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Philadelphia is officially getting a park on top of a highway. And amid the rejoicing of the waterfront being reconnected with Center City for the first time since the 1960s, a question arises: How exactly will all this work? It is, after all, a park with a manicured lawn and trees going above an interstate where thousands of cars zoom by daily, connecting Center City to a river.

Philadelphia has plenty of urban parks, from the sprawling (Fairmount) to the small (LOVE, Penn Treaty, and umpteen others). This one will be big — 11 acres between Chestnut and Walnut streets, and Front Street and the Delaware River — and lush. But the design, engineering and construction won’t be impossible. They’ll actually likely be easier than the renovation of LOVE Park, with one particular challenge: getting visitors to see the Delaware as clearly and as early as possible.

“Right now,” said Mary Margaret Jones, senior principal of Hargreaves Associates, the architecture firm working on the project, “the pedestrian bridges arc up as you go above the highway and you don’t have a good sense of the river.”

Nearly 50 years ago, Philadelphia actually had the opportunity to build the first park over a freeway. The initial discussions for I-95 in the late 1960s included the possibility of placing a six-block cap over the stretch of I-95 that passes by Center City. Instead, we got just two small areas, where you can find the monuments to Irish and Scottish immigrants and the Korean War.

Seattle thus became the first city to build a park over a highway with Freeway Park in the 1970s. Phoenix followed about a decade later with Margaret T. Hance Park.

They were still largely concrete, similar to what Philly has with Dilworth Park and the old LOVE Park. The switch to greener highway caps happened a few years ago, first with the Rose Fitzgerald Kennedy Greenway in Boston and Klyde Warren Park in Dallas.

Jim Burnett, president of OJB Landscape Architecture and who designed Klyde Warren, described it as “building a bridge for 1,100 feet” and “designing a hanging garden.”

The lawn and trees look natural on top because of what’s underneath. The trees are placed into trenches carved out deeper than the existing bridge structures and the surface of the rest of the park. The soil must be particularly engineered so that it’s not too heavy for the structure, and so it’s able to withstand heat.

This is what it looks like under the surface of Klyde Warren and what Philly’s, to an extent, will likely look like.

Credit: OJB Landscape Architecture

“Some people don’t figure it out and they end up with trees in planter boxes,” Burnett said. “We wanted to give the impression of a seamless park like you don’t even know trucks are going 70 mph under your feet. You don’t hear it, you don’t see it.”

But the similarities between Philly and Dallas, or any highway-cap park and Philly, apply only to an extent.

“The particular situation in Philadelphia,” Jones said, “is pretty unique.”

Dallas’ park connects one side of downtown to another. Boston’s park is narrow and long and meanders through the city. Philly is going from downtown to a river, from above sea level to sea level. Thus the main challenge here deals with slope.

Right now, it’s close to impossible to notice the river from the edge of Center City — even if you’re walking on one of the pedestrian bridges toward it. Those bridges tilt upward, the opposite direction for a good view. According to the feasibility study completed in 2014, the park will need to slope downward about 30 feet from the edge of Columbus Boulevard to the river. And the upward arc at least on the Walnut side from Front Street to Columbus Boulevard will likely not be as extreme as it is now when the park is complete.

“It’s not just bridging over,” Jones said. “It’s bridging over and taking a grade change down to the river that makes it graceful and inviting and easy.”

These details are being ironed out as we speak. The next three years will mostly involve setting specifics with engineering and structural options. Construction, according to the Delaware River Waterfront Corporation, should begin in 2020. The park is supposed to be complete about two years after that.

And, if everything goes right, the Delaware River will no longer be impossible to see.

Mark Dent is a reporter/curator at BillyPenn. He previously worked for the Pittsburgh Post-Gazette, where he covered the Jerry Sandusky scandal, Penn State football and the Penn State administration. His...