Slope failures can block roads, damage pavement and cause safety hazards. They can also be costly to repair.
For example, in June 2012, Carlton County received up to 11 inches of rain—causing flood damage and slope failures along Minnesota Highway 210. Reconstruction cost $21.3 million dollars, using federal and state emergency funds.
A recently funded research implementation project aims to help MnDOT identify areas that are susceptible to geohazards — such as landslides and rock falls— and better equip highway construction project managers to identify and mitigate risk.
Raul Velasquez, a geomechanics research engineer with MnDOT’s Office of Materials & Road Maintenance, is overseeing the study conducted by WSB & Associates.
The research team is analyzing satellite images to identify areas with geohazards. Geohazards include areas with shallow rock, sinkholes, and soft or sensitive soil. This information will then be added to map layers in Georilla—MnDOT’s internal web map application. Project managers can then pull this information during the scoping and planning phases of a construction project.
“The earlier we know about a risk, the better,” said Velasquez. “That way we can proactively mitigate the risk, or choose a different project location, if needed.”
This project builds on existing MnDOT statewide georisk modeling research. Phase 1 and Phase 2 were completed in 2019 and Phase 3 is expected to be completed by the end of August 2020.
Over the next six months, researchers will map northwest Minnesota (MnDOT District 2). Researchers will also refine and recalibrate existing maps. As slope failures occur, they will evaluate whether the tool accurately predicted the event.
While geohazard mapping is applied in other industries in Minnesota, this approach is novel among state DOTs. This exciting work can reduce risk of future road closures due to slides and sinkholes, safety hazards to the public, and puts MnDOT in a position of mitigating risk instead of reactively responding to geological emergencies.
“Avoiding high risk areas can help us reduce material loss, improve human safety, and save taxpayer dollars,” said Velasquez.
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
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.
Researchers identified 14 sites representing destabilized roadway slopes in Minnesota. Following site investigations, lab testing and modeling, researchers recommended eight slope stabilization techniques that local engineers can undertake without the help of outside geotechnical engineers. The methods were packaged in a simple, accessible field guide for county engineers.
“When most studies end, further research is needed. This project, however, created a user guide that local engineers can use right away to repair destabilized slopes,” said Blake Nelson, Geotechnologies Engineer, MnDOT Office of Materials and Road Research.
“This guide includes an easy-to-use flowchart that steers local engineers toward an appropriate slope stabilization technique,” said David Saftner, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering.
What Was the Need?
Winter weather and spring storms leave their mark on slopes along highways and at bridges. Erosion and other forces cut gashes and ravines into slopes. Some damage such as failing pavement at shoulders or sloughed off sections of a slope can be obvious to road users. Other, more subtle signs of creeping embankments may only catch the attention of engineers.
Slope failures must be repaired to prevent damage to roadways and embankments. When slope damage is severe, a geotechnical engineering firm must step in at some expense. By the time the first soil sample bore is pulled, county engineering departments may already be facing a bill of $20,000. But when damage is less severe, the county can often stabilize the slope using local materials and simple techniques.
Determining whether slope damage can be completed by local engineers or requires outside help remains a challenge for county road departments that often lack geotechnical expertise.
What Was Our Goal?
The Local Road Research Board (LRRB) funded a research project to determine effective methods for stabilizing damaged roadway slopes. These methods would be incorporated in a guide that local engineers could use to identify the type of slope failure and then select an appropriate repair method.
What Did We Do?
Investigators began by surveying Minnesota county engineering departments to identify sites that needed to be stabilized. Local engineers also provided details about both successful and unsuccessful stabilization methods that have been tried in the past. The re-search team inspected 14 destabilized sites identified in this effort and took soil samples from each site.
Then they conducted a literature review of slope stabilization methods, identifying 12 stabilization techniques. Based on this review, researchers tested the soil samples with direct shear tests to identify shear strength parameters such as effective friction angle and cohesion. They ran soil classification tests to measure plasticity, granularity and gradation, and moisture content. These properties were then used as inputs in slope modeling and parametric studies to examine viable repair techniques for each site.
Investigators summarized their analysis of each case and documented stabilization methods that would meet the needs identified in the case studies. Finally, the research team prepared a slope stabilization guide that local engineers could use in the field to identify the type of slope failure and the appropriate solution.
What Did We Learn?
Five of the destabilized sites featured primarily sandy soil, eight had fine-grained soil, and one was rocky. Slope failure was visible at nine of the sites. Groundwater management figured prominently in most sites and repairs.
The literature search identified approaches for specific types of failures. Managing groundwater and drainage improves shear strength in slide-prone areas; surface covers protect slopes from erosion; vegetation and plant roots stabilize soil; excavation and regrading reduce failure forces; and structural reinforcement features directly support slope materials.
Investigators identified eight slope failure mechanisms that encompassed the full range of destabilization scenarios presented in the case studies. Each method had been identified in survey responses as a technique used successfully at the local level. The site conditions that contributed to the failure were identified along with a repair solution for each failure type.
Using the findings from this project, researchers created a slope stabilization guide for Minnesota local government engineers. This field guide describes common slope failures and conditions that may contribute to each. It includes a simple, three-step flowchart that guides engineers to the appropriate repair technique by determining whether the damage is a creep or rotational failure, whether the soil is cohesive or granular, and if there are groundwater concerns.
Based on engineers’ answers, the flowchart directs them to one or more of the eight slope stabilization techniques, providing photographs and repair methods that have been successful in addressing slope problems along Minnesota roadways.
The Slope Stabilization Guide for Minnesota Local Government Engineers will be sent to each of the 87 county engineering departments. Local engineers can keep the guide on hand when they investigate slope failures along their roadways, and with it quickly identify what work needs to be done to repair the damage.
Each year, MnDOT and the Local Road Research Board solicit ideas for new research projects from MnDOT staff and city and county engineers. The ideas are then reviewed and ranked by the LRRB and MnDOT’S Transportation Research Innovation Group, which represents MnDOT’s districts and specialty offices.
“We always reach out to the specialty offices and help them develop ideas and prioritize current needs,” said Hafiz Munir, MnDOT research management engineer. “They’re in the driver’s seat. We are guiding them through the process.”
Of nearly 100 ideas submitted this year, transportation researchers will have a chance to bid on 24 ideas from seven different research areas.