Deflection-Based Design for the Capitol Crossing Support of Excavation-Paper

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The Capitol Crossing project in Washington, DC required more than 130,000 square feet of temporary support of excavation, with excavation depths reaching 72 feet. This paper examines the system beside Holy Rosary Church (HRC) and its 85-foot Bell Tower, where excavation posed a risk to historic masonry and stained-glass windows.<br /><br />The design used a deflection-based approach to estimate support-wall movements, resulting ground settlement and strain in nearby buildings, and potential damage. The design goal was to limit damage to “negligible” or “very slight.” To increase stiffness and control movement near the HRC’s foundations, the support system included a single-auger soil-mixed wall, multiple tieback levels, and underpinning. Three tieback tiers were installed within the upper 17 feet of excavation. An extensive real-time monitoring program tracked the walls, buildings, and foundations during construction.<br /><br />At the HRC’s eastern monitoring location, the measured maximum settlement was about 0.50 inches, close to the 0.56 inches predicted. Average lateral soil movement was greater than estimated, largely because wall bulging exceeded predictions. At the western location, movements were smaller, likely because it was near an excavation corner. Settlement differences between the church’s exterior wall and interior columns were negligible, and no significant cracking occurred; the stained-glass windows remained intact. Overall, the support system met its damage-control objectives.<br /><br />Monitoring also revealed unexpected eastward movement of the HRC and Bell Tower as blocks, a behavior also observed at other nearby buildings. Its cause remains unclear. The authors conclude that excavation design must account for system stiffness as well as earth and surcharge pressures, and that further well-monitored case studies are needed to improve predictions of actual performance.

Keywords

Capitol Crossing

Washington DC

excavation support

soil-mixed wall

tieback system

underpinning

ground settlement

historic masonry

real-time monitoring

Bell Tower