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The stone size that resulted from the computations was a W50 stone of approximately 6 inches in <br />diameter, which is a standard size used by the Colorado Department of Transportation. <br />Because of the small size of the reservoir and the fact that it is depressed in relation to the <br />surrounding topography, the potential for high winds is small. The criteria selected for the design is <br />likely to be much higher than those likely to be experience in reality. Even though the input data <br />were considered very conservative, the resulting stone size is quite small. <br />8.8 Piping Evaluation <br />A piping evaluation was performed using Lane's creep ratio to examine the potential for high <br />downstream seepage gradients and associated potential for a piping type foundation failure. The <br />creep ratio approach was presumably developed based on dam data from long-term performance and <br />is assumed to represent criteria for long-term piping evaluation. These failures typically occur several <br />months or years after a dam is placed into operation (NAVFAC, 1982). Furthermore, the computed <br />weighted creep ratios under dewatered conditions are higher than the values under existing <br />conditions and thus should be considered an improvement. Additionally, dewatered conditions <br />correspond to a temporary event of relatively short duration. The creep ratio approach to piping is <br />purely empirical and the procedure is based solely on statistical data. The weighted creep ratio values <br />C„, are widely scattered from the statistical average for a given soil. The actual factor of safety <br />against piping by heave on the basis of creep ratio is very high (Terzaghi et al., 1996). <br />The grain size of the foundation material at Monument Dam indicates the presence of lean clay in the <br />colluvial foundation material and in the embankment constructed of the same material. Furthermore, <br />hydraulically-placed fine sands and silts were found in the central portion of the embankment. <br />According to Terzaghi et al. (1996), the minimum weighted creep ratio for very fine sand or silt is <br />8.5. With some of the material grain sizes at Monument Dam falling in this range, a weighted creep <br />ratio less than 8.5 indicates that piping is a possibility: <br />With a head difference h of about 65 feet for the proposed geometry at Cross-Section B and a line of <br />creep length L of about 325 feet, the computed weighted creep ratio is about 5. Because this is less <br />than the minimum required ratio of 8.5, a filter at the drop structure is required. <br />The filter will consist of granular material around the drop structure. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 38 <br />