Metropolitan Community College

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Project Location

Metropolitan Community College
Omaha Nebraska
June 2007

Helical Piers

The perimeter improvements project included the construction of three (3) new signage elements/architectural landscape features. Two signage elements measure 30 feet by 5 feet and 20 feet tall. One combined sign/architectural feature measures more than 110 feet long and 20 feet tall. Construction consisted of poured concrete foundations, reinforced masonry block, brick and precast concrete.

Twenty-four soil borings were completed across the site in January 2005 as part of the geotechnical investigation for the then-proposed connector building and site improvements. The test borings identified undocumented fill soils to depths ranging from 3.5 feet to 18.5 feet. The fill was described as silty clay with varying quantities of gravel, brick, concrete and organic material. The moisture content, density, and strength of the fill were also highly variable. Soft to stiff silty clay Peorian loess was encountered beneath the fill. Stiff to very stiff lean to fat clay Loveland loess was encountered in four of the test borings at depths of 40 feet to 50 feet.

The design team determined the existing fill soils to be unsuitable for support of the structures on shallow spread footings, due to the relatively high risk of damaging differential settlements. The foundations were then designed as grade beams supported by new construction helical piers, each pier with a design load of at least 25 kips (25,000 pounds).

Based upon the soil and loading conditions, Grip-Tite determined a helical pier configuration consisting of a 2 7/8-inch outside diameter round shaft with a triple-helix lead section (10-inch, 12-inch, and 14-inch diameter helix blades) followed by one (1) double-helix extension (two 14-inch diameter helix blades). Standard extensions would advance the piers to the design depths.

Thirty-four (34) helical piers were installed to depths ranging from about 28 feet to 39 feet. The ultimate pier capacities, determined by correlation to installation torque, confirmed the initial calculated design capacities and were at least twice the design load. Despite encountering rubble fill and old, buried concrete and brick foundations, pier installation was completed in four days