Challenge

A new single-story, wood-frame dental office with crawl space was planned at the top of a steep slope on a cut/fill lot. The proposed building had plan dimensions of 60 feet north-south by 40 feet east-west with the west wall line of the office parallel to the approximate 1H:1V slope. The general subsurface profile consisted of clay fill over native clay and weathered bedrock over hard, unweathered bedrock. Fill depths increased from north to south within the building footprint. In order to utilize the available area of the lot and maintain the required parking capacity, the proposed building footprint had to be pushed out to the edge of the slope, offset about eight to ten feet from the crest. A deep foundation system was recommended to support the west exterior wall footing and the westernmost north-south interior strip footing of the office building. Piles would bear within the native clay or weathered bedrock and at elevations with safe projections to the bottom of the slope. The tight construction schedule dictated that equipment and material be mobilized quickly and installation be completed within seven days.

Solution

Helical piles were selected over other deep foundation alternatives. Helical piles could be installed with smaller equipment within the tight-access conditions of the partially developed property. A mini-excavator could also install the piles safely at the top of the steep slope. Consideration was given to deep trench excavations for the footings, but it was believed that this option could potentially cause an unstable slope condition. Fifteen (15) new construction Model 288 (2.875-inch OD by 0.276-inch wall) hollow round shaft helical piles with 10”-12” double-helix lead sections were installed to support design working compression loads of 20 kips (interior support) and 35 kips (exterior wall). The piles along the west exterior wall were spaced at eight feet on center and were advanced to depths ranging from eight to 14 feet below grade. Pile depths generally increased from north to south. The piles along the westernmost north-south interior support were spaced at ten feet on center and were advanced to depths ranging from five to 11 feet. Pile depths again increased from north to south. The ultimate pile capacities determined by correlation to installation torque were at least twice the required design loads (FOS ≥ 2). The helical piles were cut to design elevation and new construction brackets were placed at the tops of the piles to be cast into the concrete grade beams. The pile installation was completed in just one day.

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Project Summary

Architect: GBD Architects

Structural Engineer: Rogers Engineering

Geotechnical Engineer: OGD Consulting

General Contractor: Bruce Anderson Development

Certified Pile Installer: TerraFirma Foundation Systems

Products Installed: (15) Foundation Supportworks® Model 288 Helical Piles, 10"-12" Lead Section, Installed to Depths of 5 to 14 feet, Design Working Loads of 20 and 35 kips