Pacific Gas and Electric Co. (PG&E) on Tuesday released reports by separate third-party engineering/metallurgical consulting firms that concluded excavation equipment from a road-building project caused the rupture and resulting fatal fire in the utility’s 12-inch diameter natural gas transmission pipeline (Line 118B) near Fresno, CA, last April.

There is no indication of a failure by PG&E in its maintenance and operation of Line 118B, or in the pipeline materials and welds. “Mechanical testing and chemical analysis indicated that the subject pipe met 1960 and current API [industry] specifications for Grade X-42 pipe,” said the materials and corrosion engineering report from California-based Exponent Failure Analysis Associates.

“Line 118B ruptured due to a strike from the front-loader bucket operating in the area at the time of the incident,” the Exponent report said. The strike and pipe rupture were “nearly instantaneous.”

PG&E originally reported that up to 14 people were injured (one later died). Authorities closed north-south state Highway 99, a major transportation artery in California’s Central Valley (see Daily GPIApril 20). It was never alleged that PG&E was at fault.

A fireball erupted and flared for nearly an hour before PG&E field crews shut off the flow of gas. In the Exponent report there is a reference to investigators having found that there was only “minimal depth” of soil coverage at the location where the pipe was hit.

Further investigation by a separate independent consultant, however, verified that the coverage above the pipe at that location was altered by the excavation crew on the day of the incident. Redmond, WA-based Golder Associates investigated the depth of cover issue and concluded that extending the cutting into a hillside encroached on the pipeline, despite the fact that there were at least three markers in the vicinity designating the pipeline corridor.

“The excavation of the bench [cutting] to the southwest resulted in the heavy equipment damaging the pipeline with the resulting rupture of the pipeline,” said Golder Principal Engineering Geologist William Fowler and his engineering geologist colleague Donald West.

In the aftermath in April, PG&E said the Line 118B pipeline had been surveyed in 2013 and there was about 40 inches of cover between the pipe and the surface. A distance of more than three feet meets current federal and utility standards.