HP as Plan B: A Helical Alternate Turns NO into GO!

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HP as Plan B:

A Helical Alternate Turns NO into GO!

The best laid plans of mice and men often go awry! If he had been in the construction industry, Scottish poet Robert Burns most surely would have included mice, men, and machinery. Owners, their Contractors, and Engineers, are often faced with a forced move to Plan B as site issues rear their ugly heads. This was the case for a proposed seven-story, multi-use building (stores below, condos above) on Broad Street in Philadelphia, PA. Click to see Beacon Street

The original plan called for sheeting and shoring the full footprint of the property with a full basement and a slab-on-grade foundation. Soil borings led to excavation which revealed up to five separate foundations or supports from various incarnations of buildings and/or additions on the site. However, it was the issue of the proximity to the Broad Street Subway line, bordering the street, that caused Geotechnical Engineer Jianchao Li from Geostructures to call for a pow-wow with Structural Engineers, O’Donnell Naccarato. Weak surface soils required a deep excavation for the sheeting and shoring, causing the engineering professionals to rethink the original foundation plan. The challenge then became how to keep as much of the footprint of the building and basement as possible without compromising the nearby subway. The weak surface soils and a deep excavation to accommodate the basement raised red flags and sent the engineers in search of a Plan B. Click to see Westhampton

The new plan involved utilizing helical piles on the 30’ section of the property facing Broad Street and an additional 30’ section abutting the alleyway to the south of the property where utilities were located. 105 IDEAL 3.5” round shaft helical piers were installed 27’ and 37’ deep (on average) in clusters of four to support column loads. Half of the structure would retain the original slab-on-grade, while the other half of the structure was constructed with a basement and backfilled prior to the installation of the helical piles. Click to see Manhattan Apartments

As is typical of the Wissahickon Formation, weak soils overlay schist at varying depths. Accordingly, some piles went deeper, one as deep as 57’, before reaching the required 11,500 foot-pounds of torque. Urban fill and the abandoned old foundations further complicated excavation and confirmed the wisdom of the helical alternative. The sections of the building supported on helical-reinforced columns required 100 kips in compression and a 4-kip lateral load. This alternative approach allowed the owner to retain the full footprint of the original building with a partial basement. Ground-floor Broad Street Store space and the Condominiums’ square footage above were not compromised.

The General Contractor, Ferraro Construction Group, hired longtime certified Danbro Installer Steve Vaspoli (Vaspoli Builders) to serve as the helical pile contractor. Click to see Newbury Street

The helical pile world is full of stories like this: site conditions (or other issues) send Owners, GCs, and their professional consultants scurrying in search of a solution. Geostructures’ Jianchao Li and Structural Engineers O’Donnell Naccarato responded to this challenge, saving the original building footprint while eliminating subway concerns. Ferraro and Vaspoli overcame various installation challenges, involving old, abandoned footers and other obstructions. Once again, helical piles proved to be a viable Plan B when even the best laid (foundation) plans went awry.