Helical Piles Installed for Ben Bikin' Bridge in Sparta, WI

Challenge

In 2011, the City of Sparta was to begin construction of a new pedestrian/snowmobile bridge. The original design called for the bridge to sit on concrete abutments poured on top of driven “H” piles. The design included two “H” piles in each abutment driven to 140 kips with a design load of 50 kips. The project site was located near a major intersection and the city had concerns about large crane equipment blocking traffic and causing damage to the site. Soils information for the project was very limited. However it was determined that bedrock was approximately 35 feet below grade surface from a highway bridge that was located nearby. The design team then considered helical piles since they could be installed with small equipment limiting the damage to the site. It was decided that four helical piles installed down to bedrock in each abutment would give better lateral stability.

Solution

Since the soils information was limited a larger shaft size was proposed to limit buckling potential. The foundation design included eight Model 349 (3.5-inch OD by 0.279-inch wall) round shaft helical piles with 10”-12”-14” triple-helix lead sections to support the design load of 25 kips per pile with a factor of safety of 3. The abutments were excavated prior to FSW’s arrival, and footing forms were already in place. Foundation Supportworks of Wisconsin worked with the general contractor to locate and mark the pile locations. The piles were battered at a 5 degree angle to allow for proper spacing of the helices yet still keeping the pile caps within one foot of each other. The elevation of the eastern abutment was approximately one foot below the waters edge requiring pumps and temporary shoring to install the piles. The piles were advanced into the bedrock and to torque values of at least 11,000 ft-lbs, which correlated to an ultimate capacity of at least 75 kips. The piles were cut off to the design elevation and new construction brackets were tack-welded to the piles. Three feet of exposed length was left to allow caps to be embedded half way into the concrete abutment. Foundation Supportworks of Wisconsin installed the eight piles in one day to an average depth of 32 feet below footing grade.

Project Summary

Structural Engineer: Duffy Engineering & Associates

Certified Pile Installer: Foundation Supportworks of WI

Products Installed: (8) Foundation Supportworks Model 349 Helical Piles, 10"-12"-14" Lead Section, Installed to an Average Depth of 32 feet, 25 kip Design Load / 70 kip Ultimate Load