Challenge

August 2010- The Lone Star Mercedes Benz car dealership planned a renovation to their existing facility that included the construction of an approximate 1,000 square foot addition. A geotechnical investigation for the project included eight soil borings advanced to depths of 16.5 to 20 feet. The general subsurface profile consisted of 10 to 13 feet of uncontrolled fill, clayey silt with organics, over native silty sand with organics over dense sandy gravel. The dense sandy gravel layer was encountered at depths of 13 to 20 feet. The geotechnical engineer recommended against supporting the new addition with spread footings bearing within the uncontrolled fill. Over-excavation of the existing fill was also not an option due to the depth of fill and the potential for undermining the existing building and pavements without extensive shoring. Several deep foundation alternatives were considered with bearing within the dense gravel.

Solution

Helical piles were selected to support the new addition due to the relatively low (mobilization) costs compared to the other deep foundation alternatives. The smaller installation equipment could also access and maneuver more easily within the congested construction site. The foundation design for the addition included twenty-one new construction helical piles. Twelve retrofit helical piles were also installed along the foundation of the existing building to support load transferred from the addition. The helical piles were designed for a working load of 15 tons (30 kips). The helical pile configuration consisted of 2 7/8-inch O.D. by 0.276-inch wall round shaft with 10”-12” double-helix lead sections. At several pile locations extension sections with 14” helix blades were added in order to provide the torque-correlated soil capacity of at least twice the design working load (v ≥ 2). From start to finish, the pile installation and all related prep and finish work were completed in 4 days and ahead of schedule.

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Project Summary

Geotechnical Engineer: Global Engineering & Testing Ltd.

Structural Engineer: Grant Structural Engineering

Certified Pile Installer: Foundation Supportworks of Alberta

Products Installed: (33) Foundation Supportworks™ Model 288 Helical Piles, 10”-12” Double Helix Lead Section, Installed to an Average Depth of 21 feet, 30 kip Design Working Load