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5~~. V -1 K - <br />West Elk Coal Company, Ine. <br />?ost Ctlice do: 591 <br />Somerset. Colorado 81434 <br />Telephone 303 929 5015 <br />August 30, 1983 <br />Mr. James Pendleton <br />Mined Land Reclamation Div. <br />423 Centennial Building <br />1313 Sherman Street <br />Denver, Colorado 80203 <br />Re: C-007-80 <br />Dear Mr. Pendleton: <br /> <br />_. - . 198 <br />r_H4 tH, SAFE(Y & E-vat,.~~... <br />This letter and attachments should serve to resolve <br />the technical concerns which you feel were inade- <br />quately addressed in our 5/05/83 correspondence. <br />Four attachments are included which show the proba- <br />ble formation mechanism and actual cross sections of <br />the roof fractures encountered in our mine. <br />Your primary concern appears to be in relation to a <br />hypothetical mechanism relating to the formation of <br />the roof fractures. Ihave attached Figure 1 showing <br />the sequence of events which will result in the for- <br />mation of a di3genetic fracture which i9 now manifested <br />in the form of a roof crack. The mechanics relating to <br />the cracks are discussed below. <br />Initially, the coal and overlying shale are assumed to <br />have been in place. The degree of compaction and lithi- <br />fication of the coal and shale is indeterminant at this <br />stage; however, it can safely be stated that neither <br />unit is fully compacted. <br />Secondly, a channel sandstone is introduced. The shale <br />is completely scoured in the central portion producing <br />a coal/sandstone contact, and in some areas the coal <br />thickness is also reduced. It should be noted that the <br />convex nature of the bottom of this unit is of signi- <br />ficance. A lenticular thickening without channeling <br />could produce similar results. <br />Third, is a phase of continued deposition, compactional <br />deformation and lithification. The channel has over- <br />loaded and begins rapid deposition. The bottom of the <br />sand, although not necessarily totally cemented has <br />developed sufficient rigidity to fail in tension. As <br />the thickness of the overall sand body increases rapidly, <br />the gravity compactional forces deform the underlying <br />strata resulting in a change in the volume of the de- <br />positional basin. The bottom of the sandstone develops <br />extension-flexure fractures which are strongest through <br />. the sandstone/coal contact area. The probability of <br />fracture development decreases as one moves into areas <br />with thicker shale sequences. <br />AMC.?d206 <br />