Challenge

The Eastbay headquarters building in Wausau, Wisconsin experienced differential settlement of the interior floor slab with measured slab movement as much as four inches. Areas of concern included a main hallway and adjacent rooms. The hallway led to the men’s and women’s lockers/bathrooms, through the mail room and back to the shipping department. There was a two-inch elevation difference across the five-foot wide main hallway. Interior partition walls built on top of the slab were either moving with the slab resulting in gaps between the walls and ceiling, or were spanning settled slab areas and separating with the floor. There were concerns about how the movement may be affecting the roof system and any suspended plumbing lines penetrating the walls. A geotechnical investigation included seven interior hand auger borings and three exterior borings completed with a truck-mounted drill rig. The borings identified seven to eight feet of uncontrolled fill over dense sand over weathered bedrock at 9.5 feet to 17 feet. The owner wanted the slabs stabilized with a permanent solution extending through the upper uncontrolled fill. The initial design called for grout columns to stabilize the slabs, then self-leveling grout poured over the existing floors. Hydraulically-driven steel slab piers were later determined to be a more economical solution, as well as more favorable since the slabs could be lifted back toward original elevations to close existing slab/wall/ceiling separations. Voids beneath the slab would then be filled with polyurethane foam.

Solution

Slab pier locations were laid out in grid patterns with a maximum spacing generally less than five feet. Core holes eight inches in diameter were made through the floor to allow for bracket installation. As the drill bit penetrated the concrete slab, most of the cores dropped two to six inches into the voids below. One hundred twenty-seven Model 288 (2.875-inch OD by 0.165-inch wall) slab piers were installed to an average depth of 17 feet and an average drive pressure of 2,200 psi (drive force of 21 kips). The slab piers were connected in series with hydraulic cylinders to uniformly raise the floor back toward level. PolyLEVEL™ polyurethane foam was then injected under the slab to fill voids. Once injected, the two liquid urethane components react to form a rapidly setting rigid foam with a compressive strength generally greater than 75 psi. Approximately 1,600 pounds of PL250 were injected below the slab. Most of the concrete core holes were filled with concrete before installing PolyLEVEL™. A few holes were left open to confirm that the voids were completely filled. A ZIPLEVEL® was also used to verify that the slab was not lifted during void filling operations. Hydraulically-driven retrofit foundation piers were also utilized in this area of the building.

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Project Summary

Structural Engineer: EDC Management Corp.

Geotechnical Engineer: American Engineering Testing, Inc.

General Contractor: Miron Construction

Certified Pier Installer:: Foundation Supportworks® of Wisconsin

Products Installed::