Laserfiche WebLink
(3) Underdrain collector pipe cleanout: The three, 8-inch diameter sub-drain pipe outlets at <br /> the downstream toe of the dam were connected to a manifold at the time of the inspection <br /> so it could not be determined if all three pipes are flowing. Subdrain flows were similar to <br /> flows in the previous year, in the range of approximately 32 to 36 gpm(Table 1). Average <br /> flows over the past two years (June 2018-July 2020) are noticeably lower than average <br /> flows in the previous 5 years(June 2014-May 2018)where flows ranged from 37.5 to 42.5 <br /> gpm. There was an historic drought in the region in 2018 and the continuing dry conditions <br /> may explain the reduction in underdrain flows. At the time of this 2020 inspection, the <br /> slope above the pipes remains dry, and all piezometers remain dry, and it is considered <br /> unlikely that the observed reduction in underdrain flows is attributable to significant pipe <br /> clogging. <br /> It is recommended that jetting and another camera inspection of the pipes be performed to <br /> confirm that the pipes are not becoming clogged. The camera inspection completed in May <br /> 2018 showed some accumulation of sediment/precipitates was occurring at that time. <br /> Damage and partial collapse of the pipes that has been documented by previous camera <br /> inspections(2018,2014)has not been considered to be a dam safety concern as long as the <br /> pipes are flowing. It is prudent to continue maintaining the downstream ends of the pipes <br /> to ensure that they are clean of sediment and precipitates that could clog the flows from the <br /> sub-drain system,and to avoid the risk of pore pressure buildup at the toe of the dam. <br /> • Recommendation for installation of toe drain collection well <br /> To facilitate maintenance in the toe drain area, and improve conditions for monitoring and <br /> inspection of the toe drains, it is recommended BRMI consider installing a concrete structure for <br /> collecting underdrain flows from the toe drains. A concept for such a structure is provided as <br /> Figure 4. The structure shown on Figure 4 is basically an 8-ft diameter,6-ft high pre-cast concrete <br /> manhole with cover. The three toe drain pipes would be plumbed into the manhole on the upstream <br /> side. A splitter wall could be installed inside the manhole to force the combined drain flows over <br /> 1 a v-notch weir within the manhole to facilitate flow measurements,which are currently taken using <br /> a bucket and stopwatch at the end of the outfall pipe. The flows would exit from the other side of <br /> the manhole into a single pipe for delivery to the lined impoundment.The area around the manhole <br /> could be backfilled to facilitate access into the well for monitoring and maintenance of the pipes, <br /> while raising the grade to minimize ponding of surface water runoff in the toe area. The ends of <br /> the three pipes would be visible inside the well so that it could be determined if any one of them <br /> has stopped flowing or if individual flow rates are changing substantially. <br /> ' 2020 Dam Safety Inspection Report San Luis Tailings Dam <br />