Challenge

The Tuttle Building, also known as the U.S. Post Office and Courthouse, was constructed in 1910 and is listed within the U.S. National Register of Historic Places. Located in the heart of downtown Atlanta, this five-story building serves as one of the country’s 13 United States Court of Appeals sites. Proposed renovations to the structure required underpinning of an existing pyramid-shaped column footing and additional support for a new insolated column load, all within limited working space. A corner of the existing column footing to be underpinned would be cut off diagonally to allow for the proposed construction. Bringing equipment and materials to the site would be a challenge with the narrow, oneway streets surrounding the building. Access to the basement level of the building where the work was to be performed was through a single side door where judges also entered. The courthouse building could not be shut down and construction noise had to be kept to a minimum. Soil boring information was not available although certain soil conditions were anticipated by members of the design team based upon past experience in the area.

Solution

The original underpinning detail for the column footing included four helical piles with spacing as close as 16 inches. Minimum recommended helical pile spacing is typically three to four times the diameter of the largest helix plate to prevent group effects. Identifying this condition, Foundation Supportworks of Georgia proposed retrofit push piers instead. Four (4) Model 288 (2.875-inch OD by 0.165-inch wall) hydraulically driven push piers were installed to depths of 27 to 30 feet below the bottom of the column footing. The piers were advanced to a drive load of at least 37.5 kips to provide a factor of safety of at least 1.5 with the design working load of 25 kips. After driving each pile individually, the four piers were fitted with hydraulic cylinders connected in series and reloaded to 25 kips. One Model 287 (2.875-inch OD by 0.0203-inch wall) hollow round shaft helical pile was installed to support a new column load. The 12”-14” lead section was advanced to a tip depth of 8.5 feet and an installation torque of 6,200 ft-lb. The torque-correlated ultimate capacity of 55.8 kips far exceeded the design working load of 6 kips. The helical pile was installed with handheld equipment. The push pier and helical pile installation was completed in one day.

Print (PDF)

Project Summary

Architect: T.T.V. Architects, Inc.

Structural Engineer: Atlantic Engineering Services

Geotechnical Engineer: Chattahoochee Consulting Group, Inc.

General Contractor: Hitt General Contracting

Certified Pile Installer: Foundation Supportworks of Georgia

Products Installed: (1) Foundation Supportworks® Model 287 Helical Pile, 12”-14” Lead Section, Installed to a Depth of 8.5 feet Below Bottom of Excavation, Design Working Load of 6 kips; (4) Foundation Supportworks® Model 288 Push Piers, Installed to Depths of 27 to 30 feet Below Existing Footing, Design Working Load of 25 kips.