A Pacific Gas and Electric Co. natural gas pipeline ruptured Sunday afternoon during a high-pressure water test that ripped a hole in a Peninsula hillside, sending a deluge of mud and rocks onto Interstate 280 and partially closing the freeway for four hours.

The pipeline is the same one that exploded in San Bruno last year, killing eight people and destroying 38 homes.

The rupture, located about 250 feet from the freeway near Woodside, is the pipeline's second failure in four days of testing. And it follows a failed Oct. 24 test in the Central Valley that tore a 6-foot-long gash in a pipe and gouged a crater in an alfalfa field west of Bakersfield.

PG&E officials said it wasn't immediately clear why the 24-inch-diameter line failed around 3:20 p.m. Sunday, but the pipeline blew apart at what appeared to be a dent. The utility said the dent was probably caused by another company or person.

Late Sunday, chain link fencing surrounded the 7-by-7-foot hole, located about 300 feet from the nearest homes. A mat of mud and soaked grasses ran down to the freeway.

Crews worked with shovels to clear debris from the northbound lanes, which were completely closed for about an hour. I-280 was fully reopened around 7:20 p.m.

Utility officials said they had workers in the area in case of a failure.

"We'll learn from this experience," spokesman Brian Swanson said. "It's definitely a concern that this hydrostatic pressure test impacted 280. We'll do an analysis to determine how we can minimize disruptions like this in the future."

Pipe to be replaced

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PG&E has said it will pull up the pipe this morning. Local residents, who had been notified of testing in the area, will be told that extended work hours will be needed, Swanson said.

Gas service should not be affected because it had already been rerouted to other lines.

Ryan Baker, 37, was at his nearby home when the pipeline ruptured but said he didn't hear it. He also didn't recall seeing a notice about the testing from PG&E.

"I'm glad it was a situation where they were testing it and it failed rather than the alternative. The alternative is very scary," he said.

Baker, a partner in a distribution company, lives at the home with his wife and two children. He said his wife was worried about the failed test.

"I told her, don't worry, we're going to have the best stretch of pipe in town" after they replace it, he said.

The utility's test focused on a 3-mile stretch of Line 132 from Sand Hill Road in Menlo Park to Farm Hill Boulevard in Woodside. The entire transmission pipeline runs 50 miles from Milpitas to San Francisco's Dogpatch neighborhood.

Troubled pipeline

In September 2010, an incomplete weld on a longitudinal seam of Line 132 ruptured in San Bruno. PG&E had believed the pipeline was seamless. Since then, the utility has been blamed for keeping shoddy and incomplete records.

Company officials said they believe the stretch of pipe that ruptured Sunday is seamless.

For decades, PG&E avoided these high-pressure water tests, considering them too costly and burdensome. Experts say had the tests been done, the myriad problems with the pipeline under the San Bruno neighborhood would have been detected.

Instead, PG&E favored safety tests best suited for detecting corrosion. It used that inspection method even though Line 132 had at least 33 unexplained leaks since 1951.

A federal probe has since shown that one of those leaks in 1988 was caused by a flaw in a longitudinal seam weld. Federal regulations prohibit the use of corrosion-only methods on any gas transmission line with a history of longitudinal seam weld failure.

Since the San Bruno explosion, PG&E has embarked on an effort to conduct the high-pressure water tests on 150 miles of transmission pipe by the end of the year.

On Thursday, one of those tests led to the discovery of a "pinhole leak" at an unknown location along a 4-mile stretch of pipe between Palo Alto and Menlo Park. The utility has not yet determined what caused that failure.