Abstract:
Pressure-grouted helical-displacement piles were used to support a new 7-story homeless housing center in Coney Island, Brooklyn, NY; a neighborhood with noise and vibration restrictions. The contractor-proposed alternative piling system involved pumping grout under pressure through ports in the pile toe as the pile was screwed into the ground. The pile consisted of high strength steel pipe with a helical plate near the toe that helped to displace and mix the soil with the grout, creating a composite mix around the pipe. The resulting piles were used in lieu of the traditional micropiles shown on the contract drawings. Piles were installed over 100 feet deep, through interchanging layers of medium dense to dense sand and very soft clay. Two index piles were successfully load-tested to almost 1,000 kips in compression, meeting the project demand. Materials for this alternative foundation system cost approximately 60 percent of the original micropile design and production was 3 times faster than micropiles.
A case history is presented which includes a discussion of the methods used to predict pressure-grouted helical pile capacity, interpretation of the strain gage-instrumented static load tests, application of the neutral plane method for evaluation of drag loads, development of final installation parameters, production installation rates, and lessons learned.
A collaborative effort by the owner, developer, structural and geotechnical engineers, foundation contractor, and equipment and pile supplier allowed successful implementation of the alternative foundation system, for this privately contracted work. Suggestions to help designers and owners develop plans and specifications that