Helical Piles Support New Pavilion in Faneuil Hall
Introduction:
The project was part of a revitalization of Boston’s historic Faneuil Hall Marketplace, which is located near the City’s original coastline area that was filled in the 17th and 18th centuries to increase waterfront real estate.
The revitalization project included replacing an existing 1970’s structure with a new, one-story, retail glass pavilion adjacent to Quincy Market. Structural column loads ranged from about 30 to 60 kips.
Project Challenges:
Primary project challenges included: 1) construction in a sensitive, historic tourist area; 2) limited construction access; and 3) difficult subsurface soil conditions that are common along Boston’s reclaimed waterfront.
Subsurface soil conditions generally consisted of 9 to 14 feet of unsuitable urban fill (soil mixed with miscellaneous man-made debris) over up to 5 feet of soft organic silt, over thick natural marine deposits (clay, silt, sand), overlying glacial till. The upper portion of the marine layer was relatively stiff/dense and became softer with depth. Glacial till was approximately 60- to 70-feet-deep. Remnants of previous structures, such as buried timber wharfs, were present in the fill to further complicate the already challenging subsurface conditions. Groundwater was encountered at 13 to 14 feet below the ground surface.
Geotechnical Design/Build Solution:
Excavation, disposal, and replacement of the unsuitable fill and organic layers was deemed impractical due to premium costs associated with off-site soil disposal, excavation dewatering, and importing large quantities of structural fill.
The project team explored several piling options in lieu of excavation/replacement, including driven timber piles, drilled micropiles, drilled shafts, ductile iron piles, and helical piles. Driven timber piles were economically viable but were eliminated from consideration due to access issues and noise/vibration concerns. Drilled micropiles and drilled shafts both offered low-noise and low-vibration solutions, but were too expensive. Ductile iron piles and helical piles were both appealing options due to ease-of-access, low vibration, and relatively low cost. The team ultimately selected helical piles as the most suitable and cost-effective piling option.
Helical pile shafts are made of galvanized steel and are installed in short sections, each about 5- to 7-feet-long. Each pile consists of a lead helical section with welded screw-like bearing plates; subsequent straight-shaft sections are mechanically-fastened to the lead section as it is advanced into the ground. The piles are installed with a skid-steer or an excavator equipped with a high-power torque head, which is calibrated to directly correlate torque resistance with pile capacity.
The Pavilion’s final structural design required 48 helical piles with an allowable compressive capacity of 30 kips each. The final pile design was performed by Helical and featured a galvanized 80 ksi steel pipe section manufactured by The Ideal Group. The piles consisted of a 27/8-inch-diameter, 0.276-inch-thick shaft with quadruple-helix (8-inch/10-inch/12-inch/14-inch) lead sections. The piles were designed to derive end-bearing capacity in the glacial till layer below the fill, organic silt, and marine layers.
