Thursday, May 1, 2008

Corps may have to rebuild floodwall sections

The Army Corps of Engineers may have to rebuild sections of hurricane protection walls on the lakefront to meet engineering standards adopted in the aftermath of Hurricane Katrina, according to floodwall studies done by the corps.

The walls stretch from earthen levees on Lake Pontchartrain to floodgates installed after Katrina on the Orleans Avenue and London Avenue canals. Because these sections of floodwalls are outside the new gates, they are the sole defense to block storm surge from the lake from pouring into such neighborhoods as Lakeshore, Lake Vista and Lake Terrace, as well as the University of New Orleans campus.

The cost of those repairs would be about $4.3 million. Geotechnical engineer John Grieshaber, technical support chief for the corps' Hurricane Protection Office, who hadn't seen the reports, said they still must undergo internal review.

If officials conclude the threats to the walls require immediate response, repairs could be ordered immediately. Otherwise, they would be completed after the reports are finalized, and before the June 2011 deadline for completing the first phase of levee improvements, he said.

Evolving standards

The walls were strengthened after Hurricane Katrina but before the new engineering standards for walls were adopted. Both are now about 16 feet high, which is expected to be high enough to meet the requirements of the new 100-year protection system.

In the past, the walls were only required to meet standards for water rising in the canal to a pre-determined "project height," which was below the top of the wall. Now, the walls are required to withstand the pressure of water rising all the way to the wall's top, even when water shoves the wall a bit out of place, forming a crack between its hard surface and the earth into which it is built.

On each wall, one section was identified that does not meet the new engineering standards and may collapse under extreme circumstances during a so-called 100-year hurricane, or a storm with a 1-in-100 chance of occurring in any given year.

--For the Orleans Avenue Canal wall, the report cites an unacceptable reading based on soil-sample results taken close to the wall; additional sampling is likely to indicate the wall does meet the standards.

--At the London Avenue Canal wall, the report recommended that a stability berm -- a layer of clay that would add weight and mass to offset the water pressure -- be added on the protected side of the wall.

The reports also concluded that the "stick-up" of parts of the Orleans wall -- the area of the wall between its top and the earthen levee into which it was built -- was as much as a foot higher than the post-Katrina 4-foot limit for I-walls. Still, that's a foot lower than an earlier interim standard, the report noted, which might be acceptable.

But where masonry boxes containing utilities, such as electrical cables, were built a few feet from the walls, short sections of the walls were uncovered for 7 feet above the earthen embankment, which would be two feet higher than the new standard.

Overlapping demands

Studies of the I-wall segments are among dozens of investigations of features of the 350 miles of hurricane levees, either as part of continuing efforts to improve protection to pre-Katrina standards or as part of the redesign and rebuilding of the system to withstand the forces of 100-year hurricanes by 2011.

The mouths of the two canals are typical of the overlapping construction demands facing corps designers, said Grieshaber, the geotechnical engineer. The designers are scrambling to make sure that remaining immediate repairs meet the pre-Katrina design standards, while also redesigning them to meet the new 100-year standards that must be in place by June 2011.

Grieshaber said he expects these sections of I-wall and others in the system eventually will be replaced with stronger inverted T-walls as part of the 100-year improvements.

An I-wall has a concrete wall sitting atop sheet piling driven deep enough to block water from moving beneath it, while an inverted T-wall also includes a concrete platform at the base of the wall and a series of deeper, square pilings installed at angles to provide better stability.

The reports recommended replacing the concrete pad on the canal side of the Orleans wall with reinforced concrete that would be directly connected to the existing I-wall, making it into a single unit that would prevent the wall from leaning and creating a crack. It also recommended relocating the masonry boxes or building a different kind of wall at the box that would be more stable. The second alternative would cost less, it said.

The reports also recommended building a concrete cap on top of the outer layer of sheet piling on the Orleans canal and atop sheet piling that is uncapped along the London canal, which would reduce wear and make them more attractive.