CHARACTERIZING THE ENGINEERING GEOLOGY FOR 990 MILES OF PROPOSED ELECTRICAL TRANSMISSION LINE ALTERNATIVES, SOUTHWESTERN MONTANA AND EASTERN IDAHO

The Mountain States Transmission Intertie project is planned for the construction of a 500kv transmission line from west-central Montana to south-central Idaho in order to provide power grid relief in this part of the country. Although straight-line distance between the connecting substations is approximately 280 miles, nearly 990 miles of routing comprising 31 segment alternatives were evaluated to fulfill environmental planning requirements.

Engineering geology issues included in the characterization included surficial geology, mass movement potential, slope conditions, active faults, seismic zonation and liquifaction potential. To individually characterize these issues, each segment was mapped at “best-scales” using a GIS to integrate vector, raster and digital elevation information across the two state area. The GIS enabled the numeric quantification of the geological issues thereby allowing the environmental planners to compare and assess geologic and environmental impacts along the route segments.