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Assessment of In-situ Corrosion Conditions at Nevada Mechanically Stabilized Earth Wall Sites: Using Electrochemical Soil Characteristics and Linear Polarization Resistance
AuthorPearson, Nelson S.
AdvisorSiddharthan, Raj V.
Civil and Environmental Engineering
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The inability of soil to provide sufficient tensile strength presents challenges for soils being used as a structural building material. However, it is possible to improve the structural performance with the inclusion of a reinforcing system. The development of these systems has been a major advancement of the civil engineering practice. Mechanically stabilized earth (MSE) wall systems typically consist of a: concrete facing panel, specified backfill, reinforcing elements, and the retained fill. The interaction of the backfill with the reinforcements, and the reinforcements with the facing panels, produces a system that when properly designed, can be a cost effective engineering solution. In Nevada there are over 150 MSE walls that have been constructed using metallic reinforcements (Thornley 2009). Corrosion of metallic elements a naturally occurring electrochemical process is irreversible an inevitable. The rate of metal loss (corrosion) is a function of the environmental conditions and metal type. For MSE walls key parameters include the backfill’s: salt content, organic content, saturation level, as well as the metal type of the reinforcements.Nevada has two previous corrosion investigations, an extensive site investigation at I-515/ Flamingo Rd. and a statistical analysis of as-built soil records along with a preliminary investigation for I-15/ Cheyenne Blvd. These studies form the foundation for this investigation of in-situ corrosion conditions. Seven MSE wall sites were investigated using electrochemical backfill characterization and linear polarization resistance (LPR) corrosion rate monitoring. Evaluation of electrochemical backfill characteristics has resulted in the discovery of six sites that fail current NDOT/ AASHTO MSE wall backfill requirements. The in-situ soil samples collected and analyzed more than doubled the available data used to describe the corrosiveness of the backfill. Linear polarization resistance corrosion rates were obtained for more than 200 different elements. These data suggest that despite the aggressive nature of the backfill, most elements are preforming well and are below the anticipated rates. However, several elements were discovered with corrosion rates in excess of five times the design model. The use of the LPR corrosion monitoring has concluded that the conditions at I-15/ and Cheyenne Blvd. are equivalent to or worse than the conditions evaluated in 2004 at the I-515/ Flamingo Rd. complex. The discoveries at Flamingo Rd. led to remediation of the largest wall at the complex. Through the use of electrochemical backfill characteristics and LPR corrosion rates, the seven sites investigated have been ranked. The rankings are dependent on several factors such as backfill electrochemical conditions and comparison of corrosion rates data with design models. This study has confirmed that observations of conditions along the exterior of the wall are not sufficient when determining the condition of the soil reinforcements. Routine corrosion monitoring is required to monitor the depletion of the soil reinforcements and should be incorporated into a Long-term Corrosion Monitoring and Asset Management Plan (LCMAMP). It is anticipated that a program will be integrated into Nevada’s current asset management systems. The development and implementation of LCMAMP, directly reflects the federal initiative for systematic detailed evaluation of critical assets, MAP-21.