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III III III III IIII III J ~ STATL OP COIOHADO ult't~nnn n. 14MM. r•~rv ~~~~~^• <br />DEPARTMENT OF NATURAL RESOURCES <br />D. Monte Pascoe, E~eculive Director <br />A'IIN~;ll LANll fiE(;LANIA'~'lON <br />_.r~ 423 Cenlen nial Building, 1313 Sherman Street <br />"~~~~ Denver, Coloratlo 80203 Tel. (303) 839-3567 <br />David C. Shelton <br />Director <br />M E M O R A N D U M <br />28 July 1981 <br />T0: Jim McArdle <br />FROM.': Jim Pendleton <br />RE: Observations Si Visitation to Hendersen Project, AMAX Corporation <br />Mine Site: <br />_ The stope collapse pit appears to be contrvled by preexisting I:ieavage, <br />fracture or other plane of weakness. The upslope high wall clearly exposes <br />a neatly cleaned contact surface which is near vertical from bottom to about <br />75 feet below the scarp top and then rolls to about 75 degrees from horizontal <br />at t:~e top of the scarp. The downslope wall of the collapse pit ap;~ears <br />controled by a high-angle fracture inclined at about 85 degrees from hcrizor.tal, <br />dipping into the center of the pit. The side wal'_s do not appear :leanly <br />con[roled by any one preexisting plane of weakness. <br />I assume that the collapse pi*_ will continue to enlarge along she axis <br />of the ore body as extraction progresses. It may also widen upslove and <br />downslope somewhat. The clean definition on existing shear surfaces may <br />tend to limit enlargement upslope and downslope. I observed no active rockfall <br />or collapse occurring during the five minutes which I spent in clos~=_ proximity <br />to the collapse chimney. Only time will tell. <br />In general, the rock cuts at protals and other surface facilities seem <br />very stable throughout the mine and mill sues. The development waste pile <br />face does not appear particularly stable, however. The dumping of topsoil <br />down slopes at the angle of repose seems haphazard, at best. I believe that <br />AM1AX should be required to develop some workable plan for topsoiling and <br />revegetating the faces of the waste pile areas. <br />Mill Site: <br />The only problem which I noticed during our visit to the mill site <br />is the lack of a definite plan involving the face of the tailings embankment. <br />Bench top surfaces grade in different directions with no continuous drainage <br />allowance. Topsoil is dumped down slope on benches with little control resulting <br />in extreme variation in coverage. Some bare spots exist. Thickness ranges from <br />0 to 18 inches of topsoil. In places a berm is left at the crest of the bench. <br />These berms appear to be the locus of tensional settlement cracking end gullying <br />of the topsoil. AMAX should be required to formulate a detailed plan far the <br />face of the tailings embankment, including final grading, topsoiling, provision <br />of pezmaner.t armoured dewatering facilities, and revegetating plane:. Information <br />collected from piezometers existing and to be installed, as well ae; survey data <br />to verify the performance and stability of the embankment £ace, should be <br />submitted to the Division as a portion of annual reports. <br />mac: Joe Zalkind <br />