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GEOTECffiYICAL STABILITY E~~IT <br />There aze no buildings or any structures outside the permit area which could be affected by the <br />operation. The final slopes of the pit lake reclamation, as shown on Maps E-2 and E-3, show that <br />2H:1 V will be the maximum mining slope which will be later reduced to 3.OH:1.OV for reclamation. <br />The lake is fully incised into the existing terrain; no dam is located on the site. The existing gravel <br />material has few fines and the material does not get its strength from cohesion, but internal angle of <br />friction. Partly rounded gravel of this type in place has an angle of repose of approximately 40 <br />degrees. For non-cohesive soils, this is also the internal angle of friction of the material. The overall <br />slope of 2.OH:I.OV is an angle of 26.6 degrees, therefore, the factor of safety is 26.6/18.4 or 1.44. <br />This factor of safety is more than adequate to ensure long term stability of the slope, even though the <br />slope at this steepness is temporary. <br />For the backfilling of fine overburden to attain a slope of 3.OH:1.OV, this material will be stable at <br />such a mild slope only when it's compaction is assured during placement. Even if the overburden <br />backfill was not compacted properly, when the water was allowed to fill the lake, it could slump but <br />never in any way could it affect the stability of the in place gravel at a 2H:1 V slope, therefore, no <br />damage to anything off site could result. However, to further ensure that the backfilled overburden <br />does not slump when the lake is filled, it is specified that this overburden be placed in lifts no greater <br />than 1.0 foot and that compaction of the fill should not be less than 85% of the maximum dry density <br />of the material. United will test the overburden prior to backfilling to determine what is the <br />proctor/density curve of the material. <br />I, Greg Lewicki, P.E., with over 20 years of experience in mine slope safety analysis in Colorado, <br />state that this is true and that this factor of safety is adequate to prevent failure of the lake slopes. <br />~.~P'PpPY C Fs,~q b~Zc~e <br />:' 2033b ~ 5 lD~OY <br />. •. <br />.o; .u, <br />9 .~ .• k. <br />G~ <br />O~F~%U1VA~EN \ <br />North Fork Pit Mazch 04 56 <br />