Mobridge Geotechnical Study location image

Project Overview

U.S. 191 Landslide Study at RP 84 to 89 (in Charles M. Russell Wildlife Refuge)

The Montana Department of Transportation (MDT) is conducting a study to identify options to potentially stabilize a section of roadway damaged from landslides. The study area is located 60 miles northeast of Lewistown in Fergus County along the approaches of both sides of US-191 within the western end of the Charles M. Russell Wildlife Refuge.

This two-lane section of roadway is part of the National Highway System used as a major freight route, which has experienced frequent and significant pavement degradation, subsidence and bulging of the pavement, slope failures, loss of highway shoulders, and mudslides that impact traffic safety and mobility.

Factors influencing the landslides include low-strength clay/shale, concentrations of surface water and groundwater, steep slopes and highway cuts and fills

The purpose of the project is to identify potential options to improve stability and performance of the highway. Proposed work during the initial study phase includes ground surveying, aerial photography, exploration drilling, and a variety of geotechnical and engineering evaluations. Study activities are not expected to impact highway traffic.

A summary report for this first stage of studies is expected to be completed by December 2018. The initial study may be followed by supplemental work as needed to adequately determine landslide characteristics and perform feasibility evaluations of mitigation options.

Mobridge Landslide RP 84.3 to 84.8

The landslide of greatest concern is the Mobridge Slide which impacts approximately 2,000 feet of highway (see photograph). The slide pre-dates the original highway construction and experiences periods of activity and inactivity. As the highway embankment shifts downhill, the pavement distresses and cracks, creating an irregular and distressed driving surface that requires periodic maintenance.

Other Landslides

Additional landslides include locations at RP 85.5 north of the Mobridge Slide and RP 88.9 north of the Missouri River. The RP 85.5 Slide is a slump that creates a large cavity in the west embankment slope, which could eventually enlarge and impact the highway pavement. The RP 88.9 Slide is a translational slide in the east hillside slope that is pushing against the highway, causing bulging and heaving of the pavement and ditch. The highway section also includes slides that have been previously mitigated and are being monitored in the event of reactivation.