Slope Stability

Example of a slope stability analysis.

(Image Source: Wikipedia)

Stability of the upstream and downstream slopes of an earth embankment dam is essential to the dam’s safe operation. Unsatisfactory slope performance includes the following categories of slope movement: shear failure, surface sloughing, excessive deformation, liquefaction, and any other types of slope movements ^[1]. Slope movements that do not cause an immediate failure of the dam can require significant maintenance, and if left unrepaired, can lead to an eventual failure. Proper analysis and design of the embankment slopes, coupled with close monitoring and prompt maintenance or repairs to any moved sections of the slope(s) are key to preventing failure or a major incident from occurring.

Loading Conditions

“Evaluation of slope stability requires establishing the conditions, called ‘design conditions’ or ‘loading conditions,’ to which the slope may be subjected during its life, and performing analyses of stability for each of these conditions. There are four design conditions that must be considered for dams:

During and at the end of construction,
Steady state seepage (both normal and flood conditions),
Sudden drawdown, and
Earthquake loading.

"The first three conditions are static; the fourth involves dynamic loading”.^[1]

Types of Information Needed

The key inputs to any slope stability model are unit weights and soil strength parameters. “The stability of dams and slopes must be evaluated utilizing pertinent geologic information and information regarding in situ engineering properties of soil and rock materials. The geologic information and site characteristics that should be considered include:

"groundwater and seepage conditions;
"lithology, stratigraphy, and geologic details disclosed by borings and geologic interpretations;
"maximum past overburden at the site as deduced from geologic evidence;
"structure, including bedding, folding, and faulting;
"alteration of materials by faulting;
"joints and joint systems;
"weathering;
"cementation;
"slickensides;
"field evidence relating to slides, earthquake activity, movement along existing faults, and tension jointing”.^[1]

Other Guidance

A critical aspect of any engineering analysis is communication. There are a variety of slope stability modeling approaches and methodologies, and it is important to owners, consultants, and regulators that clear communication is integrated in the process. General guidance and recommendations regarding both pre- and post-modeling communication are provided on this page: Modeling Communication. Items specific to slope stability analysis that should be considered in advance of any modeling effort are summarized here: Pre-Modeling Communication: Slope Stability Model Considerations.