Rammed Aggregate Piers Defeat 75-Foot Long Driven Piles

A six-story parking garage was designed to be supported by 23 m (75 foot) long steel pipe piles driven through loose silts and silty sands. Rammed Aggregate Piers with shaft lengths ranging between 2.1 m and 2.7 m (7 feet and 9 feet) were proposed and accepted as an alternative to the driven piles. In this system, highly densified aggregate piers are incorporated within the upper weak and compressible soils resulting in a composite bearing material that is substantially stiffer than the unimproved soil, and on which shallow foundations can be supported with tolerable settlements. Methods for analyzing and predicting settlements of footings supported on aggregate pier-reinforced soils are discussed. Design parameters were verified by the performance of aggregate pier modulus load tests. Total settlements of footings were estimated to be less than 4 cm (1-1/2 inches). Measured footing settlements were less than 1.3 cm (0.5 inch). The implementation of the aggregate pier system saved the project owner $187,000.