Case Study: 1785 Massachusetts Avenue, NW, Washington, DC-Paper

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The renovation of 1785 Massachusetts Avenue NW, a 1917 National Historic Landmark in Washington, DC, expanded the building’s usable area and added a basement level. Excavation of up to 15 feet beneath the existing structure required replacing its spread-footing support while protecting the limestone façade and plaster interiors. The upgraded building also carried approximately 10% greater foundation loads.<br /><br />A deep foundation system using more than 200 micropiles was installed in restricted, low-headroom conditions. Interior steel columns were temporarily supported by micropile truss towers, with transfer beams and controlled hydraulic jacking shifting loads from the existing footings. The micropiles later became part of the permanent foundation, connected to new pile caps and concrete columns. A compression load test showed that a micropile could sustain its 200-kip design load with a factor of safety of 2.5.<br /><br />Perimeter columns and walls were supported by micropile-assisted concrete underpinning pits. Two levels of temporary tiebacks stabilized the pits during excavation. Settlement was monitored throughout construction, with differential movement limited to 0.25 inches to protect historic finishes. A second controlled jacking operation transferred final building loads to the new concrete columns and pile caps; planned perimeter jacking was not needed.<br /><br />Despite tight access, limited storage, and a busy urban setting, the project successfully transferred the building’s loads to the new foundation system while preserving its historic character.

Keywords

micropiles

foundation renovation

historic landmark

underpinning

load transfer

deep excavation

structural support

settlement monitoring

hydraulic jacking

Washington DC