2100 L Street Underpinning & Support of Excavation-Paper

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The 2100 L Street project in Washington, DC, involved constructing a new office and retail building beside the historic Stevens School, requiring deep excavation below bedrock in a constrained urban site. Challenges included wall-line shoring requirements, buried utilities, hard schist, perched groundwater, and uncertainty about the school’s foundations.<br /><br />Different systems were used to address site-specific conditions. The adjacent eight-story Gelman Building was supported with steel bracket piles socketed into rock, with tiebacks providing lateral support. This system performed well, with negligible lateral movement, average settlement of about ¼ inch, and no observed damage.<br /><br />Along L Street and 21st Street, numerous utilities—especially deep sewers—limited tieback placement. Soldier-pile-and-lagging walls therefore used upper-tier bracing, including rakers and cross-lot braces, and lower-tier tiebacks anchored in bedrock. Performance was generally satisfactory, although some soldier piles exceeded the project’s one-inch lateral-movement threshold. Monitoring highlighted the importance of temporary excavation stages and soil-berm conditions during raker installation.<br /><br />Underpinning the Stevens School proved most difficult. Soil-cement beams were mixed beneath the unreinforced brick-and-rubble foundations to create a protective roof for hand-dug piers. Perched water and exceptionally hard rock made the planned pier excavation impractical, so drilled micropiles were installed through the piers and extended into rock. Soil nails and tiebacks were also added after monitoring detected outward movement. Excavation support transitioned from lagging and tiebacks to rock bolts and shotcrete at bedrock.<br /><br />The school underpinning enabled construction but caused movement and cracking, requiring masonry repairs and interior tie rods. The project demonstrates that complex urban excavations may require multiple construction methods and careful attention to monitoring, groundwater, and temporary conditions.

Keywords

2100 L Street

Washington DC

deep excavation

urban excavation support

steel bracket piles

soldier pile and lagging

tiebacks

micropiles

underpinning

bedrock excavation