County updates Rapidan Dam removal and Casa 9 bridge design; environmental clearances under way

Ryan Tillges, Blue Earth County engineer and public works director, gave an extended update on the Casa 9 bridge replacement and the Rapidan Dam removal during the Nov. 4 meeting, describing engineering, environmental and schedule work that remains before construction.

Tillges said bridge design work has advanced into structural modeling and that the design team is leaning toward using 8‑foot‑diameter drilled shaft foundations to achieve required bearing and economy. The project team is coordinating hydraulics reviews and constructability with MnDOT and consultants and is also coordinating the bridge work with the separate Rapidan Dam removal so the projects complement each other in the river channel.

On materials, Tillges said MnDOT and the Federal Highway Administration have recommended using chromium‑alloy reinforcing bars for the bridge deck to reduce future corrosion and extend service life. "I think the recommendation is gonna lean towards, like, the including those chromium reinforced rebars to extend that bridge life," Tillges said; he noted the county was told the up‑front cost could be substantial (the transcript contained an engineer estimate in the meeting of roughly a half‑million dollars, which county staff said will be weighed against longer‑term lifecycle benefits).

The update also described environmental and regulatory steps: MnDOT and the FHWA have asked the U.S. Fish and Wildlife Service to concur that the emergency response project is ‘‘may affect, but is not likely to adversely affect’’ the northern long‑eared bat; FERC has asked the Minnesota State Historic Preservation Office (SHPO) to review Section 106 materials for the Rapidan Dam removal, and county‑hired historic resources reporting found the dam not eligible for listing on the National Register of Historic Places. Consultants are performing hazardous‑materials assessments within the dam structure and Barr Engineering is analyzing slope stability and safe access for demolition work.

Tillges said TKDA and subconsultants are preparing 30% design drawings, structural models and hydraulic analyses; he said the project team expects to continue close coordination with Barr Engineering on dam removal-related channel grading so the two efforts are compatible. The county also reported that early coordination suggested drilled shaft diameters of 6 to 8 feet, with an economy case favoring 8‑foot units for bearing, and that the state bridge engineer agreed with the deep drilled shaft pier foundation approach.

Ending: Engineers and staff said additional design, environmental concurrence and hazardous‑materials assessments are underway; the county expects to hold an open house on the bridge plan after the new year and will return with more detailed cost and schedule updates as design and regulatory review continue.