St. Charles High School Building Addition
Challenge
A building addition east of the St. Charles High School was designed to have concrete drilled shaft foundations near the existing building and shallow spread footings away from the building. The original foundation design included three 66-inch diameter drilled shafts installed to the same depth as the existing building foundations, which varied from 17.2 to 23.5 feet below the first floor elevation.
A geotechnical investigation included the advancement of two borings within the building addition footprint. The subsurface profile consisted of 1 to 4 feet of crushed rock fill underlain by soft to medium stiff silt and clay to the boring termination depth of 25 feet. However, during the site preparation phase of construction, a deeper deposit of crushed rock was encountered next to the existing building than what was observed during the geotechnical exploration. Installation of the drilled shaft foundations would then require casing to prevent the crushed rock from caving in to the excavations. Casing would significantly increase the installation costs.
Solution
Helical piles were selected to replace the drilled shaft foundations since helical piles could be installed through the crushed rock without concern for cave-in. Other benefits of using helical piles at the site included smaller equipment requirements, low vibration installation, low mobilization costs and quick installation. The revised design included two piles per pile cap with each pile slightly battered away from the other to create the required helix spacing at bearing depth. The helical piles would be installed to support design working loads of 20 kips, while also achieving minimum design depths so that the top helix plate would be below the footing elevation of the existing building.
The helical pile design consisted of the Model 350 (3.50-inch OD by 0.340-inch wall) hollow round shaft with 8"-10"-12" triple-helix lead sections. The piles were installed to termination torques of 6,500 ft-lb, or higher, to provide a factor of safety of at least 2.2 for the compression loading. The piles were installed to termination depths of 28 feet and fitted with new construction brackets to be cast into concrete pile caps. Due to the flowing rock conditions, the final pile cap excavations were performed and shored after the piles were installed. The pile leads, extensions and brackets were hot-dip galvanized for corrosion protection. The six helical piles were installed in one day.
Project Summary
Architect: Dickinson Hussman Architects
Structural Engineers: Ædifica Case Engineering; Alper Audi
Geotechnical Engineer: SCI Engineering
General Contractor: CS Construction Services
Pile Installer: Foundation Supportworks® by Woods
Products Installed: (6) Supportworks® Model 350 Helical Piles, 8"-10"-12" Lead Section, Design Working Compression Load of 20 kips, Installed Depths of 28 feet
