SH-3 Goosehaven Road Widening
Geotechnical services for an ITD project that was filled with creativity
SH-3 is a heavily trafficked two-lane highway and the main artery between St. Maries and I-90 in northern Idaho. This narrow section of road had no shoulders and many steep slopes down to the adjacent St. Joe River. To improve safety and increase transportation capacity, the Idaho Transportation Department (ITD) set out to widen SH-3 by 11 feet. GeoEngineers provided a variety of geotechnical services for ITD, including geotechnical investigation, lab testing, and recommendations.
The three-mile stretch of SH-3 sat on a levee that separated the river from adjacent farmland—and created extra geotechnical challenges. ITD quickly realized that a traditional approach to roadway construction wasn’t feasible for this project, in part because of the road’s location on the levee and proximity to the St. Joe River.
Approach
Widening the road from the east side would cause right of way issues and increase costs. Building on the west side (near the river) brought settlement and slope stability concerns thanks to soft soils and a high water table. The team needed a strategy that would allow them to build closer to the river without the increased settlement and stability risks caused by heavy fill material. They decided to recommend a lightweight fill alternative called EPS Geofoam.
Geofoam is a lightweight synthetic fill material that can be used instead of soil and other traditional fill. It provides structural stability in many engineering applications without adding significant weight, and the GeoEngineers team recommended using it in an MSE wall adjacent to the river. By including EPS Geofoam in the roadway design, ITD could avoid costly right of way acquisition and build closer to the river—without slope stability or settlement risks.
Results
Construction began in April 2022 and SH-3 reopened to traffic just six months later, despite wet weather and other complications during the summer.
