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Foundation Supportworks by Woods Case Studies: Hawkland Estates Helical Piles

Friday, April 29th, 2016 by Marianne Koiva

Challenge

The Illinois Housing Development Authority commissioned the construction of 34 single-family townhomes. Four of these homes were proposed on adjacent lots at the end of a cul-de-sac. The single-story, slab-on-grade townhomes were planned with building footprints of 1,100 to 1,800 square feet, and would be supported by continuous wall footings. A geotechnical investigation completed at the end of the cul-de-sac identified a general soil profile consisting of intermixed layers of very soft to stiff silty clay. A five to eight-foot-thick layer of weak organic silt was discovered approximately nine feet below grade. Deep excavations would be required to remove the organic layer and minimize risk of structural settlement. The geotechnical engineer, instead, recommended installing helical piles past the organic layer to bear in competent soils anticipated at depths of approximately 25 to 30 feet.

Solution

The foundation design for the four townhomes included seventy-four (74) Model 288 (2.875-inch O.D. by 0.276-inch wall) round shaft helical piles with a 10”-12”-14” triple-helix plate configuration to support a design working compression load of 15 kips per pile. The helical piles were advanced to depths from 24 to 32 feet to achieve torque-correlated ultimate capacities of at least twice the design working load (FOS ≥ 2). The tops of the piles were cut to design elevations and fitted with new construction brackets to be cast into structural grade beams. The 74 helical piles were installed in just four days.

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