Buildup of Trapped Air Caused Sewage Pipe Break : Inquiry: Report gives no specific reason for the occurrence, which the city initially dismissed as the cause.

A huge buildup of trapped air caused the rupture of a massive sewage outfall pipe in February that closed 20 miles of coastline 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 reason for the buildup, which city officials initially dismissed as the cause.

Shortly after the U.S. Coast Guard detected pools of partly treated waste bubbling to the ocean’s surface near the tip of Point Loma, city officials cited “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 Ocean Beach.

The accident resulted in one of the worst sewage spills in the history of the nation. It cost the city $10 million to repair the pipe and another $342,000 to commission the independent report.

Several claims have been filed against the city by individuals and groups who say the closure of beaches and a two-month quarantine on marine life damaged their livelihood. Thirty fishermen are among those seeking damages.

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 creating the rupture.

The firm conceded the possibility that wave action may have had some effect, but said, “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 itself, each section of which is 25 feet long, 10 inches thick and 9 feet in diameter, with 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.

“It raises as many questions as it tries to answer,” City Manager Jack McGrory said Tuesday about the report, which took eight months to complete. “The one thing we know now that we didn’t before is that entrapped air was a significant factor.”

But McGrory said city officials still disagree that trapped air alone led to the rupture.

“Based on the fact that the pipe had worked flawlessly for 29 years (since it was opened in 1963), we believe that there had to have been some external factor that caused this,” he said.

“Frankly, I think we’ll never know what really caused it.”

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

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

McGrory said Tuesday that city officials still maintain that human error played no role in triggering the explosive rupture.

“They told us (verbally) that the pipe could withstand more than 200,000 cubic feet of air,” McGrory said. “But a throttling test (used to determine the possibility of human error) could generate only 1,000 feet of cubic air. So we don’t feel (human error) created the buildup of air that led to the problem.”

Experts quoted by The Times in February expressed concern about a “water hammer” in the pipe creating the kind of buildup that Failure Analysis says caused the explosion.

Ladin Delaney, who for nine years was executive officer of the state Regional Water Quality Control Board, said he had worried about the possibility of a water hammer damaging the line.

“It sounds completely logical to me,” said Delaney, a consultant who recently retired from the board. “I was concerned about a water hammer in the outfall before. I don’t know that that’s what happened this time, but, based on past concerns, it’s highly plausible.”

Delaney said he based his concern on firsthand observations by one of his inspectors, who was touring the Point Loma facility several years ago when a water hammer blew a large cover off the top of the outfall pipe.

“One of the big things we always worried about with the plant was ‘air entrainment’--air getting sucked into the pipe,” Delaney said. “We were concerned for two reasons. (Air entrainment) cuts down on the hydraulic capacity of the line and, of course, it creates a water hammer, which was our biggest concern.”

McGrory said recently that he was aware of a water hammer incident in 1986. As a result, the city built a pipe to bleed air from the system and is now designing a separate set of pipes that will draw air out of the outfall completely.

While making frequent mention of “air entrainment” or “entrapped air” as the cause of the rupture, Failure Analysis did not say why or how air came to be trapped.

Known as the “masters of disaster,” the firm has nonetheless fielded its share of criticism.

“They’re fairly predictable witnesses for their corporate clients,” consumer advocate Ralph Nader told The Times in a recent interview. “They do what they’re paid to do: reflect the policy and philosophy of their clients.

“I would not describe them in any way as detached,” Nader said. “It’s not a ‘let-the-chips-fall-where-they-may’ kind of analysis. They’re big and have a lot of equipment, but the question is the extent to which they’ve been politicized by clients.”

McGrory said he “had a hard time buying” aspects of the Failure Analysis report. In particular, he cited the finding that two ruptures of the outfall--one at 5,800 feet offshore, the other at 3,200 feet--occurred independently of one another within 24 hours.

The city has begun plans to construct what McGrory called a “south channel” leading from the treatment plant to the pipe itself, “which will take all the air out of the outfall.”

That project will cost $12 million. An additional $60 million is being spent to extend the outfall from its present 2.5 miles offshore to 4.5 miles, and to place it 330 feet below the ocean surface. The pipe now rests 220 feet below the surface at its deepest point. The initial rupture occurred at a depth of 35 feet.

In repairing the outfall, the city ordered the placement of 700,000 tons of quarry rock on top of the pipe. As a result, McGrory said, “we’ve been assured by experts that there is no way that pipe will ever rise up.”

As for the future of the outfall, McGrory said he hopes it can continue to serve 1.7 million residents of San Diego County “indefinitely . . . for as long as we need it.”