Laserfiche WebLink
reported distances back to these features from the ends of the outlets between the <br /> 2014 and 2018 surveys. Figure 3 shows a distinctive buckling of the crown (partial <br /> collapse) of the north pipe that was documented at 72.5 ft in 2014 and at 32.2 m <br /> (105.6 ft) in the 2018 video. Figure 4 shows a partial collapse of the south pipe <br /> reported at 28.2 ft in 2014 and 14.2 m (46.6 ft) in the 2018 video. The middle pipe <br /> appears to be collapsed a short distance upstream from the outlet; reported at 8 ft in <br /> 2014 and 7.3 m (23.9 ft) in the 2018 video. The pipes were jetted clean prior to the <br /> 2014 video, but not before the 2018 video, and buildup of sediment and precipitates <br /> is evident in comparing the available images (Figures 3 and 4). It is recommended <br /> that the pipes be jetted clean again to reduce the risk of complete clogging. <br /> Because the pipes are still flowing and the observed flow rates are consistent with <br /> historic readings, the damage to the pipes is not considered to be a dam safety <br /> concern. However, it is prudent to continue to try to keep the downstream ends of <br /> the pipes clean and open to enhance collection and discharge of flows from the sub- <br /> drain system to avoid the risk of pore pressure buildup at the toe of the dam. <br /> In summary,maintenance-related recommendations include: <br /> 1. Continue to maintain the surface erosion control measures(cross berms and rock- <br /> lined down drains) in the repaired area at the right downstream slope of the dam. <br /> 2. Correct the grading on the upper left downstream slope area next to the access <br /> road where storm-water is bypassing the lined ditch and creating erosion gullies. <br /> 3. Clean the underdrain pipes by jetting to remove accumulated sediment and <br /> precipitate buildup. <br /> 4. Modify the grading in the lower right groin area to reduce erosion and sediment <br /> transport and accumulation at the toe of the dam which builds up around the <br /> under-drain pipe outlets. <br /> The overall condition of the dam is "Conditionally Satisfactory", pending completion of <br /> construction of the South Diversion Ditch drop structure. Please feel free to contact us if you have <br /> any questions regarding this report. <br /> Regards, <br /> Debora J. Miller, Ph.D., P.E. <br /> MILLER GEOTECHNICAL CONSULTANTS, INC. <br /> 9 <br /> 2018 Dam Safety Inspection Report San Luis Tailings Dam <br />