Challenge

August 2010- A $3 million dryer addition was proposed with new foundations within and adjacent to an existing one-story building with basement. The basement floor slab of the existing building is approximately 15 feet below exterior grade. An interior pile cap was planned within the basement and an exterior pile cap was planned adjacent to the existing foundation wall. The two pile caps essentially created continuous support (although at different elevations) for a new foundation wall. The bottom of the exterior pile cap was 4 feet below grade to provide frost protection. With the new exterior pile cap abutting the foundation wall of the existing building, piles would have to be installed to depths below the existing footings to prevent excessive lateral loads from being applied to the existing basement wall. A geotechnical investigation included one soil boring to a depth of 30 feet. The boring encountered very loose to loose sand fill from the surface to 8 feet, over loose sand from 8 to 13 feet, over medium dense sand from 13 to 30 feet.

Solution

Several deep foundation systems were considered, but helical piles were selected as the ideal option given the limited access to the interior pile locations and the ability to quickly mobilize equipment and product to the job site. The foundation design included two helical pile configurations. Eleven Model 287 (2 7/8-inch OD by 0.203-inch wall) round shaft helical piles with 10”-12” double-helix lead sections were included to support the design working load of 10 kips per pile, and three Model 288 (2 7/8-inch OD by 0.276-inch wall) round shaft piles with 10”-12”-14” triple-helix lead sections were included to support the design working load of 22.5 kips per pile. Seven piles were installed on the inside of the building and seven piles were installed on the outside. The exterior piles closest to the existing foundation wall were installed so the uppermost helix blades were at least 3 feet below the existing footings. The piles were installed to torque values of at least 2,300 ft-lb (Model 287) and 5,000 ft-lb (Model 288) to provide correlated ultimate soil capacities of at least two times the design working loads (FOS ≥ 2). Foundation Supportworks of Wisconsin installed the 14 piles in one day to depths ranging from 15 to 26.5 feet.

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Project Summary

Structural Engineer: Excel Engineering

Geotechnical Engineer: Nummelin Testing Services

General Contractor: The Boldt Company

Certified Pile Installer: Foundation Supportworks of WI

Products Installed: (11) Foundation Supportworks™ Model 287 Helical Piles, 10”-12” Lead Section, 10 kip Design Working Load; (3) Foundation Supportworks™ Model 288 Helical Piles, 10”-12”-14” Lead Section, 22.5 kip Design Working Load