NEW ORLEANS (AP) — Excavation work by an Army Corps of Engineers contractor weakened New Orleans floodwalls and caused them to breach in two places during Hurricane Katrina, an engineering expert testified Monday.

Robert Bea, a University of California at Berkeley engineering professor, made his statements at a trial for several homeowners’ claims against the company and the federal government. He said the excavation work performed by Washington Group International Inc. created subterranean water pressures that led to the failure of floodwalls meant to protect the city’s Lower 9th Ward and neighboring St. Bernard Parish.

“WGI’s excavations were a substantial contributing factor without which the (breaches) would not have occurred,” said Bea, a key plaintiffs’ witness.

U.S. District Judge Stanwood Duval Jr., not a jury, will decide the case.

During cross-examination by a Washington Group attorney, Bea acknowledged identifying other factors besides excavation work that also contributed to the floodwalls’ failure.

In a recent court filing, Justice Department attorneys accused Bea of performing a “deeply flawed and unscientific analysis.”

The corps hired the company to perform excavation and backfilling work near the floodwalls for a lock replacement project that began in 1999. WGI denies any responsibility for the flooding but says the corps was solely responsible for ensuring that the work wouldn’t jeopardize stability of the floodwalls.

Bea rejected claims by the corps and WGI that Katrina’s storm surge overtopped and overwhelmed the floodwalls along the east side of the Inner Harbor Navigation Canal and caused them to fail. He concluded that easterly winds prevented substantial wave overtopping.

WGI attorney William Treeby objected to this part of Bea’s testimony, arguing that it falls outside his area of expertise.

“He’s not qualified as an expert in wave overtopping,” Treeby said. “Even if he is, we have no opinions in his reports about this.”

Duval, however, overruled Treeby’s objection.

Bea said he also ruled out the possibility that the floodwall breaches were caused by a barge that broke loose during the August 2005 storm — in part because he saw no plausible way for a barge to cause both breaches.

“The barge was a victim, not a cause,” he said.

Bea testified as a plaintiffs’ expert at an earlier trial for separate but related claims over damage from the failure of New Orleans’ flood protection system during the 2005 hurricane. Duval also presided over that trial and ruled in 2009 that the corps’ shoddy work on a shipping channel left the same areas vulnerable to flooding.

But the judge also has previously ruled that the corps isn’t liable for damage from the failure of a flood-control structure like a levee or floodwall. Plaintiffs’ attorneys argue the lock replacement project doesn’t qualify for that legal protection.

Bea said he has spent more than 14,000 hours investigating the causes of Katrina-related levee and floodwall breeches and made more than three dozen trips to New Orleans as part of his research. He called it a “life-changing experience.”

“Out of the pain of Katrina has come a great deal of deep understanding of how it happened,” he said.

The trial started Wednesday and is expected to last about a month.