A difference in soil boring data transferred from one chart to another may have played a key role in engineering decisions that led to the breach on the 17th Street Canal floodwall that flooded much of New Orleans during Hurricane Katrina, National Science Foundation investigators say.
A cross-section drawing in the project design documents shows a weak layer of peaty soils running between 11 feet and 16 feet below sea level in the area that failed during the storm. But information in the individual soil borings that were used to draw the cross section show the peaty layer extending as deep as 30 feet below sea level.
Investigators said their own borings taken at the site this week confirm the 30-foot depth, leading them to believe that designers used the flawed cross section drawing to set the sheet pilings beneath the floodwalls at 17.5 feet below sea level - a choice that allowed water to migrate to the land side of the wall, causing the breach.
"It's pretty obvious the depth of the organic deposit shown on that cross section is what determined the sheet piling depth ... for the whole canal," said J. David Rogers, a member of the National Science Foundation team, and a leading authority on levee and dam failures. "They saw this marshy deposition, they recognized it as potentially dangerous, so they specified sheet piles that went just beyond the bottom of that line.