2014-03-03_REPORT - M1988112 - Laserfiche WebLink

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1.2 TAILING FACILITY DESIGN AND CONSTRUCTION The San Luis Project Tailing Impoundment is located 3.25 miles northeast of San Luis (Figure 1). The impoundment is situated in an ephemeral drainage on the west - facing foothills of the Sangre de Cristo Mountains. The facility comprises the following main features: • A 155 -feet high, 1,900 -feet long compacted embankment dam constructed on a north -south alignment across the valley. • A single - geomembrane- lined, approximately 125 -acre tailing disposal area, with sub -drain water collection system: • the tailing -water collection system includes a 2 -ft thick granular drainage blanket layer integrating a system of perforated collector pipes on top of the impoundment liner; and • the double - geomembrane lined collection pond downstream from the dam equipped with intakes, pumps, and pipelines for collecting and returning the reclaimed water back to the impoundment. • A storm -water run -on interception and diversion system comprising perimeter ditches and berms partially surrounding the tailing impoundment area and sized to collect and convey storm water up to a 100 -year, 24 -hour event from the upland contributing drainage areas. A large portion of the intercepted run -on is diverted over the left abutment of the dam via a concrete drop structure and 48 -inch diameter corrugated metal pipe (CMP). The facility was designed and built using phased construction. The initial (Phase I) impoundment was built between 1990 and 1991, with a 120 -feet high "starter" dam to crest elevation 8590. The downstream face of the Phase I dam has a slope of 3H:1 V and the upstream face has a slope of 2.5H:1 V, and dam crest width of 30 feet. Subsequent raises of the dam were constructed using the upstream method, with each raise constructed over previously deposited tailing. Phase II was built in two, fifteen -foot raises: Raise 1 to elevation 8605 was completed in December 1993, and Raise 2 to elevation 8620 was completed in August 1995. The downstream toes of the Raise 1 and Raise 2 embankments tied in at approximately the centerline of the crests of the underlying embankments, resulting in approximately 10 to 15 -ft wide benches on the final downstream slope of the dam at elevations 8590 and 8605. During Phase II a splitter embankment was constructed upstream from the main dam to manage the initial deposition of tailing and fluids. This splitter embankment runs parallel to the main embankment and is now buried in the tailing. It is no longer visible. The design intent of the tailing disposal facility was to allow for managed discharge of the tailing fluid. The tailing were deposited over the large surface area of the impoundment in thin San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 2 February 2014