Long before the new Buerger Center for Advanced Pediatric Care at The Children's Hospital of Philadelphia began making its presence known on the CHOP skyline, the $450-million project was challenging engineers and contractors below ground. The construction team, led by Turner Construction, Philadelphia, excavated 70 ft down to make way for a 1,500-car parking garage, requiring one of the largest excavations of its kind in the city, according to the team.
"That's a pretty deep hole in the city of Philadelphia," says Gregory Ryan, Turner Construction vice president, construction executive and the project director on the job. "Most holes here are between 30 and 40 ft."
During the nine-month excavation, crews hauled out roughly 300,000 cu yd of rock. Most of the rock went to the Philadelphia International Airport and was crushed for use on a runway project.
Rock excavation proved to be a delicate process given the sensitivity of neighboring buildings on the CHOP campus. The adjacent Colket Building, for example, includes an underground vivarium that could not tolerate big vibrations.
Ryan says the project team held regular meetings with the Colket staff—and the vivarium staff in particular—to discuss the excavation schedule. Turner also conducted vibration monitoring during excavation and construction.
Given the rocky site, the team was able to use conventional spread-footing foundations. Permanent tie-back retention was used to resist lateral soil pressures.
Greg Shreve, vice president at LeMessurier, Boston, the structural engineer on the project, says the team designed a concrete-braced frame in the garage to allow for great visibility. Shreve says the team originally thought it would need to use a steel-braced frame going down to the foundations, but the connections would have been more difficult.
"It ended up being a very elegant solution," he says.
Shreve says one of the main challenges of the concrete-braced frame was the congestion of the reinforcement. "We detailed the connections, how to bend the rebar and how to use terminators in order to get all of the reinforcement to fit properly," he says, adding that the entire project was planned and designed using building information modeling.
Upward efforts, Outpatient Outcome
With the project on solid footing, the Buerger Center is taking shape. The first phase, which includes eight floors and 500,000 sq ft of space, is scheduled to open in 2015.