County Road 29 Landslide Repair, Larimer County, Colorado

A known landslide along Larimer County Road 29 in Larimer County, Colorado, failed in the spring of 2021, forcing an immediate single lane road closure and resulting in a hazard to the traveling public. The landslide measured approximately 100 ft in length and 30 ft in depth and coincided with unusually high and prolonged flows in an irrigation ditch situated above the roadway. Larimer County was able to secure a design and construction team within days of the failure and utilize emergency funding to reopen the roadway in a rapid two-month design and construction schedule. The selected team of Lithos Engineering (Designer) and Harrison Western Construction (Contractor) had previous similar experience, which allowed for an abbreviated design approach prior to mobilization and a progressive design-build to reopen the roadway. The stabilization solution ultimately included a soil nail wall, micropile, shear pins, and an MSE wall to re-establish the roadway. The three-phased approach was more economical and easier to construct in comparison to other considered alternatives, including large-diameter drilled shafts or a regraded site. This case history intends to compare the selected design concept to independently developed preliminary design concepts from 2014, discuss the design origin and methodology (including back calculation) used to determine geotechnical parameters at the site, the selected design solution, construction schedule and laydown difficulties, and celebrate the ability to deliver a unique design and construction project rapidly given the critical nature of the roadway and failed slope.