Despite more than $22 million in repairs, a levee that broke with catastrophic effect during Hurricane Katrina is leaking again because of the mushy ground on which New Orleans was built, raising serious questions about the reliability of the city's flood defenses. Outside engineering experts who have studied the project told The Associated Press that the type of seepage spotted at the 17th Street Canal in the Lakeview neighborhood afflicts other New Orleans levees, too, and could cause some of them to collapse during a storm. The Army Corps of Engineers has spent about $4 billion so far of the $14 billion set aside by Congress to repair and upgrade the metropolitan area's hundreds of miles of levees by 2011. Some outside experts said the leak could mean that billions more will be needed and that some of the work already completed may need to be redone.

"It is all based on a 30-year-old defunct model of thinking, and it means that when they wake up to this one - really - our cost is going to increase significantly," said Bob Bea, a civil engineer at the University of California at Berkeley.

The Army Corps of Engineers disputed the experts' dire assessment. The agency said it is taking the risk of seepage into account and rebuilding the levees with an adequate margin of safety.

"It's always a potential, so it is a design component for every feature," said Walter Baumy, the chief corps engineer in New Orleans.

The 17th Street Canal floodwall collapsed on the day Katrina surged over New Orleans in August 2005, and the failure severely damaged Lakeview. It was one of the biggest of about 50 levee breaches that contributed to the deaths of about 1,300 people.

Fixing the 17th Street Canal has been one of the most expensive and laborious repair jobs since the storm and has served as something of a test case for scientists and engineers, who plan to apply the lessons learned there to the city's other levees.

Among other things, they repaired the wall by driving interlocking sheets of steel 60 feet into the ground, compared with about 17 feet before the storm. The sheet metal is supposed to prevent canal water from seeping under the levee through the wet, toothpaste-like soil that lies beneath the city, which was built on reclaimed swamp and filled-in marsh.

Over the past few months, however, the corps found evidence that canal water is seeping through the joints in the sheet metal and then rising to the surface on the other side of the levee, forming puddles and other wet spots.

Engineers said the boggy ground is a more serious problem than the corps realizes. Bea said there is a roughly 40 percent chance of the 17th Street Canal levee collapsing if water rises higher than 6 feet above sea level. During Katrina, the water reached 7 feet in the canal.

John Schmertmann, a retired University of Florida professor and a consultant on foundations, agreed with Bea that the corps "may still be embedding some of these not-properly-considered factors, so the new walls may not do what the corps expects."

Reducing such seepage might require the driving of sheet metal far deeper into the ground than is done now, or some other solution, said Bea, who was part of a team of experts sent by the National Science Foundation to do an independent study of the levee failures during Katrina.

Donald Jolissaint, chief of the corps' technical support branch in New Orleans, denied the problem at the 17th Street Canal is serious.

"I personally do not at all believe that this little wet spot is anything that is going to cause a breach or a failure of any kind," he said. A newly installed floodgate could be used to cut off the flow of water into the canal and reduce pressure on the levee, he said.

Nevertheless, the corps is concerned enough that for weeks, workers have been analyzing the wet spots and digging a 160-foot-long, 10-foot-deep trench to zero in on the source. "We're doing everything we can to chase this down," Jolissaint said.

The corps is also spending about $100 million by taking more than 2,000 soil borings to find out what is under the ground and determine the best design.

Timothy Kusky, a geologist with Saint Louis University and an expert on the Mississippi River, said engineering a safe levee system in New Orleans will be very difficult because of the soil.

"You've got old riverbeds and floodplain deposits all interlayered and distributed laterally in a very complex way, and then you build a levee across them," Kusky said.

As a result, a levee sinks at different rates, and the sinking creates "little cracks in them that promote seepage, and also the old river channels and floodplain deposits have different potentials for underseepage," he said.

He said the corps understands a lot of the problems, but it takes a huge amount of data to map every weakness, and the agency does not have the manpower to see that every contractor is doing the job right.

Seepage was reported at the 17th Street Canal before Katrina. The corps denies that caused the collapse. Instead, the corps contends the floodwall flexed and finally cracked under the force of water piled against it by the storm.