The MnDOT Drainage Manual refers to the California Method for estimating service life for steel pipe. The two key parameters required for use of the California Method are pH and resistivity. The current method for estimating steel pipe life is for the designer to take field samples for these two parameters. Although this method is desirable, time and expense restrictions often make sampling impractical.
The goal of this project was to develop a series of steel pipe service-life maps for the state of Minnesota. The California Method 643 is utilized to estimate steel pipe service life at locations throughout the state. Over 560 soil resistivity and pH samples were collected statewide during summer 2014 along embankments of state-trunk and county highways. Concurrent observations of soil texture, surrounding landscape, roadway type, and water presence were also made; water pH and conductivity measures were made where applicable. Data verification efforts to build confidence in field-measured soil pH and soil resistivity included comparing data to other available datasets including geology, soil pH, electrical conductivity, and soil texture, as well as observations available from district and county engineers. Field-measured soil pH data, with some exceptions in Districts 2 and 6, generally aligned with the available STATSGO soil pH data, indicating that this layer could reasonably be used in service-life calculations as it has greater resolution than provided by field data. In the absence of a statewide soil resistivity or electrical conductivity map, field-classified soil textures and the statewide STATSGO soil texture map were used to estimate soil resistivity values. Calculations of service life using the above data were completed for 18-, 16-, 14-, 12-, 10- and 8-gage galvanized and aluminized steel pipe across Minnesota. These maps were then compiled into a zone map and table that present the 90th percentile service- life estimate for various gages and types of steel pipe. Caveats and limitations to this analysis are also discussed.