A preliminary investigation indicated that the section of pipe that ruptured had been damaged by a backhoe sometime after the line was installed in 1947. PG&E is looking into when that damage might have occurred and what agency might have been responsible, Mr. Eisenhauer said.
The explosion left a 5-foot-by-5-foot crater in the hillside, and water from inside the pipeline caused a mudslide that reached northbound I-280 and blocked two lanes for about four hours, California Highway Patrol Officer Art Montiel said.
No one was injured.
The test was being conducted as part of an ongoing safety evaluation of natural gas transmission lines in "high consequence" or highly populated areas, Mr. Eisenhauer said. "That's exactly why we do these type of safety tests, to find weaknesses in the pipeline," he said.
PG&E crews have conducted pressure tests on more than 120 miles of pipeline since April.
Mr. Eisenhauer said no homes or buildings were damaged by Sunday's rupture, and that the utility employs different testing strategies on pipelines that run directly through neighborhoods, such as placing cameras or "pigs" that run inside the pipes to detect corrosion or faulty seams.
Last week, PG&E found a 1-millimeter leak in Line 132 in the vicinity of Palo Alto and Menlo Park, and last month a line ruptured in Bakersfield during hydrostatic testing.
A faulty seam on Line 132 ruptured in San Bruno on Sept. 9, 2010, causing an explosion that killed eight people and damaged 38 homes.
A driver detected Sunday's incident and contacted the Woodside Fire Protection District, saying that a water geyser was dowsing vehicles on both sides of the freeway, Battalion Chief Kevin Butler said.
PG&E, which owns the pipelines, has been testing them in the area since early July.
Moments later, someone reported an explosion in the same area and a large volume of flowing water. Witnesses reported the water shooting 30 to 100 feet into the air, Battalion Chief Bob Bender of Woodside Fire said in a phone interview.
When firefighters arrived, they found mud, rocks and debris from the hillside east of the freeway scattered across the northbound lanes, along with a faint smell of natural gas, Mr. Butler said.
The hydrostatic tests involve diverting gas service around the section of 32-inch diameter steel pipeline to be tested so as not to interrupt service to customers, then purging gas from that section and infusing it with water at much higher pressure than ever experienced in normal operation, PG&E spokesman Jim Cogan told the Woodside Town Council in June.
This particular set of tests involves 158 miles of pipeline, Mr. Cogan said. The tests include running a remote-controlled camera through the purged pipes, he said.
The last time these pipes were hydrostatically tested was in 1955, when they were installed, said Rick Salaz, a PG&E gas superintendent who attended the June council meeting. The pipes are regularly monitored above ground for methane leaks, Mr. Salaz added.
In the incident on Sunday, after the explosion and after the area tested safe for gas levels, fire crews searched the hillside and found the crater in an easement running behind homes, with the closest house about 100 yards away, Mr. Butler said.
The mudslide shut down northbound freeway traffic initially, and the far right lane remained closed while crews removed debris from the road, a California Highway Patrol spokesperson said.
While there were no injuries, flying dirt and rocks did damage one vehicle, Mr. Butler of Woodside Fire said.
— Reporting by Dave Boyce of the Almanac and Bay City News Service.
This story contains 663 words.
If you are a paid subscriber, check to make sure you have logged in. Otherwise our system cannot recognize you as having full free access to our site.
If you are a paid print subscriber and haven't yet set up an online account, click here to get your online account activated.