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1 <br />' embankment. This material will be nominally compacted by the movement of placement equipment over <br />the material. Once this initial lift has been placed, graded and stabilized, subsequent shallower lifts can be <br />' placed and compacted in a controlled manner. A diagram showing the construction sequence for the <br />embankment raise and [he impoundment expansion is presented on Drawing 5461-C5. <br />' During construction of the embankment raise, the condition of [he underlying tailings in response <br />to the superimposed load will be carefully monitored. Standpipe piezometers will be installed to monitor <br />' the phreatic surface within the tailings and settlement gauges will be installed to track overall <br />consolidation/settlement of the tailings and provide a basis for comparison of actual consolidation with that <br />predicted by the analysis. The rate of embankment material placement will be controlled to provide <br />t sufficient time for effective dissipation of pore +vater from the tailings and thereby avoid development of a <br />condition of total stress and the potential for liquifaction of the tailings deposit. Based upon the results of <br />the consolidation analysis performed on samples of the tailings materials and our experience with <br />' embankment construction under similar conditions, it is expected that the construction period will be paced <br />over a period of approximately three to four +veeks. <br />' Once it has been determined that approximately 90% of the expected consolidation of the underlying <br />tailings has been achieved, the embankment raise will be trimmed and graded to its final configuration and <br />' conswction of subsequent components of the project in this area (installation of the new geocomposite and <br />geomembrane liners, surfacing of the embankment crest, replacement of the perimeter security fencing, <br />installation of the tailings distribution piping, etc.) will proceed. In order to expedite completion of the <br />' project and commissioning of the expanded impoundment, liner installation will proceed in the uphill, <br />excavated portion of the impoundment concurrent with tailings consolidation beneath [he embankment raise. <br />' 3.2.3 Stability Analyses <br />' To assess the stability and constructibility of the proposed embankment raise, and the underlying <br />foundation materials (tailings, embankment fill, and in situ native soils) during construction and subsequent <br />operation of the facility, slope stability analyses were performed assuming the embankment configuration <br />' • shown on Drawing 5461-C5. The following scenarios were analyzed: <br />1. Rotational failure of the upstream and do++mstream slopes of the embankment post construction. <br />' 2. Rotational failure of the upstream and downstream slopes under the embankment at 50% of the <br />' [ailing storage capacity. <br />3. Rotational failure of the upstream and downstream slopes under the embankment a[ 100% of [he <br />tailing storage capacity. <br />3.2.3.1 Method of Anal}•sis <br />The stability analyses ++ere conducted using the PC sofhvare program XSTABL, Version 5 <br />(Interactive sofhvare Designs, Inc.). The program allows input of complicated geometry and differing <br />' material properties, with a choice oC several methods of analysis. For this study, a critical failure surface <br />1 9 <br />