Beneath the layers of dirt

Published July 1, 2012

In a grassy field adjacent to an earthen levee, a group of three young men uncoil a long yellow wire with metal clasps every few feet. At each of these clasps, they secure a roughly foot-long metal probe that is pushed by hand into the soil.

The wires wind their way into a black box attached to large batteries. With each blip, whir and whine from that unassuming structure, these men are saving the federal government and the American public hundreds of thousands of dollars, potentially millions.

They are deploying a multiple electrode resistivity unit – MER, for short – to create a map of changes in the soil beneath them. Each probe either sends or receives electrical impulses across the alignment. The computer within the box records the resistance, or relative ease of transmission of the electricity.

"This is the quintessential widescreen assessment tool," says Matt Glover, a geophysicist for the Omaha District of the U.S. Army Corps of Engineers. "It can pinpoint locations that we normally would have the potential to miss, and it can maximize the efficiency of our drilling process."

A team can cover between 1,500 and 3,000 feet a day, providing a wealth of information to engineers who design setback levee alignments, including important indicators of how a levee may perform over time. If the foundation has large areas of sand, that may increase the chance of seepage when the levee is loaded with water. If there’s too much clay beneath a foundation, the levee may settle, lowering its height.

It’s the first step in a process developed for the U.S. Army Corps of Engineers by engineers from the Omaha District and contractors from Terracon, a national geotechnical, environmental, CMET and facilities consulting firm headquartered in Olathe, Kan.

"For the purposes of defining ‘where is the problem, what is the problem,’ this process is more effective and efficient to determine what the critical situation is," says Joe Waxse, a senior Terracon consultant with more than 32 years in the geophysical sciences field.

Waxse would use his experience, forged in far-flung locations like Saudi Arabia, Africa and Indonesia, to help formulate the basic process with Don Moses, a civil engineer in Omaha District’s geotechnical engineering and sciences branch.

"The technologies are actually pretty old, it’s just recent advancements in digital data processing and how we applied them to make sure we get good information that was new," Moses says.

Dave Ray, the branch’s chief, would champion the fledgling process in its early days in levee construction post Hurricane Katrina in Louisiana. He would lead the push to refine and modify the process into its current form used to address the daunting repair needs for levees in the Missouri River Basin following the Flood of 2011.

"Twenty years ago, the geotechnical investigation process would be a lot more methodical – you might concentrate on only areas that were damaged," Ray says. "Whereas today, we’re looking at levees as a holistic system, rather than just concentrating on the most obvious problems. By looking at them holistically, we saw damage that likely would have been missed and are able to investigate larger areas more efficiently."

With the MER data in hand, engineers superimpose data on the maps of known performance issues on the levees recorded by flood fighters using an Android-based mobile information collection application on cell phones. Between those two data sets, cone penetrometer testing – CPT – pushes are scheduled to verify the MER data and further refine data near areas of concern. That redundancy is a needed feature as MER is not an engineer’s silver bullet.

"We know and learned that there are real limitations to the MER data when taken alone," Ray says, noting that the water table or buried materials like metal or plastic can interact with the test. "It takes a team effort by the geophysicist and the geologist and levee engineer to take a look at the data and interpret it."

Following the CPTs, drill borings collect physical samples at specific sites and depths instead of drilling at general intervals with continuous sampling – an expensive and time consuming process.

"In the old days, what we used to do was every 1,000 feet go out and drill a boring sample. And you based all your analysis and design on that one point of information every 1,000 feet," says John Bertino, the chief of engineering division for Omaha District.

While the intervals for drilling – be it 500 feet or 2,000 – would be set by a mixture of ground observations and U.S. Geological Service soil maps, the fact remained that the exact composition of the earth beneath a levee or along a proposed alignment between two drill points were nagging question marks for engineers.

"Now, we can get an actual picture of what the foundation looks like with the MER and CPTs," Bertino said. "You can really make some educated decisions based on that data. This process reduces our risk – the risk that we might miss some of the variability in the foundation."

One example stems from the work done post Katrina. A MER scan clearly showed something in a levee that required additional investigation. Subsequent borings revealed buried trees and other organic material in the levee foundation.

For Ray, part of the value of the process is obvious. "We’re able to map things a lot more closely and accurately with these methods," he says, giving an example where a levee might need only a few relief wells or a trench drain as opposed to an entire relief well field. "Instead of just assuming the worst scenario and the highest dollar fix for everything, we’re able to hone in on the type of fix needed at a particular point that might be a little more economic and easier to build. You’ve saved the government and taxpayers a considerable amount of money because you’re not overdesigning a repair."

The levee repairs in the Missouri River Basin are a prime example of this. Many levee systems are thirty or more miles in length. Applying blanket solutions across an area would be cost prohibitive and likely wasteful.

But it works both ways. Occasionally, damage or problematic areas are uncovered by the process. Left unchecked and not repaired, those features could potentially lead to significant issues or failure in a future flood fight.

"We did adjust our preliminary designs based upon the data from our geotechnical investigations," Moses says. "Where we thought we needed to widen one levee to deal with under seepage issues, the MER and CPT data showed us that repair wasn’t needed across the entire segment. It also showed us that some additional work beyond our planned repair was needed in certain areas."

For Waxse, the original work done in Louisiana compounded with efforts in the Missouri River Basin confirms the value of a process that is still gaining acceptance in the engineering world:

"It all boils down to the comfort level that you have a good picture of what’s there, and your engineering judgment is therefore better, allowing you to reduce both costs and risks."


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