The Big Break : Break Raises Questions About Best Pipe Types

Could another pipeline break?

It’s a question that provides food for thought for water managers during the day and whets their nightmares as they sleep. Generally, it cannot be predicted and, by design, it’s not supposed to happen.

But it did--nine days ago--and the debate continues over what kind of pipeline is most reliable to carry water.

Water pipelines come in two flavors: steel and concrete. The one that broke 10 days ago in Scripps Ranch, leaving nearly 400,000 persons in the lurch--was essentially a thin, steel barrel, sandwiched on the inside by mortar and on the outside by a nearly foot-thick layer of concrete that was encircled, in turn, by quarter-inch steel cords for strength. The pipe is then dressed on the outside with Gunnite.

But, for some reason, the steel liner--just one-sixteenth of an inch thick and intended not to serve as strength but just to be a water barrier--was touched with rust.

Over time, maybe years, the undetected cancer took its toll until, finally, the pipeline burst.

Such accidents occur rarely. In San Diego County, there has been nothing comparable.

Richard Balcerzak, assistant general manager of the Metropolitan Water District--which wholesales water to the San Diego County Water Authority and others around Southern California--said his agency has about 200 miles of the prestressed concrete pipe in place, accounting for 25% of its entire pipeline system. The rest is three-eighth-inch thick steel pipe.

And he said he could only think of three failures of concrete pipe in 30 years’ experience with the MWD.

“I remember one, north of Lake Silverwood (in San Bernardino County), where the pipeline experienced a sudden failure and actually blew a four-by-six foot piece of pipe out of the ground. It was pretty dramatic,” he said.

In San Diego, both types of pipe are used, sometimes along the same pipeline, as the installation jobs are put to bid in different phases.

The steel pipe, Balcerzak said, is “more forgiving,” because it takes a greater physical jolt for the steel to break than it does prestressed wire-and-concrete. Besides, internal inspections of all-steel pipelines, conducted when the pipe is temporarily shut down for routine maintenance and checks, are able to show corrosion from the inside. If a problem is spotted--the tell-tale rust signaling a pinhole leak, for instance--the area around the pipe is excavated, and new steel sheets are welded and belted to the suspect section like some huge bandage.

But the prestressed concrete pipes are sometimes less expensive and, because of their generally excellent track record, are chosen to save money.

“The more alternatives you have (in the bidding process), the greater the competition and the better the prices,” he noted.

Some people feel that the prestressed concrete pipes are actually better, he said, because they can withstand greater pressure than steel--not only from within, but from the pressure of the backfilled earth around it.

But the steel cords--which provide the ultimate strength for the pipe--and the interior steel water liner itself can be damaged ever so slightly, either in the manufacturing process, or when it is installed, or even in settling--or an earthquake--afterward. And, unlike its all-steel counterpart, it’s virtually impossible to spot the damage either to the steel liner or the steel wrap-around wire because they’re covered on both sides by mortar or Gunnite. At best, suspicious-looking, short hairline cracks running the length of the pipe might telegraph a problem--if it’s even visually detected in the first place.

Lester Snow, general manager of the County Water Authority, says he has no reason to lose faith in prestressed concrete pipelines, despite the Scripps Ranch experience.

“Just because you have a cavity, you don’t assume all your teeth need to be filled,” he said.

Balcerzak said the industry as a whole is still undecided on which pipe is better.

“It’s an issue we’re revisiting--whether to go steel or prestressed concrete. You don’t want to discard the economy of having a choice between the two of them, but on the other hand, you don’t want to put something in the ground that you’re going to lose sleep over.”