An elevator addition was planned within the original Wilson Middle School building constructed in 1933. A soil boring completed outside the building identified mixed fill material to a depth of 11 feet over medium dense sand to a depth of 31 feet. Exterior grade at the test boring was approximately 2.5 feet below finished floor elevation and three feet above the proposed elevation for the elevator pit excavation.
A deep foundation system was needed to transfer the new loads of the elevator addition to competent bearing soils below the fill. The deep foundation system would also minimize influence that the elevator loads would have on the existing shallow footings. Machinery used to install auger-cast piles, drilled shafts or driven piles could not access the proposed elevator location. The deep foundations would also have to be installed within the tight working space of the elevator pit excavation, which further limited installation equipment and foundation options.
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. Twelve (12) Model 287 (2.875-inch OD by 0.203-inch wall) hollow round shaft helical piles with a 10”-12”-14” triple-helix lead section were selected to support the design working load of 25 kips per pile. A hand held drive unit, powered by a remote hydraulic source, was used to install the helical piles to at least 5,600 ft-lb of torque to correlate to ultimate pile capacities of at least 50 kips (FOS ≥ 2). Standard extensions advanced the piles to depths from 14 to 20 feet below the bottom of the excavation. New construction brackets were bolted to the tops of the helical piles to be cast into the poured mat foundation of the elevator shaft. The twelve helical piles were installed in one day.
Architect/Structural Engineer: Shive-Hattery, Inc.
Geotechnical Engineer: Terracon Consultants, Inc.
Certified Pile Installer: MidAmerica Basement Systems
Products Installed: (12) Foundation Supportworks® HP287 Helical Piles, 10"-12"-14" Lead Section, Installed to Depths from 14 to 20 feet, Design Working Load of 25 kips