Challenge

A new 379,800 sq. ft. warehouse was proposed on a site that had been previously used to dispose of mine waste (slimes). A geotechnical investigation discovered fill soils over slimes and waste clays as deep as 40 feet within the proposed building footprint. The surficial fill soils were determined to be competent to support the relatively light design compression loads. However, deep foundations were considered to resist the uplift loads, thereby eliminating the need for massive concrete column pads. The design uplift loads were 58, 64 and 70 kips for the interior column pads, columns along two exterior wall lines and the center column line, respectively. The failure criteria was very stringent, no more than 0.25 inch of movement (uplift) would be allowed at service load conditions.

Solution

Helical piles were proposed to be cast into the concrete column pads to support the design tension loads. One or two piles would be used at each column location to reduce individual pile loads to 30.2 to 58 kips. Spacing efficiency modifiers were used to determine capacities of piles in the two-pile caps where pile spacing was less than three times the diameter of the largest helix plate. Thirty-two (32) Model 288 (2.875-inch OD by 0.276 wall) round shaft helical piles with a custom helix plate configuration consisting of either four or five 12” helix plates were installed to resist design working tension loads from 30.2 to 33.3 kips. One-hundred twenty-seven (127) Model 350 (3.50-inch OD by 0.313-inch wall) round shaft helical piles with a helix plate configuration consisting of either 12”-14”-16”-16”-16” or six 12” plates were installed to resist design working tension loads from 36.8 to 58 kips. The helical piles were installed to an average depth of 46 feet below grade to achieve torque-correlated ultimate capacities of at least twice the design working loads (FOS ≥ 2). Prior to installing the production piles, six tension load tests were performed at the site on sacrificial piles to verify pile capacities and monitor deflections. The pile load tests were planned within days of the test pile installations, which would not allow for full redevelopment of soil strengths around the helix plates and along the pile shafts. The tension piles would also not be pretensioned following installation. Therefore, 3/32-inch shims were added within the extension couplers during installation of both the sacrificial test piles and the production piles to control vertical movements and meet the stringent deflection criteria.

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Project Summary

Architect/Structural Engineer: Bergmann Associates

Speciality Foundation Engineer: Ramos Engineering & Associates, LLC

Geotechnical Engineer: Professional Services, Inc.

Construction Manager: Freund Andrus Construction

Certified Pile Installer: L.R.E. Ground Services, Inc.

Products Installed: (32) FSI HP288 Helical Piles, (127) FSI HP350 Helical Piles, Average Pile Depth of 46 feet, Design Working Tension Loads from 30.2 to 58 kips