Case History of Two Mechanically Stabilized Earth Walls on Steep Slopes

The Beartooth Highway crosses the Beartooth Mountains on the Montana-Wyoming border and is a principal access to Yellowstone National Park. On May 20, 2005, snow melt and a severe rainstorm on the 3,345 meter pass triggered large debris flows that damaged the roadway in 13 locations. As part of the emergency repair, two mechanically stabilized earth (MSE) retaining walls up to 7.9 m tall and more than 46 m long were constructed to restore the roadway. The foundation below one wall was prepared by replacing 4 vertical meters of highly fractured rock in the center of the debris flow chute with reinforced concrete and by installing rock dowels to reinforce adversely dipping rock joints. To span highly weathered rock and a fault within another debris flow chute, the central 15 m of the second MSE wall was founded on a 1 m thick, micropile-supported, tied-back, reinforced concrete slab. Soil nail reinforcement with a temporary wire mesh facing was used to retain the excavation backslope during foundation slab and MSE wall construction. This paper discusses MSE wall design and construction, including, access and equipment limitations, foundation preparation, temporary soil nail excavation support system, rock foundation stability, micropile design and construction, and MSE wall reinforcement.