Laserfiche WebLink
Henderson 3 Dam Buttress Design <br />January 26, 2018 <br />Page 2 <br />The Henderson tailings impoundments are being raised using the upstream method of dam <br />construction. The current function of 1 Dam and 3 Dam is to impound tailings that comprise <br />hydraulically deposited sand, silt, and clay sized particles. Deposition at Henderson began in 1976 <br />and was implemented to allow both dams to be raised independently until the time that they were <br />joined at the crest to form one continuous dam. The current permitted height for the Henderson <br />tailings dam is about 300 feet, with a crest elevation of 8,900 feet and a crest length of about 11,000 <br />feet. The Henderson tailings dam is designed to impound an estimated 300 million cubic yards of <br />tailings and currently has a crest elevation of approximately 8,878 ft. <br />To operate a safe upstream method tailings dam, the design requires that a downstream shell of <br />essentially unsaturated tailings sand be constructed as the primary structural element supporting the <br />finer -grained tailings upstream. To accomplish this, the sandier portion of the tailings is deposited <br />within the wide beach adjacent to the crest. Each raise berm is constructed of dry sandy tailings <br />across the downstream portion of the beach to achieve an overall slope of about 4H:1V. Drainage <br />measures are provided by underdrains and horizontal drains to maintain a relatively low phreatic <br />surface through the impoundment. Deposition with leadoffs in the winter and spigots in the summer <br />months is closely monitored throughout the year, and a suite of instrumentation is used to verify <br />compliance with the design intent. <br />Between 2016 and 2017, field investigations conducted on 3 Dam (the original dam and stepback) <br />included advancing four Cone Penetration Tests (CPTs), drilling 17 boreholes, and performing 10 <br />vane shear tests as shown on Figure 1. This field program was to complement previous fieldwork <br />that had been conducted at 3 Dam. The previous field investigation locations are also noted on <br />Figure 1. <br />A4:M5WRQIEGTSIWA7 Cft1494_NENJ BUTiRL40.OEU%MRQLESCRUS RZPORrJtEPoR7WENDERSON_]DMI BUTMESSDESIGN LEMRRE?OW-REV B_FINALDOCX <br />