This 4-story office building is leading the way for tech companies to be part of Pittsburgh’s Strip District revitalization. DLA+ Architects designed 105,000 square feet of space with tech companies in mind. It features a high-bay first floor layout, and the open-floor plan with wide column spacing is ideal for companies that require research and development space. Social media powerhouse Facebook recently signed a lease in this highly desirable location which is being referred to locally as the gateway to “Technology Row.”
The soils underlying the site were less desirable. Construction Engineering Consultants, the Geotechnical Engineer of Record (GER), identified Uncontrolled FILL in all seven soil borings they performed on-site. This urban fill ranged from 6 feet to 15 feet deep and consisted largely of silty sand mixed with slag, brick, and debris. The FILL is underlain by an alluvial medium dense silty sand and sandy gravel. Rock, consisting of shale and siltstone, was found approximately 70 feet deep.
Column loads of 35 to 300 kips coupled with the presence of the variable nature of the FILL on-site posed significant total and differential settlement concerns. Therefore, the GER recommended deploying a deep foundation system such as drilled shafts or auger cast-in-place piles or a shallow foundation supported on Rammed Aggregate Pier (RAP) ground improvement system to support the structure.
The Owner, RDC Design-Build (RDC), contacted GeoStructures, Inc. (GSI) to provide turnkey ground improvement solution for their site. They required a system that would provide the required bearing pressure and mitigate the risk of settlement due to the heavy column loads and presence of uncontrolled FILL. Additionally, a foundation support system that minimized spoils was highly desirable because of the urban location of the site. The displacement RAP system, also known as Impact Pier, was selected for the site. This system provided the required settlement control, and no spoils were generated during installation.
Approximately 260 Impact Pier elements were installed. The depth ranged from 8 to 15 feet based on loading and settlement requirements, and commensurate with the FILL depth. By nature, uncontrolled FILL is manmade, and its compression characteristics are highly variable. Terminating elements within the FILL poses a significant settlement risk. District 15 houses high bay research and development wings and high-profile tenants. Therefore, RDC and their design team chose to mitigate settlement risk by requiring that elements be installed through the FILL and terminate in natural soils. GSI provided a design build solution that met the project’s technical challenges and saved money versus traditional deep foundation systems.