Challenge

March 2011 - ITT Technical Institute planned to renovate a three-story building in the Sheridan Park district. Renovations included new construction helical anchors for tiedown support at existing deep foundation locations. Each helical anchor would support a design working tension load of 20 kips. The engineer of record specified a helical anchor design depth of 29 vertical feet below grade. The geotechnical investigation consisted of two soil borings outside the existing building footprint. The borings showed variable soil conditions including dense silty sandstone to a depth of 14 feet and very dense claystone at 19 feet. Perched groundwater was observed at a depth of 5 feet. Because all work had to be performed within the existing building, work space and access would be limited.

Solution

The foundation design included helical anchors embedded into concrete pile caps that would be doweled into the existing foundations. Each concrete cap incorporated four (4) helical anchors with two (2) vertical anchors and two (2) battered anchors at 45 degrees from horizontal. Twenty-four (24) Model 150 (1.5-inch round corner square bar) helical anchors with an 8”-10”-12” triple-helix lead section were installed and fitted with new construction brackets to be cast into the concrete caps. A “V-style” cut was used on the leading edge of the 8-inch blade in order to penetrate the dense soils more easily. One of the many benefits of helical anchors, especially for a site with variable soils, is that anchor capacity can be estimated by correlation to the installation torque. While the ultimate, torque-correlated anchor capacity of 40 kips (FOS=2) was achieved at depths of 20 to 22 feet, all helical anchors were installed to a depth of 29 feet (lengths of 29 and 41 feet for vertical and battered anchors, respectively) per the engineer of record’s design specifications. Four Model 288 (2.875-inch OD by 0.276- inch wall) hollow round shaft helical piles with 8”-10” double-helix lead sections were used in a tension anchor application at vertical anchor locations where the Model 150 could not achieve the specified required depth of 29 feet.

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Project Summary

Architect: PBV Architects, LLC

Structural Engineer: Mountain Design Group, LLC

Geotechnical Engineer: Terracon

General Contractor: Brinkman Construction, Inc.

Certified Pile Installer: Complete Basement Systems of Colorado

Products Installed: (24) FSI Model 150 Helical Anchors, 8V”-10”- 12” Lead Section; (4) FSI Model 288 Helical Piles, 8”-10” Lead Section; Installed to Design Depth of 29 feet, Design Working Load of 20 kips (Tension)