City officials Tuesday said it could take a while to determine what caused a gaping sinkhole on 10th Street in Downtown Pittsburgh and noted it’s important to remember the age of the city when making that determination.

“We’re building on top of 250 years of development, and there are vaults all throughout not only Downtown but throughout Western Pennsylvania when it comes to mining and other types of things that were never reported 100 years ago, let alone 200 years ago,” Mayor Bill Peduto said. “There’s a lot of surprises underground.”

A day after the collapse created a hole nearly 100 feet long and 20 feet deep, contractors were removing pieces of concrete and other debris from the scene.

Peduto said the main concern in the immediate aftermath is public safety.

“Now the engineering begins,” he said.

He said two main questions remain: Why and how did the sinkhole happen and how does the city make sure the road is stable in the future?

Part of the issue, experts said, is aging infrastructure.

“There will never be sufficient funding to buy our way out of this one,” said Julie Vandenbossche, an assistant professor of geotechnical and pavements engineering at the University of Pittsburgh’s Swanson School of Engineering.

City spokesman Tim McNulty said once debris is removed and below-ground fiber-optic cables are safe, utility companies will begin assessing damage to water, gas and electric lines.

Simeon Suter, a geologist supervisor with the Pennsylvania Geological Survey, said those underground utility lines are among the things that can cause sinkholes.

“Anytime there is a mass of material taken out of the ground, there will be some subsidence — even if it’s just an inch,” he said.

That can include mining, dissolving bedrock and shifting soil.

The geology of Pennsylvania lends itself to the development of sinkholes. Much of the rock found underground in the area is carbonate bedrock — types of rock easily dissolved by acid. Rainwater, Suter said, generally trends to the acidic side in terms of pH levels.

Beyond mining and geology, the general structure of urban areas lends itself to swaths of ground opening up.

“There are a lot of things we put underground — water lines, sewer lines, cable lines, electric lines,” he said. “When we excavate and put those lines in and backfill, some settlement takes place. Now what you have under the street is not as compacted as soil.”

The idea of putting utilities underground isn’t new and it isn’t unique to Pittsburgh, said Lev Khazanovich, a University of Pittsburgh professor of engineering.

“It’s a logical place for almost any location,” he said. “The problem is that, in many cases, those utilities themselves are pretty old, and there are difficult geologic conditions that make it worse.”

That aging infrastructure isn’t just a Pittsburgh or Pennsylvania problem. The American Society of Civil Engineers grades the country’s infrastructure every four years. The overall grade for the United States in 2017, the most recent year the country as a whole was graded, was a D-.

The local chapter of civil engineers graded the state in 2018. Its roads received a D+, its drinking and storm water systems both received Ds, and waste water received a D-.

Most of the country’s infrastructure is already built, Vandenbossche said, so the issues lie not in building new roads or bridges or waterways, but finding better solutions to repair and renew what already exists.

“The aging infrastructure is not on our side,” she said. “The needs aren’t increasing linearly but exponentially.”

Staff reporter Tom Davidson contributed to this report. Tony LaRussa and Megan Guza are Tribune-Review staff writers. Contact Tony at 724-772-6368, tlarussa@tribweb.com or via Twitter @TonyLaRussaTrib. Contact Megan at 412-380-8519, mguza@tribweb.com or via Twitter @meganguzaTrib.

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