Anatomy of a Water Main Break
You know it's winter in Pittsburgh when your car is getting beat up by pot-holes, the streets are chalky with salt, and water main breaks proliferate. But what exactly is going on below the pavement?
Clogged pipes, flooded basements and sheets of ice on roadways are some of the visible signs of water main breaks. But many leaks and breaks go undetected - including sewer line breaks which filter through the soil and along side the pipes for months or years.
At least a quarter of Pittsburgh’s water is lost annually – much of it to cracked pipes - but it is the big breaks, the show stoppers, that make it into the news. Pittsburgh Water and Sewer Authority spokeswoman Melissa Rubin said there are times you can see the water shooting into the air.
“Unfortunately people aren’t typically aware of their infrastructure, they go in turn on the faucet and there’s water and flush the toilet and that’s great and not thoughts ever put into it until you see water shooting in the air. And oh my gosh you can’t take a shower,” said Rubin.
Miles of Pipes, Countless Breaks
Pittsburgh has 1,200 miles of pipes, many of them around a century old. When they are replaced the moldering cast iron pipes are changed out for ductile iron pipes for water and PVC for sewers. But Rubin says it is rare that they replace an entire street’s worth of infrastructure because pipes and paving are expensive and the work is disruptive. Instead the process is often piece meal with pipes being patched or repaired with clamps multiple times before they are finally replaced.
Rubin explained there are two types of breaks, “splits where there’ s a hairline fracture on the side of the pipe, and there’s complete blow outs where it cracks in half.”
Breaks have many culprits. The freezing and thawing cycle of the ground shifts the soil which pushes against the pipes. The make-up of the soil plays a part too – whether it is clay, rocky, or acidic - they all pose problems for long buried pipes. Then there is the engineering standard by which the pipes were placed in their earthen bedding in the first place. Additionally, many of the pipes are tenuously thin from decades of degradation as water wears down the metal. Carnegie Mellon University Professor of Civil Engineering Dave Dzombak said this is common problem in city’s with old infrastructure.
“All metals except for gold and silver corrode when in contact with water and that’s a process we call oxidation – long term contact with water.”
Dzombak explained that humans also have a hand in certain water main breaks. He points out that breaks frequently occur in clusters which are caused by the very people who are trying to fix the original break.
“There’s a pressure distribution and it’s variable across the system. And if we are fixing one pipe, and we have to close off part of the system, it changes the pressure in other parts of the system and weak spots will fail when we do that. Parts of the system will see somewhat higher pressures, and just enough to take a weak-walled pipe and have it fail. So it’s frustrating – we get one break and fix that and boom, boom, boom around that we start getting other breaks,” said Dzombak.
Yet another complicating factor to repairing water main breaks is the maze of subterranean lines that keep the city humming. PWSA spokeswoman Melissa Rubin says the presence of other utilities sharing trenches with the water and sewer pipes is some times indicated by industrial graffiti marking the pavement.
“You see the green arrows and the red, the yellow and the blue. The blue is the water line, green is sewer, red is cable, and yellow is gas – so they all come out and mark so that you’re not damaging other infrastructure in the process of your repair.”
In January a major water main break under Ft. Pitt Boulevard could not be repaired until a near-bye gas leak was fixed first.
But short of water pouring onto streets and sudden drops in water pressure, it can be tough for utilities to know when and where a break has occurred. CMU Professor Dzombak technology will meet that need eventually.
“Better monitoring out in the distribution network, better monitoring for pressure, for flows, for quality of water. New technologies are being developed for monitoring that are less expensive. And we’re going to see more and more of that in the years ahead.”
Until then, the city will continue to be on guard for winter’s inevitable disruptions both from the sky and from under the ground.