Continued growth of the North Carolina State University’s Non-Woven Institute (NWI) drove the development of a new industrial building. The new 3-story, 105,000 sf for Technology & Innovation on North Carolina State University’s Centennial Campus will house both high-bay industrial laboratory and office space. The development also consists of a new 4-level, 26,000 sf parking deck.
Subsurface explorations on the site encountered up to 14 feet of loose to medium dense uncontrolled fill underlain by residual soils consisting of very loose to medium dense silty sand and soft to stiff silt. Partially weathered rock (PWR) was encountered at depths varying from 6 feet to more than 50 feet (maximum explored depth) across the site. In addition, the heavily loaded footing for the new parking garage would be imposing new loading on an existing underground storm water storage system.
Geopier® ground improvement was used to control settlement in the loose uncontrolled fill for the heavily-loaded parking deck foundations. However, the presence of an existing underground storm water storage system located along the perimeter building wall presented design and construction challenges.
Footings bearing at elevations above the storm water storage system would impart loads on the system so alternative solutions were explored. One Alternative was to lower footings in adjacent to the storm water system. The problem with this solution is that it would require significant shoring to excavate for the footing and the shoring would be in the granular fill zone of the pipes, making it difficult to retain using conventional shoring (i.e. the gravel would spill out).
A second alternative was to support the footings on driven Ductile Iron Pile solution. This provided support for the parking deck footings adjacent to the storage system without lowering the footings or requiring shoring. The piles were designed to penetrate the fill and residual soils and terminate after achieving “set” on the weathered rock. Pile working loads up to 40 tons were developed by end bearing on rock. The small diameter piles generated limited vibration during installation and effectively transferred foundation loads to the rock bearing layer to minimize impacts on the existing storm water system.
GeoConstructors, Inc, installer of both Geopier® ground improvement and DuroTerra’s Ductile Iron Piles, offered up a combined foundation solution to provide overall foundation support. The Ductile Iron Pile solution consisted of 118 mm (4.65 inches) diameter piles with a 7.5 mm (0.3 inch) wall thickness installed to terminate on very dense weathered rock. A total of 24 Ductile Iron Piles were installed in 3 days. Pile lengths ranged from 14 to 29 feet before reaching “set” of 1 inch or less of movement in 50 seconds.