When people think of slope failure or geohazards, they think of landslides and mudslides in mountainous regions like California. Those of us living in the Midwest don’t typically worry about property damage or disruptions in public services due to slope failure. Unfortunately, slope failure impacts a wide range of landscapes, even those considered relatively level. In fact, in the Twin Cities there have been increasing numbers of slope failures that significantly impacted infrastructure and property. The most recognizable example is probably the 2014 slope failure along the West River Parkway in Minneapolis. This slope failed after more than 11 inches of rain fell in two days, impacting a popular recreational trail as well as a major health care facility. Repairs were completed in 2016, and cost $5.639 million [i].
Slope failure is a geohazard that impacts many types of infrastructure, from individual homes to municipal storm sewer networks to oil and gas pipelines. In fact, the Pipeline and Hazardous Materials Safety Administration requires that natural gas and hazardous liquids pipelines develop risk assessment programs for slope failures in their systems. Likewise, many municipalities are beginning to incorporate these types of risk assessment programs into their own planning activities.
So what causes slope failures? Like all geohazards, the causes are myriad and complex. Establishing a framework of how the physical processes behind slope instability function is crucial in determining risk.
Simply put, slope stability is based on the interaction of two forces: driving forces and resisting forces. Slope failures occur when driving forces overcome resisting forces. The driving force is typically gravity, and the resisting force is the slope material’s shear strength.
When assessing a slope’s stability look for indications that physical processes are decreasing shear strength. These can include:
- Weathered geology: Weak, weathered bedrock, jointed rock, or bedrock that dips parallel to the slope can decrease stability.
- Vegetation removal: Droughts, wildfires and humans can remove vegetation from the slope, decreasing stability.
- Freeze/thaw cycles: Water in rock joints or in soils can decrease slope stability.
- Stream action: Rivers can erode the bottom of the slope, called the toe, decreasing stability. This can occur over time through normal stream action or catastrophically during flood events.
- Human modifications: Humans modify stability through actions such as excavation of the slope or its toe, loading of the slope or crest, surface or groundwater manipulation, irrigation, and mining.
- Slope angle: Steeper slopes tend to have greater risks for instability.
- Soil type: Soils have variable amounts of shear strength, dependent on factors such as soil texture, pore water, and particle cohesion.
- Water sources: Water works in many ways to reduce shear strength. For example, pore water pressure in soils decreases shear strength, and saturated soils are more likely to lead to slope failure. Perched water tables, groundwater seeps, and excessive precipitation are some examples of water sources that may lead to slope failure in certain conditions.
Many things can impact the stability of a slope. Just like with stream crossings, all geomorphic factors affecting slope stability should be considered when determining the risk of slope failure.
After the geomorphic factors for each slope crossing have been adequately assessed, these indicators can be fed into our geomorphic framework of slope stability to determine how likely slope failure is at a particular location.
An example of a risk matrix developed for slope stability is below. This matrix is determining the likelihood that a slope failure will occur and multiplying that by a known consequence to derive a risk factor (from the formula above). For this type of risk matrix to work, robust rational and consequence definitions should be developed to support the risk estimation. In this example, geomorphic analyses have resulted in a specific set of justifications for the likelihood of slope instability. These categories are then assigned risk factors. Very Low stability slopes, as defined by the rational in the matrix, have an Almost Certain (5) risk factor.
Detailed definitions have also been determined for the Failure at Road consequence, and those definitions are assigned risk factors. A slope failure at a road is considered Critical (5) if the road is a critical evacuation route, major transportation corridor, or restricts access to emergency facilities. Almost Certain (5) slope failures at Critical (5) roads have a Risk Factor of 25 and require mitigation.
While this example matrix only lists one consequence category (Failure at Road), a risk matrix can be designed to include as many consequences as necessary to capture the complete risk profile. Additionally, the application of five risk factors is merely an example. Risk matrices can be designed with as many or as few risk factor categories as necessary.
The outcome of this analysis is a set of risk factors that pipeline operators, city planners, engineers, or transportation officials can use to prioritize capital spending in a non-biased way, proactively estimate capital budget, manage interim risks, and more accurately estimate maintenance budgets.