In a recently completed pilot study, researchers developed maps for two Minnesota counties that rank the failure potential of every slope using a geographic information system (GIS)-based model.

“GIS mapping has been applied to very small watersheds. The two counties in this study are huge areas in comparison. We used a physics-based approach that shows engineers where slope failure is likely to occur,” said Omid Mohseni, Senior Water Resources Manager, Barr Engineering Company.

What Was Our Goal?

The goal of this study was to determine if slope failure models could be developed to help counties anticipate where failures may occur. Researchers used publicly available data, research findings and geotechnical theory to develop failure models that could then be mapped with GIS in two topographically dissimilar Minnesota counties. These maps would identify slopes susceptible to failure so that county highway departments could develop preventive strategies for protecting roadways from potential  lope failure or prepare appropriate failure response plans.

What Did We Do?

Researchers began with a literature review of studies about the causes of slope failure, predictive approaches and mapping. They were particularly interested in research related to potential failure mechanisms, algorithms used for predicting failures and slope-failure susceptibility mapping.

Then investigators collected data on known slope failures in Carlton County in eastern Minnesota and Sibley County in south central Minnesota to identify failure-risk factors not found in the literature. Researchers reviewed various statewide data sets, identifying topographic, hydrologic and soils information that could be used in GIS-based modeling. Next, they developed a GIS-based slope-failure model by incorporating the available data with geotechnical theory and probability factors from hydrologic data, and writing computer code to allow the data to be input into mapping software.

Researchers tested the software on known failure sites to refine soil parameter selection and failure models. The refined models and software were then used to identify and map slope failure risks in Carlton and Sibley counties.

What Did We Learn?

After analyzing the literature and the failure and geotechnical data, researchers identified the following key causal factors in slope failure: slope angle, soil type and geology, vegetation, land use and drainage characteristics, soil moisture, and rainfall intensity and duration.

Researchers then developed mapping models for the two counties using three key data sets. The first was data from 3-meter resolution, high-quality lidar, which measures distances with laser range finders and reflected light, available through Minnesota’s Department of Natural Resources website. The team augmented this data with U.S. Department of Agriculture soils survey data, and with National Oceanic and Atmospheric Administration and National Weather Service hydrologic data for precipitation and storm duration information.

Based on research in geotechnical theory, researchers developed algorithms for anticipating failure and built these into the lidar-based topographic mapping model. They also developed input parameters based on the failure factors and established output parameters representing five levels of failure susceptibility: very low, low, moderate, high and very high.

After testing the GIS-based model against a slope along County Highway 210 in Carlton County, researchers confirmed that failure potential correlated well with documented or observed slope failure. The team further validated the model by applying it to several small areas in the adjacent Carver and Sibley counties, finding similarly effective correlation with identifiable failure sites.

Independent geotechnical experts examined the modeling software and further refined geotechnical, soil and hydrologic elements. Finally, the team developed maps of Carlton and Sibley counties that assigned failure susceptibility levels to slopes in the two counties. Viewing maps through the software remains the most useful way to examine slopes, although large-format maps are available.

“If county engineers have higher slopes adjacent to roadways, they can use this basic tool to predict slope failures and then hire a geotechnical consultant to investigate the site.” – Tim Becker, Public Works Director, Sibley County

What’s Next?

With additional funding, mapping could be extended to every county in Minnesota to further refine failure modeling. Maps may also be useful in identifying structures such as roadways, ecological features, transmission lines and pipelines, bridges and culverts that may be threatened by slope failure susceptibility. Potential risks could be used to prioritize slope treatment plans.

This research effort is part of a slope failure risk mitigation strategy that includes the recently released Slope Stabilization Guide for Minnesota Local Government Engineers. Another project, underway at MnDOT, is identifying, mapping and ranking slopes vulnerable to slides that could affect the state highway network. The project

This post pertains to the Local Road Research Board-produced Report 2018-05, “Storm-Induced Slope Failure Susceptibility Mapping,” published January 2018. More information is available on the project page.