Challenge

January 2011- In 2010, the Bruce Howard Construction Company began construction of a new 5,400 sq. ft. office and warehouse to serve as the company’s new headquarters. The new building is a wood-framed, metal-sided structure supported by wood columns. The laminated wood columns extend about four feet below floor elevation and are embedded within 16-inch and 24-inch diameter concrete footings. Signs of settlement were observed soon after construction started, evident by racked windows, buckled siding and distress in the roof. Settlement up to four inches was measured. JES Construction, Inc. was contacted to review the conditions and propose a solution to stabilize the structure. The general subsurface profile, determined from three test borings to depths of 40 feet, consisted of sandy and clayey fill soils to depths of 13.5 to 17.5 feet over native sand with isolated clay layers. A drainage swale was reportedly filled in to develop the property. The fill soils were variable in strength with standard penetration test blow count values (N-values) ranging from 4 to 16 blows per foot. The native sands were generally medium dense (N = 10 to 19 bl/ft) above 30 feet and medium dense to dense (N = 16 to 40 bl/ft) below 30 feet. The clay layers were described as stiff to very stiff in consistency.

Solution

The structural settlement was likely caused by consolidation of the loosely placed fill. One hundred fifty-two (152) helical piles with retrofit brackets were installed to re-support the existing building by extending through the fill to bear within the native medium dense to dense sands or stiff to very stiff clay. The retrofit pile system consisted of the Model 287 (2.875-inch OD by 0.203-inch wall) hollow round shaft with 8”-10”-12” triple-helix lead sections to support a design working load of 15 kips. The retrofit piles were installed to torque-correlated ultimate capacities of at least 30 kips to provide factors of safety (FOS) ≥ 2.0. Hydraulic cylinders were used to lift the foundations to near the original, pre-settlement elevations. Since the concrete floor had not yet been poured, a new structural slab was proposed. The structural slab was supported on new construction helical piles spaced at eight feet center to center in a grid pattern. The helical piles supporting the structural slab consisted of the Model 287 hollow round shaft with 10”-12” double-helix lead sections to support a design working load of 10 kips. The piles were installed to torque-correlated ultimate capacities of at least 20 kips to again provide FOS ≥ 2.0. The retrofit and new construction piles were installed to depths (below bracket bearing elevation) ranging from 22 to 40 feet.

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Project Summary

General Contractor: Bruce Howard Construction Company

Drilling Services: ECS Mid-Atlantic, LLC

Certified Pile Installer: JES Construction, Inc.

Products Installed: (152) Foundation Supportworks™ Model 287 Retrofit Helical Piles, 8”-10”-12” Lead Section, (82) Model 287 Helical Piles, 10”-12” Lead Section, Installed to Depths of 22 to 40 feet, Design Workings Loads of 10 and 15 kips