Inspection of Failed Roadway
Montana highway MT 49, known locally as Looking Glass Hill Road, is a significant traffic corridor for Glacier National Park traffic that links the Two Medicine recreation area with St. Mary Lake and the east entrance to the Going to the Sun Road. Even though it is only seasonally open, the National Park Lodge and local businesses rely heavily on travelers using the highway.
The slope stability problems within the project limits include numerous landslides and rockfalls that are detrimental to the function of the roadway and require extensive and regular maintenance. Maintenance includes reestablishment of the road's horizontal and vertical alignments through landslide sections, and removal of substantial accumulations of rockfall debris annually and as part of routine road patrols.
Based on the FHWA emergency relief evaluation performed in August 2002, eight of the slope stability problem areas between MP 4.6 to 10.0 were identified as emergency sites. Based on this finding, Landslide Technology was retained to perform the field analysis and generate landslide and rockfall mitigation measures.
The project was conducted in two phases. Phase 1 consisted of a preliminary aerial and ground reconnaissance and an initial subsurface investigation and instrumentation program to identify the types of landslide movements including typical depths and rates in 2004. Phase 2 included an expanded exploration program, instrument monitoring, geotechnical analyses, and development of mitigation design concepts. Phase 2 commenced in the summer of 2005. Overall, 29 inclinometer and piezometer borings representing over 1,800 feet of drilling were installed and monitored for the duration of the project.
Recommended landslide mitigation measures included rock buttresses, MSE walls, tieback walls, stone column shear keys, and bench cut realignments. Rockfall mitigation measures included scaling, modified rockfall fences, ditch improvements, rock bolting, and roadway realignment.