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<br /> <br /> <br /> <br />3 <br /> <br />Photo 1. Severe longitudinal cracks in downstream <br />slope <br /> <br />Photo 2. Seepage exiting dam face <br /> <br />Photo 3. Slope failure on downstream slope <br />Triggers for More Quantitative Analyses <br />Besides a change in conditions resulting from <br />rainfall/flooding or other events, triggers requiring <br />stability analysis be performed may include: <br /> Designing a new dam. <br /> Raising an existing dam. <br /> Construction of a berm. <br /> Potential reclassification of a dam to high <br />hazard. <br /> Deterioration of existing conditions, i.e. <br />oversteepening of embankment slopes for any <br />reason. <br /> Reassurance that a latent, undetected issue <br />has not developed – indicators of such an issue <br />may include embankments with steep slopes <br />(greater than 2H:1V), soft foundation <br />conditions, high phreatic surface within the <br />dam and/or foundation, seepage at the face or <br />toe, depression/sinkhole formation or <br />observed scarp or bulge. <br /> Indications from field observations that <br />instability may be developing – i.e. observed <br />scarps, toe bulges, longitudinal cracking along <br />crest or slope. <br />Slope Stability Analysis Requirements <br />The analyzed stability of a slope is expressed as a <br />Factor of Safety (FS). FS values greater than 1 indicate <br />the estimated driving forces are less than the <br />resistance forces. However, due to inherent <br />uncertainties in the behavior and characterization of <br />earth materials, regulations and good practice require <br />FSs greater than 1 for most loading conditions. Each <br />regulatory agency has its own FS requirements; <br />however, the following table provides some commonly <br />adopted values: <br />Loading Condition Min. Factor <br />of Safety <br />Steady State Drained 1.5 <br />End of Construction 1.3 <br />Rapid Drawdown 1.2 <br />Post-Seismic 1.2 <br />Pseudo-Static (where <br />applicable) <br />1.0 <br />