After years of landslide activity along a portion of Highway 73 in the Missouri River Badlands, a repair was implemented but unsuccessful. The North Dakota Department of Transportation (NDDOT) turned to Barr and another consultant to permanently stabilize the roadway embankment. The soils at the site include high-plasticity clays with limited long-term shear strength and coal seams carrying groundwater that can contribute to slope failures.
We conducted a geotechnical field investigation, installing piezometers and inclinometers that could be read remotely to allow more frequent tracking of groundwater and slope movement and provide a better understanding of the problem. One inclinometer, developed by Measurand, Inc., was the first of its kind and capable of measuring larger displacements like those observed at the site.
The instrumentation data, combined with geologic field mapping and an extensive laboratory testing program, enabled us to better characterize the soils’ shear strength and determine the nature and extent of the landslide, including the significant impact that drainage conditions and water pressures had on movement. Instrumentation and site reconnaissance showed that the landslide was much larger than previously identified.
Testing and investigation results were used in seepage and slope-stability analyses to develop and assess stabilization options, which were presented to NDDOT. The option selected for final design involves installation of large-diameter concrete drilled shafts to pin the sliding mass on one side of the road and a deep interceptor trench to collect groundwater on the other.
The Highway 73 project won a 2020 Engineering Excellence Award from the American Council of Engineering Companies of North Dakota.