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OCT-OA-AB 02:15PL1 FROM-McCULLEYFRICKiGILIIAN 3034471836 T-OIA P.03/15 F-154 CASH MINE /GOLD HILL MILL COLD HILL, COLORADO TAILINGS IMPOUNAMENT EXPANSION TECHNICAL IZEV1S10N A. EMBANKMENT STABU.ITY AND LINER FOUNDATION The original design for the expansion of the tatlmgs impoundment at the Gold Hill Mill includes a 13-foot mtse of the embattkmen[, based on a modified upstream construction technique. The footprint of the embatdmtent rarse extettds approxunauly 40 feet out from the upstream face of the ongina] embankrttent onto the staface of the existing tarltngs. Approximately 20 to 25 feet of the extsring tttiltngs remnm exposed beyond the upstream ice of the embartlmtrnt terse. A biaxial geognd was installed on the surface of the existing tatlmgs, within the footprint of the cmbarikmrnt rarse, prior to construction of the new embatilanent, to facilitate placcmrnt of the mtnal fill layers. The geogrid was not specifically designed as a rcinforcmg member in the embatrkmrn[ mtse, al[hough some reinforcement benefit is realized. The stabtliry of the raised embattlement was analyzed as part of the desrgn for the new facility. The rotational failure potential of the upstream and downstream slopes of the factltry was evaluated at venous stages of filling of the impoundment. The soil parameters used in the computer model analyses were based, in pan, on results of laboratory analyses of representative soil and tailings samples collected from the sere and, in part, on conservative estimates from published hteranirc and previous experience with similar matrnals. All of the stabthry analyses mdtcated factor of safety greats than the minimum acceptable values. The indicated govettung cotidltton was determrncd to be the post~onstructton condition, with saturated tatlmgs as the foundation for the embankment raise and no bunressing by newly placed tatlmgs. Mtnunum calculated factors of safety were 13 for static condtttons and 1.14 for pseudo static (sersmic) conditions. Throughout the analyses, it was assumed that the existing tailings would be lefr tit place. It was subsequently proposed that a portion of the exposed saturated tailings upstream of the embankment raise be removed to unprove foundation conditions below the new liner system for the expanded impoundmrnt. Observation of the embanlo=tent rarse following completion of construction has revealed no evidence of distress that would indicate potential instability of the foundation. However, initiation of mechanical removal of the exposed tarlmgs and established vegetation in two azeas of the existing unpoundmrnt gave rise to concerns the[ such removal may compromise the stability of the embankment through removal of the existing materials that aze providing passive resistance against movemrnt of the foundation tailings under the embankment raise. Analysis of possible block failure mechanisms to the embattktnen[ and foundation tailings was condttcted using the soil property and soiUlincr interface fricaon value employed in nc~ original stabiltry analyses. This analysts of possible block failure mechanisms indicated potrntiel instability, turn with the existing tailings left in place. The factors of safety calculated through the analysis were m the range between 0.85 and 0.91. 7~~5461~rECH-RQVi WPD 1 Ocrobcr 9, 1998