The exact corner where the San Bruno gas pipeline blew a crater in the ground was singled out by PG&E nearly two decades ago as an area of “possible slope instability” because it skirted the powerful and active San Andreas fault so closely.
But despite identifying it as a hazardous site, Pacific Gas and Electric chose not to replace that section of pipe in the mid-1990s when they replaced almost all the pipe around it.
In fact, it replaced Pipeline 132 right up to San Bruno Avenue, less than a quarter-mile south of the rupture site, and then picked up the project again a quarter-mile north of the rupture site, at Plymouth Way. Left untouched was the half-mile section through Crestmoor, the very section destined to one day turn that neighborhood into a panorama of charcoal and ash.
Click on the photo to the right to see a map of the pipeline replacement in San Bruno.
While most of the National Transportation Safety Board’s investigation has focused on other major flaws in the pipe construction, agency spokesman Peter Knudson said the investigators are also considering possible earth movement as a factor in the explosion.
According to California Geological Service maps, the San Bruno pipeline runs less than a block away from the San Andreas fault, as it cuts northwest through the Peninsula, and a fraction of the fault even veers right under Glenview Road, stopping perhaps 100 feet shy of the explosion site.
The fault was close enough to the pipeline that PG&E seismologist William Savage met with then-planning
director of San Bruno George Foscardo in 1992 about the potential dangers. In a letter to Foscardo a few months after that meeting, released by the NTSB last week, Savage explained that much of the pipe must be replaced, and “added safety will be provided by using heavier-walled pipe and long-radius elbows” to mitigate any earth movements.
An attached fact sheet about the work explains that PG&E is in the process of a 25-year program “that will replace all aging natural gas pipelines in the system” by 2000. One of the two San Bruno locations singled out for “possible slope instability” is “an area of low hazard near Glenview Drive south of its intersection with Earl Avenue” — the intersection that would later explode.
However, that portion was not ever actually replaced, and instead was left where it was.
PG&E spokesman Paul Moreno said the company completed all of its planned pipe replacement, and could not explain why the half-mile through Crestmoor was left untouched.
UC Berkeley engineering professor Robert Bea noted that in the early days after the explosion, he and other engineers were concerned that earth movements could have caused the explosion. But since then, it’s become clear that even though there were almost certainly earth movements over the years in the vicinity, that wasn’t the primary culprit for the explosion. While earth shifting or ground settlement may well have contributed to the pressure on the pipeline, there has been no evidence presented that the pressure had accumulated to the point that it would be a dominant cause of explosion.
“Another horse won that horse race,” he said.