Smithsonian National Postal Museum – The Challenge
The Smithsonian’s William H. Gross Stamp Gallery, located at 2 Massachusetts Ave. NE was undergoing a renovation, which included the construction of a new elevator shaft. Due to the confined interior space and conflicts with existing foundations, as well as the presence of old fill and poor bearing soils, the project structural engineer and architect specified that the elevator mat foundation is supported by helical piles.
Other challenges included limited geotechnical engineering data, restrictive access to the work area, and equipment size limitations. The helical pile installation equipment had to be electric or pneumatic powered (no combustion engines allowed). Equipment had to cross the marble-tiled floor of the existing museum and pass through hallways and standard doorways, with the additional requirement of being moved at night when the building and museum were closed.
Smithsonian National Postal Museum – The Solution
The helical pile system was recommended because of its relative ease of installation and adaptability to erratic site conditions. The helical piles required to meet the structural specifications were designed by the JES engineering team and incorporated numerous Helical Piles. This hollow shaft round pile has a 2.875” outside diameter and ASTM A572 Grade 50 x 3/8-inch thick helix blades.
The piles were designed to provide 46.6 kips of ultimate compression. Tension and uplift were not a design factor. The torque required to achieve the design capacities was 5,508 ft-lbs. The helical pile components were hot-dipped galvanized in accordance with ASTM A123 [Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products].
To supplement the subsurface soil data collected from a geotechnical boring which was performed outside the building, JES performed a “Helical Test Probe” at the elevator location using a Compact Electric Excavator to support the hydraulic torque motor used to install helical piles.
Actual production piles were installed using specialized helical pile equipment configured for limited site space, and capable of running off of battery power and hydraulic service fed from outside the building through a window. All equipment was mobilized from the museum loading dock in the garage to the work area using hand-operated pallet jacks.
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Smithsonian National Postal Museum – Project Summary
JES installed eighteen (18) FSI Model HP288 Helical Piles. The lead pile section was 7’ long with 8,”10” and 12” Helices. To reach acceptable bearing depth 5’ shaft extensions were used. The piles were installed to a typical depth of 21.5 feet and the required torque (using a safety factor of 2) was achieved. In order to verify individual pile capacity, a “Pro-Dig C440 Torque Transducer” (Intelli Tork) was used to determine that the required torque was being delivered to each pile during installation.
The piles were installed from the partially excavated elevator pit. Once the pit was excavated to design base elevation, JES cut each pile to the required elevation and capped them with an FSI 6”x6” steel plate, which was embedded in the concrete mat foundation.
JES Project Engineer: David E. Stinnette, P.E.
Architect: Cho Benn Holback + Associates
Structural Engineer: Spiegel Zamecnik & Shah
Geotechnical Engineer: ECS/Hillis-Carnes
General Contractor: Clark Construction Group, LLC