Abstract:
A challenging earth retention project presented itself in Washington, DC in the backyard of the historic House of the Temple (The Scottish Rite of Freemasonry, Supreme Council 33°). Located at 15th St NW and S St NW, a forty foot deep underground basement was to be installed adjacent to the existing Temple, the historic carriage house structure dating back to the 1800’s and across a narrow alley from a 4 story brick apartment building. The unique geology of the site included typical urban fill overlying a mixture of loose alluvial soils underlaid by dense marine deposits of the Potomac Formation. The clay, sand and gravel of the Potomac included nested boulders with diameters up to 3’. Design of the earth retention system took into account ground conditions, installation methods and estimated movement of the system and adjacent structures. Anticipated ground water and unanticipated leaky sewer water also proved obstacles to overcome during construction. Soldier Beams were installed using a variable moment-vibratory hammer to minimize disturbance to the historic and sensitive structures. Monitoring of the existing structures included both seismograph and optical targets during installation of soldier beams and as the excavation progressed. Micropiles drilled into the Potomac Formation were a part of the permanent foundation of the new structure. This local case history will delve into the details of the successful design, installation and performance of an earth retention system and specialty foundations in a historic and highly congested urban environment.