United States Department of Transportation
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Abstract:NTL Engineering & Geoscience was requested to conduct a statewide research project and provide information and recommendations to assist the Montana Department of Transportation (MDT) with design of metal culverts. Under current sampling, testing, and analysis methods, MDT has experienced numerous failures within the design life of metal culverts. NTL has identified some factors they believe to be a part of the discrepancy between design and in-service pipe life. This report provides details of their research project, and provides recommendations based on data and analysis. Selection criteria for buried metal culverts varies widely between state agencies and private consultants primarily since a standard methodology for identifying environments corrosive to metal culverts, assessing corrosion potential, and selecting materials that will perform in accordance with design criteria have not been clearly defined in standard engineering literature. Although fundamental to design, these tasks are often difficult to perform and are subject to debate and variation within the design community. Corrosivity is dependent upon many variables and the interaction of soil with metal is likewise complex and multi-variant. Their research has focused on observed, in-place culvert conditions and selective testing of soil samples from these locations. Resistivity and pH were selected as the main indicators of culvert corrosion for this research. Although resistivity is a standard measure of soil corrosivity, numerous methods of preparing soils and measuring soil resistivity are accepted in practice with data then applied in a fairly generic manner to a relatively "standard" scale for assessing soil corrosion potential. NTL has performed corrosivity analysis based on the current MDT procedures for resistivity testing and pH along with other published test methods to evaluate the current MDT testing and analysis methods for selected sites in the various Districts. Their research has attempted to begin the framework for a more consistent and applicable methodology for corrosion assessment for the MDT. They recommend, in this regard, that the AASHTO T288 Minimum Soil Resistivity test method be adopted as the basis for resistivity determination. It is further recommended that resistivity testing address the soil conditions expected to exist for backfill, foundation, and the adjacent drainageway as part of a culvert site investigation.
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