2018-09-10_INSPECTION - M1988112

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construction should include correcting the drainage in this area to direct storm-water flows into the lined ditch. (2) Underdrain collector pipe outlets: The three, 8-inch diameter sub-drain pipe outlets at the downstream toe of the dam were found to be blocked at the time of the initial dam safety inspection in 2013 due surface runoff sediment deposition at the toe of the dam and sloughing of saturated embankment materials from above the blocked pipes. The pipe outfalls were exposed and cleared of debris in 2013, but new sloughing had partially re-covered the pipes by the spring of 2014. As reported in the 2015 second annual inspection report,the pipes were uncovered again in August 2014, cleaned out by jetting, and a camera inspection was completed on August 19, 2014. The first camera inspection report was attached with the 2015 annual inspection report. All three pipes were flowing at the time of the 2014 camera inspection, and all three were found to be partially collapsed at varying distances up- gradient from the outlets.Although partially collapsed,the pipes are conveying water collected by the gravel underdrain system that sits above the liner. To prevent blockage of the sub-drain collection pipe discharge area by surface sediment buildup and sloughing at the toe of the dam, in 2014 the three pipes were extended and connected to a manifold. The combined pipe flows are conveyed by a single pipe extension from the manifold at the dam toe to the downstream pump-back pond. The underdrain collection pipe manifold/extension appears to have been effective in reducing pore-pressure buildup resulting in a seepage exit emerging on the embankment face above the pipes. At the time of this 2018 inspection the slope above the pipes remains dry. The pipe ends were exposed (manifold removed) and all three pipes appeared to be flowing. It was not possible to estimate the flows from each pipe, as the pipe ends were partially submerged (see Photo 13). There continues to be sediment buildup in the area due to surface runoff that appears to originate from the downstream right groin area,where evidence of erosion was observed(Photos 15 and 16). It is recommended that area be graded to direct storm-water runoff away from the toe area and minimize the maintenance required to keep the pipe discharge area open for inspections. A second camera inspection of the pipes was conducted in May 2018. The videos from that inspection were reviewed and compared to the 2014 camera survey report as part of this 2018 dam safety inspection. The north and south pipes were flowing partially full at the time of the 2018 survey and distinctive collapse features were observed as were documented in the 2014 survey,as shown on Figures 3 and 4. Due to the pipe extensions that were completed in 2014,there is a difference in the 6 2016 Dam Safety Inspection Report San Luis Tailings Dam