Cities Battle Growing Risk of Flying Manhole Covers

Sent aloft by underground explosions, manhole covers have shattered a toilet inside a home near Minneapolis, dented cars parked at Cleveland’s Rock and Roll Hall of Fame, and knocked over a sanitation worker and a parked car in Manhattan.

In San Diego, St. Louis, San Francisco, Newark and New Orleans, they have blown high and landed loudly, narrowly missing people.

In Georgetown this year, three manhole covers blew on busy M Street. One in downtown Washington slammed into a car, and another was wrestled to the ground by Secret Service officers near the White House. Fatalities are rare, but one of the worst accidents happened in Pittsburgh in 1991, when an underground explosion sent a manhole cover through the windshield of a mother’s car, beheading her in front of her children.

The phenomenon of flying manhole covers has been seen increasingly over the past decades in major cities, the result of aging underground infrastructures that sizzle, leak, flame, spark and sometimes explode and pop like deadly pressure cookers.

Utilities are looking for other ways to keep the lid on streets. Their search for stopgap measures has led to one place: California’s Silicon Valley, where the Electric Power Research Institute, based in Palo Alto, has studied the problem for years. EPRI holds manhole seminars, where utility company engineers pay to work alongside scientists and engineers who are experts in manhole explosions.

EPRI’s work is a bit blue-collar for the high-tech area, so the tunnel where manhole explosions are detonated almost daily is tucked away in a working-class suburb in Massachusetts. There, engineers set off hundreds of explosions and videotape everything.

“Generally speaking, we take our underground system for granted,” said Ralph Bernstein, the technical leader for EPRI. “It is not seen. It is forgotten until a major manhole event occurs.”

Manhole events, in the parlance of this tiny world of scientists, are classified into three categories: smokers, fires and explosions. Explosions happen when an electrical spark ignites a gaseous underground cocktail and sends cast-iron lids--most manufactured in India and weighing 100 to 300 pounds--at least a foot in the air.

The record for a flying manhole cover was set in Wisconsin, where one was able to leap a tall building and land on the other side of the block, Bernstein said. He remembers a Manhattan explosion that flummoxed Consolidated Edison workers who couldn’t find the manhole cover for weeks--until a construction worker discovered it on the roof of a 10-story building.

“They really can fly,” Bernstein said.

There is one explosion for every 10,000 manholes, Bernstein said. They happen primarily in major cities, where cables are generally much older and other factors tend to aggravate the situation underground. Cities also have more manholes--there are about 57,000 in Washington and about 225,000 in New York City.

The tiniest crack in an underground cable, a nick or a tear, can turn the gases that naturally occur underground into an explosion. Cracks can be made by a utility digging up a street. Ceramic ducts can crack from the weight of thousands of trucks rolling over them for decades. Poor workmanship on a splice, which is roughly the size and shape of a car’s muffler, can allow insulation to fail.

The first step in prevention is a rigorous maintenance schedule, something Potomac Electric Power Co. agreed to under duress after the Georgetown explosions. So far, Pepco has inspected 1,700 manholes and found faults in 23 splices, said William Gausman, Pepco general manager for power distribution.

In New Orleans and Pittsburgh, utility companies tethered their manhole covers to the ground. Pepco may try switching to grated manhole covers, which allow gases to escape, Bernstein said.

The problem with those covers, besides the obvious accumulation of more debris underground, was illustrated this month when a fireball roared through a tunnel and shot through the grate and into the street in Georgetown.

Under development are computer chips that can cut off electricity when the gas buildup inside a tunnel is at a dangerous level and systems that can sense an electrical arc coming through a cable and cut the power to it, Bernstein said.

“But that is far off in the future,” he said. “For now, we have to find better ways for mitigation.”