-predicted movement at the top, likely due to structural yielding or non-composite behavior of the casing, rather than inadequate rock-socket resistance.<br /><br />The results support the LPile rock parameters used for design. They also show that depth to rock and shaft structural stiffness strongly influence performance: small differences in rock depth can affect top-of-shaft deflection more than substantial changes in assumed rock properties. For shafts with rock less than about 10 feet below ground, overburden soils and intermediate material had little influence on lateral response. Structural yielding observed in the test shafts is not expected in production shafts, which will use more conservative reinforcement design.

Keywords

Dulles Silver Line

Phase II

elevated guideways

drilled shafts

siltstone

lateral load tests

LPile modeling

rock socket resistance

shaft deflection

foundation design