When a sinkhole cratered Mulberry Street from one sidewalk to the other last week, there was no telling how long it had been growing.
Where might similar disasters be waiting to happen?
City public works officials say they are stepping up efforts to find out — but acknowledge they can't prevent them all. Transportation officials meanwhile couldn't say how widespread similar problems might be.
A leak in an 8-inch water pipe feeding a single property on Mulberry caused the 30-foot-deep sinkhole to open up between Greene and Paca streets on the Fourth of July. It was the city's latest infrastructure failure, following breaks in city-owned water and sewer mains on York Road in Cockeysville last month and Centre Street in Mount Vernon in April.
It went undetected because that section of what is a major west-to-east artery across downtown is paved with concrete. While more durable than asphalt, concrete is also less prone to showing signs of trouble beneath it.
Underscoring the difficulty of noticing such issues before they become major problems, public works crews did not notice any leak under the roadway even as they replaced a water main under the same stretch of road last month.
"You use a certain amount of judgment," spokesman Kurt Kocher said of the city's efforts to root out fragile or broken pipes.
In this case, the water main was a higher priority than the leaky pipe because it carries more water to more customers.
The city has contracts with several companies to scan its water system for vulnerabilities, using electromagnetic, acoustic or video probes, Kocher said. Officials factor in a pipe's age, material and any history of breaks to prioritize it for inspection, he said.
The efforts have helped reduce the number of water main breaks in the city by almost 40 percent since fiscal year 2014, though they still numbered almost 800 in the fiscal year that ended June 30 — occurring more than twice a day, on average.
The department aims to repair or replace 15 miles of water mains each year, and they finished 19 miles of mains in the last fiscal year.
"It's not a perfect science," Kocher said. "You cannot do 4,000 miles of pipes all at once."
If breaks and leaks do occur, they are easier to detect beneath asphalt than under concrete, engineers said. It's like covering a hole with a blanket versus a piece of plywood — the asphalt is more likely to show a depression, if not allow water to bubble up.
City transportation spokeswoman Adrienne Barnes could not say how many miles of city roads are paved with concrete, nor why the material is used on some roads and not others. Engineers said it is designed to last decades, while asphalt requires more frequent maintenance.
That section of Mulberry Street likely has been paved with the same concrete slabs for at least 50 years, and perhaps a century, Barnes said.
"That was the standard practice back in those days," she said.
The Mulberry sinkhole is expected to take weeks to repair and refill.