SRF led a team of 10 subconsultants for the $100 million reconstruction project of the roadway approaches to I-90, which spans the Mississippi River. The team for this highly complex project provided final plans in an aggressive schedule. As part of the final drainage design, SRF designed nearly 25,000 lineal feet of storm sewer and culverts which presented a number of unique challenges including: Steep topography, narrow roadway and active railway corridors, and many retaining walls.
The drainage design included:
- Existing culvert rehabilitation: The resulting design included both new and lined pipes with up to 50 feet of cover, and pipe jacking of up to 54-inch diameter pipes to cross the existing railroad corridor.
- Bridge deck drainage analysis: The design ensured the Mississippi River Bridge met Minnesota and Wisconsin standards while also avoiding critical structural elements, which required multiple inlet sizes.
- Energy dissipation measures: Where it was not feasible to replace steep existing pipes, which accommodated up to 45 feet of elevation change, pipe lining techniques were proposed. SRF then designed special hydraulic drop structures and riprap measures to mitigate the increased flow velocities due to the lining.
- Steel casing for pipes near retaining walls: Over 2,000 feet of steel casing, up to 18 inches in diameter, were required. SRF worked with the concrete pipe industry on the special design of cased concrete pipe in locations up to 50 feet deep.
- Two permanent wet detention ponds: These ponds provide water quality treatment and greatly reduce peak discharges to the Mississippi River.
- Permitting: The Minnesota NPDES permit and the Wisconsin Trans 401 Permit requirements were met. Also met were FAA requirements within the runway protection zone.
These many unique design elements work together in a drainage system that meets both states’ trunk highway criteria, provides water quality treatment, and that fit within the project construction stage.