Resistivity Imaging At Mn/Dot: ‘Building Bridges’ In Duluth

Electrical resistivity imaging (ERI) has become an effective supplement to the subsurface investigation process at the Minnesota Department of Transportation (Mn/DOT). Mn/DOT has employed ERI on a variety of projects related to scoping, preliminary design and post-construction failures. ERI profiles were the only source of subsurface information utilized in the foundation design of Bridge #69656 (Kohl’s Bridge) over Miller Creek in Duluth, MN. Mn/DOT’s conventional methods of acquiring soil and rock data such as standard penetration testing, cone penetration testing, and rock coring could not be performed due to the sensitive nature of the project site. Consequently, Mn/DOT bridge, foundation and district engineers elected to incorporate deep foundations into the design of the bridge with no design option for shallow foundations. ERI was performed and revealed shallow bedrock depths (<10 feet) on the east side of Miller Creek suggesting that spread footings on bedrock could be utilized for at least a portion of the project area. Footing excavations prior to construction ultimately revealed that shallow foundations on bedrock could be incorporated for all project structures (roadway retaining walls and bridge) resulting in construction of the proper foundation design for the site conditions and modest savings in terms of cost. The Kohl’s Bridge Project marked the first time a Mn/DOT bridge design had ever been based solely on geophysical data. Because of these accomplishments, the Duluth District was recently awarded an American General Contractors Bridge Construction Award for the structural design and investigative approach. The effectiveness of ERI instilled a sense of confidence at Mn/DOT towards the use of geophysics and opened up opportunities for geophysical applications on subsequent projects in the Duluth District and throughout the state.