Trapped Air Caused Sewer Pipe Rupture, Report Says : Environment: Inquiry offers no cause for buildup. Accident off San Diego closed 20 miles of coastline for more than two months.

A huge buildup of trapped air caused the rupture in February of a massive sewage outfall pipe that closed 20 miles of coastline here for more than two months, according to an independent report released Tuesday.

The long-awaited findings from the same firm that investigated the explosion of the space shuttle Challenger and the collapse of a hotel walkway in Kansas City, Mo., gave no specific cause for the buildup of trapped air.

Shortly after the Coast Guard detected pools of partly treated waste bubbling to the surface near the tip of Point Loma, city officials pointed to “wave action” as the probable cause of the rupture and spill, which had serious environmental effects.

The discharge of up to 180 million gallons a day of effluent sent bacterial counts soaring and forced the closure of beaches from the international border to the San Diego River, near the community of Ocean Beach.

Several claims have been filed against the city by individuals and groups who say the closure of beaches and a two-month quarantine on area marine life damaged their livelihood.

The Menlo Park-based firm of Failure Analysis Associates, which conducted the investigation, made clear that seismic activity and damage from a passing ship or anchor--two other theories posed by the city--played no role in the rupture.

The cost to the city of San Diego for the investigation and report by Failure Analysis Associates is $342,000.

The firm conceded the possibility that wave action may have had some effect but stated: “Wave action, coupled with the most extreme tides and currents, could make only a minor contribution to an uplift failure of the outfall.”

The report also ruled out corrosion of the pipe, each section of which is 25 feet long, 10 inches thick, 9 feet in diameter and has an in-water weight of 55,000 pounds. Thirty-one sections of the pipe split open less than three-quarters of a mile from shore.

Workers at the E. W. Blom Wastewater Treatment Plant told The Times that an accident at the plant two days before the rupture was detected may have led to the buildup of air that caused the pipe to come apart.

But the report said only full-scale testing could determine whether human error triggered the rupture.