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Foundation Supportworks by Woods Case Studies: St. Louis, Missouri Elementary School Supported with Helical Piles

Monday, February 27th, 2017 by Desiree Bainbridge

Challenge

The Hickey Elementary School building was constructed in 1965. An elevator addition was planned for the school to comply with ADA requirements. The soil profile at the proposed elevator location generally consisted of uncontrolled fill with rubble and debris underlain by competent clay. A deep foundation system was therefore needed to transfer the new loads of the elevator addition below the fill to the competent bearing soils. The deep foundation system would also minimize the influence of the elevator loads on the existing shallow footings. The deep foundations would also have to be installed within the tight working space of the elevator pit excavation, which included low overhead conditions due to an eight-foot high ceiling.

Solution

Helical piles were selected as the ideal deep foundation solution for this project given their ability to be installed with relatively small equipment within the confined space of the existing building. The design engineers required three (3) Model 288 (2.875-inch OD by 0.276-inch wall) hollow round shaft helical piles with a 10”-12” double-helix lead section be installed to support a total service load of 80 kips (27 kips per pile) in compression. A hand held drive unit, powered by a remote hydraulic source, was used to install the helical piles to torque-correlated ultimate pile capacities of at least 54 kips (FOS ≥ 2). The limited overhead clearance prompted shorter (5-foot) extensions to advance the piles to depths from 21 to 31 feet below the bottom of the excavation. New construction brackets were fitted to the tops of the helical piles to be cast into the poured mat foundation of the elevator shaft.

During installation, old concrete foundations were encountered below the pit excavation. While the helical piles effectively “churned” through most of the surficial ruble and debris, they could not penetrate this large obstruction. A mini drill rig was brought in to core through the existing footing and allow for the helical pile installation.

About the author
Desiree is a digital marketing coordinator at Woods Basement Systems.
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