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As reported in the 2015 second annual inspection report,the three, 8-inch diameter sub-drain pipe outlets <br /> were uncovered again in August 2014,and the pipes were cleaned by jetting and a camera inspection was <br /> completed on August 19, 2014. The camera inspection report was attached with the 2015 annual <br /> inspection report. All three pipes were flowing at the time of the camera inspection, and all three were <br /> found to be partially collapsed at varying distances up-gradient from the outlets. Although partially <br /> collapsed, the pipes are conveying water collected by the gravel underdrain system that sits above the <br /> liner. Based on analyses performed as part of the Detailed Inspection Report (MGC, 2014) phreatic <br /> conditions within the dam embankment are anticipated to remain low and static factors of safety <br /> unchanged even with the pipes fully blocked. Because the pipes are flowing and observed flow rates are <br /> consistent with historic readings,the damage to the pipes is not considered to be a dam safety concern. <br /> To prevent blockage of the sub-drain collection pipe discharge area by surface sloughing at the toe of the <br /> dam, in 2014 the three pipes were connected to a manifold and the combined pipe flows are conveyed by <br /> a single pipe extension from the manifold at the dam toe to the downstream pump-back pond. The <br /> underdrain collection pipe manifold/extension appears to have been effective in reducing seepage <br /> emerging on the downstream toe of the dam in the vicinity of the pipes. At the time of this 2016 inspection <br /> the slope above the pipes remains dry, but there was some ponding of water in the vicinity of the pipe <br /> outfalls(see Photo 12)that may be due to surface water and some minor leakage from one or more of the <br /> sub-drain pipes up-gradient from the manifold. It is recommended that this area be graded and backfilled <br /> to reduce ponding of surface water at the dam toe in the vicinity of the pipe outfalls. <br /> The Detailed Inspection Report (MGC 2014) included recommendations for design of improvements to <br /> the South Diversion Ditch and drop structure to minimize risk to the dam during major flood events. In <br /> 2015 to early 2016,MGC completed hydrologic and geotechnical studies and prepared final engineering <br /> design,construction drawings and technical specifications for a project that will substantially increase the <br /> hydraulic capacity of the South Diversion Ditch. The project includes improvements to the channel and <br /> replacement of the existing conduit drop structure with a more robust reinforced concrete drop structure <br /> at the left dam abutment. It is recommended that the South Diversion Ditch system improvements be <br /> constructed as soon as funding can be secured. <br /> The overall condition of the dam is "Conditionally Satisfactory", pending construction of improvements <br /> for the South Diversion Ditch and replacement of the drop structure. Please feel free to contact us if you <br /> have any questions regarding this report. <br /> Regards, <br /> Debora J. Miller,Ph.D.,P.E. <br /> MILLER GEOTECWCAL CONSULTANTS, INC. <br /> 2 <br /> 2016 Dam Safety Inspection Report San Luis Tailings Dam <br />