Case Studies Case Studies: Industrial Building Underpinning

A 279,000 square-foot industrial building, constructed in 2013, exhibited significant wall and floor slab settlement, with more than 12 inches of movement at the southeast corner. Before the building was constructed, the site was initially used for mining sand and gravel. It was later used as an inert debris landfill for about 13 years. Records from the landfill operations indicated that the fill was composed primarily of soil, rocks, gravel, concrete and asphalt debris. A geotechnical investigation performed in 2007 identified about 50 feet of debris fill east of the subject property. Based on the recommendations in the report, deep dynamic compaction was selected for soil improvement before construction, to allow the use of a shallow foundation system. After the settlement had occurred, another geotechnical investigation performed in 2018 showed about 60 feet of debris fill at the southeast corner of the existing structure. The building construction consisted of exterior concrete tilt-up panels and interior steel columns, both supported on shallow foundations. As a result of the settlement, the tops of adjacent tilt-up panels separated by as much as 8 inches, and their connections to the roof joists were compromised. A temporary shoring system was then required to support the walls and roof, in case of further misalignment. Given the amount of settlement and misalignment of the tilt-up panels, the ideal remedial method would first stabilize the structure and then potentially lift the walls back toward the original elevation. The interior column pads and floor slab in the southeast corner of the building would also require stabilization and minor lifting. Underpinning was required beneath 43 of the wall panels, with each panel being 26 to 41 feet long. Estimated service loads of the panels ranged from 7 to 14 kips per lineal foot.

 Compaction grouting was chosen for the slight lift and stabilization requirement at the selected interior column pads and slab areas. Micropile and push pier systems were both evaluated as possible solutions for underpinning and lifting the exterior footings and tilt-up panels. Micropile installation would be difficult in the debris fill and would require casing to prevent grout loss within the anticipated void space. Casing would also be required to minimize downdrag forces on the micropiles, should consolidation of the fill continue to occur. Additionally, the micropile capacity would be dependent upon the bond strength below the debris fill, which was not well identified, and capacity could not be verified during installation. Push pier system capacity, on the other hand, is primarily dependent upon end bearing and can be easily determined by monitoring the drive pressure during installation. The Model 288 (2.875-inch OD by 0.165-inch wall) push pier system with side-load retrofit brackets was ultimately selected to support and lift the tilt up panel foundations. Given the installation challenges posed by the debris fill, installation would also include predrilling with a 3.5-inch diameter downthe-hole hammer to a minimum depth of 60 feet. Due to the cavity created by predrilling being larger than the outer diameter of the pier sections, each section of pier tubing would require field welding to connect them as they were lowered into the predrilled hole. The push piers would be installed on both sides of the footing (inside and outside) at the heavier panels to meet the service load requirements and to also maintain stability during the lift process. Before the lift, compaction grouting would be performed around the piers to densify and stabilize the soil mass. Pier installation was completed in two phases; first to install enough piers to stabilize the exterior panels and minimize further movement, and then to install additional piers to have sufficient capacity for the lift process. After the second phase, the 303 push piers were generally spaced at 3- to 5-foot centers. The push piers had a 60-foot minimum depth requirement (to bear below the estimated debris fill depth) and a minimum final drive force of 54 kips to achieve a factor of safety of 1.7 or greater. Piers were required on both sides of the tilt-up panel foundations in most areas to achieve the required factor of safety. Pier depths generally ranged from 65 to more than 100 feet. The lift operations recovered most of the initial settlement; however, a fair amount of finesse with minor adjustments was required to bring the tilt-up panels to plumb. The work was performed over two years, between the initial evaluation, pier installation, grouting and lifting operations.

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