SEPTA’s Frankford Terminal, Philadelphia, Pennsylvania

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Overview: Modernization of the 84-year-old Frankford Transportation Center was the largest single construction project ever undertaken by the Southeastern Pennsylvania Transportation Authority (SEPTA). The final component of the $650 million upgrade was a 5-story parking structure. GeoStructures provided site characterization, foundation design, and inspection and materials testing during construction for the project.

The foundation design called for socketing the caissons into mica schist bedrock below a zone of highly weathered rock. Recognizing that the weak material controlled the socket lengths (which, in turn, controlled pre-assembly of reinforcement cages on site), the general contractor sought socket refinement from GeoStructures before starting work. This task was accomplished by performing 30 core borings and 80 percussion probesone at each proposed caisson location.

Because it is not a standardized technique, percussion probing must be calibrated to core borings and compressive strengths. For this project, three separate rock classes, each with an assigned percussion drilling rate, weathering, fracturing, and compressive strength, were used to determine an appropriate rock socket length for each caisson.

GeoStructures followed through with inspection and materials testing during construction. As expected, the design efficiency gained by accurate rock mass characterization translated into construction efficiency and cost control by minimizing drilling overruns and reinforcement waste.

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