After a series of significant rainfall events in early 2019, a landslide occurred on the existing hillside slope between CSAH 14 and CSAH 21. The hillside slope bound between the two roadways is approximately 80 feet in height at a location where an existing 18-inch diameter culvert discharged stormwater runoff from above the CSAH 14 roadway down the slope.
SRF performed a hydrologic and hydraulic analysis of the larger drainage area and the existing roadway storm sewer system, and prepared a hydraulic engineering report that informed the alternatives analysis and supported the permitting process. Gale-Tec completed the subsurface exploration, testing, and geotechnical evaluation associated with the slope failure. The analysis identified four viable alternative solutions to stabilize the slope and improve drainage conditions. A two-tier RSS wall, along with implementation of storm sewer, was the preferred alternative.
A temporary stabilization alternative and construction plan set were developed in the fall of 2019 to ensure the roadway could safely remain open during the winter. The temporary condition was implemented and allowed the road to maintain open during final design.
On July 26th, 2020, a day before bid opening, heavy rains caused additional washouts and expanded the project limits. The SRF Team jumped into action, conducted a site visit, and set forth a plan for updating plans and the Project Memorandum for State Aid review. Within 5 weeks, the plans were updated, the contractor ordered the additional materials, and the project obtained all required permits. The additional slide increased the project size and materials by approximately 100 percent.
The project was funded through the FEMA disaster funds and required the NEPA process to be completed in addition to the USACE, MnDNR, and NPDES permitting.