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SHEET PILES TOO SHORT FOR SAFETY, EXPERTS SAY |
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by Bob Marshall
October 28, 2005 Deputy director of the LSU Hurricane Center, has been investigating the failures since shortly after Hurricane Katrina tore through the region Aug. 29, but said it has been denied access to official corps documents. After reviewing 17th Street Canal levee construction records the newspaper obtained, including soil profiles and engineering drawings, the LSU team listed mistakes it said were obvious in the design and construction of the floodwall: The levees and the floodwalls are built on highly organic soils one would expect to find in the wetlands that once existed here: compressed plant material, layers of wooded swamp debris including root systems, treetrunks and branches and organic clays. Each of those ingredients is subject to decomposition, a process that leaves soils “as porous as a sponge cake” and with a high content of water, the team said. Like a sponge cake Engineers say moist, porous soils generally are weaker than dry soils and provide less support for structures. The LSU team said such soils failed to provide the wall with support beneath it and from the side, turning the structure into the equivalent of a stick floating in mud. The jumble of different types of weak, porous soils that were layered at the levee location should have raised more alarms among engineers because it meant water could travel through the material, setting up conditions allowing it to move easily. The effect was like icing between layers of a sponge cake; it would take even less water pressure from the side for layers to move, threatening any vertical structures — like a floodwall — embedded within it. The sheet piling used to support the floodwall’s concrete top was driven to a depth of 17 feet, while the canal bottom was 18.5 feet deep. That meant water from the canal could easily migrate through the porous soils beneath the sheet pilings, further eroding the passive support system engineers counted on. The levee walls were built atop “spoil banks” that had been dredged from the canals when they were dug in the late 1800s and early 1900s, and when they were later deepened. That meant some of the support for the piles and the concrete cap were made up of a mix of the same weak soil found below them. Attaching a concrete top to the sheet piles in weak soils further reduced the margin of safety because it created a joint requiring even more support. Using only sheet pilings would have been safer, the team said. The team emphasized that while driving a single layer of sheet pile 50 or 60 feet deep to harder, less porous sands would have provided more support, the single-wall design still might not provide the factor of safety needed to withstand a Category 4 or stronger hurricane. Solution under pressure Arthur Theis, a consulting engineer with the team who once worked on similar drainage canals for the state, said the safest design would be a larger, wider levee made of more suitable soils. “But that wasn’t an option because the area had already been developed with homes and businesses adjacent to the canals,” Theis said. “They tried to squeeze a solution into a narrow space. The safety factor is much lower with these types of walls, especially in these soils. ”The investigation into why the levee walls failed is underway even as the corps moves to temporarily repair the canal walls and other levees through-out the area, and prepares to restore them to pre-Katrina strength. The corps has awarded a dozen contracts totaling $89,231,062 for levee reconstruction. Included is a $6.2 million contract to Boh Brothers Construction Co. of New Orleans to install a new sheet pile wall on the east side of the 17th Street Canal near the Old Hammond Highway Bridge, where the wall failed. Corps officials have said the new sheet piling will be sunk to a depth of 51 feet. Along the London Avenue canal, which also suffered a breach during the storm, the piling will be sunk to 65 feet. Those contracts have not yet been awarded. A $4.6 million contract also has been awarded to M.R. Pittman of Harahan for work along the west side of the Industrial Canal, stretching from south of the France Road ramp to 700 feet north of Benefit Street, where part of the levee wall failed after being topped by Katrina storm surge. That job includes constructing I-shaped walls similar to those that failed. Bob Marshall can be reached at (504) 826-3539 or rmarshall@timespicayune.com. Mark Schleifstein can be reached at mschleifstein@timespicayune.com or (504) 826-3327. |
