On the Relative Purity of Dirt

There’s a great deal of talk about debris found in levees along Lake Pontchartrain.

The skinny is this: Levees are made of dirt.  Prior to Katrina, the Army Corps of Engineers made some levees larger by adding additional dirt.  Back then, it is said, contractors were told to use what they call the “visual inspection” method.  This is the technical term for “eyeballing”.  Contractors were required to look at the dirt they were using to build levees and make judgment calls about how much debris was there.  If they saw debris, they were supposed to remove it.  The reason for this is that levees require a type of clay-based soil that falls within certain requirements in terms of liquid content, plasticity, and consistency.  Too much debris can break up the tensile strength of the soil.

The problem is, dump trucks carry tons of dirt to a site when building a levee.  Just eyeballing, one can’t possibly see all the chunks of concrete and bricks (or as one informant put it, “kitchen sinks and everything”) inside the dirt.  Post-Katrina requirements on levee testing revealed that debris content exceeded restrictions in these locations.  Now, all dirt must be formally tested for debris content; the Corps says it is only using trusted borrow sites (a borrow pit is where they get their dirt); and the compromised portions of levee are being replaced.