Sinkholes, another byproduct of Philly’s aging infrastructure

 Workers examine a sinkhole in Kensington. (Bobby Allyn/WHYY)

Workers examine a sinkhole in Kensington. (Bobby Allyn/WHYY)

James Stuckey is a deliveryman at Franco’s Pizza at the corner of Tulip and East Huntingdon Streets in Philadelphia’s Kensington neighborhood.

Here’s how he described the giant fissure that spontaneously spread out in front of the shop earlier this week.

“I would say you can fit a car in there, or a truck,” he said. “It looks like a meteorite hit it.”

As crews continue to plug away at the sinkhole, city officials are describing the phenomenon of chasms opening under city roads as a growing problem with few preventive defenses.

There have been 2,019 reported sinkholes across the city this year, according to the Philadelphia Streets Department. That’s up nearly 20 percent from last year with four months left to go in 2015.

Streets Commissioner David Perri says that means there’s roughly one sinkhole every mile of city road.

“We attributed the large increase we’re having to extreme bouts of cold weather,” he said.

Perri notes that this year has experienced some of the coldest recorded weeks in the city’s history.

Which means water freezes longer deep under the roads, creating ice formations that push away soil. If the ice creates a big enough space, it can trigger an open space. Over time, that can cause a collapse.

“In the springtime, when the ice pockets thaws out, it leaves basically a hole under the street. And that hole can stay as a latent defect for many months, even for several years,” Perri said.

“Eventually a heavy truck will go down the street, it’ll hit that latent defect below the surface, at the right spot, and, all of a sudden, the truck will punch through and you’ll left with a large hole in the street,” he said.

Temple University geologist Laura Toran said besides colder weather sticking around longer, the human-made stuff below ground is partially to blame.

“We developed some of the first sewer systems. We were ahead of our time. And now we’re paying the price for that,” Toran said.

The water main that broke in Kensington on Monday dated back to 1886. Other Grover Cleveland-era pipes in Philadelphia are bound to give out soon.

“Maybe an old pipe had sediment in it, the water came and pushed the sediment along — water’s a very powerful force, it can move boulders — so the sediment weakens the competency of the existing formation,” Toran said. 

Busting pipes often provoke sinkholes. But not always. They sometimes start with cracks deep in the ground that began millions of years ago.

But the earth is now giving up more than it has in recent years.

According to Streets Commissioner Perri, Philadelphia averaged 1,200 sinkholes a year a decade go. Now, the typical number is closer to 2,000. In 2014, there were 1,699 reported sinkholes. That was an improvement from 2012, when the city saw 2,103 sinkholes, city records show.

“Freeze-thaw cycles,” Toran said. “They weaken, just like they weaken and create potholes. They can weaken these bridges that are over existing openings. So you might not have a specific event. But year after year after year and finally the last straw and something will collapse.”

Sinkhole might sound like a menacing situation, but it just describes when dirt below concrete falls, compared to a pothole, which is just a bowl-shaped hole on the top layer of a road. 

Responding to a sinkhole typically brings out multiple agencies — Water Department, PGW, PECO, cable utility workers, whereas a pothole can be patched up in 90 seconds, with a pothole killer machine

Geologists have done autopsies of Philadelphia craters and here’s what they’ve found: Down to 30 feet is what they call “urban fill,” and it’s composed of all sorts of stuff.

“We find doll arms, pots, bricks and foundations and then regular soil,” she said when surveying what’s underneath the city.

You’ll find lots of different types of stone, like Wissahickon rocks, where shiny mica comes from, used in buildings, asphalt and roof shingles.

It’s generally not as prone to sinkholes as, say, limestone, which dissolves faster than other types of rocks. Perri says Philadelphia’s lucky not to have an abundance of limestone.

Not so with some communities nearby, like the King of Prussia area, which is under a limestone patch. In 2011, a 75-foot sinkhole opened up there.

Limestone swaths, or carbonate rock, as geologists call it, cut through Harrisburg and Allentown, too, but largely bypass Philly.

But nowhere in Pennsylvania compares to amount of limestone found in Florida. Officials there call sinkholes “a fact of life.”

“This is not Florida, where our cave-ins our swallowing whole houses and vehicles. That’s not a phenomenon that you’re going to find in Philadelphia. These things are a nuisance, but not the type of thing that you’re going to fall into an underground cave underneath the city streets,” Perri said.

And the city’s water department is working on replacing some of its dusty pipes, at a pace of 22 miles per year. But there are more than 3,100 miles of underground infrastructure, so it’ll take some time.

The department has yet to meet its goal of replacing all pipes by the time their 100 years old, but officials say workers are moving as fast as they can given limited resources.

As for the factors city officials can’t control: colder weather and ancient rock movements. They’ll continue to conspire against Philadelphia, exacting unpredictable earth holes around the city. 

But Streets Commissioner Perri says the vast majority of sinkholes — he prefers the term “cave-ins” — are manageable, about 3 feet by 3 feet. That’s not capable of swallowing a car, but maybe a small dog.

Geophysics technology can detect subterranean areas that are vulnerable, but predicting the next sinkhole is nearly impossible. Underground cracks or voids can stay dormant for decades or longer.

“I often think about places that I would absolutely refuse to live. I’m afraid the state of California is off. That’s for me. I do not want to be in an earthquake,” Toran said. “And they look at us and see the risk that we have, and everybody chooses their regional risk. I think I’m OK living with a sinkhole risk.”

Want a digest of WHYY’s programs, events & stories? Sign up for our weekly newsletter.

Together we can reach 100% of WHYY’s fiscal year goal