Connecticut Basement Systems Case Studies: New Maintenance Storage Building - Helical Piles
Challenge
The Springfield Housing Authority planned the construction of a new maintenance building. The one-story, slab-on-grade building had plan dimensions of 20 feet by 36 feet with construction consisting of concrete block walls with pre-manufactured wood roof trusses. Springfield was once a city heavily involved in manufacturing, with many of the old factories burning coal for heat.
Areas around the city were then designated as coal ash dump sites. The property selected for the proposed maintenance building was one of those sites. A single test pit was excavated on the property to a depth of 8.5 feet, exposing fill soil, ash, cinders and brick the entire depth.
Supporting the building on shallow spread footings was not an option since bearing within the upper loose fill soils would likely result in damaging differential settlements.
Solution
Deep helical pile foundations were proposed to minimize the risk of structural settlement. The intent was to either penetrate the existing fill soils or bear within deeper, dense fill. With limited subsurface information available, the original foundation detail included helical piles with 10"-12" double-helix lead sections extending to a depth of 19 feet to support a design working load of 55 kips.
The foundation details and pile configuration were later modified following the installation of a test pile and completion of a load test.
The revised foundation details included fifteen Model 350 (3.5-inch O.D. by 0.313-inch wall) round shaft helical piles with 10"-12"-14" triple-helix lead sections and 14"-14" double-helix extensions.
The design working load was reduced to 45 kips. The piles were advanced to depths ranging from 14 feet to 83 feet. At the termination depths, the torque-correlated ultimate capacities were at least twice the design working load. The pile installation was completed in 4 days.
Project Summary
Architect: Reinhardt Associates
Structural Engineer: Engineering Design Associates, Inc.
Geotechnical Engineer: Design Associates, Inc.
General Contractor: Five Star Building Corporation
Certified Pile Installer: Foundation Supportworks Northeast
Products Installed: (15) Foundation Supportworks Model 350 Helical Piles, 10"-12"-14" lead section with 14"-14" Extension installed to depths of 14-83 feet, 45 kip design working load.
