Laserfiche WebLink
APPENDIX 2.05.4-1 <br />• MINE BACKFILL STABILITY <br />SUN COAL COMPANY - MEADOWS N0. 1 <br />ROUTT COUNTY, COLORADO <br />Original grading plans have called for a final mine backfill slope not to <br />exceed 3:1. However, in order to make it possible to provide relatively flat <br />areas (requested by J. Eilts, 1/83) on the reclaimed surface, it is proposed to <br />locally steepen backfill slopes to 2.5:1. Intermediate slope transitions <br />between the locally steepened 2.5:1 sections will be at about 4:1 to 5:1 so that <br />an overall slope of approximately 3:1 is maintained and also so that no impound- <br />ment of runoff will occur on the backfill surface. The purpose of the following <br />analysis is to document the geotechnical stability of the proposed 2.5:1 slope <br />sections. <br />Previous geotechnical studies have established that the mine backfill will <br />• be dry and essentially free from development of an internal phreatic surface <br />(Appendix 2.06.5-1). These same studies have estimated a likely mine backfill <br />strength ranging from 27° to 35° friction angle and 500 psf to 1500 psf cohe- <br />sion. In the following analysis, any possible cohesion component of backfill <br />strength has been neglected in the interest of conservatism. For dry, cohesion- <br />less slopes, the development of deep-seated failure surfaces is unlikely; and <br />the most probable mode of failure consists of development of shallow failure <br />masses more or less parallel to the slope, as illustrated on Figure 3.3-1 of <br />Appendix 2.06.5-1. For these conditions, the stability of the slope can be ade- <br />quately modeled by evaluating so-called "infinite slope" failure conditions. <br />Factors of safety computed from well-known relationships are tabulated as <br />follows:(1) <br />• 111U.S. Army Corps of Engineers, 1970, "Engineering and Design, Stability of <br />Earth and Rock-Fill Dams," EM-1110-2-1902. <br />- 345 - (Rev. 5/86) <br />