Cedar Rapids, IA - Wilson Middle School Elevator Addition Uses Helical Piles for Foundation Support

Challenge

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.