Document Type

Conference Proceeding

Publication Date



The addition of passing lanes has been a long standing need on Idaho Route 55. This narrow, winding road is one of the few routes that links Boise, Idaho, with destinations to the north. The roadway is bounded by a steep rising slope on the east, and a precipitous drop to the Payette River on the west. Additionally, the area is known for landslide hazards. Given these hazards and uncertainties in the subsurface geology, Idaho Transportation Department (ITD) commissioned an engineering geophysical survey to aid in the design of the passing lanes just south of Banks, Idaho.

The engineering design required that several issues be addressed. These included the following:

  1. Slope Stability
  2. Amount of blasting required.
  3. Need for retaining walls.
  4. Volumes of soil and rock to be excavated

To help design engineers address these questions, P-wave refraction mapping was used to produce cross-sections perpendicular to the roadway. A novel approach was needed, since traditional in-line shooting would have required offsets greater than were available. The solution was to perform a 3-D, ground consistent, delay-time inversion. Soil and rock properties, and variations in the thickness of overburden were determined. These results were found to be in agreement with track-hoe trenching done at selected locations along the roadway.

The geophysical work revealed that the site differed significantly from assumptions used in the initial design stages of the project. The potential for landslides was found to be significant. Given the updated geologic picture and projected costs, ITD decided to abandon this site for the passing lanes. The project was terminated, thus avoiding excessive costs that would have been experienced due to the differing site conditions.

Copyright Statement

This is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online at Geotechnical Special Publication No. 113, published by the American Society of Civil Engineers. Copyright restrictions may apply. doi: 10.4133/1.2922604