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