Use Of Engineering Geophysics In The Design Of Highway Passing Lanes

The addition of passing lanes has been a long standing need on Idaho Route 55. This<br>narrow, winding road is one of the few routes that links Boise, Idaho, with destinations to the<br>north. The roadway is bounded by a steep rising slope on the east, and a precipitous drop to the<br>Payette River on the west. Additionally, the area is known for landslide hazards. Given these<br>hazards and uncertainties in the subsurface geology, Idaho Transportation Department (ITD)<br>commissioned an engineering geophysical survey to aid in the design of the passing lanes just<br>south of Banks, Idaho.<br>The engineering design required that several issues be addressed. These included the<br>following:<br>1. Slope Stability<br>2. Amount of blasting required.<br>3. Need for retaining walls.<br>4. Volumes of soil and rock to be excavated.<br>To help design engineers address these questions, P-wave refraction mapping was used to<br>produce cross-sections perpendicular to the roadway. A novel approach was needed, since traditional<br>in-line shooting would have required offsets greater than were available. The solution was<br>to perform a 3-D, ground consistent, delay-time inversion. Soil and rock properties, and variations<br>in the thickness of overburden were determined. These results were found to be in<br>agreement with track-hoe trenching done at selected locations along the roadway.<br>The geophysical work revealed that the site differed significantly from assumptions used<br>in the initial design stages of the project. The potential for landslides was found to be significant.<br>Given the updated geologic picture and projected costs, ITD decided to abandon this site for the<br>passing lanes. The project was terminated, thus avoiding excessive costs that would have been _<br>experienced due to the differing site conditions.