Ongoing repairs at the infamous site of New Orleans’ 17th Street Canal levee breach have an added wrinkle that sets the project apart from other hurricane protection repairs around the city: The site is a potential treasure trove of forensic evidence for lawsuits filed by victims of the flooding.
Scientists and attorneys still hunting for clues into what happened at the failed, I-wall-fortified levee, hover in the wings, scrutinizing excavated materials as the project proceeds. An inquest into the performance of the flood protections around the city funded by the U.S. Army Corps of Engineers has identified a design flaw that allowed the flood wall to flex, opening up a vertical crack in the top of the levee that separated the front and back halves and initiated a slip fracture in a clay layer beneath the levee—but that explanation is not stopping investigators from sniffing for further clues.
Boh Bros. Construction Co. LLC, New Orleans, has built a cofferdam and shoring system to permit excavation of tons of debris from the temporary plug in the breach site. Boh is extracting toppled panels from the failed I-walls, as well as hundreds of massive wire-rope harnessed sand bags, rip-rap and whatever other material or debris that was flushed, dropped or bulldozed into the hole to staunch the deadly flow of water.
“The only thing different here is the way we’ve got to go about getting the failed wall out of the way,” says Robert S. Boh, Boh Bros. president. “The trick is preparing to excavate to get the walls and pieces of sheet pile out. Of course, there is also a lot of scrutiny on this particular project because of ongoing lawsuits involved with the site.”
The first round of forensics examinations were conducted in Dec., 2005, when Brig. Gen. Robert Crear, Corps commander of the Mississippi Valley Division, gave the order to pull the original sheet pilings at the base of the failed walls so they could be measured to see if the contractor followed the Corps’ design specifications. Boh Bros installed the original sheet piling, which literally “measured up” to the prescribed lengths and specifications.
“People were looking for someone to blame and thought someone cut corners somewhere,” Boh says. “No one predicted those I-walls would fail. They weren’t supposed to fail.” Cleaning out the debris is necessary before the Boh can drive the steel pilings that will support the new T-wall construction, says Ernie Murry, construction representative for the Corps. “Besides that, the forensic engineers want to see the old walls, sheet-piling debris and all that stuff to examine it,” Murry says.
Boh Bros. won the $14.7-million contract to excavate the temporary plug and replace it with a 720-ft length of T-wall. Work started March 3, 2006, with an original scheduled completion date of September 2006. But the project has experienced “numerous delays” because of reconfiguration of the temporary restraining structure (TRS), says Boh project superintendent Clifford Fenerty. The TRS is the wall of sheet piling that was driven in after the breach was sealed. It juts about 30 ft into the canal.
Before excavating the temporary plug, the contractor was instructed to place a second line of sheet piling roughly 65 ft inland from the TRS, boxing in a cofferdam work area.
One of the first revisions to the TRS plan involved moving the new wall farther back than originally planned. “They had us push it back to 85 ft,” Fenerty says. “They didn’t know exactly how far back things were pushed with the surge.” The revision meant Boh could not begin excavations until early February.
The excavation is within a site about 75 ft wide and 600 ft long, an area determined by seismic testing to locate the sub-surface debris. “We can only open up a certain amount of area at a time,” says Fenerty. “We created a bracing system to ensure the cofferdam doesn’t move.”
The contractor fabricated seven frames of interlocking steel beams as rangers and whalers to hang between the cofferdams and shore them during excavation. “You put the sheeting in where the top is at elevation minus 2 [ft],” Fenerty says. “Then we dig to minus 7 [ft] to install the whaler. You hang the beams and put in roughly six inches of blocking between the beams and sheeting to create pressure.”
Crews pumped water into the cofferdam on the north end of the site, referred to as TRS1, to equalize the pressure during excavation. As the excavation is completed and backfilled with sand, the blocking is removed and the bracing lifted out. “If you didn’t have the blocking, you would have to cut it to release the rangers,” Fenerty explains. Although the excavation depth across the site will be a consistent 18 ft, an additional level of bracing will be used on the southern end to avoid having to use water to equalize pressure. “On the south end, we will have two rangers—a top and a bottom—so we can excavate and remove the failed sheeting for forensics.”
Although sand bags, cables, rip-rap and other debris are liberally mixed in the soil at the north end, an estimated 30 concrete panels 30-ft-long by 25-ft-high are expected to emerge with the south end debris. The total volume of excavated material is expected to exceed 22,000 cu yd.
T-wall replacement construction will proceed from north to south as Boh backfills the excavation. The contractor will drive steel H piles to support the new T-wall construction. The wall will have a 3-ft deep by 19-ft wide footing braced on batter piles on both sides, Fenerty says. A rebar loop on top of the water-side pile will tie with rebar in the wall.
The design is typical of T-wall construction going in elsewhere in the hurricane protection system, including about 4,000 ft of new T-wall replacing I-walls that failed on the east side of the Inner Harbor Navigation Canal, says Murray, who was construction representative on that project as well.
The tie-in of the beefy T-walls to the remaining miles of relatively flimsy I-walls on the 17th street canal makes an incongruous match, but Murry explains that the Corps eventually plans to replace all of the walls with the T-wall design. “Since we have to repair this I-wall, we will go ahead and do it as a T-wall,” he says.
The Corps has no schedule for converting the remaining walls, but officials say they will not replace any failed areas with a design that has proven to be substandard. “We are not going to make a mistake again,” says Crear. “We’ve got to construct it based on the best science and technology…You in fact are going to build it stronger and better. Later on, if you decide you are going to invest in those walls, then you’ve got a section already done and can build to that.”