SRT Communications, the largest telecommunications company in the state of North Dakota, is headquartered in the city of Minot within the area of “North Hill”. This area of Minot is known for foundation and slab-on-grade instability issues due to local soil conditions, seasonal wetting and drying of “active” soil, and a high groundwater table. At the SRT headquarters building, the issue was differential movement of six-foot and nine-foot wide exterior concrete sidewalks.
The sidewalks experienced cyclical heave and settlement with changes in moisture content of the subgrade soils. The differential movement created trip hazards between sections of the concrete and at the entrance/ exit locations of the building. The bottoms of doors would also wedge against heaved sidewalk sections, preventing the doors from being opened fully.
The project included removing the existing sidewalks and replacing those with new structurally-reinforced sidewalks designed with deep foundation support. Proximity of the sidewalks to the building, tight working spaces, and landscaping that could not be disturbed during construction, were a few of the challenges associated with installation of a deep foundation system. Subsurface information was also not available for design of the sidewalk foundations.
Helical piles were an ideal deep foundation option given the limited working space. Helical piles could be installed with smaller equipment that could mobilize quickly to the site. Without soil information for design, helical pile capacities could also be estimated by correlation to installation torque.
Sixty-one (61) Model 288 (2.875-inch O.D. by 0.276-inch wall) round shaft helical piles with 10”-12”-14” triple-helix lead sections were installed with a tracked skid-steer. Design working loads were 18 kips and 24 kips for the 6-foot and 9-foot wide sidewalks, respectively.
The piles were installed to lengths of 13.5 to 20 feet and to installation torques of at least 4,000 ft-lb and 5,400 ft-lb to achieve the respective torque-correlated ultimate capacities of at least 36 kips and 48 kips (FOS ≥ 2). After the piles were installed, new construction brackets with 6-inch square cap plates were bolted to the tops of the piles and embedded within the poured concrete slabs. The helical piles were hot-dip galvanized for corrosion protection. The 61 helical piles were installed in just four days.
Architect: Ackerman – Estvold Engineering and Management Consulting, Inc.
Structural Engineer: CW Structural Engineers
General Contractor: Mattson Construction Company
Certified Pile Installer: Innovative Basement Authority
Products Installed: (61) Model 288 Helical Piles, 10″-12″-14″ Lead Section, Installed to Lengths of 13.5 to 20 feet, Design Working Loads of 18 and 24 kips