Abstract:
An approximately five-mile-long section of Many Glacier Road on the east side of Glacier National Park traverses the edge of Lake Sherburne and across a landslide complex extending from the reservoir up over 1000 feet in elevation to the ridgeline. A report summarizing the original road construction in 1927 and 1928 noted fills in multiple locations were brought up several times before any stability was realized. The landslide deposits are disturbed glacial drift mixture consisting of clay, silt, sand, gravel, and cobbles with boulders. Lake Sherburne is a reservoir used for irrigation with a surface elevation fluctuating 25 to 30 feet throughout the year, on average. In 2013 the Western Federal Lands Highway Division (WFLHD) identified six active areas of landsliding causing damage to Many Glacier Road for future project programming. In 2015, the National Park Service decided to complete landslide mitigation at milepost (MP) 6.2 as a trial to test the effectiveness of horizontal drains as mitigation for these landslides. The MP 6.2 site was interpreted to be the fastest moving landside causing the most recurring roadway damage. Two water tables were observed at the site, a lower water table that stayed relatively constant near the reservoir spillway elevation, and a near-surface water table sensitive to precipitation. Data collected at MP 6.2 indicated landsliding in the profile’s upper 15 to 40 feet averaged about 14 inches of displacement per year. Deeper seated, relatively slower landsliding was occurring at depths between 35 to 55 feet, averaging about 1.5 inches per year. It was hypothesized that providing drainage for the near-surface water table would slow down shallower, faster moving landsliding. The trial horizontal drain mitigation was constructed in the fall of 2017. Reduced movement rates and road maintenance costs have been documented with continued site monitoring.