Distribution of Chloride, PH, Resistivity, and Sulfate Levels in Backfill for Mechanically-Stabilized Earth Walls and Implications for Corrosion Testing
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Distribution of Chloride, PH, Resistivity, and Sulfate Levels in Backfill for Mechanically-Stabilized Earth Walls and Implications for Corrosion Testing

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      Final project report.
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      The ultimate goals of this research were to improve quality, speed completion, and reduce risk in mechanically-stabilized earth (MSE) wall projects. Research objectives were to assure (1) that variability in the corrosion properties of soil (pH, minimum resistivity, chloride, and sulfate levels) due to sampling and analytical technique was much lower than variability in these levels within and between soil sources and types and thus did not inflate the risk of emplacing a corrosive soil as MSE wall backfill, (2) that the number of soil type samples analyzed prior to acceptance of a backfill was appropriate, based on the expected distribution of corrosion properties within the backfill, and (3) that the corrosion properties of backfill material did not change appreciably over time, especially after emplacement and over the design lifetime of an MSE wall. Corrosion properties of soil were tested with Florida Methods (FMs) 5-550, 5-551, 5-552, and 5-553 for pH, minimum resistivity, water-soluble chloride, and water-soluble sulfate, respectively. Changes to the quality assurance (QA) plan for acceptance quality characteristics (AQC) of backfill corrosion properties were recommended based on research outcomes. These changes had as their focus an improvement in the buyer’s statistical power to accept good backfill material through reductions in test error associated with material properties, sample processing, and laboratory procedures. Recommended changes included (1) proposed revisions to the FMs for pH, minimum resistivity, chloride, and sulfate, (2) an increase in the number of independent samples tested for pH and minimum resistivity, (3) a revised acceptance limit for minimum resistivity, (4) method operator training and independent audits, (5) an inter-laboratory study post-implementation of the revised methods, and (6) re-evaluation of the need for chloride and sulfate testing for backfill above a pre-determined minimum resistivity. Geochemical modeling with the U. S. Geological Survey (USGS) model PHREEQC for a conservative case of a low ionic strength and poorly buffered sandy backfill revealed that in a few years’ time the pore water of emplaced backfill could equilibrate with infiltrating rainfall. The model results suggested that a soil’s buffering capacity might be important consideration when metal is used as reinforcement in MSE wall backfill. Field and laboratory testing of candidate sands to calibrate the model were recommended ahead of any proposed changes to the QA plan based on model results.
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