Foundation Supportworks, Inc. Case Studies: Retail Store Slab Settlement

Tuesday, July 26th, 2022 by Brooke Donnelly

Challenge

Located in Canton Michigan, a major retail store was experiencing as much as 3 inches of floor slab settlement in the grocery department. The extent of the settlement was over an area of about 2,250 square feet and included the slab beneath several cold storage units. A geotechnical investigation was performed that indicated the slab was constructed on debris fill, mainly consisting of large pieces of concrete. Small diameter holes were also drilled into the slab at various locations to estimate the void depth below the slabs. Based on this analysis, void depths as much as 6 inches were observed. The settlement had become a safety hazard for customers and the store owner was looking for an immediate slab leveling solution rather than removal and replacement of the floor slab. To address the settlement in the cold storage area, the units would need to be removed and relocated to allow access for mitigation. During the slab repair operations, customer access to the rest of the grocery department was required and the work could only be performed between the hours of 10:00 p.m. to 3:00 a.m.

Solution

Slab reconstruction was not a preferred option and slab underpinning was not feasible without predrilling due to the debris fill below the slab. PolyLevel® polyurethane injection was selected as the preferred option based on the cost effectiveness, speed of repair and the ability to use the facility immediately after placement of the material. PolyLevel® is a two-part urethane that expands into a rigid foam used to fill voids, stabilize slabs, and lift concrete. PolyLevel can be injected below the concrete slab through 3/8-inch diameter drilled holes. Once the two parts are mixed, a chemical reaction converts the liquid urethane components into a strong, rapidly setting foam material. In its foam state, PolyLevel® is extremely light, with different formulations weighing between 2 to 4 pounds per cubic foot (pcf). Other void-filling or lifting materials can weigh upwards of 120 pcf, adding significant weight to supporting soils or base materials and potentially contributing to further settlement. With a typical compressive strength of 60 to 70 pounds per square inch and an in-place density of about 4 pcf, PolyLevel® 400 was selected to relevel and support the concrete slabs. The work was performed with the operating equipment located outside the store by using 300-foot-long hoses to access the work area. The PolyLevel® injection process filled voids and lifted the slabs back to level. A total of 3,175 pounds of PolyLevel® 400 was injected below the concrete slabs. The cold storage units were able to be re-installed immediately after the slabs were leveled. The PolyLevel® project took four days to complete.

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