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1 <br />' 3.2 Proposed Embankment Raise <br />The proposed embankment raise +vill be a conventional homogeneous earth-fill type embankment, <br />similar to the existing embankment. The design of the raise is based upon a modified upsteam/centerline <br />construction concept. The analyses indicate that the downstream face of the completed embankment could <br />t be steepened slightly, compared to the existing embankment, without unduly compromising the stability of <br />the structure, and there is a desire to maximize the impoundment capacity of the completed facility, without <br />' encroaching into the downhill portion of the site, which has been designated for future development of a <br />larger tailings impoundment to accommodate full-scale mill production. Therefore, the proposed raise has <br />been aligned such that the do++•nstream toe of the raise essentially coincides with the downstream toe of the <br />existing embankment. Atypical cross-section through the completed impoundment (See Drawing 5461-C4) <br />illustrates that the majority of the proposed raise will be founded on the impounded tailings beach adjacent <br />to the upstream face of the existing embankment and upon the underlying existing embankment, which <br />' serves as the starter structure. Construction of the proposed raise will increase the storage capacity of the <br />impoundment facility by approximately 13,000 tons. The proposed construction will increase the <br />embankment height by 12 feet, to a design crest elevation of 8,420 feet AMSL. The maximum height of the <br />embankment crest above existing ground will be approximately 22 feet, and the maximum elevation <br />difference behveen the embankment crest and the existing ground at the toe of the do++mstream face will be <br />' approximately 28 feet. <br />3.2.1 Design Criteria <br />' The functional requirements for the proposed impoundment expansion works include the following <br />criteria: <br />' Provide an incremental disposal volume of approximately 274,000 cubic feet (equivalent to <br />approximately 13,000 tons of tailings), which is expected to accommodate a mill start-up period of <br />' three months; <br />• Maintain a minimum operating freeboard allowance of two (2) feet, such that there is sufficient <br />capacity in the impoundment to accommodate the incident precipitation and runoff from tributary <br />areas that would result from a 100-year/24-hour storm event. <br />' The design criteria for the proposed embankment include the requirement that the completed <br />structure be stable, with appropriate factors of safety, under static and pseudo-static loading conditions. <br />The stability analysis for the embankment, +vhich is discussed in detail in Section 3.2.3 of this Design Report, <br />' indicates that acceptable slopes for the upstream and downstream faces of the embankment raise are 1.5:1 <br />and 2:1, respectively. The stability of the steeper upstream face of the embankment will be enhanced, almost <br />' immediately upon completion of construction, by the deposition of new tailings in the impoundment, which <br />will effectively buttress the embankment. Embankment stability will also be enhanced by a biaxial geogrid <br />installed on the surface of the existing tailings prior to placement of fill for the embankment raise. The <br />' primary function of the geogrid is to act as a separator between the tailings and the embankment fill and to <br />reinforce the initial lifts of fill and, thereby, improve trafficability for construction equipment during <br />construction. In addition, the geogrid ++•ill also enhance the long-term stability of the embankment by <br />' interrupting potential shear failure surfaces through the upstream region of the embankment. <br />7 <br />1 <br />