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New Cost and Time Efficient Approach Produces Better Results

 

MER testing after 2011 Missouri River floodsDeveloping the Approach

Natural disasters have impacted our aging levee systems nationwide, and the failures have been catastrophic. After Hurricane Katrina hit the Gulf Coast in 2005, the U.S. Army Corps of Engineers (USACE) Omaha District contacted Terracon to team with them to develop and perform an advanced geotechnical exploration and testing program for the USACE New Orleans District to evaluate subsurface conditions as part of the New Orleans levee reconstruction program. For an initial technical demonstration project, Terracon and the Omaha District evaluated several geophysical technologies in an effort to identify potentially critical subsurface soil anomalies that might have otherwise not been detected using traditional geotechnical investigation methodologies. Geophysical technologies that were evaluated during the demonstration project included ground-penetrating radar, electro-magnetic profiling and multi-electrode resistivity (MER) profiling. Only the MER method proved to be effective in the local heavy clays.

Another objective of the New Orleans demonstration project was to compare the effectiveness of cone penetrometer testing (CPT) and field vane shear testing (VST) in-situ methods to conventional drilling, sampling, and lab testing. It was shown that the in-situ methods provided reliable soil characterization information much more quickly and cost-effectively than could be achieved with conventional borings alone. The demonstration project results allowed USACE geotechnical engineers to greatly reduce the distance between test locations (from 2,000 to 200 feet) for a comparable cost and schedule, while also reducing the risk of missing critical subsurface anomalies, such as buried stream channels.

“In the old days, we used to drill and sample every 1,000 feet,” said John Bertino, USACE Omaha District Engineering Division chief. “And you based all your analysis and design on that one point of information every 1,000 feet. Now, we can get an actual picture of what the foundation looks like with the MER and CPTs. You can really make some educated decisions based on that data.”

The successful results of the New Orleans demonstration project became the catalyst for developing a new phased subsurface investigation approach that is faster, more thorough, and less expensive than previous methods. By performing the geophysical screening first, the scope of in-situ CPT testing can be focused more in the areas of known anomalous soil conditions and spread out over larger distances in areas where the MER data indicate uniform subsurface conditions. Relatively expensive and time-consuming conventional drilling, sampling, and vane shear testing can then be done on a reduced and targeted basis, to correlate the in-situ test data with soil classification and strength testing lab data from conventionally obtained soil samples.

“We’re able to map things a lot more closely and accurately with these methods,” said Dave Ray, USACE Omaha District Geotechnical Branch chief. “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.”

Approach Application

The 2011 Missouri River floods resulted in extensive damage to federal levees in Missouri, Iowa, Nebraska, North Dakota, South Dakota, and Montana. The floods created major challenges to quickly repair damage before the spring 2012 flooding season. The December 2011 Disaster Relief Appropriations Act appropriated almost $300 million for repair of the damaged Missouri River federal levees. Faced with the daunting assignment of implementing the huge reconstruction program, the Omaha District established the “Systems Restoration Team” (SRT) to focus and use the skills and resources of the entire USACE to execute the work.

The Omaha District informed Terracon in early January of their plan to apply the geotechnical investigation methodology the USACE and Terracon developed in New Orleans. The goal was rapid subsurface characterization of critical repair areas in northern Missouri and southern Iowa, some of which had already been subject to emergency construction repairs completed without the geotechnical investigations that are normally performed prior to design and construction.

Terracon was tasked with the geotechnical investigation and data analysis for four levees and lab testing for 11 levees. The first phase of work consisted of MER screening of each levee. MER data was typically posted to an FTP website within 36 hours of data acquisition. Omaha District geotechnical engineers analyzed MER data and identified and assigned CPT test locations. Terracon performed CPT testing, and these results were superimposed on the MER profiles to allow the USACE personnel to check that the MER and CPT results correlated, and to help identify potential soil boring locations, which involved using split spoons (SPT) for sample collection.

The iterative geotechnical investigation approach yielded significant project benefits. The Omaha District was able to save substantial time and money on the levee reconstruction projects. According to Ray, “Twenty years ago, the geotechnical investigation process would be a lot more methodical – you might concentrate on only areas that were damaged. Today, we’re looking at levees as a holistic system, rather than just concentrating on the most obvious problems. Using this new geotechnical investigation approach, we saw damage that likely would have been missed and are now able to investigate a larger area more efficiently.”

The schedule for the geotechnical investigation project was very aggressive, but Terracon’s nationwide network of offices and deep reservoir of equipment and trained personnel made it possible to meet the schedule. “We were able to quickly and efficiently come together from several Terracon offices to overcome the logistical challenges and meet the aggressive schedule,” said Project Manager Steve Bunting, who also manages Terracon’s Construction Materials Engineering Testing Department in Omaha. “We were able to work cooperatively to apply new processes, methodologies, and techniques to solve project challenges.”

Personnel and equipment were drawn from six offices. Terracon ran two MER crews, two cone penetrometer rig crews, and two drill crews. A total of 15,000 linear feet of MER survey, 348 CPT soundings totaling 12,725 linear feet, and 50 soil borings totaling 2,500 linear feet were completed during the field effort. Terracon also completed 2,400 laboratory tests at our USACE-validated labs. The lab testing was primarily completed at Terracon’s Omaha office, with our Cincinnati and Kansas City (Lenexa, Kan.) laboratories providing support.

During the geotechnical investigation, several USACE construction projects competed for work space on the levees where Terracon was to perform field work. Bunting and his team worked closely and attended meetings twice a week with the Omaha District SRT and its construction contractors, to coordinate our investigation activities, reschedule our crews to avoid impacting construction operations, and expedite the completion of our investigations whenever requested.

Moving Forward

The phased subsurface investigation methodology has now been refined to the point that it’s being used widely by the Omaha District and its contractors for multiple Missouri River levee system projects and disseminated throughout the USACE. Also, not only did the Omaha District SRT and USACE meet their most important objective, which was to be in a position to award construction projects to repair the damaged Missouri River levees this fiscal year, but they have developed and confirmed the viability of a geotechnical investigation methodology that will save time and money on future projects.

 


MER and CPT

Multi-electrode resistivity (MER) is a geophysical investigation method that allows for the imaging of subsurface soil and rock materials based on differences in electrical resistivity measured by an array of electrodes positioned at uniform intervals along a line.

The cone penetration test (CPT) is an in-situ method in which an instrumented cone is pushed into the ground using a hydraulic ram at a slow constant rate. It is used to determine the geotechnical engineering properties of soils and to understand soil stratigraphy.