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1.0 Introduction <br />This report documents an evaluation of rockfall hazards associated with undermining the <br />Twentymile sandstone cliff at Cyprus-Amax's Foidel Creek mine near Oak Creek, <br />Colorado. Longwall workings from the Southwest mining district will extract up to 8.5 feet <br />of coal from beneath portions of the cliff potentially causing failure of the cliff, release of <br />boulders from the failure zone(s), and hazards to traffic on County Road #27 whose <br />alignment is located from 300 to 1200 feet horizontally from the base of the cliff. <br />1.1 Background <br />The layout of longwall panels that will affect the stability of the Twentymile sandstone cliff <br />is shown in Drawing 1. Predicted subsidence limits indicate that the cliff will be affected <br />• by mine workings below Zones 1 to 8 and 13 to 14. Maximum strains and displacements <br />will be experienced in Zones 1 to 5, Zone 8, and Zones 13 to 14. <br />An analysis of Twentymile sandstone cliff stability has previously been presented in a <br />report by SAIC (1992) along with an assessment of hazards to County Road 27. The <br />potentially affected area was divided into 6 "hazard zones" in the SAIC study. Several <br />summary statements have been excerpted from the SAIC report to promote an <br />understanding of the overall cliff stability and rockfall hazard issues: <br />Large slides that have occurred moved slowly and have not been damaging <br />to man-made facilities <br />Toppling failures may occur at the western end of the cliff <br />• Failure in SAIC Zones 5 & 6 (Zones 3 through 9 in this study) is controlled <br />by joint orientation and continuity. The cliff is currently stable as joints are <br />discontinuous (i.e., they do not intersect to form individual blocks). <br />