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'!i3:e Savaoe -2- Januzzy 14, 2982 <br />The vlti.-ate reason for detern!n_rc the overall overburde,~ si.•eIl factor <br />is to project the volu.:.e o° bacl:filled spoil. f:no::ing this volume <br />relative to pre-c'ninc pit volume is necessary to assure proper con- <br />ficurztion of the final bact:`i1_'ed surface. sur_`ace gradie.^.ts and land <br />fora r.,a_ have to be adjusted ir. order to acco-.-odate increases or <br />decreases in spoil c~olume. During this desicn exercise, care rust be <br />to?:en to avoid ~~~ cr_ation c° potentially structurally- or erosionally- <br />urstable Land surfaces. A s:•steaatic error in os~erSurden si.e1_' pro- <br />jection could result in 2000mlll2ted errors during pit life. Under <br />es tircation of the sc.•e1i factor, in the worst case, could result in an <br />incompletely bac}:filed final cut. O•:er-es tim~ation of t'^.e swell factor <br />could result in surplus baci:filI and elevation o_` the reclaimed surface, <br />particularly durino later secments of the pit development. <br />In defense o° their o-noosed technicue for overburden s::el1 factor <br />determ_ration, Utah I::ternat'_o:al cites pzst pit performance as verifi- <br />cation of their projected o':erburde.a s:e11 factor. They theorize that, <br />if they had under-es ti rated the overburden swell fac*_or, their operational <br />pit e~.•ould systematically constrict and t.^.ey n~ocid begin ex~~riencing <br />di`f_cclty :r. exposi;q coal for extraction. Converselg, if they ove_- <br />est:-aced the overburden s::el1 factor, the bac.;filled surface e,•ould be <br />lo::er than projected. <br />The Division recocnizes that the determination of precise overburden <br />s~:ell :actors represents a d_fficult site-soeci_`ic task. The purpose <br />for deterr..ination of that sr: ell factor is to facilitate the best passible <br />design and plannine of bacY.filled and reclaimed post-.:.fining surfaces, <br />in order to avoid the manifestation of potentially detrimental erosional <br />and slope instability phenomena, In Light of the as yet unverified <br />nature of the applicant's proposed method of the overburden swell :actor <br />determination, the Division considers it appropriate to require the <br />applicant to verify the actual post-mi-ing topography achieved at the <br />Trappar .:fine. Further, the Division also considers it appropriate to <br />recuire the applicant to utilize pre-mri no and post-r.~ning tooocraphic <br />survey data to calculate the actual overburden swell factor encountered <br />on an annual basis. <br />/mt <br />cc: Fred Banta <br />David Shelton <br />Jerry Zimpfer <br />Dave Craig <br />