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2 Embankment Dam Slope Stability 101 Introduction Design of new embankment dams, and the more common scenario of reviewing the conditions of existing dams, should, as general practice, include evaluating the stability of the embankment structure. Stability, in the simplest definition, refers to the ability of a slope to resist the driving forces tending to move earth materials downslope. The stability of an embankment can be adversely affected by excessive stresses on the crest or slopes, sudden addition or loss of water in the reservoir, changes in internal water pressures, or loss of materials due to erosion (both internal, such as piping, and external, such as surface erosion). Stability conditions of a dam can be assessed using both visual and analytical methods. Recently, the central Front Range and surrounding areas in Colorado experienced historic rainfall that led to extensive flooding in the region. The rainfall and flood imposed loading conditions that many dams, both large and small, had never experienced. These events may have created changes of conditions, internally, in embankment dams. The Colorado State Engineer’s Office recently completed emergency inspection reports for affected dams, some of which will require quantitative slope stability analyses to further assess their conditions and levels of safety. The purpose of this article is to describe visual inspections of stability performance and identify triggers that may indicate the need for a more quantitative or analytical approach. This article is not intended to be prescriptive and provides only a general overview of assessing embankment stability. Future articles will provide more details in terms of strength characterization and specific analysis methodology for different loading cases. Visual Inspection and Monitoring for Stability For many western states, State Engineers have waived the requirements of performing stability analyses for low hazard dams if it can be demonstrated that the dams have conservative slopes and were constructed of competent materials. Generally, upstream earth embankment slopes should be no steeper than 3H:1V (horizontal to vertical), and downstream earth embankment slopes no steeper than 2H:1V. Regular visual inspections are always required, even if stability analyses have been waived, and such inspections can provide efficient means of monitoring embankment performance with respect to stability. Regular visual inspection is the best tool an Owner can use to assess the safety of an embankment dam. Benchmarking photographs (those taken of the same feature from the same perspective, inspection to inspection) are invaluable to the monitoring process. Photos can be compared across multiple inspections to identify subtle changes in conditions, which may be an indication of a developing adverse condition that affects the stability and safety of the dam. Visual indicators of developing instability may include:  Longitudinal cracks on the dam crest or slope (see Photo 1).  Wet areas on the downstream slope or toe (see Photo 2) indicating an adverse internal phreatic level within the embankment. The relationship between reservoir level and seepage quantity and quality should also be established and used to compare successive observations.  An apparent slope failure or slump (see Photo 3).  Erosion or sloughing of the downstream slope which results in oversteepening of the overall slope.  Displaced riprap, crest station markers, or fence lines indicating movement.  Bulges at or downstream of the toe.  Depressions or sinkholes in the dam crest or slopes.  Changes in the appearance of the normal waterline against the upstream slope at multiple water levels.