Use of Settlement Reducing Auger Cast-in-Place Pile below a Mat Foundation

Abstract

As the tallest building in Cincinnati, the 45 story Great American Tower at Queen City Square is not only changing the downtown skyline, but is also contributing to the advancement of the state-of-the-art use of auger cast-in-place (ACIP) piles as settlement reducers. Predicted settlements of a traditional mat foundation were excessive, and the costs and construction time associated with traditional ACIP piles extending to bedrock were not preferred. A mat supported on a limited number of ACIP piles, with the piles primarily acting as settlement reducers, was evaluated as the preferred foundation type. Traditional standard penetration test boring data was initially used to develop a numerical analysis of the soil-structure interaction using FLAC 3D software. The model was further modified with cone penetration and pressuremeter testing, along with actual load test results on several ACIP pile elements of varying lengths, but all tipping above bedrock. Through numerous project team discussions, and detailed numerical modeling analyses, ACIP piles were recommended for reducing mat foundation settlement. The process used advanced geotechnical analysis and design to mesh traditional mat and ACIP pile foundations into a unique soil-structure interaction based foundation system.