Settlement Reducing Auger Cast-In-Place Piles Below a Mat Foundation of a 41-Story Structure-Paper

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The Great American Tower in Cincinnati, a 41-story, 203-meter building, was constructed over variable soils that included thick, highly compressible lakebed deposits. Analysis showed that a conventional mat foundation would settle excessively, while supporting the structure on piles extending to bedrock would be costly and slow.<br /><br />The design team developed a piled-mat system in which a limited number of 18-inch-diameter auger cast-in-place (ACIP) piles act primarily as settlement reducers, rather than carrying the building’s full load. Three-dimensional FLAC 3D modeling accounted for soil variability, excavation and construction sequence, structural stiffness, and interaction among the mat, piles, and ground. Field investigation and pile load tests informed and refined the model. The final design used 281 strategically placed piles terminating in a stiff transitional soil layer above bedrock.<br /><br />Predicted settlement for the mat-and-building-core system without piles remained unacceptable. Adding the settlement-reducing piles lowered predicted settlement to less than 1 inch—an estimated 80–90% reduction compared with the unpiled mat models—and brought settlement within project tolerances.<br /><br />Instrumentation during and after construction included pile strain gauges, earth-pressure cells, a settlement sensor, and monitoring of excavation and vibrations. Measurements generally agreed with model assumptions and predictions, supporting the design approach.<br /><br />The project demonstrated that advanced soil–structure interaction analysis, validated by field testing and monitoring, can successfully combine conventional mat and ACIP pile foundations. The approach reduced the number of piles required compared with a bedrock-supported system, saving construction cost and time while providing acceptable foundation performance.

Keywords

Great American Tower

Cincinnati

piled-mat foundation

settlement reduction

auger cast-in-place piles

soil-structure interaction

FLAC 3D modeling

lakebed deposits

foundation design

pile load testing