Highway embankments, specifically referring to where earthen material is placed and compacted to raise the grade of a highway above the level of the existing surrounding ground surface, may be constructed with cohesive materials that can become dried and desiccated causing shrinkage of the soil during the summer. This can result in cracks and fissures near the surface of the slope. During heavy rains, water infiltrates into the cracks and fissures, saturating the slope surface materials creating a perched water table. The increased water pressure has the effect of reducing the effective shear strength of the soil while simultaneously increasing the driving forces. In addition, cohesive materials near slope surface can soften over time and lose strength. The soils along the slope face can swell and develop seepage parallel to the slope. This can result in surficial slope instability. Surficial slope instability is also dependent on regional climate and vegetation which can change over time.
Surficial slope stability issues in highway embankment slopes have been reported and typically occur during heavy rains. Only some state and local agencies have incorporated requirements to evaluate surficial slope stability in their design guidelines. It is important to incorporate surficial slope stability analysis which includes a saturated depth which is appropriate for the specific site conditions. Site conditions that can impact stability include geometry (e.g., steepened slope), fill materials, and climate. When the analyses show a potential for instability, slope stabilization techniques such as mechanical, chemical, and biological stabilization methods may be required to address the surficial slope instability.
Surficial slope failures can create safety concerns, traffic congestion, and increased maintenance. A proactive approach for the mitigation of surficial slope instability is generally not considered in the design. A reactive approach to fix the failed slopes as a maintenance activity is more common and significant resources are required to fix surficial slope failures.
The objective of this synthesis is to document state DOT practice on identification, investigation, design guidance and requirements, prevention, and mitigation of surficial slope failures.
Information to be gathered includes (but is not limited to):
• Design guidelines and analysis techniques for new embankment slopes, specifically related to surficial stability;
• Proactive mitigation methods (including standard and special slope protection drawings such as rock plating, geocell, etc.) used to address surficial instability;
• Construction standard typicals and specifications for new embankment slope material placement;
• Impact of regional climate focusing on rainfall and temperature;
• Material properties and site conditions of failing and/or failed existing embankments;
• Methods to identify embankment areas susceptible to surficial slope failure; and
• Reactive maintenance mitigation techniques to repair surficial slope issues.
First Panel: October 10, 2022
Teleconference with Consultant: November 16, 2022
Second Panel: July 12, 2023
Shunyi Chen, North Carolina Department of Transportation
Ari Menitove, Utah Department of Transportation
Remesh Neupane, Virginia Department of Transportation
Nicole Oester Mapes, Colorado Department of Transportation
Ve’Niecy Pearman-Green, Illinois Department of Transportation
Krystle Pelham, New Hampshire Department of Transportation
Khalid Mohamed, Federal Highway Administration
Nancy Whiting, Transportation Research Board