Newhouse News Service

A team of engineers says what it calls weak Army Corps of Engineers levee safety standards may explain why some New Orleans canal floodwalls breached during Hurricane Katrina.

If borne out, the theory could have major implications for the future of New Orleans’ hurricane protection. It suggests that the Corps’ current design practices may be inadequate and large portions of the levee system – not merely breached areas – may not measure up to design specs.

Engineers at the University of California, Berkeley studying the levee system say the Corps’ design standards for levees require an unusually low safety margin. Other engineers agree, saying that large structures designed to protect lives are typically built with significantly higher safety factors – and are thus less likely to fail under stress.

When Katrina’s storm surge came along Aug. 29, the Berkeley engineers say, it weakened the soil under the walls of the 17th Street and London Avenue drainage canals, possibly in ways unaccounted for in the Corps’ design process. That would have erased the already-small safety margin altogether. The walls breached, flooding large portions of the city.

So far, this is just an educated guess. Engineers studying the levee system say that although soil failures appear to be a common factor in the canal breaches, they don’t have enough evidence yet to say exactly how each wall failed, and some may have failed for different reasons than others.

In many areas of the city, levees gave way after being overtopped. But the city’s 17th Street and London Avenue canal walls were not overtopped but failed anyway, suggesting a design or construction flaw.

The Berkeley investigators, who are sponsored by the National Science Foundation, say evidence collected so far, including soil boring data and design documents, supports their theory as a good general explanation that may apply to more than one wall – and to other parts of the levee system.

Their central point is that the Corps’ safety standards for floodwalls may be inadequate. In engineering terms, a safety factor is a kind of cushion incorporated into a design to account for unexpected problems. It assures that a structure can take all the punishment it’s designed for – plus a little more. The higher the safety factor, the bigger the cushion and the more secure the structure.

Corps manuals and design documents for the 17th Street and London Avenue floodwalls require a safety factor of 1.3. That means the cushion is 0.3: The wall must be 30 percent stronger than the maximum forces it’s built to endure.

Berkeley engineering professor Robert Bea, who has helped formulate the working theory, said that number jumped out at him when he saw it.

“The reason it immediately shows up on my radar screen as being horribly low is because the soils are so variable,” he said. Bea explained that when something is uncertain – such as the strength of foundation soils – engineers will raise the safety factor. New Orleans soils are notoriously weak and unpredictable.

Several independent engineers said that a safety factor of 1.3 is unusually low for structures so important in protecting lives and property, especially those that are “dynamically loaded” – built to endure powerful, rapidly changing forces like a storm surge.

“A 1.3 safety factor is exceedingly low for a dynamically loaded structure. I don’t know of any dynamically loaded structure that is lower than 2.0,” said Thomas Eagar, a professor of engineering at the Massachusetts Institute of Technology who studied the collapse of the World Trade Center.

Eagar reeled off a list of typical safety factors. For the structural frame of a building, the safety factor is 1.67. For a bridge, 2. For ladders or scaffolding, 4. Generally, he said, if there is a lot of uncertainty about a structure’s stability or its failure would kill or injure people or damage property, the safety factor will be higher.

Joseph Wartman, a geotechnical engineer at Drexel University who is on an American Society of Civil Engineers team investigating the levees, agreed that 1.3 was unusually low, and that typical safety factors for most structures vary between 1.5 and 2.0.

“This is a generally accepted guideline that is widely accepted as the standard of contemporary practice,” he said. If there is a lot of uncertainty about soil strength – a common problem in New Orleans – then engineers tend to set the factor closer to 2, he said.

Tony Young, a civil engineer in the Mississippi Valley Division of the Corps in Vicksburg, Miss., defended Corps procedures, saying that engineers meticulously assess variable soil conditions and design walls conservatively.

“When you compare 1.3 for a levee safety factor with another safety factor that may be used in a different type structure, you don’t always have an apples-to-apples comparison,” he said.

Cost is also a factor, Young said. Increasing the safety factor by even one- or two-tenths would cost a lot more, he said, and require more materials and more space. “If you are in an inhabited area, in an urban area, that becomes an even bigger issue, and the cost factor could be even greater,” he said.

The biggest source of uncertainty for New Orleans levees and floodwalls is the soil. That’s where part two of the Berkeley engineers’ theory comes in. Bea and his colleagues say that they believe Katrina’s storm surge weakened soil under the floodwalls’ sheet pile foundations in ways that designers did not anticipate.

Soil boring data taken post-Katrina show generally weaker soils under the floodwalls than data from the 1980s, before the walls were built, Bea said. He calculated safety factors using the new data, and says that in some places the safety factor is now below 1 – meaning Katrina had erased the safety factor and the wall is in danger of collapse.

Copy the Story Link